Design For Manufacture and Assembly

Howard Gibson C.E.T., hgibson@eol.ca

2018/07/02

Contents

1 Introduction 1

2 General DFMA Principles 1

3 Design Methodology 2

3.1 Part Count . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3.2 Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3.2.1 Fasteners . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.2.2 Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2.3 Poka Yoke . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.3 Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.3.1 Machining . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.3.2 Sheet Metal . . . . . . . . . . . . . . . . . . . . . . . . 13

3.3.3 Welding . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.3.4 Casting . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.4 Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.5 Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.6 Subcontracting . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.7 Infrastructure . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.7.1 Design Change Rules . . . . . . . . . . . . . . . . . . . 19

3.8 Communication . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.9 Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.9.1 Concurrent Design . . . . . . . . . . . . . . . . . . . . 21

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4 Design Skills and Tools 214.1 Drafting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.2 Bills Of Material (BOMs) . . . . . . . . . . . . . . . . . . . . 244.3 3D CAD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

5 Notes 265.1 Design of Covers . . . . . . . . . . . . . . . . . . . . . . . . . 275.2 Style . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.3 More on Subcontracting . . . . . . . . . . . . . . . . . . . . . 285.4 Office Politics . . . . . . . . . . . . . . . . . . . . . . . . . . . 295.5 Lean Manufacturing . . . . . . . . . . . . . . . . . . . . . . . 29

5.5.1 The Seven Wastes . . . . . . . . . . . . . . . . . . . . . 295.5.2 The Five Questions for Continuous Improvement . . . 30

List of Figures

1 Two Handed Assembly . . . . . . . . . . . . . . . . . . . . . . 42 Ratchet Drive Kit . . . . . . . . . . . . . . . . . . . . . . . . . 63 Please don’t . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Box Assembly, No Cables . . . . . . . . . . . . . . . . . . . . 75 Box With Connectors . . . . . . . . . . . . . . . . . . . . . . . 86 Poka Yoke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Part In Lathe . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 Machined Assemblies . . . . . . . . . . . . . . . . . . . . . . . 129 Sheet Metal Box . . . . . . . . . . . . . . . . . . . . . . . . . 1310 Who Has the Biggest Influence? . . . . . . . . . . . . . . . . . 2011 Bad Drafting . . . . . . . . . . . . . . . . . . . . . . . . . . . 2212 Hex Socket Head Cap Screw . . . . . . . . . . . . . . . . . . . 25

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1

1 Introduction

Designing something that can be assembled and used, may be enough for one-off shop equipment and proof-of-principle. In a cost competitive productionenvironment, it is not enough. We need product that that works well, thatis reliable, and that is easy and cheap to manufacture. This requires signif-icant effort by design and engineering, and an understanding of engineeringprinciples, such as Design For Manufacture and Assembly (DFMA).

2 General DFMA Principles

There are a number of websites and articles that summarize rules for DesignFor Manufacture and Assembly.[1]

1. Simplify the design and reduce the number of parts.

2. Standardize and use common parts and materials.

3. Design for ease of fabrication.1

4. Design within process capabilities and avoid unneeded surface finishrequirements.

5. Mistake-proof product design and assembly (poka-yoke)

6. Design for parts orientation and handling.2

7. Minimize flexible parts and interconnections.

8. Design for ease of assembly.

9. Design for efficient joining and fastening.

10. Design modular products.

11. Design for automated production. 3

12. Design printed circuit boards for assembly.1 The reference provides a long list of tips. Most of Product Design for Manufacture

and Assembly [2] is about fabrication processes.2 Again, the reference provides lots of suggestions.3 Product Design for Manufacture and Assembly [2] has a chapter entitled “Design for

High-Speed Automatic Assembly and Robot Assembly”.

2 3 DESIGN METHODOLOGY

3 Design Methodology

Congratulations, you are the mechanical designer for your company’s newHandy Dandy Super Duper Nutating Widget! The widget you are to design,requires custom parts to be fabricated. It requires catalogue parts and ma-terials to be ordered. All parts must be stored in inventory until they areneeded. The inventory must be searchable, and tracked by accounting. Theparts must be kitted. The widget must be assembled and tested. It must bemoved safely around the plant.4 It must be packaged and shipped. The wid-get must get sold. The widget must be useable by the customer. The widgetmay require service, possibly by the customer. The widget must eventuallybe disposed of.[3] You must communicate and work with co-workers fromother technical specialties, managers, and manufacturing. You are part ofa team. Mechanical design must account for all of this. All of your designdecisions must be made in context of a design strategy.

DFMA requires a top-down design process, in which the designer looks fora solution that meets all the requirements. The design review team shouldinclude the other designers on the project, and manufacturing, and sales, andany other stakeholders. You should model and document several designs, tothe point that you can evaluate performance and cost. Your very first ideaought to work, but it probably is not the best idea you can come up with.

3.1 Part Count

DFMA manuals tell us to reduce the number of parts. Fewer parts meansfewer assembly steps, fewer fasteners, and reduced inventory.

A typical widget contains parts that accomplish its basic functionality,and then a bunch of other pieces that connect everything together, and pro-vide protection, and possibly handling and/or mounting to other equipment.

There are a number of causes of excessive parts. . .

• Designers start from a base plate or some other existing structure, andthey design a mount bracket for each and every part. You should betrying very hard to mount everything directly to some primary base orchassis.

4 MIL-STD-1472G[4] states that a male or female can lift a 16.8kg (37lb) object fromthe floor and place it on a surface not more than 152cm (5ft) from the floor. This is usingtwo hands, with back straight, lifting with the legs.

3.2 Assembly 3

• Designers are pressured to re-use existing parts and designs. This cansave tooling and inventory costs. It can also result in kludges. All sortsof cost is generated by the design and fabrication of brackets to connectcomponents to holes that were not intended for those components.

• Parts are designed in sequence instead of all-together. A part designedearly on is finalized, and fabricated, and cannot be modified to solvethe DFMA and other design issues that come up. As noted below ,concurrent design is good practice.

Each mechanical part should be made to do as many things as possible.Any part called “bracket” probably can be eliminated from your design.

3.2 Assembly

Good DFMA practice is for all parts to be mounted on a stable base. Thestable base can be your widget’s primary structure. It can be your primarystructure mounted to an assembly fixture.5 The assembler now has two handsto handle parts and tools.

It is very desirable that there be no need to flip the stable base over. Itis nice if the base does not have to be rotated on the work bench. Partsattached to the structure should be retained by gravity or by configuration.The assembler now has both hands to manage fasteners and tools. If thepart is retained by snaps, then there is no need for fasteners, which is evenbetter!

Failing all that, installation of a part should require no more than twohands. If your assembly is a stable base as noted above, you use one handto hold the part, while the other holds a fastener and driver. The part mustbe light enough to be lifted with one hand. There must be some satisfactoryhand-hold on the part. If fasteners are oriented vertically, they are retained intheir holes. Hex socket, Torx, and Robertson6 screws can hang horizontallyoff their drivers, as shown on Figure 1 , allowing one handed installation ofthe fasteners. Long fasteners will be retained horizontally in deep holes. Ifa part is a two-handed lift, it could still be retained one-handed by a pin orsome integral feature.

5 Note how the assembly fixture is not designed by manufacturing. It is an integralpart of your mechanical design strategy. You may even need features in your widget tosupport fixturing!

6 A Robertson drive has a square bit. This was invented in and is popular in Canada.

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Figure 1: Two Handed Assembly

Okay, you failed again. Your part requires two or more people to beinstalled. Make sure your two or more people can get at the part at assemblytime. Make sure there are hand-holds.7 If your part has lifting points, it canbe lifted and manoeuvred with a winch or crane. Locating pins or otherfixturing features make it easier and faster to line up and install fasteners.Did we mention the advantages of snap retention?8

Some products are inherently big and heavy. Lifting and moving aresignificant parts of the manufacturing process. As the designer, you need tounderstand these as much as you need to understand fabrication.

Figure 1 shows a device fairly conveniently assembled with screws, usingtwo hands.

• The base part either is clamped to a work bench, or it is heavy enoughto stay in place while other parts are installed.

• The hex socket head cap screw hangs horizontally on the key. A Robert-son or Torx screw would work too. A slotted or Phillips screw wouldbe very much less easy to manage.

• It would be nice if the installed part had some sort of retention feature

7 This is not just a cost issue. Lifting clumsy, heavy parts is a good way to damagecomponents, and get people injured.

8 Do not overestimate the advantages of snap retainers. As the designer, you mustdo some structural analysis to ensure they work. Snap retainers can be difficult to dis-assemble. Screws and bolts are way stronger than snap retainers.

3.2 Assembly 5

like a pin9 or flange, but not absolutely necessary. The assembler hasa free hand to hold the part in place.

• This part is small enough to not need hand holds, however, an indenton either side would make it more graspable, and would probably haveno effect on fabrication cost.

If the installed part is too heavy for easy one-handed handling, thenthere must be retaining features. The assembler needs one or both hands tomanage fasteners and tools.

The assembly in Figure 1 works fine with the manual driver. If theworker were using a power driver, both hands would be required, and theinstalled part would have to retain itself in place.

3.2.1 Fasteners

Try to standardize on a small number of fasteners and other small, standardhardware. A worker with thirty two different fasteners kitted, must selectthe correct one for each attachment. They take more time and/or theymake more mistakes. Reducing the quantity of fastener types, simplifieswarehousing and kitting.

If power tools are being used to install fasteners, we10 do not want tochange the bit or reset the torque.

Each and every hex socket cap screw requires a different sized key.11 Thismust be searched for. The set of hex keys must be kept complete. If youspecify Phillips, Torx, or Robertson sockets, the assembler will spend lesstime searching for tools, or using the wrong tool. The ratchet drive kitin Figure 2 belongs to the author. There ought to be more slotted andRobertson drives in it. Otherwise, we see four Phillips drives and sevenTorx drives, one of which is on the extender. There is a total of seventeenhex drives, in both English and metric sizes.

9 Dowel pins must of course, be must be documented, ordered, purchased, stored, kittedand installed at fabrication time, at extra cost.

10 Manufacturing is not “them”. You and manufacturing are part of a team.11 Hex socket flat and button head cap screws smaller than M5 or #10 have tiny sockets

that strip easily. Stripped flat head screws in particular are difficult to remove and replace.Do not specify them.

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Figure 2: Ratchet Drive Kit

Figure 3: Please don’t

3.2 Assembly 7

Figure 4: Box Assembly, No Cables

3.2.2 Cables

Cables are the responsibility of electronics. They connect intimately withthe mechanics, so mechanical and electrical designers must work as a team.

Cabling strategy number one is to not have cables. DFMA manuals rec-ommend minimizing flexible objects. Get all your circuits onto one printedcircuit board (PCB). There are PCB mounted connectors that mount throughpanels, thus eliminating the need for additional fasteners, and cables connect-ing between the PCB and panel. If you have a second PCB, mount it to thefirst PCB directly through board-to-board connectors.

In Figure 4 the big PCB sits on the base, and is attached to the frontpanel through its connectors. Retaining nuts are not absolutely necessary.The second, smaller PCB is mounted to the big one using a board-to-boardconnector. Its LEDs extend through the cover. The cover can be designedto snap in place. It can also help retain the top PCB. There is no need forthreaded fasteners on this box.

Cables are expensive to build and test. They must be routed throughyour system, and probably tied down somehow. Cable ties, like fasteners,must be documented, ordered, purchased, stored, kitted and installed.

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Cable assembly

Figure 5: Box With Connectors

Connectors should be inserted through the inside face of a panel, as shownto the right on Figure 5 . The two connectors and their wires are a sub-assembly that can be built and tested outside the box. Manufacturing hasthe option of subcontracting the cable assembly to an outside vendor whospecializes in this. The cable is built in parallel with the chassis, so it is notpart of your assembly schedule critical path. Your service people have theoption of replacing the cable, rather than repairing the wires in situ.

If the connector is installed through the front, as shown on the left inFigure 5 , the wiring will have to be done in place. This is less flexible formanufacturing.

Unfortunately, front mounted connectors look better. If looks matter,you may have to live with the inconvenience of a front mounted connector.Sometimes, you can treat the panel as part of the cable!

Consider designating an ugly panel in your otherwise stylish product. In-stall your ugly external components on the ugly panel. Methodically followall the DFMA rules on the ugly panel, including the rear mounting of con-nectors. If your rear, ugly panel connector is small enough to fit through thefront hole and the front connector threaded ring, you have a removable cableassembly, with all the advantages noted above.

3.3 Fabrication 9

A. B.

Figure 6: Poka Yoke

A handy point is not shown on Figure 5 . Connectors with screw-onnuts, as shown, usually are designed to be inserted into “D” shaped holes.This constrains the connector from rotating. All the assembler needs to dois wrench the nut. This is good DFMA practice.

3.2.3 Poka Yoke

Design your parts so that there is obviously only one way to install them.This prevents mistakes. It minimizes time examining drawings, and phonecalls to the design office.

• Make things obviously asymmetric as in Figure 6 A.

• Add orienting features. This can be a flat on a diameter as in Fig-ure 6 B. On a pair of castings, it probably costs nothing to add orientingarrows.

• If you have a rectangular hole pattern or a pitch circle, move one holeout of the pattern, also shown in Figure 6 B.

3.3 Fabrication

Note how we design for assembly, before we design for fabrication. When youare building your widget, obviously, you do fabrication first. When you aredesigning, you solve all the requirements, you work out the assembly process,then you design the parts and work out fabrication.

Any parts you design must be fabricated. In theory, you prepare a draw-ing of the part, and manufacturing figures out how to make it. In reality,

10 3 DESIGN METHODOLOGY

the designer selects a fabrication process,12 uses its advantages, and copeswith its limitations. As a designer, you must understand fabrication pro-cesses. These are described in quite a bit of detail in Product Design forManufacture and Assembly [2].

The following list is nowhere near complete. It provides a general un-derstanding, and it shows off the strategic thinking you must do to selectprocesses.

3.3.1 Machining

Tips on Designing Cost Effective Machined Parts, by Joe Osborn[5] is anexcellent reference on dealing with machine shops.

Machining ranges from accurate to extremely accurate. Machining wastesmaterial, especially with large, cut-from-billet parts. Part by part, machiningis expensive. There is minimal tooling required to do it, so machining is aneconomical way to produce single parts. If accurate features are required,consider designing your widget around one accurate part. All the otherpieces in your design can be fabricated from cheaper processes.

Generally, if you use any process other than machining, you need to workaround loose tolerances. In any sort of production, this is worthwhile.

Corners of machined pockets must be rounded. A larger corner radiusallows for a larger, faster cutter. Through holes should be rounded too,although there are ways to fabricate sharp, through corners. Study yourpart and count the number of setups on the mill or lathe. Each setup isexpensive. Orient your machining radii to not force extra setups. Almostall machined parts today are fabricated with CNC, so tool changes are doneautomatically, quickly and cheaply. Machining easily produces orthogonalparts. Non-orthogonal angles require tricky setups, and/or very careful de-sign and documentation.

The material cost and machining time of a housing machined from billetis a function of the cube of its size. Small housings can be very cheap. Largehousings probably should be fabricated from pieces, from sheet metal, orcast.

Beware! Machining allows bad drawings, since the tolerances requiredto make your part work are well within the process’ capability, even if the

12 In over thirty years, the author cannot recall not knowing how a significant partwould be fabricated.

3.3 Fabrication 11

Cutting

Tools

Inaccessible

Face

Jaws of Chuck

Material

Final machined part

Figure 7: Part In Lathe

drafter did not call them up. If you have been machining all your parts andyou have started using other processes, your drafting may have to improve.13

In Figure 7 , the part shown can be fabricated in one set-up on a lathe.The front and side features can be fabricated by the cutting tools, shown. Thepart can be cut off by the tool shown. Any other features on the inaccessibleface require the part to be re-chucked in an extra process, at extra cost.Material is often fed through the rear of a lathe chuck. The part can befabricated and cut off. The material is then automatically fed, and the nextpart fabricated. According to Joe Osborn[5], round parts often are CNCmachined by vertical mills. The rear face still is not accessible without theextra set-up. When you design round parts, try to ensure that all yourfeatures can be machined from one side.

The assemblies in Figure 8 show a number of design issues. Both as-semblies hold the same printed circuit board, contained in a machined box.

13 If your machinists figure out how your parts assemble, they may ignore your tolerancesand make your parts assemble. On a one-off assembly, this is good. When your designmoves to production, the parts may be split up between shops. Now your as-built designdoes not work!

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Figure 8: Machined Assemblies

3.3 Fabrication 13

Figure 9: Sheet Metal Box

The box on the left requires at least four set-ups on a vertical mill. Thefirst set-up machines out the insides. The second and third set-ups do theholes on the sides. The fourth set-up drills and countersinks holes on thebottom. The box on the right requires one set-up only. The tapped holesand access to the connectors all are machined from the top.14 On the right,the pan head screws through the cover, do not require countersinks. Thecovers are flat, and they can be fabricated more cheaply by punching fromsheet metal. The countersinks on the left would be a rapid, automatic toolchange on a CNC mill, but they are an extra process in a sheet metal shop.The bottom screws and the hexagonal standoffs on the left hand view areparts that must be must be documented, ordered, purchased, stored, kit-ted and installed. The extra features on the right are done as part of theCNC machining process, so the cost is minimal.

3.3.2 Sheet Metal

Thin, flat panels are cut using punches, water-jets or lasers. Often, theflat panel is bent into some 3D shape as with Figure 9 . Holes with sharpcorners are easily punched. Pockets must be machined, in an extra process, atextra cost. Countersunk or centre bored holes are an extra process, at extra

14 If a batch of these boxes are to be machined, the machinist may start from a largethick piece of material, and CNC machine nine or twelve pieces on a thick base. Havingdone this, they flip the material over and fly-cut the bottom material, releasing everything.With the assembly on the right, they completely fabricate nine or twelve pieces in twoset-ups.

14 3 DESIGN METHODOLOGY

cost. The panel can be cut out accurately. Sheet metal bending is muchless accurate than punching or machining. You must talk to your fabricatorand ensure you understand their tolerances. If the bends are not specifiedand fabricated properly, they crack.15 Make sure you specify proper bendradii. Make sure the fabricator follows your drawings. Sheet metal partscan be assembled in the shop by riveting or welding to create more elaborate3D shapes.

I have the following tolerances from a precision sheet metal shop.[12]

Operation TolerancePunching repeatable to .004”.

Punch feature to punch feature .005”Formed bends 1/2◦

Hole to edge ±.010”“Standard” sheet metal tolerances ±.060”

Punched holes ±.003”Hole to bend ±.015”

Bend to bend ±.020”

Sheet metal allows the fabrication of large, hollow, covered structures,with minimal waste of material. A carefully designed sheet metal box canbe extremely strong and rigid. Attachment points should be located close togussets and walls.

You can specify accurate hole patterns, but you cannot locate them ac-curately from bends. Read up on composite positional tolerances in yourGD&T text.[7] Make sure the composite tolerance works in your design. Itis possible to machine an accurate hole pattern, after bending, as an extraprocess, at extra cost.

If you are making lots of components, investigate the packing of yourflat punch-outs on the metal sheet. If you design your components to packtightly, you reduce your scrap rate. 16

15 For any given material, there is a recommended minimum bend radius. You need tolook this up. Optimal materials for machining often are quite different from optical mate-rial for sheet metal fabrication. For example, the usual grade of aluminium for machining is6061-T6. For a 14 gauge panel (.064”), the bend radius is .096”. For aluminium 5052-H32,the radius is .064.[6] Aluminium 5052-H32 is popular with sheet metal fabricators.

16 3D CAD software like SolidWorks, allows you to design bent sheet metal parts, andto flatten them out to see the as-punched pattern. Take advantage of this.

3.3 Fabrication 15

3.3.3 Welding

You can weld sheet metal. You can burn, water-jet or laser cut metal platesand weld them. You can weld structural sections like tubes, angles, I beamsand wide flange beams into a space frame.

Welding is not an accurate process. You need to talk to your welder andensure you specify fabricateable tolerances on your drawings. If you needaccurate tolerances, you need to machine your weldment, which is an extraprocess, at extra cost.

Make sure the welding equipment fits into the space where you want yourweld.

Many commercial metals either are heat treated or work hardened. Whenyou weld them, they become annealed. Stress relief and heat treating each arean extra process, at extra cost. These processes are not necessarily feasible.If the material is tricky to weld, it must be fabricated by a better trained,more expensive worker. Know your metallurgy.

If your part is a safety critical structure, you may have to worry aboutworkmanship, quality standards, and leaving a paper trail.

Welding allows all sorts of elaborate non-orthogonal structures at lowcost. If these are carefully designed, they can be extremely strong and rigid.

3.3.4 Casting

Casting requires expensive tooling. This must be amortized over your pro-duction run. Design changes require modifications to tooling, or new tooling.Casting patterns can be produced by rapid prototyping, making one-off pro-totypes feasible.

Compared to permanent moulding and die-casting, sand and investmentcastings use cheaper tools, making them suitable for shorter production runs.The latter two processes can cast high melting temperature materials such assteel. Casting dies usually are made of steel, restricting the cast material tolow temperature metals such as aluminium and zinc, plastic, or investmentcasting wax. Try to make the dies two-piece, and make sure your part canbe ejected from them.

Once you have paid for tooling, castings are cheap. Castings can be verycomplex with little impact on cost, at least once the tooling is paid for. Acast or moulded housing can be styled with all sorts of weird, cool lookingnon-orthogonal shapes.

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Castings can be machined to provide accurate features and small holes,but this is an extra process, at extra cost. Try to make your design workwith the as-cast features. For example, probably, you cannot cast clearanceholes for M4 screws. Maybe you can cast holes for M8 screws!

Castings and mouldings can be made from anything that can be meltedinto a liquid, including thermoplastics. Thermo-setting plastics and fibrereinforced plastics also are fabricated with moulds, and the comments above,apply.

When you do castings, you should do aggressive DFMA, reducing thepart count, and looking for an optimal layout for assembly.

3.4 Service

Many products require service and maintenance. Your widget may requireextensive testing as part of the production process. Service may be doneunder warranty. Either you pay for this directly, or the service is part ofyour customer’s total cost of ownership. This may be done by your ownhighly trained personnel, or it may be done by untrained customers.

• Service components should be located in accessible positions.

• Fasteners, adjusters, and anything that requires observation, should beoriented towards where they will be accessed.

• Probably, covers should be convenient to remove.17 Try to minimizethe number of screws.18 Consider using quick release fasteners.

• There should not be any component other than a cover between yourservice people and the components to be serviced.

• You should be able to turn the system on and operate with covers on oroff, and you should be able to install or remove covers while it works.

• The covers should not affect critical system alignments.

17 Possibly, covers should be difficult for unauthorized people to remove.18 On a sealed enclosure, maintenance people often forget to replace all the screws. At

best, your enclosure does not comply with the NEMA and IP standards you promised. Atworst, you have a hazardous fail condition.

3.5 Documentation 17

3.5 Documentation

At the end of the project, a designer submits documentation. Documentationis not the only skill a designer must master, but it is a link in a chain.Documentation is necessary for manufacturing, and to support continuedengineering. It also is a valuable tool during the design process. You needto inform the rest of the project team what you are doing. If you use theofficial documents for this, you have done the official documents, and youdon’t need to re-do them. Your team must conduct effective design reviews.If assembly drawings, even very preliminary ones, are provided, the assemblyprocess can be reviewed.

With 3D CAD, you can generate fabrication drawings early in the designprocess with all the critical tolerances. You can review the tolerances and dotolerance stacks. These do affect your design.

Manufacturing can cope with bad or non-existent documentation. Youcan telephone back and forth and hold meetings. You can show your fabrica-tor how the assembly works, allowing them to work out functional dimensionsand tolerances. Ultimately, manufacturing can generate its own documenta-tion. You, the designer, will not be in control of it. Is that a good thing?

3.6 Subcontracting

When you subcontract work, you need a well defined work statement. Youhave a purchase order. Your purchase order calls up your drawing or doc-ument. Your purchase order is at least approximately a contract, and yourdocument is a clause in that contract. Prepare your documents accordingly.

Your vendor, however much they like to do the work, needs to makemoney. If you are difficult to work with, they either will refuse your business,or increase their prices. Either way, you lose.

All of the comments above about things that must be documented, or-dered, purchased, stored, kitted and installed, apply whether you do the workin-house, or subcontract.

There is more about subcontracting under the Notes.

3.7 Infrastructure

In anything more than one-off fabrication and assembly, you probably havea department that orders material and parts, that maintains a warehouse,

18 3 DESIGN METHODOLOGY

and that kits parts for whoever is doing the assembly.

Any item moving in and out of a warehouse is a financial transaction.Warehouses are controlled by accounting, not engineering. The people whodo the actual work are low-level clerks with very limited authority. Theyfollow procedures. It can be amazing to see how much stuff they are keepingtrack of. Your design and documentation must work with these procedures.

The following is a limited production scenario designers need to under-stand.

• Parts are stored in the warehouse by part number or stock code.

• When a widget is to be built, manufacturing reads the BOM, andsearches through the warehouse for the parts.

• Any parts not found, must be ordered.

• Anything with a vendor and part number will be ordered from thatvendor.

• Anything with your company part number on it will have a documentkeyed to that part number. The document must be examined for or-dering instructions.

• A fabricated part will be ordered from the appropriate vendor, whichmanufacturing must identify somehow.

• An assembly will have its own BOM, which will be processed.

• A specification control may have ordering instructions on it.

• All the parts will be kitted for assembly.

If you have documented sub-assemblies, manufacturing can assemble theseahead of time and put them back on the warehouse shelves. This is conve-nient when the sub-assembly is used on multiple products. It is convenientif the sub-assembly requires a different specialized worker than the mainassembly.

Mass production processes are very much more elaborate.

3.8 Communication 19

3.7.1 Design Change Rules

Any modification to your widget must be performed and documented me-thodically, if you are not to create chaos in manufacturing. You need tofollow design change rules.

• For any given part or assembly with a drawing and part number, donot change form, fit or function.

• If you must change form, fit or function, you must create a new partnumber.

• The revision number on your drawing should not be contained in yourpart number.19

Anything called up by part number on a BOM must function in whateverassembly the BOM belongs to. Obviously, if you change the length or youdelete holes, the part is no longer functional in existing assemblies. If youadd holes, your new part probably is functional in existing applications. Onyour new assembly that requires the holes, the old parts are not functional.If you have these in stock, these are what will be kitted.

If some of your revisions result in functional changes to the parts, manu-facturing must assume that all revisions are functional changes, and separateall stock by revision number.

3.8 Communication

You are part of a team. Most engineering and design projects are multi-disciplinary. You need to talk to co-workers. You need their co-operation.

You and your co-workers need to share information. You can set upblog sites and wikis. Microsoft Project stores and manages documents. Theauthor likes to set up websites in his project folders.20 It would be nice if thedesign team all sat in the same area, where they could see each other, andget in the habit of talking.

19 ASME Y14.100[10], Section 6.8 provides a more detailed list of changes requiring anew Part Identification Number (PIN).

20 The HTML language is idiot simple. HTML, sufficient for communication with co-workers, can easily be written using a text editor. There is no need for fancy, expensive,hard to learn, web development tools. It helps if someone in-house knows how to writeCascading Style Sheets (CSS).

20 3 DESIGN METHODOLOGY

MATERIAL

LABOUR

OVERHEAD

COST

5%70%

50%20%

15%

5%

5%

30%

DESIGN

INFLUENCE ON COST

Figure 10: Who Has the Biggest Influence?

Manufacturing is part of your design team. They need to sit in on thedesign reviews. You need to understand their requirements.

3.9 Management

Design For Manufacture and Assembly means that engineering does some-thing more than just make the thing work, somehow. Management mustcommit to the engineering time needed to make the thing fabricateable andassembleable.

Figure 10 , Who Has the Biggest Influence, comes from Product Designfor Manufacture and Assembly.[2] Their reference is Munro and Associateshttp://leandesign.com, with no mention of a publication.21 This is genericdata that must vary wildly from company to company and product to prod-uct. Regardless, it shows that the design department’s direct cost is verymuch less than its effect on the overall product cost, especially if there issignificant production involved. Extra time and effort taken by designers toassure efficient manufacturing will reduce cost and improve quality.

Mechanical design time should take some fixed percentage of the totalproduction cost of the widget. It does not matter if you make a hundredthousand assemblies at ten bucks each, or one piece at a million dollars. Thechart in Figure 10 shows 5% of total cost.

A ten dollar assembly may seem simple, but every cent saved will bemultiplied by 100,000. Design and evaluate multiple versions of the widget.For each version, thoroughly study and plan the process. This should notwait for the completion of the design.

21 I redrew the reference’s Fig 1.4 as a bar chart.

21

The mechanical design department must develop design rules, standard-izing hardware, and anticipating requirements that occur over and over againin your industry.

3.9.1 Concurrent Design

If design is done in sequence, then task A is completed. Task B starts sometime later. It is not possible for task A to recognize and solve task B problems,or capitalize on opportunities created by task B.

Concurrent design means that all the design tasks are performed simulta-neously. This is messy, and it requires a lot of communication and teamwork.That communication and teamwork creates all sorts of opportunities to beclever and implement DFMA.

4 Design Skills and Tools

4.1 Drafting

Your drawings are how you communicate with the outside world. When yousubcontract work, you need clear specifications, and you don’t want shopswasting time trying to interpret your stuff. For anything done in-house, youneed to set an example of how work ought to be done, and you still don’twant the shop wasting time trying to interpret your stuff.

Joe Osborn spends over a third of his article Tips on Designing CostEffective Machined Parts [5] on Drawings and Prints. This should not besurprising. In a jobbing shop, the time spent reading your drawings is partof the fabrication cost. Time is wasted when they cannot find informationquickly. Time and material are wasted when they make mistakes. The shopmay be willing to do free re-work for goodwill’s sake, but they will accountfor it when they quote future jobs.

Are you still using 2D CAD? Change your background screen from blackto white. Do it now. We’ll wait! This is the only possible explanation forFigure 11 , an issue noted by Osborn.

Drawings now are sent out electronically. With 3D CAD like SolidWorks,a common process is to send the drawing out as PDF, accompanied by aSTEP file of the 3D model. Full sized prints are nice, but they do not fitthrough email, and you may not have a 36” plotter.

22 4 DESIGN SKILLS AND TOOLS

.500

1.500

1.000

.7501.250

1.000

Figure 11: Bad Drafting

If you are sending out files in PDF format, assume your vendor has a let-ter sized22 black and white printer. Your colour may generate an unreadablehalf tone. Generate PDF files in black and white mode. There are printersout there that do B or A3 size. Maybe your fabricator has one! The authorhas not seen a machine shop with a full sized plotter.

Watch out for sheet and font sizes.

• 3/32” or 2.5mm fonts are readable on A or A4 sized drawings printedletter size.

• 1/8” or 3mm fonts are readable on anything up to C or A2 sized draw-ings printed letter size.

• 5/32” or 4mm fonts are readable on D or A1 sized drawings on aB or A3 size printer. They might be readable on a good letter sizedprinter.

• 5/32” or 4mm fonts are fairly readable on E or A0-sized drawings on aB or A3 sized printers. They are not readable on a letter sized printer.

• 5/32” or 4mm fonts on D, E, A1 or A0 sized drawings are readable ona full-sized plot hanging on a wall two meters (six feet) away. Theselarge drawings do not fit on desks and workbenches.

22 Letter sized either is 8 12 × 11 inches or metric A4 size, 210 × 297mm. In Canada,

people could be using either size paper in their printers.

4.1 Drafting 23

Be careful sending out DXF files. These provide reliable geometry. MostCAD packages use their own special font to implement GD&T symbols.These may or may not work on your fabricator’s CAD. but they are nottransittable by email.

Engineering offices these days seem to use printers that do A size andB size, or A4 size and A3 sized prints. 36” plotters are becoming rare.

If you do not have a full sized plotter, consider creating a A size or A4 sizetitle block, and a B size or A3 size title block, and leave it at that. Optimizeyour fonts for the smaller sized drawings. 2.5mm? Make the titleblock itselfas small as possible to optimize space on the sheet. The larger drawing sizesare useful only if you plot full size. E size is a valuable resource when youare working on a drafting board, you have a blueprint machine, and a wallto hang prints on. Most of us are in cubicles now, with limited acccess towalls.

Your fabrication drawings must be inspectable. You must be able toinspect your part and confirm that either it conforms to the drawing andyour requirements, or it does not. Drawings are uninspectable if. . .

• . . . dimensions are missing.

• . . . tolerances are missing or obviously unachievable. If the shop ismaking a best effort, you have no control over the final part.

• . . . drafting standards are not followed. Standards such as ASME Y14.5[7]provide an unambiguous definition of the drafting terms and symbols.

• . . . there is no apparent way to inspect the part. If a critical featurecannot be accessed for inspection, then you cannot be confident that itconforms.

In The Quest for Imperfection[8] Charles Murray states that Japanese carmanufacturers do not inspect their components as carefully as their Americanand European counterparts, yet they achieve higher quality and reliabilityat assembly. This was presented as a counter-intuitive example of orientalinscrutability. This author suspects that this is an example of good draftingpractice. The Japanese send out drawings with achievable tolerances. Thevendors charge less. Everybody inspects less. The assemblies are tweaked tocope with the realistic, looser tolerances.

24 4 DESIGN SKILLS AND TOOLS

4.2 Bills Of Material (BOMs)

The bill of material is part of your design and documentation. Obviously,the BOM will be used to create requisitions and purchase orders. Not soobviously, information moves from engineering, to the purchasing/accountingdepartment. It would be nice if information did not have to re-typed at eachtransfer of responsibility. If your BOM is set up in the same format asyour requisition form, you can copy and paste. If your BOM is in the sameformat as your MRP/ERP database, you can copy and paste, or electronicallytransfer the data. This saves time and it eliminates errors.

Type once, only.

You do not want non-technical clerks re-interpreting your BOM entries.You and your accounting department must agree on a data format. Ac-counting must trust you to fill in BOM entries correctly. Talk to them. Evenif they don’t want to copy and paste or electronically transfer, create theopportunity.

Databases tend to have fixed length fields. They tend to have separatefields for vendor and/or part number and/or description. BOM entries mustconform to this. The author would like it if BOMs and databases had a singledescription field sixty characters long, but he has not seen it. If your owncompany part number is called up, someone must pull out a document, read itand follow the instructions. If the manufacturer, part number and descriptionfits in the database field, everything is simple. Sometimes, the manufacturer’sinformation is too long, or their ordering process is too complicated to fit inyour BOM entry. You can write up ordering instructions on your specificationcontrol, and call the part up by your own number.

ITEM DESCRIPTION1 CAP SCR HEX SOCK .190-32X0.63 SST2 HSCS SS 10-32X5/83 MCMASTER CARR 92196A271 HSHCS4 HX SCK CAP SCR ST STL 10-32UNC X 5/85 SPAE NAUR 370-053 HX SK CP SCR 10-32X5/86 HEX SOCHET CAP SCRW STSTL 10-32X5/87 HXSCK KPSCR 1032UNFX.62 SS8 CAP SCREW ST STL 10-32X5/8

All of the above BOM entries23 describe the same thing, a hex socket head

23 Typos are deliberate.

4.3 3D CAD 25

Figure 12: Hex Socket Head Cap Screw

cap screw, stainless steel, 10-32UNF×5/8. Some people have eccentric waysof writing things. Some people are trying to fit things in small database fields.Some people do not like typing. Some people can’t spell. A clerk must orderparts, writing a requisition that may have to be interpreted by another clerkworking for the vendor. Yet another clerk will set up MRP/ERP and assignstock codes and warehouse storage, possibly to each and every part above.The person doing the actual assembly must be able to identify the parts onthe work bench in front of them. Probably, they don’t have a McMaster Carrcatalogue at hand. Something called a “cap screw” could be a hex sockethead cap screw, or it could be something with a hexagonal head.

If BOMs are prepared manually, create copy-and-paste lists for standardparts like screws. This assures consistent BOM entries, and it reduces huntand peck typing. In 3D CAD, set up libraries of standard components withcorrect BOM entries.

4.3 3D CAD

Obviously, 3D CAD provides 3D visualization of the design. Lots of peopleare not good at reading drawings.

A big benefit of 3D parametric CAD, as opposed to 2D CAD or draftingboards, is that drawings update automatically as the models are changed.Assembly and fabrication drawings can be generated early, and used to com-municate with co-workers. An exploded assembly drawing shown off in earlydesign reviews, shows manufacturing how you intend to assemble your wid-get. Your assembly procedure gets reviewed early in the project, when it iseasy to make changes. On fabrication drawings generated early in the de-

26 5 NOTES

sign process, you can apply tolerances to the critical features. Early in thedesign process, tolerance stacks tell you whether or not your design can befabricated and assembled. Tight, difficult tolerances are identified, and thedesign changed to eliminate them.

3D CAD generates Bills Of Material (BOMs). Again, these can be gen-erated early, providing a view of parts to be ordered. This is absolutelysuperior to bills of material written out on the backs of envelopes, used togenerate purchase orders, then discarded. It is also absolutely superior tobills of material generated several months after you delivered your product,or several months after manufacturing has worked up it own BOMs.

Insert fasteners and other standard hardware into your CAD model.These must be called up on the BOM. Attaching the fasteners reveals allsorts of design and assembly problems. Sometimes, they are inaccessible.Sometimes, there is no room to install them. When you see how many dif-ferent fasteners you have, you can change your design to standardize them abit.

Do not cut corners when modelling and drafting. You can send 3D modelsout to the shop and skip the drawings, but Joe Osborn[5] recommends draw-ings. Model Based Definition (MBD) is not an efficient process in limitedproduction jobbing shops.

In higher production shops, fabricators read your drawings, and preparetheir own fabrication drawings. Your drawings should not specify fabricationprocedure.24 Your drawings should specify what you will accept from thevendor. Let the vendor figure out how to do stuff. An alternate MBDprocedure is to mark only the critical dimensions and tolerances on drawings.The fabricator receives the drawing and the 3D model, which is enoughinformation for them to prepare their own drawings. It is understood thatthe specified dimensions will be inspected, and that everything else is lesscritical.

5 Notes

This is all stuff not directly pertaining to DFMA, but interesting and relevant.

24 The author has been told by a machinist that specifying tap drills is an insult.

5.1 Design of Covers 27

5.1 Design of Covers

Most of the author’s experience has been with laser optics and remote sensing.These are complex, expensive devices requiring all sorts of test proceduresto get them working. They require maintenance and repair, later. Some ofthis is done under warranty. Optics require alignment, which must survivecontinued assembly, and use in the field. Your industry probably is different,but similar design rules will be feasible, and a good idea. Regard the followingas an example, not as a set of hard and fast rules.

Covers are surprisingly difficult to design. Do not leave this for last.

• Covers have multiple functions, providing protection from dust, liquid,EMI/RFI, and people’s fingers. The cover may shield people fromthings like class IV lasers. The cover may have to look good. Theserequirements must all be managed together.

• Covers are an integral part of your structural design, as well as yourplan for assembly and access.

• You should be able to power your system up and run it, and be able totake the covers on and off without disturbing any of this. Dumb piecesof metal or plastic accomplish this. You can attach electronics or othercritical components to a hinged door.

• If things, such as optics, must be kept aligned, the structure must bevery much more rigid than the covers. Alignment must not be affectedby the installation and removal of the covers.

5.2 Style

Making it look cool can impact fabrication and assembly.

• Style costs more. For any given product package, there is a way todo it that is cheap, functional and ugly. Pretty things are expensive.Good styling may (have to) compromise easy assembly and access. Getmanagement to commit to a budget.25

25 Management may surprise you here and hand you a large budget. Sometimes, stylematters.

28 5 NOTES

• Keep it simple. You are engineers and mechanical designers, not artists.Don’t put curves on things for the sake of putting curves on things.

• Bad styling looks much worse than unstyled functional. You tried. Youscrewed up.

5.3 More on Subcontracting

This is an expansion of some ideas, presented above under Subcontracting .When you send drawings out for subcontract, you need them to be as

clear as possible. On a one-off project, you can talk to the vendor, andprovide extra information. In production, the clerical staff in the purchasingoffice must reproduce the conversations, and identify and produce any extradrawings required by the fabricator.

There was a discussion on Eng-Tips[11] on critical dimensions on draw-ings. Some manufacturing engineers described their need to understand thefunction of the parts they were fabricating and inspecting, so that they couldidentify the critical features. Let’s assume we send out the assembly drawing.

1. Purchasing needs to know that the assembly drawing must be sent out.This is not obvious from the BOM. The purchasing database probablylacks the resources to store information like this. This process could bea lot of effort.

2. If fabrication requires the assembly drawing, the entire order must go toone shop. This is not very flexible for purchasing, especially if the fab-rication processes are varied. Shop A may be good for your machinedparts, and shop B good for sheet metal.

3. Both shop A and shop B can cope with the order, and both receivepurchase orders. Do they solve assembly problems the same way? Arethe resulting parts interchangeable?

4. Your assembly drawing may contain information that the fabricatordoes not need to know. It could be your proprietary technology. Itcould identify your customers. It could be something classified as anational secret. Do your fabricators and their employees understandyour data security?

All of this hassle goes away if you are good at drafting.

5.4 Office Politics 29

5.4 Office Politics

Design For Manufacture and Assembly means enabling people outside ofdesign engineering. You need to establish respect and trust with co-workersall around the company. Part of this means doing your job. If manufacturingneeds documentation, they will get it, if not from you, then by some othermeans. If by some other means, then you and engineering no longer controlthe design. Manufacturing may no longer respect your opinions. This isespecially true if they have had to debug your design.

If manufacturing or accounting are too lazy or process driven to do theirjobs, you are stuck. This is a management issue. DFMA requires teamwork.

5.5 Lean Manufacturing

Lean Manufacturing is manufacturing’s job. Much lean manufacturing re-quires design to be done properly. Lean principles can be applied to engi-neering and design.

According to Lonnie Wilson[9], a lean manufacturing process:

1. Has a focus on quantity control to reduce cost by eliminating waste.

2. Is built on a strong foundation of process and product quality.

3. Is fully integrated

4. Is continually evolving

5. Is perpetuated by a strong, flexible, and appropriate culture that ismanaged consciously, continuously, and consistently.

The bolds and italics are Wilson’s.

5.5.1 The Seven Wastes

What follows is a quick regurgitation of Wikipedia. . .

Transport moving products that are not actually required to perform theprocessing

Inventory all components, work in process, and finished product not beingprocessed

30 5 NOTES

Motion people or equipment moving or walking more than is required toperform the processing

Waiting waiting for the next production step, interruptions of productionduring shift change

Overproduction production ahead of demand

Over Processing resulting from poor tool or product design creating ac-tivity

Defects the effort involved in inspecting for and fixing defects

Lonnie Wilson[9] lists them in a different order. Any inventory that doesnot support sales, is waste. Waiting means simply that your workers are notworking. Defects are scrap, upon which you expended time and cost.

Careful study of manufacturing during design creates opportunities toplan an efficient production with minimal transport, inventory and motionaround the shop.

If the manufacturing process you select requires expensive tooling andset-up, there will be a need to do a large production run, creating lots ofinventory, and the above mentioned overproduction. The production toolprobably is part of your inventory.

Over processing happens when your design requires extra manufacturingsteps. Try to design your castings and weldments so that they do not requireclean-up machining. Minimize the number of set-ups on your machined parts.

Defects happen when the manufacturing process is marginally capable ofmeeting your tolerances. Do your tolerance stacks. Open up your clearanceholes and loosen critical tolerances. Difficult tolerances require the manufac-turing process to run more slowly and carefully. If a significant number ofparts do not conform, the inspection process itself becomes expensive.

Defects also happen when your documentation is bad, and people cannotsee how to assemble and test stuff.

5.5.2 The Five Questions for Continuous Improvement

Continuous improvement is a useful concept for design and engineering, aswell as manufacturing.

According to Lonnie Wilson[9]. . .

REFERENCES 31

1. What is the present state or condition?

2. What is the desired future condition?

3. What is preventing us from reaching the desired condition?

4. What is something we can do now to get closer to the desired condition?

5. What is our expectation when we do this?

(a) What will happen?

(b) How much of it will happen?

(c) When will it happen so we can “go see”?

References

[1] NPD Solutions http://www.npd-solutions.com/dfmguidelines.

html

[2] Product Design for Manufacture and AssemblyGeoffrey Boothroyd, Pe-ter Dewhurst, Winston Knight. Marcel Dekker Books

[3] Institute of Scrap Recycling Industries http://www.isri.org/

about-isri/awards/design-for-recycling https://www.google.

ca/search?q=design+for+recycling

[4] MIL-STD-1472, Department of Defence Design Criteria Standard Hu-man Engineering. As of 2016/07/13, this seems to be at revision G. Youcan Google and find PDFs of this online. Since this is a US military pub-lication, you should assume that their people are younger and physicallyfitter than your people.

[5] Tips on Designing Cost Effective Machined Parts by JoeOsborn, OMW Corporation, http://www.omwcorp.com/

tips-on-designing-cost-effective-machined-parts/

[6] Cumberland Diversified Metals http://www.cumberlandmetals.com/

aluminum/minimum-bend-radii/

32 REFERENCES

[7] ASME Y14.5-2009 Dimensioning and Tolerancing. American Society ofMechanical Engineers This is the ASME standard for geometric dimen-sioning and tolerancing (GD&T). There are textbooks out there, butthis official standard is very readable. Earlier versions of the standardare ASME Y14.5M-1994 and ANSI Y14.5M-1982.

[8] Design News article The Quest for Imperfection, by Charles J. Mur-ray. 2005/10/10. http://www.designnews.com/document.asp?doc_

id=220035 As of 2016/11/30, this is no longer available. :( I can-not find it on https://web.archive.org. I Googled "The Quest for

Imperfection" Murray and found http://connection.ebscohost.

com/c/articles/18572445/quest-imperfection.

[9] How to Implement Lean Manufacturing, by Lonnie Wilson, McGrawHill

[10] ASME Y14.100-2004 Engineering Drawing Practises, American Societyof Mechanical Engineers. This an updated of the old DOD-100 standard.

[11] Eng-Tips.com post and discussion http://www.eng-tips.com/

viewthread.cfm?qid=322065 Critical Dimension[s]. “. . . what isconsidered as ‘Critical Dimension’ and how should I select it?” Theauthor participated in this discussion as Drawoh.

[12] Vendor communication with Christie Digital.