Tuesday, 22 September 2009

Extruder Part 1: the Heater

Although I can see the advantages of Nophead's heater, I'm not at all sure that I can machine stainless steel, so I decided to make the standard heater from the RepRap website. I remembered to order insulated nichrome wire and a thermistor when I ordered my electronics from Makerbot, I bought some PTFE rod on eBay, and my local village hardware store sells fire cement, so I just needed the 6mm brass bolt. No-one seems to sell brass bolts any more (or if they do, they want huge amounts of postage), and I searched all my neighbours' junk boxes without luck (I assume they've all thought I'm mad for years, so what's one more eccentricity?) but I did find an old ball-cock valve with a 7mm brass arm. Close enough; stick it in a drill and file it down a bit.

My only machine tool is a cheap, slightly rattly, pillar drill. I have a machine vice and an old set of taps and dies, and I bought four pin vices, an Archimedean drill and a set of miniature drill bits on eBay for about 7 quid inc postage (why are tools so cheap?), so it was time to set up the poor-man's lathe.

If you put a rod in a vice, and try to drill a hole down the centre, your chances of getting it concentric are negligible. Even if you get it started right, it will probably drift off centre and break through the side of the rod half way down. The trick is to put the drill bit in the vice, and the rod in the drill chuck, and that way it stays central when you drill.

Here's how to set it up. First, put the drill bit in the pin vice, and do it up tight. Then grip the cutting end of the drill bit in the drill chuck, push the drill down, and set the drill table so that the machine vice can hold the pin vice. With the drill down, tighten the machine vice on the pin vice, and bolt it to the table. Then release the drill chuck, and let the drill up. You can then put the brass rod in the chuck, and drill it out.

I followed the instructions on the RepRap web page: slight dent in one end using poor-man's lathe, drill 0.5mm hole with Archimedean drill (and sewing machine oil), taper off end until dent is gone, drill it out from other end with poor-man's lathe (more sewing machine oil) and finally, M6 thread. I think next time, I'll thread it first. It makes it harder to hold in the drill, but I managed to bend it a bit with the die (not enough to matter, but enough to annoy me), which happens far too often.

Drilling and tapping the PTFE rod, winding the nichrome wire, adding the fire cement and fitting the copper pipe were straightforward, but how to dry out the cement without a variable power supply? I tried baking it on the lowest shelf of the oven on slow-cook, but I wasn't sure it was reaching the right temperature, and I certainly wasn't going to turn it up to 250C.

Here's my method of avoiding the damp cement short circuit problem. Connect a 100K pot to the thermistor terminals on the extruder board, the heater to the heater terminals, and the thermistor to your multimeter, set to 100K. With the thermistor resistance table in front of you, launch the RepRap software (I used ReplicatorG) and set the temperature you want. Now you have to continually adjust the pot so that the temperature shown on the software matches the resistance shown on your multimeter. As soon as it's dry enough to insulate the heater from the thermistor, you can replace the pot with the thermistor, and leave it to software control.

Here is the final part, complete with the first piece of oak! A bit wonky perhaps, but plastic filament pushes down to the end without any trouble, so we'll see.

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