Monday, 21 December 2009

Reliable extruder at last?

... well only time will tell but I have now fixed all the teething problems on my "no compromise" extruder.

The first problem was it was leaking plastic. I simply tightened the thread about another quarter turn while hot. The problem started when I had to dismantle it to replace the first resistor that I damaged. When I put it back together I didn't get it tight enough as it is difficult to judge when full of plastic and hot. The seal relies on the fact that the relatively sharp edge of the stainless steel tube can bite into the softer aluminium. It seems to work when tightened enough.

The other problem was that the motor would skip steps in the middle of a build for no apparent reason. It seems the amount of force required to extrude varies wildly for which I have no explanation, but I did find some mechanical issues that were reducing the torque available.

I noticed the gear would always be in the same position when the motor skipped. I found that the grub screw was catching on the bearing housing. You would expect it just to grind the PLA away, but PLA is very hard, so it would take a very long time to do so. I increased the clearance around the wheel hub and also around the moving part of the ball bearings.

Another issue was that both the worm and the gear were slightly off centre on their shafts, so when the two high points coincided they would bind. The hole in the Meccano gear is slightly bigger than the 4mm shaft it is on, not sure why. The hole I drilled in the worm is 5mm but the MakerBot motors have imperial shafts about 4.75mm, so that was even more eccentric. Added to that was the fact that the motor bracket has a slight warp to it angling the shaft down a little. All these things conspired to make it stiff to turn once per revolution. I fixed it by tightening the bottom motor screw tight and slackening the top two a little. That was enough to reliably extrude PLA. Making the motor holes into slots would make things less critical.

Although the extruder was working reliably for PLA I wanted more torque in reserve, so I switched to a higher torque motor more suited to my driver chip. The Lin motor I was using was rated at 0.3Nm holding torque for 2.5A, but my controller can only manage about 1.5A without some better heatsinking. I switched to the Motion Control FL42STH47-1684A-01 which gives 0.43Nm at 1.7A. So at 1.5A I have gone from 0.18Nm to 0.4Nm, i.e. doubled the torque and also got the right shaft diameter to fit the hole I drilled in the worm.



The only downside is that it is bigger and heavier, not really an issue on HydraRaptor.

To give it a thorough test I printed off a couple of Mendel frame vertices.



These take about 2 hours each with 0.4mm filament, 25% fill, double outline at 16mm/s, infill at 32mm/s. Six are needed in total.

I still have to test it with HDPE and PCL., I know it works with ABS.

14 comments:

  1. *action clip!*
    *action clip!*
    *action clip!*
    *action clip!*
    *action clip!*
    :)
    //Fan in sweden

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  2. The one on the right looks even better.

    To me it looks like the best quality print you have had from these pictures.

    Do you know why they have the 3 extra holes in the mid section..?

    I could not see why they were there so omited them in my build.

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  3. Swedish fan,
    It looks and sounds just the same as the last clip, the only diffrence is the motor is a bit longer.

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  4. Dude,

    i want before and after fotage... side by side, flexing / unflexing.. :)

    //sweden

    (btw i relly enjoy reading this blog!)

    ReplyDelete
  5. I have already posted before and after flexing videos, this is just a bigger motor.

    http://www.youtube.com/watch?v=RXzQIa6ZUf8
    http://www.youtube.com/watch?v=8tYzMw7bkMk

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  6. "Mendula" wants to have HydraRaptor's baby!
    Hhehe

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  7. How could i have missed that!!! Sorry for bothering you!

    /sweden

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  8. What sort of extruder is this? I assume not one of Wades and is the design documented anywhere? Out of interest where did you source the springs from?

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  9. It is my own design documented in the article at the "no compromise" link at the top.

    The springs came from the hinges of the lid of an A3 flatbed scanner.

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  10. Hi nophead - I checked that link but could not find any design of the mounting block to hold stepper, gears etc. Did I miss something?

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  11. No I haven't published the STL because it doesn't fit anything other than HydraRaptor and it needs a re-design to allow a pulley with a boss on it. To make the existing design work I had to use a splined shaft and put a steel hub in the brass pulley. A much better idea is to put a boss on the pulley but that makes the whole thing wider so it wouldn't fit HydraRaptor either.

    I am trying to think how I can adapt it for Mendel but the carriage is very restrictive.

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  12. Pity I liked the idea of using a worm gear I assume the arrangement would give good pressure to push plastic.
    Also the gears being Meccano, so good for people like me who cannot print parts at this stage.

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  13. Pity I like the idea of using Meccano gears, especially a worm gear that I assume would greatly increase the force on the plastic being inserted. Good luck with adapting it to Mendel - hope you succeed

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