chapter 23 machining processes used to produce various shapes

1

Chapter 23 Machining Processes Used to Produce

Various Shapes

Alexandra Schönning, Ph.D.Mechanical Engineering

University of North Florida

Figures by Manufacturing Engineering and Technology

Kalpakijan and Schmid

2-2

Introduction

What is milling?A manufacturing process in which a rotating,

multitooth cutter removes material while traveling along various axes with respect to the workpiece.

Other processes will be discussed, such as Planing, shaping, broaching, sawing, filing, and gear

manufacturing.

3

Milling operations

Slab milling (Figure a) Arbor, cutter

Face milling (Figure b) Spindle and cutter

End milling (Figure c) Spindle, shank, end mill

Figure 23.3 A typical part that can be produced on a milling machine equipped with computer controls. Such parts can be made efficiently and repetitively on computer numerical control (CNC) machines, without the need for refixturing or reclamping the part.

4

Slab Milling

Also called peripheral milling The axis of cutter rotation is parallel to the workpiece

surface to be machined The cutter has a teeth along its circumference where

each tooth acts as a single point cutting tool Conventional vs. Climb milling

Conventional milling• Also called up-milling: the rotation of the cutter is such that it first

engages the workpiece at the bottom. • The cut is not a function of the surface characteristics• Common method of milling.• Proper clamping is necessary to prevent the upward rotation of the

cutter. Climb milling

• Also called down-milling: the rotation is such that the cutter first engages the workpiece at the top.

• Cutting forces holds the workpiece in place. However, a rigid setup is important since there are high impact forces.

5

Conventional and Climb Milling

Figure 23.4 (a) Schematic illustration of conventional milling and climb milling. (b) Slab milling operation, showing depth of cut, d, feed per tooth, f, chip depth of cut, tc, and workpiece speed, v. (c) Schematic illustration of cutter travel distance lc to reach full depth of cut.

6

Milling Parameters

The velocity at the point of contact

The chip thickness can be found

Feed per tooth

Cutting time

Material Removal Rate

Definitions of symbols Definition of Symbols

tc: chip thickness f: feed per tooth of cutter D: depth of cut N: angular speed in rpm n: number of teeth on cutter periphery v: linear speed (feed rate) t: cutting time L: length of the workpiece Lc: extent of the cutter's first contact

with the workpiece (illustration on next page)

w: width of cut

v D N

v r

tc2 f d

D

fv

N n

rad

slength

N = angular speed in rpm. This version of the formula is used in the book. It includes the unit conversion from rpm to radiance per minute. The units of the velocity are distance/min.

It is assumed that lc<<lt l lc( )

v

MRRl w d

tw d v

As tc gets larger the forces on the cutter tooth increases

7

Illustration of Lc

Lc

Lc: extent of the cutter's first contact with the workpiece

8

Summary of Milling Parameters and Formulas

TABLE 23.1N = Rotational speed of the milling cutter, rpmf = Feed, mm/tooth or in./tooth

D = Cutter diameter, mm or in.n = Number of teeth on cutterv = Linear speed of the workpiece or feed rate, mm/min or in./minV = Surface speed of cutter, m/min or ft/min

=D Nf = Feed per tooth, mm/tooth or in/tooth

=v /N nl = Length of cut, mm or in.t = Cutting time, s or min

=( l+lc ) v , where lc =extent of the cutter’s first contact with workpiece

MRR = mm3/min or in.

3/min

=w d v , where w is the width of cutTorque = N-m or lb-ft

( Fc ) (D/2)Power = kW or hp

= (Torque) ( ), where = 2 N radians/minNote: The units given are those that are commonly used; however, appropriate units mustbe used in the formulas.

9

Example A slab milling operation is being

carried out on a 12-in long, 4 in wide annealed mild steel block at a feed f=0.01 in/tooth and a depth of cut d=1/8 in. The cutter is D=2in in diameter, and has 20 straight teeth, rotates at N = 100rpm, and is wider than the block to be machined. Calculate the material Removal rate, estimate the power and torque required for this operation, and calculate the cutting time. Given:

• w=4in• L=12 in• f=0.01 in/tooth• d=1/8 in• D=2 in• n = 20 teeth• N = 100 rpm• Cutter width > block width

• Specific energy: 1.1 hp*min/in3

○ Table 20:2 (annealed mild steel)

Find• Material Removal Rate• Power• Torque• Cutting time

secondst 37.5tL Lc( )

n60

inLc 0.5 Lc D d

lb ftTorque 577.7TorquePower

N

33000

2

hpPower 11Power spec_enrg MRR

in3

minMRR 10MRR w d v

in

minv 20v f N n

10

Face Milling

Cutter is mounted on a spindle having an external axis of rotation perpendicular to the workpiece surface

Workpiece moves along a straight path at a linear speed, v.

Direction of cutter Conventional milling (Fig. c)

• up-milling Climb milling (Fig. b)

• down-milling

Leaves feed marks on the machined surface

Terminology in figure Lead angles: 0-45o. Low angle

low vertical force

Figure 23.8 Terminology for a face-milling cutter.

11

Face Milling

Figure 23.6 A face-milling cutter with indexable inserts. Source: Courtesy of Ingersoll Cutting Tool Company.

v

d w

12

Cutter and Insert Position in Face Milling

Figure 23.10 (a) Relative position of the cutter and insert as it first engages the workpiece in face milling, (b) insert positions towards the end of the cut, and (c) examples of exit angles of insert, showing desirable (positive or negative angle) and undesirable (zero angle) positions. In all figures, the cutter spindle is perpendicular to the page.

Third example in figure c: the insert exits the workpiece suddenly as opposed to exiting with an angle.

13

End milling

The cutter, called an end mill, rotates about an axis perpendicular to the workpiece surface (typically – can be at an angle)

Ball nose: A type of end mill in which the bottom surface is rounded Used in the production of

curved surfaces for dies and molds

14

Other Milling Operations and Cutters

Straddle milling: two or more cutters are mounted on an arbor and are used to machine two parallel surfaces on the workpiece Easier to keep tolerances than if

milling one surface at a time Form milling: produces

curved profiles. Also used in machining gear teeth.

Circular cutters can be used for slotting and slitting. Slitting saws are typically <

5mm. T-slot cutters: used to mill T-

slots which are used in clamping workpieces to the work table. (figure next page)

15

Other Milling Operations and Cutters

T-slot cutter Note order of cuts:

• 1st: cut slott so the T-cutter can move

• 2nd: finish the T-slot

Shell milling Has a hollow inside.

Mounted on a shank.• Allows the same shank to be

used for different size cutters

Figure 23.12 (a) T-slot cutting with a milling cutter. (b) A shell mill.

16

Tool holders

Milling cutters are classified as Arbor cutters

• Mounted on an arbor • Used in slab, face, straddle, and form milling

Shank cutters• The cutter and the shank are one piece.• Examples include end mills• Straight shanks

♦ Mounted collet chucks or special end mill holder

• Tapered shanks○ Tapered for better clamping.○ Common on larger end mills○ Mounted in tapered tool holders

17

Figure 23.13 Mounting a milling cutter on an arbor for use on a horizontal milling machine.

Arbor

18

Milling Process Capabilities

Process capabilities include Surface finish, dimensional tolerance, production

rate, and cost considerationFeed rate (typical): 0.1 mm/tooth – 0.5 mm/toothDepth of cut (typical): 1-8 mmCutting speeds (varies much): 30m/min to

3000m/min

19

Capacities and Maximum Workpiece Dimensions for Machine Tools

TABLE 23.2 Typical Capacities and Maximum Workpiece Dimensions forSome Machine Tools

Machine toolMaximum dimension

m (ft)Power(kW)

Maximumspeed

Milling machines (table travel) Knee-and-column 1.4 (4.6) 20 4000 rpm Bed 4.3 (14) Numerical control 5 (16.5)Planers (table travel) 10 (33) 100 1.7Broaching machines (length) 2 (6.5) 0.9 MNGear cutting (gear diameter) 5 (16.5)Note: Larger capacities are available for special applications.

20

Approximate Cost of

Selected Tools for Machining

TABLE 23.3 Approximate Cost of Selected Tools for Machining*Tools Size (in.) Cost ($)Drills, HSS, straight shank 1/4 1.00–2.00

1/2 3.00–6.00Coated (TiN) 1/4 2.60–3.00

1/2 10–15Tapered shank 1/4 2.50–7.00

1 15–452 80–853 2504 950

Reamers, HSS, hand 1/4 10–151/2 10–15

Chucking 1/2 5–101 20–251 1/2 40–55

End mills, HSS 1/2 10–151 15–30

Carbide-tipped 1/2 30–351 45–60

Solid carbide 1/2 30–701 180

Burs, carbide 1/2 10–201 50–60

Milling cutters, HSS, staggered tooth, wide 4 35–758 130–260

Collets (5 core) 1 10–20*Cost depends on the particular type of material and shape of tool, its quality,and the amount purchased.

21

General Recommendations

for Milling Operations

TABLE 23.4General-purpose starting

conditions Range of conditions

Workpiecematerial Cutting tool

Feedmm/tooth(in./tooth)

Speedm/min

(ft/min)

Feedmm/tooth(in./tooth)

Speedm/min

(ft/min)Low-C and free-machining steels

Uncoated carbide,coated carbide,cermets

0.13–0.20(0.005–0.008)

120–180(400–600)

0.085–0.38(0.003–0.015)

90–425(300–1400)

Alloy steels Soft Uncoated, coated,

cermets0.10–0.18

(0.004–0.007)90–170

(300–550)0.08–0.30

(0.003–0.012)60–370

(200–1200) Hard Cermets, PCBN 0.10–0.15

(0.004–0.006)180–210

(600–700)0.08–0.25

(0.003–0.010)75–460

(250–1500)Cast iron, gray Soft Uncoated, coated,

cermets, SiN0.10–10.20

(0.004–0.008)120–760

(400–2500)0.08–0.38

(0.003–0.015)90–1370

(300–4500) Hard Cermets, SiN,

PCBN0.10–0.20

(0.004–0.008)120–210

(400–700)0.08–0.38

(0.003–0.015)90–460

(300–1500)Stainless steel,austenitic

Uncoated, coated,cermets

0.13–0.18(0.005–0.007)

120–370(400–1200)

0.08–0.38(0.003–0.015)

90–500(300–1800)

High-temperaturealloys, nickel base

Uncoated, coated,cermets, SiN,PCBN

0.10–0.18(0.004–0.007)

30–370(100–1200)

0.08–0.38(0.003–0.015)

30–550(90–1800)

Titanium alloys Uncoated, coated,cermets

0.13–0.15(0.005–0.006)

50–60(175–200)

0.08–0.38(0.003–0.015)

40–140(125–450)

Aluminum alloys Free machining Uncoated, coated,

PCD0.13–0.23

(0.005–0.009)610–900

(2000–3000)0.08–0.46

(0.003–0.018)300–3000

(1000–10,000)High silicon PCD 0.13

(0.005)610

(2000)0.08–0.38

(0.003–0–015)370–910

(1200–3000)Copper alloys Uncoated, coated,

PCD0.13–0.23

(0.005–0.009)300–760

(1000–2500)0.08–0.46

(0.003–0.018)90–1070

(300–3500)Thermoplastics andthermosets

Uncoated, coated,PCD

0.13–0.23(0.005–0.009)

270–460(900–1500)

0.08–0.46(0.003–0.018)

90–1370(300–4500)

Source: Based on data from Kennametal Inc.Note: Depths of cut, d , usually are in the range of 1–8 mm (0.04–0.3 in.). PCBN: polycrystalline cubic boron nitride;PCD: polycrystalline diamond.Note: See also Table 22.2 for range of cutting speeds within tool material groups.

-22

General Troubleshooting Guide for Milling Operations

TABLE 23.5Problem Probable causesTool breakage Tool material lacks toughness; improper tool angles; cutting

parameters too high.Tool wear excessive Cutting parameters too high; improper tool material; improper tool

angles; improper cutting fluid.Rough surface finish Feed too high; spindle speed too low; too few teeth on cutter; tool

chipped or worn; built-up edge; vibration and chatter.Tolerances too broad Lack of spindle stiffness; excessive temperature rise; dull tool; chips

clogging cutter.Workpiece surfaceburnished

Dull tool; depth of cut too low; radial relief angle too small.

Back striking Dull cutting tools; cutter spindle tilt; negative tool angles.Chatter marks Insufficient stiffness of system; external vibrations; feed, depth, and

width of cut too large.Burr formation Dull cutting edges or too much honing; incorrect angle of entry or

exit; feed and depth of cut too high; incorrect insert geometry.Breakout Lead angle too low; incorrect cutting edge geometry; incorrect angle

of entry or exit; feed and depth of cut too high.

23

Surface Features and Corner Defects

Figure 23.14 Surface features and corner defects in face milling operations; see also Fig. 23.7. For troubleshooting, see Table 23.5. Source: Kennametal Inc.

24

Design and Operating Guidelines for Milling

Similar to the guidelines for turning Use standard milling cutters Chamfers should be used instead of radii because of the

difficulty of smoothly matching various intersecting surfaces

Internal cavities and pockets with sharp corners should be avoided since cutters have a finite radius

Workpieces should be sufficiently rigid to minimize any defects resulting from clamping and cutting forces.

Minimizing vibrations Cutters should be mounted close to spindle to reduce tool

deflection Tool holders and fixtures should be as rigid as possible If vibration occurs

• Modify tool shape and/or process conditions• Used a cutter with fewer teeth or with random tooth spacing

25

Milling Machines

Column-and-knee type machines Most common The spindle may be horizontal or

vertical Machine consists of

A work table: on which the workpiece is clamped.

A saddle: supports the table and moves in a perpendicular direction

A knee: supports the saddle and allows for vertical movement (depth of cut)

An overarm: in horizontal machines. It can accommodate different arbor lengths

A head: contains a spindle and cutter holder

Plain miling machines: 3 axes Universal column-and-knee milling

machines: 4th axes (rotational)

Horizontal spindle

vertical spindle

26

Other types of milling machines

Bed-typed milling A bed is used instead of the knee. No vertical

movement is possible. The work table is mounted directly on the bed. Used in high production runs for simple parts.

Figure 23.17 Schematic illustration of a bed-type milling machine. Note the single vertical-spindle cutter and two horizontal spindle cutters. Source: ASM International.

27

Planing and shaping

Cutting operation by which flat surfaces, grooves, and notches are produced along the length of the workpiece.

Typically performed on large workpieces. The workplane is mounted on a table that travels along a straight path

A horizontal cross-rail can be moved vertically Shaping: similar to planing but for smaller parts

Used to machine notches, keyways, and dies

Figure 23.20 Typical parts that can be made on a planer.

28

Broaching and Broaching Machines What is a broach?

A long multitooth cutting tool. Used for what?

To machine internal or external surfaces, such as holes of different sections, the teeth of internal gears, multiple spline holes, and flat surfaces.

Terminology and geometry: Rake angle typically from 0-20

degrees. Clearance angle typically from

0-4 degrees.

Pitch = k*sqrt(L) (to find appropriate pitch)• k = 1.76 when L is in mm• k = 0.35 when L is in inches• L = length of surface to be cut

At least two or more teeth should be in contact.

Different type of broaches exist• Surface broaches:

○ Slab broaches (for cutting flat surfaces), slot, contour, dovetail, straddle, pot (precision of external shapes)

• Internal broaches include hole, keyway, internal gear, and rifling

Figure 23.22 (a) Cutting action of a broach, showing various features. (b) Terminology for a broach.

29

Broaching and Broaching Machines

Turn broaching The workpiece is rotated between centers, and the broach passes

tangentially across the bearing surfaces and remove material Broaching Machines

Either pull or push the broaches and are made either horizontal or vertical. Forces required depend on workpiece material, total depth and width of

cut, and cutting speed. Broaching process parameters

Cutting speeds range from 1.5m/min for high strength alloys to 15m/min for Aluminum alloys

Ceramic inserts may be used for finishing operations Design Considerations for Broaching

Parts should be designed such that they can be clamped securely Blind holes, sharp corners, dovetail splines, and large flat surfaces should

be avoided Chamfers are preferred over round corners

30

Chipbreakers and a Broaching Machine

(a)

(b)

(c)

Figure 23.23 Chipbreaker features on (a) a flat broach and (b) a round broach. (c) Vertical broaching machine. Source: Ty Miles, Inc.

31

Internal Broach and Turn Broaching

Figure 23.24 Terminology for a pull-type internal broach used for enlarging long holes.

Figure 23.25 Turn broaching of a crankshaft. The crankshaft rotates while the broaches pass tangentially across the crankshaft’s bearing surfaces. Source: Courtesy of Ingersoll Cutting Tool Company.

32

Sawing A saw consists of a blade with a series of small teeth. The width of the cut is called the kerf and is typically thin

resulting in little waste material. At least two or three teeth should be engaged for the blade to

prevent snagging (catching the saw tooth on the workpiece). The thinner the stock, the finer the saw teeth should be, and the

greater the number per unit length.

Figure 23.27 Examples of various sawing operations. Source: DoALL Company.

33

Figure 23.28 (a) Terminology for saw teeth. (b) Types of tooth set on saw teeth, staggered to provide clearance for the saw blade to prevent binding during sawing.

Sawing

Types of saw teeth

34

Sawing

Types of saws Hacksaw: Straight blades, reciprocating motion. Cutting only

takes place during one of the two reciprocating strokes. Circular saw: Also called cold saw. Used for… Band saw: continuous long, flexible blades and have a

continuous cutting action. (both ways) runt band Diamond edge blades and diamond wire saws: high strength

wire is coated with diamond particles. Used for cutting hard metallic, nonmetallic, and composite materials.

Friction sawing A steel blade, or disk, rubs against the workpiece at speeds up

to 7600m/min. The frictional energy is converted into heat, which softens a

narrow zone in the workpiece. Used to cut hard ferrous metals and reinforced plastics. Commonly used to remove flash form castings

35

Filing and Finishing

Filing: Small scale removal of material from a surface, corner, or hole, including removal of burrs.

Shapes: flat, round, half round, and triangularRotary files and burs:

Used for removing material in die making, deburring, scale removal from surfaces, and producing chamfers on parts

Figure 23.30 Types of burs. Source: The Copper Group.

36

Gear Manufacturing by Machining

How are gears manufactured? Casting, forging,

extrusion, drawing, thread rolling, powder metallurgy, blanking sheet metal, injection molding (non metallic), casting (non metallic), and machining.

Gears can be machined by Form cutting (future

slides) Gear generating (future

slides) Gear terminology

Figure 23.31 Nomenclature for an involute spur gear.

37

Form Cutting

The gear-tooth shape us reproduced by cutting the gear blank around its periphery.

After each tooth is cut, the cutter is withdrawn, and the gear blank is rotated (indexed).

Form cutting can be done on milling machines, with the cutter mounted on an arbor and the gear blank mounted in a dividing head.

Limitation: Form cutting can only produce

gear teeth that have constant width (spur and helical but not beveled gears.

Figure 23.32 (a) Producing gear teeth on a blank by from cutting. (b) Schematic illustration of gear generating with a pinion-shaped gear cutter. (c) Schematic illustration of gear generating in a gear shaper using a pinion-shaped cutter. Note that the cutter reciprocates vertically. (d) Gear generating with rack-shaped cutter.

38

Gear Generating

The cutting tools in gear generating may be one of the following: A pinion-shaped cutter

• The cutter is one of the gears in a conjugate pair. The other gear is the blank.

A rack-shaped straight cutter• The generating tool is a

segment of a rack which reciprocates parallel to the axis of the gear blank.

39

Gear Generating

Hob: A worm or screw made into a gear-generating tool. It consists of slots that will

be used to cut the teeth. The angle between the

axis of the hob and the gear are about 90 degrees apart.

40

Cutting Bevel Gears

Figure 23.34 (a) Cutting a straight bevel-gear blank with two cutters. (b) Cutting a spiral bevel gear with a single cutter. Source: ASM International.

41

Gear Finishing Procedures

Grinding Form Grinding

• The shape of the wheel is identical to the tooth spacing. (Figure A)

Generating• The grinding wheel acts as

shown in Figure (b)

Honing• The tool is a plastic gear

impregnated with fine abrasive particles.

• Improves surface finish. Lapping

• To further improve surface finish.

• Uses an abrasive compound

42

Gear Manufacturers Association

AGMA: American Gear Manufacturers Association Sets the standards for gears in the US

chapter 23 machining processes used to produce various shapes

Documents