Monday 13 April 2009


This may be an evolutionary dead end, with the move to stepper motors and pinch wheels, but I wanted to try a couple of things that have been on my "to try" list for a long time.

The main issue that I have had with the pump part of the original extruder is that the bearings wear out fairly quickly. Both the half bearings themselves and the lands on the shaft. One problem is that being only half bearings, any lubrication soon gets carried away by the plastic.

The best lifetime I have had is with stainless steel bearings and a stainless steel shaft. The downside of a stainless steel shaft is that you cannot solder a nut on to provide the drive. I have found two ways round this:-
  1. Use a hex head bolt. For some reason stainless steel bolts never seem to have thread all the way to the top. Since the thread needs to be sharpened with a die anyway, it can be extended at the same time. It is hard work tapping stainless steel though. You need a split die, set to its biggest diameter to start with, and you need cutting compound. The hex head allows you to get a good grip to stop it turning and the original thread makes it easy to start off square.
  2. Drill through the nut and shaft and insert a pin. If, like me, you break lots of drills then broken drill shafts make perfect pins. I now buy drill bits in packs of five or ten!
I replaced the two half bearings with three ball bearings. At the top is an M5 bearing to take the axial thrust. At the bottom I use two M4 bearings as rollers to take the radial load.

The downside of this arrangement is that you still need to turn a land on the bottom of the shaft. It could probably be done with a file and drill though. It actually works without removing the thread, but I expect it might wear away the rollers.

This design works but there are a few things I would change if I built another: -
I made it compatible with the existing filament guide to avoid having to reconfigure my machine for HDPE. Ideally the screw holes at the bottom end need to move out to allow longer bolts to hold the rollers and the size needs increasing from M3 as the threads strip eventually.

I left clearance to allow the top bearing to be inserted from below, but left no access to the nuts. Consequently it was very difficult to assemble and I had to make undersized nuts.

I used the smallest outside diameter bearings I could find for the given inside diameter. That was a mistake because it is hard not to foul the outer part of the bearing with a washer as the moving part is so small. Star washers seem to just grip the inner and provide enough standoff to clear the outer. I used counter sunk heads to clear the outer face of the rollers. I expect larger diameter bearings use bigger balls, so perhaps have higher ratings.
All easy things to put right with a design iteration.

Another thing I have been meaning to try is the GM17 gearmotor. I have had some for a long time, but without a second shaft, adding a shaft encoder is not trivial, as it is with the GM3. Solarbotics now sell a cheap magnetic encoder that fits inside the casing, making it a more attractive proposition.

To fit the motor in place of the GM3 a new mounting bracket and a shorter version of the shaft coupler is needed.

Here is the completed pump: -

And here it is built up into an extruder: -

I am waiting for the magnetic encoder to come from Canada so I tested it open loop with a couple of bench power supplies.

The GM17 is a bit quieter than the GM3, but not that much when heavily loaded. It extrudes at a similar rate, but the speed seems to vary a lot with load, so it would be useless without closed loop control. It seems to labour and get quite hot at 12V, so I don't imagine its life would be a lot better than GM3. It overruns a lot when the power is disconnected, so it would need a full H-bridge and reverse thrust to get decent stopping.

I still have lots of things to try: stepper drive, a roller instead of the filament guide, an offset screw drive to avoid the rollers.


  1. Nice work. I don't think the screw drive is dead: it seems to provide the best grip, and the more grip/power you have, the more variation you can accept with heaters/insulators/nozzle sizes.
    My current drive is a modified BfB screw thread, so there's a lot of this type out there. Moving from this to a stepper/pinch wheel is a moderate investment: a new stepper driver board and motor, plus a couple of RepRapped parts are required. Improvements to the existing extruder are a lot easier.

    Nice use of the bearings at the bottom landing. I was looking for something like that at the bottom of my coach bolt.

    Thanks mate.

  2. I was wondering if you had though of using a much larger, fine thread bolt? If you had a bolt that was larger in diameter than your bearing, you can just use one bearing on each side. Perhaps something like an M24-1.0 bolt? Or here in the US a 1"-14 bolt. You could us a bolt much smaller but with a M24 you could use skate bearings.

    You would still have to turn down the ends of the screw to fit the bearings. Or you could drill a 8mm hole threw the M24 bolt(you would only need the section of the M24 bolt to be 20mm long or so...) and attach the M24 bolt onto a 8mm drive shaft for the bearings.
    The downside to this option is that you will loose a little more power to the friction work of the screw against the plastic as the surface speed of the screw against the plastic will be higher with a larger diameter screw.

    Just an idea.

  3. I had thought of using a very small bearing at the bottom end ,but I must admit I hadn't thought it through to use them at both ends.

    It would require a lathe and as you say it would increase the frictional loss greatly and also the torque required. That is why I chose this method as it only seems to have advantages over the original design. The rollers do rotate with the shaft so the wear on it should be minimal. Also it starts at 4mm diameter compared to the original 3mm, so more surface area to spread the wear over.

    The offset drive method allows two proper bearings and needs no lathe work, so that would be preferable if it gives the same grip.

  4. To fix a nut on the end of the SS threaded rod, you can always lock a couple of nuts off against each other.

    Vik :v)

  5. > It is hard work tapping stainless steel
    > though. You need a split die, set to its
    > biggest diameter to start with, and you
    > need cutting compound.

    I would really like some pictures about this "split die", or better a short tutorial or photos of the tapping process. I am asking because (maybe im lame) but I never successed tapping steel. I even bought four different die kit, and I destroyed the steel rod at the end.

    So Im certainly lacking some knowledge/skills when working with hard materials (steel). I would like to see a walkthrough of the process. I hope it is not much to ask;)


  6. Vik,
    I did consider that but thought the anti-clockwise torque would unscrew the nuts. But I suppose if I extend the coupler so that it goes over both nuts it will lock them together.

    I will try that method next time, thanks.

  7. > I had thought of using a very small bearing at the bottom end ,but I must admit I hadn't thought it through to use them at both ends.

    In my Bits From Bytes 1.0 extruder, the one with the thread crossing the filament slightly diagonally, I've added a very small bearing around the end of the drive screw that I threaded (sharper, more grip). I filed the end of the thread a bit smaller (now in a drill press, using the vice to push the file against the end of the rod). A first it would be retained by a 'retainer washer'. I thought it was an awful way to keep the drive screw from pushing out. Now it is held at all sides with minimal friction. The thread seems to have such a good grip that I can file off the side of my filament when the nozzle is blocked. After solving the block I need to push it through manually.

  8. Love the blog.
    If you want a thread the whole length of the 'bolt' then you don't actually want a bolt, you want a 'set screw'