RepRap: Blog

An idea I've been kicking around is to use the resistance of the nichrome heater element in the MkII Polymorph Extruder to measure its own temperature.

Adrian's circuit chops the current going through the coil. It might, I thought, be possible to measure the resistance of the coil while there is no heating current going through it. A quick experiment with a lighter showed a nichrome coil varying from 4.9R to 130R when I waved the lighter underneath it.

This isn't really my area of expertise, and I wondered if anyone else might have some thoughts on how it might be done. If we can remove the need for a thermistor, connectors, added assembly steps and wiring, then the device gets simpler - even if we have to add another regular component or two to the board.

Vik :v)

I've put a picture of the state of my MkII extruder construction in my personal page on the RepRap blog if anyone is interested:

http://reprapdoc.voodoo.co.nz/bin/view/Main/VikOlliver

Vik :v)

I've designed a spider to hold the gears together. The holes around the large central hole for the centre gear allow for the mounting of the stepper motor, some stiffening (if necessary), and the attachment of a turntable or linear axis across the top:

Here's the actual gears as fabricated by the Stratasys. They came out quite well and mesh neatly:



You can't see the nut-holding collars in this photo because they're underneath. Now to create a "spider" to hold everything together.

Vik :v)

I've been experimenting with the idea of using plain copper wire for conductors, as "bend alloy" has so far been shown to be unobtainable in NZ. The L-sectioned posts I'd blogged earlier don't look easy to fabricate, so I wondered about creating a 'trap' out of wire to hold chip legs and components in. I wrapped wire into 3 figure-of-eight shapes around some posts (the remainder were just to keep the wire out of the way) and pressed a PIC into the gaps, taking care not to actually touch the posts themselves:



Even the single strand on the far left produced a contact resistance of <0.1 Ohms. A heated hypodermic could trail a wire around on a RepRap platform, and terminate the wires by pushing them into a Polymorph surface. Some form of cunning nipper would be needed to cut the wire off, a few mm clear of the end of the needle and fold it off to the side. I think the next stab at the Polymorph surface would then form the wire into a J-shape, creating a primitive barb to anchor the wire in the setting Polymorph. Once the wire is in place, of course, the RepRap can bury it in plastic if necessary.



Do the assembled think it would be worth pursuing this concept? I can find a hypodermic and warm it up a bit to test the Polymorph anchoring. The wrapping would need some FDM'ing of suitable plastic posts of various shapes for straightforward wire-wrapping (junctions) and making sockets with. Suggestions on potential post shapes welcome.

Vik :v)

As Vik mentioned below, I have got a new extruder design working (though he was too modest to say it is based on an original idea of his). Full documentation and downloads are available from the Extruder Version 2 section of the RepRap wiki.

Here's a picture:





A geared electric motor (A) drives a screw chamber (B) in which a threaded rod is forced against a rod of polymorph. As the thread turns, the polymorph is pushed downwards to a heated nozzle (C) and is extruded out of the bottom. The control electronics is shown at (D).

This Version 2 design has many fewer parts than Version 1, those parts are cheaper and easier to obtain, and the overall mass of Version 2 (just under 200 g) is less than half the mass of Version 1.

Here's a picture of it building a small thin rectangular block.

Simon has set up a wiki for all the RepRap documentation, for which many thanks to him (especially as he did it in a few hours flat...).

We will be putting all future documents up there, and gradually migrating the existing ones across too.

Here in New Zealand, there is a lull before the happy, normal, sane people of Laingholm turn into rabid pyromaniacs and make life busy for us volunteer firefighters. So, time for an update.

Adrian has successfully made a screw-driven Polymorph extrusion head, having solved the problem of the Polymorph binding up the driving screw. The MkII head is about half the weight and a quarter the complexity of the Mk I design. I'll let him have the joy of filling in the details on how he did it.

I've been preparing presentations for the last fortnight (including my first mention of RepRap at NZCS) but have now started designing some of the RepRap-able parts for the raising and lowering of the turntable. Despite having a nice machine and lots of memory, desigining such things as 79-toothed gears still takes quite a while. Ed's FDM design guide is invaluable for this. Here's the state of the work so far, the larger gears being around 100mm across:



I've also taken apart an inexpensive but effective cordless 3.6V screwdriver as a possible power source for driving extruders etc. The one I have was NZ$11 from Dick Smith Electronics (about four quid) and has a nice little planetary gearbox that should provide all the oomph we need - complete with "wall-wart" charger. Adrian's flexible MkII design will be able to cope with it. Here's the guts exposed: