Deviant Z axis

The RepRap Darwin Z-axis has four screw thread drives linked by a timing belt and toothed pulleys, driven by a large stepper motor.



There are a few things about the design that I am not keen on: -

• The beefy motor and timing belt make it expensive.
• The belt tension puts lateral force on the threaded rod, which causes a lot of friction and looks like it will cause the plastic bearings to wear.
• Making the pulleys and splicing the belt seem like tricky things to get right.
  

When Forrest Higgs came across a small fast stepper motor I decided try an alternative scheme using four motors wired in parallel, eliminating the pulleys, belt and lateral forces. The four small motors are about the same price as the big one, so the cost of the timing belt is saved.



When the motors arrived I got a bit of shock at how small they were. Although I had seen photos from Forrest's blog they were about half the size I had imagined. They are lowish inductance and large step angle (15°) so they can go very fast.

I designed a bracket to hold the motor and mate up with the Darwin corner bracket: -



As you can see there is vast discrepancy between the shaft sizes and one quarter of the weight of the table is born by each of the tiny motor bearings. I was staring to get a bad feeling about the idea.

I had several unsuccessful attempts at making a flexible shaft coupling from ABS: -



None of these were flexible enough or concentric enough. My final design used a piece of plastic piping to get the flexibility.



It has a captive nut for the M3 set screw. The piping is just a friction fit and the rod screws into it.

Even with this design I had a problem with the eccentricity of the hole for the motor shaft.
When you make a hole with fused filament fabrication, the outline of the hole has a start and an end. This causes a bump in the perimeter. Possibly, the outline should end one filament diameter short of where it started, rather than being a full circle. Also each layer should start and end in a different place. When I get chance I will try this.

To remove the bump I ran a drill though the hole. When the hole is as small as this (2mm) the bump displaces the drill leaving the resulting hole off centre. I ended up having to drill them on the lathe, which is cheating.

I mounted the four motors and wired them up in parallel to a micro-stepping chopper drive and a 36V power supply.



I don't like the RepRap scheme of distributing the electronics around the machine so I mounted mine all together at the bottom of the machine on a sheet of perspex. The perspex rests on one of the base diagonals and is held in place by four brackets which clamp around the lower frame.



As soon as I powered it up I realised that the motors had nowhere near enough torque to turn the M8 threaded rods. It wasn't a big surprise, two things that were though:

The motors got ridiculously hot, well over 100°C before I switched them off. The coil resistance is 27Ω which is smaller than some much larger 12V motors, giving a dissipation of about 10W. These look more like 5V motors to me, either that or they are not continuously rated.

I found that my micro-stepping drives don't work well with tin can motors. The micro-steps are very uneven in size. Micro-stepping assumes that the torque displacement curve of the motor is sinusoidal, which doesn't seem to be the case for large step angle tin can motors. Not a big problem in this case as I don't need the extra resolution. I will replace the drive with something simpler when I have got the machine working.

So all in all a big failed experiment! I should have wired up one of the motors before I wasted the plastic making all the mounts.

My fall back plan was to use some larger tin can motors I rescued from a skip recently.



The one on the left is bipolar and the one on the right is unipolar. I decided to try the bipolar ones first, I may switch to the unipolar to simplify the electronics, if they have enough torque.

The shaft coupling was much easier to make because the shaft is bigger (4mm) and has a pin through it. I didn't need to resort to the lathe this time.




I designed and made a new set of motor brackets, they took about 8 hours to print in total.



Here is one motor installed: -



It seems to have plenty of torque for the job. I am waiting for more bolts to arrive to mount the others.

These are not low inductance motors so they won't be as fast as the original single motor design. The large step angle (7.5°) and my 36V supply will help to mitigate this. I originally thought the z-axis speed was unimportant because it moves so rarely, but actually on HydraRaptor I use the z-axis to lift the head 0.4mm when moving between filament runs so it does need to be reasonably quick.

This scheme certainly simplifies the mechanical construction but may not make economic sense. The motors are cheap in large volume (£2-3) but I haven't found a retail price.

Bespoke bracketry

A friend of mine makes underwater cameras from plumbing accessories so that he can observe the fish in his pond. He needed a bracket to hold a piece of pipe with an adjustable tilt angle so he asked me to make one. Here it is: -









I don't know how long ABS will last in a pond, but as long as it is years rather than months it is no trouble to print it again.

From illusion to creation

A few days ago I spent a frustrating evening trying to create this test shape in ArtOfIllusion: -

No matter what I did, I could not get a manifold object that could be exported as an STL file. Eventually I reduced the problem to the fact that AOI cannot do a simple boolean subtraction of two rectangular cubes correctly.



The result looks OK but it is non manifold, I think some of the triangles are the wrong way round.



Fixing it with the Solid Editor produces this: -

So AOI is not really usable for engineering. It is open source, so theoretically I could fix it myself, but life is too short to fix my own bugs, let alone other peoples. I posted a bug report and moved on.

Speaking to one of my colleagues who does mechanical design for a living, it would seem that professional tools are much easier to use and you don't have to worry about operations on coincident faces, etc. He recommended CoCreate Modeling Personal Edition, which is free for non professional use. It is limited to 60 parts in one design and can only save designs in its own proprietary format, but it can export STL and VRML. It is Windows only and needs an internet connection every three days. It is however, very easy to use. I had a quick look at Google Sketchup and Blender but they did not seem as easy.

The way you model in CoCreate is that you start by drawing in 2D on Workplanes. Workplanes can be arbitrary, but generally are created on a face of the part you are building. The 2D drawing tool is called CoPilot. It shows lots of hints when lines are parallel or line up with things already drawn, and shows dimensions to nearby features. This makes it very easy to create 2D geometry with precise dimensions, or geometrical alignments. You can also draw construction lines to help you line things up.

When you have a 2D profile on a Workplane you can then Extrude it or Turn it to make a solid. This is similar to AOI, except that the 2D drawing in AOI is very primitive and it is hard to get exact dimensions. As well as adding material you can remove it with familiar machine operations like Mill, Bore, Punch, Stamp, Section and Shell. These would all take multiple steps in AOI. CoCreate also has the boolean operations: Unite, Intersect and Subtract, but whereas you have to do almost everything in AOI with booleans, I have not needed to use them so far in CoCreate.

Once you have your basic 3D form it is very easy to add chamfers, fillets, blends, etc, and surprisingly it is also easy to remove them again.

Nothing I have done so far, (including filling a hole with a cylinder of the same dimensions), has managed to create a non manifold shape. It is very quick and easy to make practical objects. Here are a couple of parts I modeled for my experimental z-axis as they appear on screen: -



I have no idea how I could have created these in AOI.