Z-axis for 3Drag/K8200My solution: zero wobbling, zero backslash, 4 options.

Abstract:A new follower nut support reduces wobbling and backslash. Here the design for a better z-axis kinematic using stainless steel threaded rods or trapezoidal threaded rods, single or double motor. Theoretical analysis, comparison of performance, parametric scad files, step by step guide, for a complete solution tailored according to the needs.

1 – Using one z rod

Rational:I. The threaded rod has to work only in traction, not in

compression or in a mixed way. That eliminates any risk of rod backslash.

II. The NEMA motor must not be subjected to longitudinal loads (z), all cargo must be backed up.

III. The threaded rod should handle only vertical movement. Movements in other directions (x, y), due to crooked bars, eccentricity, etc... must not be transmitted to extruder arm and even to supporting structure (wobbling).

IV. Good indicator of proper assembly is quiet operations, using different speeds.

ABOVE: The bar top is locked to hold the weight of the arm with the extruder. Small displacements from the vertical are allowed.

CENTER: The arm is resting on the following nut, ensuring traction only. The nut can not rotate, and its (eventual) lateral movements (x, y) are not transmitted to the arm.

BELOW: Nema engine should not bear any weight: a flexible coupling ensures vertical decoupling. The engine can make small horizontal movements (x, y) but not rotate: no efforts are transmitted to the structure.

Original 3Drag performancesHeight of the extruder arm measured (10 times) by a micrometer, at 17 cm from the threaded rod.a) coming from down: avg: 0.035, min: 0.01, max: 0.05b) coming from up: avg: 0.169, min: 0.15, max: 0.18

difference: 0.134 mm

Z-axis, 2 dimensions schema

1a) Cheap solution,stainless steel threaded rod, M8 (pitch 1,25 mm)

The threaded rods of stainless steel are more rigid and with a threading more precise with respect to the rods of galvanized steel.

steps/mm, driver A4988 = 2560steps/mm, driver DRV8825 = 5120layers: n x 0,00625

more mnemonic: k x 0,025 mm(e.g. 0,100; 0,125; 0,150; 0,175; 0,200 ... )

ABOVE LEFT

You can use the original support, but it is better to replace it with top.stl, for easier access to the fixing bolt, as required in the assembly.Using the original bearing, the threaded rod has an upper M8 washer and the original self-locking nut, nothing below.Note: maybe better to use here a cradle bearing, but they are difficult to find. See also 1b) for the use of a flange bearing.

CENTER LEFT Replace the original support with l_center.stl.A large stainless steel washer rests on a brass nut (not stainless !, to avoid risks of seizure).

The lateral movement (x, y) are amortized from the "spider" of the nut. The nut has ±1 mm of clearance x, y.

That structure applies at arm only the very low forces of the "spider" springs.Block with a soldering iron the brass nut and the steel washer, paying attention to the centering and flatness.

Join the two parts with two M3 bolts, using the spacers or some 3mm washers, so that the brass bolt will pave well on the steel washer. Ensure that washer and brass nut are centered before tightening.Note: in place of the stainless steel washer, can be used a 10mm collar bearing, with the top washer reversed. This reduces friction for lateral displacements.

LOWER LEFT

Print the part l_bottom.slt, the elastic support (x, y) for the NEMA motor.

Use four M3 bolts and 2 washer each to secure the motor to the "spider", which allows small lateral movements to the motor.One M5 bolts with square nuts will secure the support to the frame.Holes are provided for fastening the zero microswitch using M3 bolts and spacers, if necessary.Use a flexible coupling 5x8 to secure the threaded rod to the NEMA motor, reducing the vertical stresses.

Materials:● Stainless steel threaded rod, M8 (Brico, 7 € / m)● Flexible coupling, 5x8 (Banggood, $ 3.4)● Stainless steel washer, 10x30● Brass nut, M8● M3, M4, M5 bolts.

PerformancesLess noise than the originalHeight of the extruder arm measured (10 times) by a micrometer, at 17 cm from the threaded rod:

a) coming from down: avg: 0.006, min: 0.00, max: 0.01b) coming from up: avg: 0.086, min: 0.07, max: 0.11

difference: 0.080mm

1b) Using a Tr8x8 trapezoidal threaded rod

The trapezoidal rods are with tolerances lower than the common threaded rods. This reduces the causes of many problems, resulting in smooth and precise operations.The used rod diameter is 8mm with 2mm pitch and 4 starts: the lead is 8mm, so the minimal vertical increment is 40 μ.

steps/mm, driver A4988 = 400steps/mm, driver DRV8825 = 800layer: k x 0,04 mm

(e.g. 0.080; 0.120; 0.160; 0.200; 0,240 ... )

ABOVE LEFT

The support uses a flange bearing in a pillow block, which ensures the required fixing. This bearing is essential with trapezoidal bars, but it's also great for fixing the standard M8 threaded rods.Print the part top_trap.stlScrew the bearing to the support with 2 M4 bolts, washer, washer, spring washer and nut. The assembly has a certain clearance, which allows a fine adjustment. Secure it to frame using a M5 bolt and square nut.

CENTER LEFTReplace the original left support with l_center_trap.stl.Screw the brass nut at the center of the "spider" with two parker screws : make 2mm pilot holes. Check that the nut protrudes a little from the other sideAdd a 10mm brass washer to ensure a flat contact surface.Block with the heat of a soldering iron the steel washer, making sure the centering.

Join the two parts with two M3 bolts, using spacers or 3mm washers, so that the brass washer will pave well on the steel washer. Ensure that steel washer and brass nut are centered before tightening.

LOWER LEFTPrint the part l_bottom.slt, the elastic support (x, y) for the NEMA motor.

Use four M3 bolts to secure the motor to "spider".One M5 bolt with square nut will secure the support to the frame.Holes are provided for fastening the zero microswitch using M3 bolts and spacers, if necessary.Use a flexible coupling 5x8 to secure the threaded rod to the NEMA motor, reducing the vertical stress.

Materials:

● 8x2mm trapezoidal 304 steel rod 300mm long, with brass nut (Banggood, $ 9.59)● Flexible coupling, 5x8 (Banggood, $ 3.4)● Stainless steel washer, 10x30● Brass washer 10mm● Pillow block and flange bearing KFL08 (Banggood, $ 1.78) ● M3 flat head bolts ● M3, M4, M5 bolts.

PerformancesNoise quasi null.Height limitation: with the used trapezoidal rod (30 cm) the max. extruder height is 15 cm.Height of the extruder arm measured (10 times) by a micrometer, at 17 cm from the threaded rod.

a) coming from down: avg: 0.006, min: 0.00 max: 0.01 b) coming from up: avg: 0.061, min: 0.06, max: 0.7

difference: 0.055

same, at 41 cm from the threaded rod.a) coming from down: avg: 0.051, min: 0.04 max: 0.06 b) coming from up: avg: 0.185, min: 0.18, max: 0.19

difference: 0.134

Note the low dispersion: min and max are close.

1z) Mounting one z rod

1. Secure the NEMA motor, the microswitch, mount the upper support.2. Temporarily fit the threaded rod, by controlling the distance from the frame. Adjust if

necessary the position of the upper bearing.3. Remove the threaded bar, and remount it with the central support, no spider, but with the

steel washer. Check that the bar is centered in the hole of the washer, both with the arm at the bottom than at the top. It can be necessary to adjust the washer position.

4. Final assembly, centering the bolt hole in the brass washer steel. Tighten all, making sure that the motor shaft and bar penetrate only the bare essentials in the flexible coupling.

5. Loosen the bolt M5 which supports the upper support and shrink it by lifting: the flexible coupling must be slightly tensioned. Now the weight is supported by the upper bearing.

2 - Using two z rods

Rational:The extruder stops at different heights from the expected value depending on whether the movement is from the top or from the bottom.

In the case of a trapezoidal rod I measured:

at distance d1 = 17 cm: Average of 10 measures, Δz1 = 55 μ

at adistance d2 = 41 cm: Average of 10 measures, Δz2 = 134 μ

This is not a phenomenon of rod backslash, but a problem of mechanical hysteresis.In fact, the Δz are proportional to the distance (in the case of true backslash the Δz would be independent from the distance):

∆ z1

d 1≈

∆ z 2

d 2≈3.2

Furthermore, by adding a weight of 1 kg at 41 cm from the axis, the extruder arm is lowered to 0.210 mm (measured at 41 cm.). This decline is not very important in 3D printing (the load on the arm is constant) but instead can disturb using 3Drag as CNC, with varying loads.

To remedy this, the solution I prefer is a second Z-axis to the right, substantially as the left one, with a second NEMA motor, connected in parallel with that on left.So the extruder arm is 'resting' on both ends, right and left, and 2 Nema motors support the arm weight

Extruder arm

2a) Cheap solution, using two M8 stainless steel rods

ABOVE RIGHTAs for left support, print the file top.stl.

If you have difficulty finding a ball bearing 8x22 (608xx) as the original, you can use the flange bearings used with the trapezoidal bar. In this case, print top_trap.stl

CENTER RIGHTThe center right support extends the of the extruder bar, and is composed of

• two cheeks, • the support for the sliding bearing on the guide bar sized 8mm, • the support in two pieces for the brass nut on the threaded rod.

Two solutions are availables: 1. aluminum cheeks (20x152x2), see

r_arm_alu.png for holes position, and print r_center_alu.stl

2. printed cheeks, use r_center.stl

Remove the old support (loosen the vertical slider bar from the frame) and mount the two cheeks with two bolts and two M5 square nuts each.Then mount the bearing slide (2 bolts M4) and fix the

right slider, and tighten gradually checking the correct operation.Secure the brass nut (not stainless!) in the center of the "spider" and block it using heat. Block with the heat of a soldering iron also the steel washer, making sure the centering.

Join the two parts with two M3 bolts, using spacers or 3mm washers, so that the brass bolt will pave well on the steel washer. Ensure that washer and brass nut are centered before tightening.

Note:To drill with precision the two aluminum cheeks, after being cut to size with hand nibbler, I used http://www.thingiverse.com/thing:951383 with a 3mm two-edged mill, using Repetier with manual control and measures in file alu_arm.jpg.The holes were passed through with a screw gun and a 5mm tip.

LOWER RIGHTUse a flexible coupling 5x8 to secure the threaded rod to the NEMA motor, and use the part r_bottom.slt to secure the motor to the vertical frame.

NEMA ENGINEJoin the 4 cables from the new engine in parallel to the old, paying attention to the colours.Adjust the corresponding driver for higher current (0,7-0.9 V ref)

Materials (Right side):

● Steel threaded rod, M8 (Brico, 7 € / m)● Flexible coupling, 5x8 (Banggood, $ 3.4)● Stainless steel washer, 10x30● Brass nut, M8● ball bearing, 8mm● NEMA 17 step motor (Banggood, $ 15.99)● (optional) 20x152x2 Alu (Brico) 2 pieces● M3, M4, M5 bolts.

PerformancesAcceptable noise.Height of the extruder arm measured (10 times) by a micrometer, at 17 cm from the threaded rod, using aluminum arms:

a) coming from down: avg: 0.001 min: 0.00 max: 0.01 b) coming from up: avg: 0.026, min: 0.02, max: 0.03

difference: 0.026mm

same, at 41 cm from the threaded rod:a) coming from down: avg: 0.004, min: 0.00 max: 0.01 b) coming from up: avg: 0.059, min: 0.05, max: 0.07

difference: 0.054mm

Good results, but with some dispersion.The threaded bar on the right fluctuates more than the left one (it appears to the eye): this can justify the still acceptable but greater dispersion of values on the right.

Placing a weight of 1 kg the extruder arm is lowered 0.04 mm.

2b) Using two Tr8x8 trapezoidal threaded rods

ABOVE RIGHTAs for left support, print the file top_trap.stl.

Screw the block to the support with 2 M4 bolts, washer, washer, spring washer and nut. Secure with M5 bolt and square nut to frame.

CENTER RIGHTPrint r_center_trap.stl, consisting of two cheeks, support for the sliding bearing and the two pieces for the trapezoidal brass nut.Two solutions are availables:

1. aluminum arms (20x152x2), see r_arm_alu.png for holes position, and print r_center_trap_alu.stl

2. printed arms, use r_center_trap.stl

Remove the old support (loosen the vertical slider bar from the frame) and mount the two cheeks with two bolts and two M5 square nuts each.

Then mount the bearing slide (2 bolts M4) and fix the right slider, and tighten gradually checking the correct operation.Screw the brass nut at the center of the "spider" with two

parker screws : make 2mm pilot holes. Check that the nut protrudes a little from the other side

Add a brass washer 10mm to ensure a flat contact surfaceBlock with the heat of a soldering iron the steel washer, making sure the centering.Join the two parts with two M3

bolts, using as spacer some M3 washers, so that the brass washer will pave well on the steel washer. Ensure that steel washer and brass nut are centered before tightening.

LOWER RIGHTUse a flexible coupling 5x8 to secure the trapezoidal bar to the NEMA motor, and use the part r_bottom.slt to secure the motor to the vertical frame.

NEMA ENGINEJoin the 4 cables from the new engine in parallel to the old, paying attention to the colours.Adjust the corresponding driver for higher current (0,7-0.9 V ref)

Materials (Right side):

● 8mm trapezoidal 304 steel rod 300mm long, with brass nut (Banggood, $ 9.59)● Flexible coupling, 5x8 (Banggood, $ 3.4)● Stainless steel washer, 10x30● Pillow block and flange bearing KFL08 (Banggood, $ 1.78) ● NEMA 17 step motor (Banggood, $ 15.99)● (optional) 20x152x2 Alu (Brico) 2 pieces● M3 flat head bolts ● M3, M4, M5 bolts.

PerformancesVery low noise, but higher than that of a single trapezoidal rod.Height of the extruder arm measured (10 times) by a micrometer, at 17 cm from the threaded rod, using aluminum arms:

a) coming from down: avg: 0.00 min: 0.00 max: 0.00 b) coming from up: avg: 0.04, min: 0.04, max: 0.04

difference: 0.040mm

same, at 41 cm from the threaded rod:a) coming from down: avg: 0.00, min: 0.00 max: 0.00 b) coming from up: avg: 0.04, min: 0.04, max: 0.04

difference: 0.040mm

The difference has stabilized at 40 μ, which is the distance corresponding to one step (precision limit of the trapezoidal rod used).Measurement repeatability is excellent.

Placing a weight of 1 kg the extruder arm is lowered by 0.01 mm.

2z) Installation with two z rods

First, mount the left axis as indicated in §1z, then the right axis:I. We need to free the extruder arm:

• Unscrew one support of the 8mm vertical bar.• Loosen the two brackets that attach the arm to the left carrier and remove the arm.• Unscrew the front of the arm.• Pull out one seeger and remove the sliding bearing LM8U

II. Mount the arm extruder:• Fix the horizontal arm on the carriage to the left.

Note:using 2 extra supports in the original packaging, I mounted a 8mm aluminum tube at the rear of the vertical frame, which serves as a guide for extruder cables.

III. Fit the two cheeks (printed or aluminum)IV. Insert sliding bearing and put the snap ring on the printed support.V. Insert the bearing in the 8mm bar 8m, mount it on frame.VI. Check that the arm slides as before. Tighten.VII.Mount the right rod by following the same method used for the left: first the top and bottom,

then stand and washer stainless, finally complete.VIII.To install and connect the motor Nema Nema 17. The new motor has cable colors that

differ from the original:Original New ConnectorBlue Green YellowRed Red OrangeGreen Yellow RedBlack Blue Brown

note: better to use two identical motors for the z axes, the new one can be used for the extruder.

IX. Using a micrometer fixed at the extruder arm, adjust the print plate.X. Moving now the micrometer along the arm, adjust the levelness by manually turning the

threaded rods.

ConclusionsAs might be expected, the results with two trapezoidal rods are the best ever. Also good results with 2 rods M8.Unfortunately my trapezoidal rods limit the usable height only to 15 cm.A trapezoidal rod, 35/40 cm long, with fewer starts, should give even better results.In my 3Drag I finally installed the two trapezoidal rods, despite the reduction in height.However I cherish the two stainless steel M8 rods, with their central supports (the upper and lower supports can stay the same). If I need to print over 15 cm, to change threaded rods takes a few minutes, an additional benefit of this project.

Notes on STL filesThis project can be downloaded from http://www.thingiverse.com/thing:1104836:The STL are optimized for Slic3r using polyhole() and largehole() modules, Experimental to get big holes printed in right sizes.

Test: Print top.stl for only z=7mm then measure the bearing hole: it MUST be ≈ 22.1 mm, else modify largehole()in 3Drag-z-azis.scad, and remake required STL

All the supports respect the original distance profile-threaded rod of 3Drag: new supports can then be mixed together and with the original supports.Z-axis_plate_TOP.stl used by do_spider.scad, do not print it

nema_spider.stl used by 3Drag-z-azis.scad, do not print it

nut_spider.stl used by 3Drag-z-azis.scad, do not print it

------------------------------------------------top.stl 1 hole 5mmtop_trap.stl 1 hole 5mml_bottom.stl no brim, 1 hole 5mm, 4 x3mmr_bottom.stl no brim, 1 hole 5mm, 4 x3mmr_bottom.stl no brim, 1 hole 5mm, 4 x3mml_center.stl no brim, 2 holes 5mm, 2 x4mm + 2x3mm (spider)l_center_trap.stl no brim, 1 hole 5mm, 2 x4mm + 2x3mm (spider)

Better to print the following files separating the objects (netfabb), support and spider no brim:r_center.stl holes: arms 5mm, blocks 4mm, spider 3mmr_center_alu.stl holes: arms 5mm, blocks 4mm, spider 3mm r_center_trap.stl holes: arms 5mm, blocks 4mm, spider 3mmr_center_trap_alu.stl holes: arms 5mm, blocks 4mm, spider 3mm

CreditsZ-axis_plate_TOP by eastbrothers http://www.thingiverse.com/thing:361064Z axis bearing support by Brandano http://www.thingiverse.com/thing:443103