MAUS V0.04 Axis Driver Construction (Deprecated)
The 3D printed parts for an Axis Driver are on two 3D print sheet (maus_axis_driver.stl flexure_coupling.stl). Note that the Flexure Coupling will likely need to be printed with a brim, and so is kept separate. The mechanism's essentials are a spacer and mount for a NEMA17 stepper, a coupling and drive screw that fits a lever arm and bearing block, and a structural box with a parallelogram-like flexure on the end. Three of these completed assemblies are needed, and they are identical.
Two fasteners need to be prepared, and these are the most complex machining operations in the assembly process. That said, they're not hugely sophisticated. Also, some wiring.
Select the most pristine M3 x 50mm screw available. 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. 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 Bearing Nut, 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. Clean away any fragments of swarf. This nut will be used for the Bearing Block.
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.
Fitting Flexure Coupling to NEMA17 - note coupling is held by static face of the clamp
Drive 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, and you may find it convenient to mark this off on the shaft. Gentle pressure from squeezing with a clamp will suffice, or a more percussive approach may be taken with careful deployment of a soft-faced mallet.
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.
Insert 6 captive nuts into Motor Pillar Assembly
Insert 6 M3 nuts into the captive nut cavities in the Motor Pillar Assembly, shown in the image above.
Insert 3 captive nuts into Platform Assembly
Please DO NOT trim the small bits of filament on each side of the Drive Nut Holder. These stabilize the flexures during assembly.
Take two M3 nuts and insert one into each captive nut slots on either side of the Platform Assembly.
Inserting a tight captive nut into the Drive Nut Holder
Select a pristine M3 nut to insert into the cavity in the central Drive Nut Holder until the nut is fully enclosed. This hole has a very tight fit and a taper. Take care not to snap the small bits of filament on either side. This is best done by carefully but very firmly inserting the nut slightly (your mallet and an M3 x 50mm screw may be needed), and then inserting an M3 screw and washer from underneath and tightening the screw to seat the nut. It is important to hold the Drive Nut Holder with pliers when doing this, as holding any other part of the Platform Assembly will cause excessive torque and damage the flexures.
Insert Bearing Nut into Bearing Block
Insert the drilled-out Bearing Nut into the Bearing block and push it in as far as it will go with the head of an M3 screw.
Limit Switch's crossed wires (white Metriccano Strip used for clarity)
Take the limit switch wires and thread them through the two widely-spaced holes in the Motor Pillar Assembly. Using a 2-hole Metriccano Strip and two M3 x 12mm screws, secure the wires so that the bare wire is level with the top of the panel in the Motor Pillar Assembly. The top of the Metriccano Strip should also be flush with this surface. Fold the two wires flat over the Metriccano Strip so that they cross but do not touch. Trim the wires flush with the exposed face of the Metriccano Strip.
Completed Motor Pillar Assembly
Place the NEMA Plate on the stepper motor, and the Motor Pillar Assembly on top of that. Secure in place with four M3 x 20mm screws and washers. If your NEMA17 motor's threaded holes are too shallow, you may need additional washers.
Checking initial Drive Screw and Flexure Coupling alignment (Note: nut should fit inside coupling)
Press the Drive Screw vertically into the hexagonal hole in the top of the Flexure Coupling so that it does not wobble, while avoiding impaling yourself on the Limit Switch wires. Most of the nut will go into the coupling. Rotate the coupling by hand and make sure the Drive Screw can rotate vertically and has not been accidentally bent. If it does not remain vertical when rotated, even when its position is corrected, chances are that the Flexure Coupling is not properly fitted to the motor shaft. Press down hard on the coupling away from the direction of lean to fix this.
Remove the Drive Screw and screw it into the Drive Nut Holder until 16mm of thread remains between the Drive Nut Holder and the head of the Drive Screw. Place the Platform Assembly on the Motor Pillar Assembly, push the Drive Screw firmly back into the Flexure Coupling and rotate the Flexure Coupling until the two assemblies meet, barely pushing the Drive Nut Holder up. When you rotate the Flexure Coupling back and forth by half a turn, the downward pressure should be only just sufficient to keep the Drive Screw locked inside the Flexure Coupling.
Connect the Platform Assembly to the Motor Pillar Assembly with four M3 x 50mm screws - you may need to rotate the Flexure Coupling again to relieve any stress added to the Drive Nut Holder when the screws were tightened. When this is done the Drive Screw should be firmly held vertical.
Try-fitting the Bearing Block
Screw two M3 nuts onto the exposed Drive Screw leaving approximately 6mm of thread, and place two M3 washers on top. Avoid rotating the Driver Screw at this point. Temporarily fit the Bearing Block in place with two M3 x 15mm screws. Adjust the nuts below so that they are finger tight against the Bearing Block. Remove the Bearing Block.
Carefully remove the two M3 washers. Raise the top M3 nut by one-half a turn. Hold the nut from above with pliers. Do not rotate this nut again. Using needle-nosed pliers (or heaven forbid, a proper 5.5mm spanner) lock the lower nut into the upper nut. With your fingers, check they are tight.
Replace the two M3 washers on top of the locked nuts. With a small drop of oil each, lubricate the top washer, Drive Nut, and Bearing Nut. You may now clip away the fine pieces of PLA on each side of the Drive Nut Holder.
Loom Bands on the Bearing Block
Loom Bands are weak elastic bands approximately 18mm in diameter, made from a non-latex material that does not readily degrade. They can be obtained online, or from toy and craft shops where they are used for making woven jewellery. They are used here to provide anti-backlash tension on the Drive Nut. It is helpful to use a variety of colours.
Place five loom bands on each side of the bearing block. Re-attach the Bearing Block taking care not to trap any of the Loom Bands.
Working from the centre bands, hook the bands under the hooks on the side of the Drive Nut Holder alternating left and right to reduce sideways stress. Use blunt tweezers or a dental tool for this, as a sharp object will damage the bands. This completes the Axis Driver assembly process.
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.
When testing an axis for the first time, it is not uncommon to see the Drive Screw moving back and forth as it rotates. This indicates the Flexure Coupling is not quite seated and will cause retrograde movement of the platform. Use the CNC controls to rotate the Drive Screw so that the screw is moved closest to you in its orbit, place a stout screwdriver or similar lever under the edge of the Flexure Coupling, and pry it away from you against the Motor Pillar Housing. Try a few test rotations. Repeat the process until Drive Screw eccentricity is minimal. Perfect centring is not necessary, but overall displacement should be less than 0.4mm
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.
Remember that you will need to complete three of these to build a μRepRap.