Maus Axis Driver Construction - VikOlliver/RepRapMicron GitHub Wiki

The 3D printed parts for a V0.05 Axis Driver are on two 3D print sheets. The required files are maus_parallelogram_axis_driver.stl and flexure_coupling.stl which are built from the RepRapMicron Github repository or can be downloaded as printable STLs here. Ensure that you pick the MAUS or PIKA variant as appropriate. Parts are printed with 2 or 3 shell layers, and 5 top and bottom layers. The latter ensures that all horizontal flexures are printed with fully solid infill. Plain PLA filament is adequate.

PIKA Has its own version of the Axis Driver in development here. Assembly and performance is identical to the MAUS version, except that both NEMA14 and NEMA17 stepper motors are supported.

The earlier V0.04 Axis Driver Instructions still available on the wiki.

If there should be any dangling pieces of filament on the Axis Driver flexures, do not attempt to remove them. They should not affect the operation of the flexure and removal may inflict damage.

Note that the Flexure Coupling will likely need to be printed with a brim, and so is kept separate. The mechanism's essentials are spacers and mounting for a NEMA17 stepper, a coupling, a supported drive screw, an endstop, and a frame containing a pantograph-like flexure mechanism. Ideally the NEMA17 should have 400 steps/rev. 200 steps/rev will give adequate horizontal spacing but will not allow as much flexibility in creating the desired thickness of a resin layer.

Three of these completed assemblies are needed, and they are identical. However PIKA and MAUS drivers have one different part: The Motor Pillar (top right of image below). On MAUS this has three rows of holes on its Metriccano blocks, and on PIKA 2 rows. The assembly process is exactly the same for either variant.

Two or three fasteners need to be prepared, and these are the most complex machining operations in the assembly process. That said, they're not hugely sophisticated.

If you can get M3 x 50mm screws with a hexagonal or knurled head which fit in the Flexure Coupling, use them. Otherwise the instructions below show you how to machine a screw to fit. Also, some wiring.

Select the most pristine M3 x 50mm screw available, one with thread along the full length. Use a zinc plated or plain steel screw - avoid screws with a black finish and stainless steel. "Pan head" screws will be easier to shape.

Wrap the thread with a non-gummy soft tape and secure in a drill chuck with the head sticking out. Run the drill and simultaneously file the side of the screw head - this procedure is much easier with a pan head screw (one that is slightly domed). The objective is to reduce the diameter of the screw head to less than 5.2mm - slightly smaller the width of an M3 nut from flat to flat. Be very careful not to damage the thread. If you damage it, pick another screw and start again. This 50mm screw will be used for the Drive Screw. It is VERY IMPORTANT that the screw thread not be bent, scuffed, scratched, spalled, or mutilated in any way.

Remove the tape and clean the thread.

Wind an M3 nut onto the screw to about 10mm from the head. Apply a small quantity of thread locking adhesive to the thread adjacent to the head. Turn the nut all the way down to the head and tighten. Wipe off any excess adhesive.

To make the two Bearing Nuts, fit the 3mm drill bit to the drill. Hold an M3 nut in pliers or a vice and cleanly drill the thread out of the nut. If you have a 2.7mm bit (7/64"), running that through first will make the 3mm bit less likely to bind. A little oil (preferably a cutting oil) will help prevent the drill bit binding too. Clean away any fragments of oily swarf. These nuts will be used for the Nut Bars.

Preparing limit switch wires

Take the Limit Switch Wires you will be using on the CNC Controller for this axis. Bare both ends approximately 10mm. Take a 50mm length of 28ga stainless steel or nichrome wire, place it alongside a switch wire so the ends are flush. Bend the switch wire and 28ga wire out at 45 degrees to ensure a tight twist, and twist together 10 times or so - it is important that both wires twist, rather than just one wraps around the other. Trim off any uneven ends on the twist. Fold the twist down against the insulation on the switch wire, point the 28ga wire forward, and shrink heatshrink over the twist to secure it to the insulated wire (image shows this process from left to right). Repeat for the other switch wire.

Forcibly fitting Flexure Coupling to NEMA17 - normally hand pressure is sufficient

Push the bottom (flat with maze-like patterning) end of the conical Flexure Coupling onto the NEMA motor shaft until the shaft contacts the flat shelf inside the coupling's cavity. This is recessed approximately 7mm maximum, and you may find it convenient to mark this off on the shaft. The shaft's hole is gently tapered, and the exact distance will vary depending on the quality of your print and the exact diameter of the shaft.

If the 3D print suffers from "elephant's foot" where the first layer is overly thick, ream the first millimetre or two with a 5mm drill bit. Hand pressure for a snug fit is generally sufficient. Failing that, gentle pressure from squeezing with a clamp should suffice to push the Coupling onto the motor shaft. If you crack it, prise it off and print a new one.

Note: Clamps have a swiveling face and a static face. If a clamp is used, the coupling must be pushed by static face of the clamp.

Rotate the coupling, and by eye ensure it is correctly centred on the shaft. It may be necessary to push down on one side or the other to achieve this if the initial fitting was not perfectly square on.

Motor with coupling, Motor Pillar Assembly, and optional Spacers

The Motor needs to fit into the Motor Pillar Assembly such that the top of the Coupling protrudes from the Motor Pillar between 11mm and 14mm. The number and size of Spacers required to achieve this will depend on the length of the Motor's shaft. It may be necessary to print additional Spacers, or scale spacers vertically to suit your Motor. In this example a 4mm and 5mm Spacer were required for a Motor with a shaft length of 24mm.

Clearing nut cavities and installing nuts in the Motor Pillar Assembly

Each captive nut cavity in the Motor Pillar Assembly has a built-in support which will need to be picked out with a pointed tool before the nuts can be installed. Once cleared, insert 4 M3 nuts into the captive nut cavities in the Motor Pillar Assembly, shown in the image above.

Securing Motor and Spacers to Motor Pillar Assembly

Adding any necessary Spacers, secure the Motor to the Motor Pillar Assembly with four M3 screws. Keep the Motor centred as well as practicably possible. The exact length of the screws will vary depending on the Spacers used. In this case 29mm M3 screws were required, as the threaded holes in the Motor were too shallow to accept a 30mm M3 screw. Rather than shorten the screw, washers were added under the screw heads.

Removing built-in support from Nut Bars and Frame Trio

Remove the support material from the five captive nut cavities on the Frame Trio and from the pairs of holes on the Upper and Lower Nut Bar. Note that the Upper Nut Bar has octagonal holes. This is intentional, and they do not hold nuts.

Inserting nuts in Lower Nut Bar

Insert two M3 nuts into the two hexagonal cavities in the Lower Nut Bar, and press them home with a 50mm M3 screw.

Inserting nuts into the underside of the Frame Trio

Insert five M3 nuts into the five hexagonal nut cavities on the Frame Trio. Be careful to avoid stressing the flexures in the Frame Trio when inserting nuts; it is best to lay the Frame Trio upside down on the edge of a bench, supporting the overhang with one hand. Use a 50mm M3 screw to press the nuts home.

Not all axes will use all nuts, but they are much easier to insert now than when the Axis Driver is assembled.

Insertion of nuts into the top side of the Frame Trio

At the other end of the Frame Trio is a cross-shaped structure. This contains a nut cavity which holds the Drive Nut, and it is important to select an M3 nut that is free from defects for this purpose. Insert this nut taking care not to damage the thread (i.e. screw a 50mm M3 screw all the way through the nut for leverage) and to insert it as fully and evenly as possible. Note that this cavity is particularly tight by design and significant force will be required.

Take two M3 nuts and insert them into the slots indicated above. Place them in with pliers, tweezers, or forceps to ensure they present flat sides to the edges of the cavity when they go in. A printable Nut Tool is provided to push them in straight with.

The nuts should click into place, and when fully seated it should be possible to access their internal thread through the holes at the end of the framework. If not, hit them again with a little more enthusiasm.

Insert Bearing Nuts into Nut Bars (drill bit is a reminder that these are the drilled-out nuts)

Insert the two drilled-out Bearing Nuts into the Upper and Lower Nut Bars, and push them in as far as they will go. Temporarily thread an M3 nut onto a 50mm M3 screw to use as a punch for this purpose.

Fitting The "Loom Bands" for anti-backlash

Take a bunch of 3 or 4 silicone "Loom Bands" or a thin 'O' ring approximately 25mm-30mm diameter. Thin silicone bracelet string loops may be used. Do not use ordinary rubber bands or rubber string. Stretching the bunch gently, slip them under the hooks near the Drive Nut on the Frame Trio. Then temporarily use a long M3 screw to hold them out of the way. Repeat with more bands and hook on the other side of the Drive Nut.

Screwing Drive Screw and Lower Nut Bar into Drive Nut

Using as little oil as possible, lightly lubricate the Drive Screw. Thread it on to the Lower Nut Bar as shown. Loosely screw this into the Drive Nut from the underside of the Frame Trio.

Stacking nuts and washers on the Drive Screw

Onto the protruding screw, thread two M3 nuts approximately 10mm down the shaft. Lightly lubricate two 7mm or 8mm M3 flat washers and also put these on the shaft. (The Drive Screw and washers shown in the image have previously seen significant use in another Axis Driver).

Fitting the Upper Nut Bar to the Axis Driver

Take the Upper Nut Bar and lightly lubricate the Bearing Nut. Insert two M3 x 50mm screws via the octagonal cavities. Insert the screw shafts through the Frame Trio into the Lower Nut Bar pillars such that the Upper Nut Bar pillars are inside the loops formed by the bundled Loom Bands. Do not tighten the screws yet. The end of the Drive Screw should be in the Bearing Nut at this point.

Lifting the Loom Bands and positioning the top lock nut

While maintaining some tension on the Loom Bands to prevent them slipping out of their hooks, lift the Bands onto the rounded areas of the Top Nut Bar. Ensure the Bands are not trapped under the Nut Bar's pillars. Loosely fasten the Nut Bars' 50mm M3 screws so that some movement of the pillars against the Frame Trio is possible. Pull the head of the Drive Screw approximately 1mm away from the Lower Nut bar. Spin the top nut on the drive screw until it contacts the Top Nut Bar (a small screwdriver is useful for this).

Try-fitting the Drive Screw into the Flexible Coupling

Hold the top nut steady and unscrew the Drive Screw a little. It should start to protrude a little from underneath the Lower Nut Bar. If you screw it in, less should protrude. Place the Frame Trio Assembly on top of the Motor Pillar Assembly and see if the head of the Drive Screw engages with the cavity in the Flexible Coupling. There is some compliance in the Flexible Coupling, but the amount of protruding Drive Screw should be adjusted so that it can only just be felt to engage fully with the Flexible Coupling.

Securing Frame Trio Assembly To Motor Pillar Assembly

Secure the Frame Trio Assembly to the Motor Pillar Assembly with four 50mm M3 screws, then back each screw off 1/4 turn. Rotate the Flexible Coupling back and forth a few degrees to check that the Drive Screw head is fully captive and there is no excessive friction.

Fastening the lock nut on the Drive Screw

Until the lock nuts are secured, avoid rotating the top nut on the Drive Screw or the Drive Screw itself.

Using a small tool, rotate the lower lock nut up the Drive Screw until it is pressed against the upper nut. Using a pair of fine pliers, haemostats, or even 5.5mm spanners etc., tighten the two upper nuts together. When they are successfully tightened it should be possible to rotate the Drive Screw back and forth by turning only the lower nut. If this causes the lower nut to come undone, the nuts were not tightened together sufficiently.

Connect the Motor to a convenient controller and drive the Motor so that the Drive nut travels up and down the shaft a couple of millimetres. Repeat this two or three times. If the Drive Screw does not rotate evenly it may be necessary to re-centre the Nut Bars and/or the Frame Trio. Then carefully tighten the screws in the Nut Bars and Motor Pillar Assembly. Do this on alternate screws to ensure even loading.

Endstop components and captive nut holes

Turn the Limit Switch Support so that the two grooves beside the central hole are visible. These are where the Limit Switch wires will go. Insert two M3 nuts into the Limit Switch Clamp captive nut cavities as indicated above.

Limit Switch Support showing correct placement of Limit Switch wires

Insert two 12mm M3 screws into the outer pair of round holes in the Limit Switch Support from the underside. Rest a pair of Limit Switch Wires in the grooves, such that the bare wires just protrude into the U-shaped central void. Try to maintain this alignment as you put the Limit Switch Clamp on the screws and securely tighten them.

Folded Limit Switch Wires

Fold the bare wires over the Limit Switch Clamp such that they cross. The lower wire should be as close as possible to the Limit Switch Clamp, the upper wire should be separated from the lower wire by approximately 1mm. Trim off any bare wire that goes beyond the Limit Switch Clamp.

Mounted Limit Switch

Using two 16mm M3 screws fitted with M3 washers attach the Limit Switch to the end of the Frame Trio as shown above. The limit switch should be level and positioned so that the lever protruding into the U-shape contacts the wires before the Drive Nut bottoms out on the Drive Screw. Be conservative at this point. Adjust it for maximum range of motion after testing.

Optional Tension Band Booster

If the bands or 'O' Rings are not supplying adequate tension to remove backlash when the Drive Nut is at its maximum distance up the Drive Screw, a Tension Band Booster may be required. This can also happen in use if the Bands stretch slightly.

Raise the Drive Nut to the maximum elevation. Raise the cluster of Bands on one side with a utensil and insert the Tension Band Booster under them with the flanges pointing down. Repeat for the group of bands on the other side. No fastening is required as the booster is held in place by Band tension and the flanges. The Booster has holes that permit adjustment of the Nut Bar screws without removing the Booster.

Completed Axis Driver

Manually test the mechanism by rotating the Flexure Coupling and ensuring that rotation is smooth. Connect the motor and limit switch to the CNC Controller. Verify both work, movement is in the expected direction, and that adequate tension is applied on the Drive Nut and Flexure Coupling to ensure backlash-free movement. The mechanism will initially be inconsistent and make occasional small cracking noises while all the flexures wear in. Run the mechanism back and forth for approximately 10mm on the Drive Screw four or five times using the CNC Controller's manual jog capability.

You may need to bend the limit switch wires slightly, so be observant on the first limit switch test.

Smooth movement of the platform and lack of backlash can be verified by examination with a USB microscope. Approximate movement calibration can be achieved by observing the movement of the 3D printed layers on the platform's surface with a USB microscope and counting the 0.2mm layers as they pass a fixed point on the monitor. There may be some lateral movement, but this will be constrained once the Axis Driver is fitted to its axis on the μRepRap.

Remember that you will need to complete three of these to build a μRepRap.