Yet another Prusa Z-coupling

I finally got around to building the Holiday Prusa Mendel I printed over Christmas. I had a few problems with some of the comedy parts and had to revert to using some of the more up to date ones that I sell.

I didn't find the Z couplings worked very well. The requirements are to couple the M8 threaded rod to the 5mm motor shaft exactly coaxially and with no vertical play, but with some angular flexibility to cater for slightly bent threaded rods or any slight angular misalignment.

The rods are not held very coaxially because the clamp is not symmetrical. The alignment depends on how much the two independent clamps are squeezed, which depends on the exact diameter of the shafts relative to the printed diameter of the part.

They are not very flexible either because they have to be strong enough to support half the weight of the X-axis and the extruder. The direction of pull is in the weak direction of the part that tends to de-laminate it, consequently I print them 100% fill to make them strong enough. I would imagine that if there is any wobble in them the constant flexing would eventually fatigue the part and cause it to break.

I looked around at the various attempts to improve these, but I wasn't happy that any satisfied all the requirements above. I did find two sources of inspiration though:

This one by keegi uses a piece of tubing to provide the angular flexibility and it also helps to grip the smooth motor shaft.

This one by Griffin_Nicoll has the strong direction of the part in the right direction, but suffers the same problem as the original because it has two independent clamps. That is easily solved by removing the split in the top section, but then it would be difficult to grip the smooth motor shaft without the clamp halves being exactly parallel, which would depend on the exact shaft and part sizes. It also has no obvious flexibility. Putting the tubing on the motor shaft solves both these problems.

I hacked Griffin's script to make this version: -

I removed the split, changed the holes and the nut traps to fit M3 and changed the motor shaft diameter to 7mm, which is for a 5mm shaft with tubing on it.

Here it is mounted: -

Both halves are identical inside so not matter what the shaft size is they will always centre and align the shafts automatically. The sleeving allows the shaft to flex angularly and also makes a very firm grip on the motor shaft. The part bears weight along its strong direction and is not required to flex at all, so should last forever. Another possible benefit is if the part is made from PLA it is somewhat insulated from the motor shaft by the tubing, so there is less chance it will melt.

I haven't run the axis yet, but it turns very easily manually and there is no wobble at all. I will include these in my kits from now on and I will include the short piece of tubing as it would be annoying to have to buy just 30mm. Note it does require four extra M3x20 bolts, nuts and associated washers.

The files are here on Thingiverse.

FR4 fail

Well it seems that FR4 only lasts for about a week. The grip slowly fades making the parts very easy to remove. In fact they all pop off as the bed cools below 60°C and slide about due to the fan and the bed's final movement to the front. The odd small part falls down inside the machine.

If I mounted my machine so it was inclined at 45° they would all fall out the front and could be directed by a chute into a hopper and the machine could then build continuously unattended. Who needs a conveyor belt! The only problem is the grip is now not enough to hold the bigger parts during the build.

I have tried cleaning with acetone but it doesn't seem to help. I suspect the high temperature is making the epoxy more brittle and less sticky. I will be able to prove that when the FR4 without copper on HydraRaptor fails. If I then turn it upside down and it still works on the under side then it is not a temperature ageing effect. If the other side is still working then it must be a reaction to the ABS or the acetone that is the problem.

It is shame because I much prefer a solid substrate to tape. Something like polyimide and fibreglass laminate would probably be ideal but it is hundreds of dollars for a piece big enough.

Wolfgang has posted a mystery material to me that sounds promising, so back to PET tape until it arrives. My friend Tony found that Farnell sells it in wider rolls. It seems to be a bit thicker as well, so is easier to apply, but a lot more expensive than the stuff from BestOfferBuy.com.

ABS on FR4

I have been printing both ABS and PLA on PET tape for more than a year now. It works well and lasts for many months, but eventually the silicone adhesive fails and it blisters. Applying it is fiddly to avoid any overlap but also not leave gaps between the adjacent runs of tape. I have been on the lookout for a solid material to avoid these pitfalls.

Stoffel15 (Wolfgang) told me that FR4 fibreglass PCB material works well. FR4 is the most common PCB material and is a glass fibre and epoxy resin laminate. It will handle solder re-flow temperatures (~ 240°C) for short durations and can be used continuously at 140°C. As I haven't worked on single sided PCBs for many years, I had forgotten what the surface of the raw material looks like. It is actually smooth and glassy, so ideal as a bed material.

I ordered some single sided PCB material from Farnell. It works fantastically well. It seems to have a bit more grip than PET and has the advantage that there are no lines on the part from the joins in the tape. It also has no give in it, so I don't get any blistering at sharp corners like I did with tape, sometimes leaving shallow dimples.

Another advantage is that when the object cools it tends to break free because it contracts more than the bed does. With tape there is some compliance, so it usually stays stuck when the object cools and it is often hard to remove parts. With FR4, if you get the layer height spot on, the parts break free of their own accord, and if not, are very easy to snap off. This vertex bracket was loose after the bed cooled to 50°C.

Yet another advantage is that I stick the tape to a steel plate 0.9mm thick that weighs 280g. The FR4 is 1.6mm thick but it only weighs 134g, so less than half the mass.

I also tried some plain FR4 without copper and that seems to work just as well. It is 0.9mm thick and weights only 75g. The disadvantage is it is bright yellow, which makes it hard to see the white plastic on it.

I have printed a full set of Mendel parts so far on FR4 and every part has come out perfectly flat, and was easy to remove.

I don't know if it will degrade over time, but there is no sign of surface damage so far. The dark features on the picture above are marks on the aluminium plate underneath.

The nice thing about the z - probe I have on HydraRaptor is that I can change the bed without any calibration.

This is what the underside of an object looks like.

I used the same temperature I used for PET tape, which is 140°C for the first layer and 110°C after that.

I haven't tried PLA yet, but my guess is it will stick because it seems to stick to a superset of things ABS sticks to.

Great tip Wolfgang!

In the past I tried FR2 (SRBP, Paxolin) but that did not work, probably because it had a matt surface. I also tried some CAT7FR, which is another type fibreglass PCB material, but again it had a matt surface and did not work very well. I was able to build a flat object on it, but the first layer outline did not stick properly, so some holes were a bit scrappy.

The copper on the bottom of the single sided material could be used as a heater like the Prusajr heated bed design.