X & Y

I have finished building my Darwin Cartesian bot. It went together fairly easily although I did cheat when it came to making the pulleys. The idea is to cast toothed pulleys in PolyMorph using a mould made from RP components and lined with a piece of belt. I had one go at this and decided it was not going to produce an accurate result: -



The RP mould, when made on my machine, is not round enough and for some reason the diameter of the mould is too small, making the resulting pulley very thin walled and flimsy. It produced a 13 tooth pulley but 16 teeth is the correct number for 0.1mm per motor half step and makes a chunkier pulley. I bought three aluminium ones from Farnell for £5.90 each: -



These are ridiculously expensive for what they are but I think it is worth spending a bit of money in an area that increases accuracy. It is also one of the few places where the accuracy of the parent machine affects the accuracy of the child.

The big problem is that they only have a 4mm hole in them so you have to bore out the x-axis one to 1/4" and the two y-axis ones to 8mm. To do that accurately really requires a lathe. As mine is only a tiny watchmaker's lathe I had to use every drill from 4.5mm to 8mm in 0.5mm steps. I found that dipping the drill bits in Trefolex cutting compound made it much easier to drill. This was recommended to me for tapping but it great for drilling as well. It is a sort of jelly, so not too messy.

When you use a twist drill to make a hole it comes out a little small and not perfectly round. It needs to be finished off with a reamer to get a nice fit onto the motor shaft and the 8mm rod. I happened to have a 1/4" reamer but I had to improvise for the 8mm ones with a piece of emery paper wrapped around a 7mm drill shank. Not ideal, so I ordered an 8mm reamer as I expect I will be making lots of 8mm bearing holes in the future.

I also had to drill and tap M3 set screw holes in the pulleys. Easily done with a drill press and it means I don't have to file a long flat on the y-axis drive shaft.

I tested the axes with a signal generator connected to the step input of my stepper motor drivers to find the pull in step rate, i.e. the maximum rate at which the motor will start with no acceleration. Here is the x-axis running at 150mm/s: -


RepRap Darwin x-axis from Nop Head on Vimeo.

Any jerkiness seen is the video, not the axis, which runs very smoothly. The axis does not have the mass of the extruder on it yet but I have run it at the same speed with a reel of solder on top. I expect with a bit of an acceleration ramp, like I use on HydraRaptor, I will be able to get it to go two or three times faster. This is not too surprising because it is a similar design to a 2D printer carriage but with a much more powerful motor. It will be interesting to see what effect it has on stringing if I speed up the head moves from 32mm/s to 150mm/s or more.

I have the motors wired bi-polar parallel, which is the fastest configuration. The inductance is four times less and the voltage halved so I think that is 8 times faster than bi-polar serial. Added to that I am using a 36V supply instead of 12V and FETs rather than Darlingtons. The voltage on the motor will have gone from about 9V to about 36V, so all in all about 32 times faster current rise rate I think. I am using expensive drives but the only aspect I don't know how to do cheaply is anti-resonance, so unless I am stepping through the resonant frequency I should be able to recreate this performance cheaply.

The rated current in this mode is 3.4A per phase but I am only using 1A per phase at the moment so that they don't get too hot. Given that the average supply current from the 36V rail will be correspondingly less than this, it should be fairly easy to generate the 36V supply from 12V to keep to the original goal of using PC power supplies or car batteries.
It would be good to use electronics that can boost the current while accelerating and decelerating.

Here is the y-axis running at 100mm/s: -


RepRap Darwin y-axis from Nop Head on Vimeo.


A few things I have noticed about the design that I would do differently: -

There is a bit of runout on the y-axis motor coupler leading to the shaft wobbling a bit and the motor bracket flexing to accommodate that. I think it would be better to have another bearing at this end of the shaft and a flexible coupling to the motor.

Several of the bearings are made with an RP insert, in my case ABS. I don't know how long these will last. I will have a go at making them from HDPE some time as that should make a better bearing and possibly replace the y-axis ones with 0608 skate bearings.

The rod that carries the Y-axis idler pulleys is held in place by tight fitting "jam" bearing inserts. I can't see the point of these, other than making all the y bearing housings the same. I would replace two of the bearing housings with a smaller part with an 8mm hole through it to carry the rod and possibly a set screw to lock it in place.

The X and Y axis opto tabs enter from the top. The opto has a 0.8mm vertical slit which is the optical aperture. A tab coming in from the side blocks all the slit at once making its resolution several times better than when the tab enters from the top. This graph, taken from the datasheet illustrates the difference: -


The z-axis opto endstop is at the top whereas I prefer to home away from the workpiece so that homing is always a safe operation when z is homed before x and y.

I will leave these tweaks until I have the machine up and running. All I have to do now is make a new extruder, hook my stepper drives to a micro and port my firmware. I will then have a Darwin that I can directly compare with HydraRaptor and see how it differs in performance. I will then look at replacing the electronics with something much cheaper.

Every little helps

The RepRap Darwin design has 10 diagonal tie bars across the corners of all but the top face of the cube, making it very rigid. These are attached by 20 diagonal tie brackets.



The brackets are held onto the protruding 8mm stubs by M5 set screws through a captive nut. The diagonal bars are then held in place by M8 nuts either side of the bracket.

When fitting them I noticed that the set screws and nuts are not necessary. All the holes I make come out a little undersized and stringy so I clean these out with an 8mm drill. This makes them an interference fit onto the M8 rods. The force exerted by the M8 nuts is enough to squeeze the bracket to make it a tight fit. This is the case when they are made from ABS with 25% fill. Other plastics may be too strong or brittle.

This shortcut saves 20 grub screws and nuts and the time to fit them (inserting the nut can be quite fiddly). Not only that, the bracket can be simplified and made smaller because it does not need space for the nut and grub screw. This optimisation is well worth doing because, although these brackets are quite small, there are 20 of them so they are a significant part of the time taken to replicate.

Here is my smaller design which uses 21% less plastic and reduces the time to make 20 from 11.5 hours to 9 hours on my machine :-



I also used a truncated teardrop for the lateral hole. This relies on the fact that filament can span gaps as well as being able to build out at 45°. The drawing below illustrates that, even for an 8mm hole, the difference between a proper circle, which would require support material, and this truncated shape is very little. It also shows where the full teardrop would extend to.



Here is a picture of it installed alongside the old design: -



I think this is a beneficial mutation that will slightly increase the rate at which Darwins reproduce in the wild. The new DNA can be found here.

A bit of a contraption

As I was adding the diagonal tie bars my wife said "it's becoming a bit of a contraption". I am not sure if that is a good thing or a bad thing!

My alternative z-axis using four tin can stepper motors works reasonable well. I am running them from 36V with a constant current chopper drive. They are all wired in parallel and the total current is set to 1.25A. They have 22Ω coils so that corresponds to about 7V. I am guessing they are rated for 12V, but if you run steppers at their maximum rating they get very hot, especially when mounted on plastic rather than a metal chassis. Under running them, as I am, they only get to about 40°C, which should be fine even for PCL and PLA brackets. Total power used by the axis is about 9W.

The standard Darwin z-axis can carry a small child: reprap-prints-child. Not having any small children, I tested it with a vice that weighs 3.3Kg. As it would take 300 hours to print anything that size so I think it is a reasonable worst case test.

The pull-in step rate (i.e. the maximum rate that the motor will start at with no acceleration) is about 400 steps/second. It runs reliably at 320 steps/s, which is 8.33 mm/s. If I understand the settings page on the wiki then this is more than 10 times faster than people are running the belt drive version. Still much slower than HydraRaptor's z-axis though.

Here is a video of it in action: -


Alternative RepRap Darwin Z-axis from Nop Head on Vimeo

As you can hear, it doesn't make a lot of noise, something that is a big improvement on HydraRaptor, which has a very noisy z-axis.

The total travel is 230mm, which is also a bit better than the standard Darwin I think, but you have to subtract the length of the extruder barrel to get the maximum work height.