Sunday, June 12, 2005


Feeder head for polymorph extrusion

I've done a different design to Vik's for a feeder head to go with the polymorph extruder in the blog of Saturday 4 June. Mine's more complicated than his, but is a bit stronger. Here's a close-up of it working:

It consists of two pinch wheels driving a 3mm diameter polymorph rod into the top of the PTFE tube (see the previous blog). The pinch wheels are 40mm M4 cap screws being turned by gears at the back. The knurling on the cap screws gives a good grip on the polymorph, though it still slips a bit, so this is not the final design. However, if you want to download the STL file for it (1.6MB), click with the right mouse button here. Here's a close up of the feed mechanism:

The cap screws run in bushes embedded in the head. The two M6 screws that you can see the heads of on the right serve to clamp the pinch-wheels together, and the one half out of the bottom of the picture clamps the PTFE tube of the extruder. Ignore the blue scribble - that's just me making notes to myself...

Here is a view of the gearing at the back:

Just to the left and above the small gear you can see the shiny end of the bush insert.

The whole device works quite well, but needs a few improvements. These are:
  1. Double or triple up the pinch wheels in a totem-pole up the polymorph rod to reduce slippage; they're only cap screws with gears on the back, so that's not going to cost much more.
  2. Use Vik's much better solder nozzle design (see the blog for 8 June; improved further by his making it detachable on 11 June). If one used the higher temperature solder that would allow the polymorph to be worked at a higher temperature. One could also bury a loop of one end of the heater nichrome wire in it and use the brass extruder barrel to make one of the heater connections. This would automatically make a thermal fuse to prevent temperature getting to the point where PTFE starts to decompose (250 C). The problem Vik found with the solder not wetting the brass may resolve with additional flux. More experiments needed...
  3. Switch to steel or brass gears. The nylon ones are plenty strong enough, but I had to drill and pin them to the shafts. It would be much simpler to file flats on the shafts and to attach the gears with grub screws. This'd need metal gears to hold the grub screws strongly enough.
  4. Fit two optical sensors, one to measure pinch-wheel speed, and one to detect when the polymorph rod runs out so the machine can go fetch another.
  5. Control the whole thing (including the extruder temperature) with a local PIC, rather than by hand...
Here's the parts list:
  1. The RP part from the STL file,
  2. 1 off: 40mm M4 cap screws with 20mm unthreaded shaft at the top,
  3. 1 off: 30mm M4 cap screws with 20mm unthreaded shaft at the top (or saw 10mm off Part 2 above...),
  4. 5 off: thin (2mm) M4 nuts,
  5. 2 off: 25mm M6 cap screws,
  6. 1 off: 30mm M6 cap screw,
  7. 4 off: M6 nuts,
  8. 4 off: M6 washers,
  9. 1 off: 35mm M6 brass screw,
  10. 2 off: 5mm M3 countersunk screws (for attaching the motor),
  11. 45mm x10mm diameter PTFE rod,
  12. 400mm 0.2mm diameter nichrome wire,
  13. A reel of PTFE plumber's tape,
Plus these parts from RS Components:
  1. 1 off geared 12v motor - 245-6118,
  2. 1 off 80-tooth gear - 745-270 (this needs an internal diameter of 4mm, which it doesn't have so you'll need a sleeve to make it fit),
  3. 3 off 20-tooth gear - 286-2355,
  4. 1 off thermistor - 484-0149,
  5. 4 off bushes - 262-1939 (two in each hole).

It's fun watching this evolve. The gears and motors are a bit expensive though. I'm thinking of getting a cheap corddless screwdriver or two and seeing if thaat couldn't be adapted.

How about a boring old microswitch for the Polymorph sensor? Fewer components and a simpler interface. We'll stand a better chance of fabricating one too, further down the line.

Vik :v)
Yes - you're right about the cost of the gears; I must search more widely for a cheaper supplier.

As to the microswitch, Ed already has wood's metal end-stop contacts in his axis design...
I was thinking that the design of cheap tools such as electric screwdrivers is a kind of biomimetic evolutionary process in itself: The cheap ones that are easy to make win.

We like "easy" :)

I remember seeing some switch contacts on your robot too.

Vik :v)
I'll try the trick of filing a notch in the M6 bolt/extruder and see if it'll tap its way into the PTFE. The 3mm drill should be useful for aligning the bolt and the hole down the middle of the PTFE.

Vik :v)
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