Saturday, 4 April 2009

All torque and no traction?

As promised, I have tested two more drive methods. The first was a 13mm knurled wheel that I had lying around. Handily it was on the end of an 8mm shaft, so I just pushed it through a skate bearing and pressed that into a bearing block.

The results were: -





Surprisingly, a bit better than the same diameter timing pulley that I tested previously. I did have to set the gap quite small for the softer plastics, so the filament comes out quite squashed, which may cause problems downstream. The torque is much more even than with a timing pulley.

The final test was a threaded pulley made by the method aka47 blogged here. Following Andy's instructions, I milled a 6mm slot into a block of steel mounted in my lathe's tool post.

I removed the lathe's chuck and backplate and mounted a collet directly in the spindle taper for best centering and stiffness.

I used the shank of an M4 cap head bolt as an axle and some oiled steel washers for spacers, rather than PTFE as Andy's recommendation.

The next bit is magic. You put a tap bit in the lathe's chuck and advance the pulley towards it by 0.05mm each time the pulley revolves. This is viewed from above: -

You would imagine that the inner diameter would have to be exactly an integral multiple of the thread pitch, and the same for a knurling tool. Oddly it doesn't seem to matter, and I can't explain why, even having observed it.

My first attempt was with a M3 x 0.6 tap. I got the height a bit wrong but is was still usable.

The inner diameter of the thread is only 2.4mm, so the filament did not sit in it easily. I made another with an M4 x 0.7 tap, which has an inner diameter of 3.3mm. Perhaps the best fit would be M3.5 x 0.6 but I don't have one of those.

I mounted the pulley on the splined shaft that I had tested before and reprapped yet another bearing block.

I picked the pulley inner diameter as 13mm to get comparable results with my previous tests. Ideally it should be smaller to reduce the torque required. For all but the 4mm splined shaft test I had to use a socket wrench to wind the shaft.

This gave the best result of all the pinch wheel tests, but not as good as screw drive on PCL.





I tried the M3 pulley and that was better still, raising PCL to 8Kg. Here is a summary of all the tests: -

4mm splined shaft 2.5 Kg 3.0 Kg 5.0 Kg 7.5 Kg
13mm timing pulley 4.0 Kg 10.0 Kg 8.5 Kg >8 Kg
13mm knurled wheel 5.0 Kg 10.0 Kg 12.0 Kg >12.5 Kg
13mm M4 worm pulley 6.0 Kg >12.5 Kg >12.5 Kg >12.5 Kg
13mm M3 worm pulley 8.0 Kg >12.5 Kg >12.5 Kg >12.5 Kg
M5 thread 9.0 Kg >12.5 Kg >12.5 Kg >12.5 Kg

The red figures are lower or marginal compared to the force required to extrude 0.5mm filament at 16mm/s.

My conclusion is that the worm pulley is the best pinch wheel drive method. It also does the least damage to the filament. It does require a lathe though. On the other hand, using an M5 hex head bolt, a couple of ball bearings and some RP parts requires no lathe and should have better grip. That is the direction I am going to go.


  1. I have a copy of an old article about a Meccano hobbing machine that uses a tap the way that you do. They had the rotational rate of the gear blank linked to the rotation of the tap. I'd wondered if that was really necessary and it appears that you've answered the question quite nicely.

    What sort of rotational rate were you using to cut that brass worm pully?

    Wouldn't it be cool if we could make worm gears that way? I'm really tempted to try to make some in HDPE.

  2. My spindle speed has no calibration but I used a slow speed, probably 200-300 RPM.

    People make high resolution worm gears for telescope mounts using this method

    I can't see any reason why it would not work with HDPE with a bigger thread pitch.

  3. Nice work.
    I've been having trouble with my BfB screw drive so yesterday I built a drive with bigger sharper thread from a coach bolt. The thread is larger and sharper, and seems to have a lot more 'bite'.
    The BfB design has a diagonal intersection which gives a shorter contact area. I'll post some pics today.

  4. Renoir,
    Thanks. My M5 thread is very sharp because I made it with a die (the original bolt only had a short threaded section). I will try it mounted diagonally and with a skate bearing roller to see how it compares. I would like to have one design that will do both hard (inflexible) plastics and soft ones. If diagonal feed is less effective then I may end up with inline screw feed for soft plastics and pinch wheel for hard.

  5. Chris! Thanks for the link! :-D

  6. How about two of your building block units stacked vertically?

  7. Freds,
    That was Adrian Bowyer's first design, see It was abandoned in favour of the much simpler threaded drive that Vik Olliver proposed, so we are going round in circles!

  8. I think the best solution would be combining the off-the-shelf solution (timing pulley) with a custom made one (worm pulley) which requires a lathe.

    The idea is make the pulley the exact same size as the timing pulley. So it can be a drop-in replacement of the timing pulley.
    In this way everyone wins.
    For those who cant manage to be made the pulley by a lathe, he can use a timing pulley instead for the beginning (or if he do not want to work with PCL).

    I do not like the M5 thread design, because it makes the biggest damage to the filement, and I suspect the thread wears out from times to times. (needs resharpening, like bughunting in software world).

    Oh and it produces dushes too, what can lead some unwanted color traces in final print. (its more serious problem when we arrives at transparent printing).

    Just my two cents.


  9. Hi, All:

    I've used the screw thread opposite two ball bearings-- it seemed to work well. Here's a picture:

    I used v-groove rings pressed onto the bearings to minimize sliding friction as the filament moves.

    It worked fairly well-- definitely plenty of grip. I did notice that the filament comes out with a set of gashes from the screw that 'helix' their way around the filament-- twisting it a bit.

    The lashup pictured worked ok, but I noted that a shorter screw contact area and perhaps only one bearing would work.

    The test results above would seem to confirm that's indeed the case.

  10. Hi Khiraly,
    The RepRap 2.0 design will accommodate various pulleys so should be able to do what you want.

    I prefer the screw design because it gives more grip and needs less torque. I don't know what you mean by "dushes" but the contamination comes from the half bearings. Once I replace those by closed ball bearings I expect the filament to come out clean.

    I don't think the thread will wear out any time soon. Stainless steel is very hard compared to plastic.

  11. Dave,
    Yes I like your grooved rollers. I will try a plain skate bearing first though as I am trying to make a lathe-less design. If I need the groove I might try forming a plastic rim onto the bearing. I.e RepRap it and drop the bearing in at the right layer.

  12. Nop,

    i made the grooved roller, but if it turns out it works well enough that it is worth it, you can purchase v-groove or u-groove bearings-- they are sold as wire guides.

    Prices were a bit salty for me, about $8each was the cheapest i found, so i made them instead.

  13. That's good to know, thanks Dave.

  14. Do you have 2 mm filament? What is the smallest diameter worm wheel you could make?

  15. Enrique,
    No I haven't got any 2mm filament yet.

    I don't know what smallest worm wheel I could make is. As it gets smaller the teeth would get more wedge shaped. Andy Hall made a smaller one here

    I would guess that the smaller it is the less grip you would get due to the smaller contact area.

  16. > I don't know what you mean by "dushes" but the
    > contamination comes from the half bearings.

    By "dushes" I meant dust. It was a typo (I should look up in a dictionary).

    Although I cant see how do you want to mount a ball bearing on the M5 thread (the old bronze bearings was cutted at half).

    I dislike the M5 thread idea, because I wasnt able to cut thread on my M6 rod, I needed a lathe at the end. (I buyed 4 separate tap sets, spended more then 200$;-\)

    But it is just a personal preference. However I expect lot of twisted M5 screw;)


  17. I forgot to say, that I love how do you solve the extruder problem.

    First you modularizated the design, and after you solved each module separately. The heater, the heatsink problem and now the driver mechanics.

    It is brilliant and extensive work.

    There are not much work left. Do you plan to examine closer the different driver options? (stepper motor vs. DC motor (Zach's new finding)). The stepper motor is quite heavy...


  18. I like the GM17 which is cheap and light and now has a shaft encoder option. I don't know how long it will last though.

    The other thing I will try is a very small stepper motor with some gearing. A NEMA17 is far more power than is required, but without gearing it is needed for the torque. Stepper have best efficiency when operated in the mid speed range, which requires gearing.

  19. I was just thinking about all the gearing required for a stepper motor to power the extruder without overheating and other problems associated with running steppers at low speeds, and suddenly it occured to me, that what you have here is a worm *gear*, no less. It should be trivial to attach a short section of brass studding to a motor shaft and mount the motor at some convinient angle relative to the filament. Such a contraption should be able to run in the optimal speed range for a given motor, while providing a filament feed rate and force comparable with the threaded drive.

  20. Wow

    Excellent work Nop. I am impressed. The worm wheels looked promising but your bearing and bracket have taken it to the next level.

    Enleth, that was one of the ideas I was working towards. More specifically the motor drive can be direct and offset (or at an angle to allow the filament to feed in without fouling the drive.

    Something to consider though with the direct motor drive as you suggest is that the motor needs to be able to take the force which is applied to the driving motor in an endwise direction ie the motors weakest. This force will be equal to the force that the plastic is extruding at......

    I guess a fix for this should be to use a thrust bearing of some type and should keep the friction down.

    Nop's design doesn't have this problem but does need either gearing on the drive input or a chunky motor.

    Thoughts for what they are worth.

    PS I was thinking also of the potential of using worm wheels elsewhere on the machines axes maybe to replace some belts with studded rod.

  21. Hi Andy,
    Yes I also had the thought that you could use studded rod as a rack and the worm wheel as a pinion. If you also turned the rod with a second motor you could have high speed and high resolution with cheap motors. The problem would be supporting the rod in that case though.

  22. I wonder if something that has higher coefficient has been considered, like rubber wheel? I am referring to how printer feeds the paper.

    P.S. I am really new to RepRap, so new that I might be speaking non-sense.

  23. Yes things like that have been tried The problem is that some plastics we extrude are extremely slippery. HDPE has a coefficient of friction almost as low as PTFE so I don't think anything will grip it without making indentations.

  24. Hello nophead. I am looking to purchase a lathe capable of supporting my reprap research, which would include making one of these pulleys, and similar tasks to your no compromises extruder work. Do you think something like this would do the trick?

    Or is it too small?

    - gavilan

  25. I think you could make the pulley but you might have problems with other parts. The issue is that I think the hole through the centre of the chuck would not be big enough to take the 12mm aluminium rod I used. You could drop down to 10mm though to solve that.

    I also use a 4 jaw chuck a lot, I am not sure if you can get one that size.

  26. Just excellent! Thanks for sharing this!