Sunday, December 31, 2006


First low thermal inertia extruder barrel fabricated for Tommelise...

I'm just going to make a few comments and show you the pics on this little adventure. I'll write it up in considerably more detail over at the Tommelise website a bit later. Since I decided to go for HDPE and HPP polypropylene I decided that the extruder barrels were going to have to handle a lot more pressure than a crimped end was likely to handle since those two plastics have rather low melt flow indices.

Because of that I decided to braize a piece of 0.635 mm copper over the end of the 5/32 inch copper barrel. Some of the advantages of American mix and match standards is that a 5/32 inch hard copper tube just about exactly seats a 3 mm plastic filament. That really cuts down the metal mass in the extruder barrel.

I'd planned to learn braising for some time. This morning after reading up on how it's done I got a propane torch, some copper/phosphorus braising rods and a bbq lighter. The braising rods were rated at 40,000 psi so I figure they ought to handle whatever I throw at them in terms of pressure. Interestingly, I'd read that I needed to use flux, but when I bought the rods the instructions said that flux wasn't necessary. It wasn't, either.

Here you can see the 0.635 mm copper end plate with braising flange lying beside the 3/16 inch hard copper tubing.

Here you can see a piece of copper/phosphorus braising rod wrapped around the join between the end plate and the tube. Note that I use larger alligator clamps to hold things together. That works rather well. As an aside note that the copper/phosphorus rod is rather brittle. You have to be careful when you bend it around tight corners.

Here the tube is set in my vise and ready to heat.

Now I begin to apply heat to the join.

I faffed around for a while with the torch running at too low a flow rate. When I cranked it up the braising rod immediately melted and connected the join with no trouble.

Another pic of the braised join.

Here I've sawed off the support flange.

Here I've cleaned off the oxide from the braised end cap and knocked off the rough edges. It now looks exactly like a hollow copper nail.

Here is the Demel set up with a #76 wire gauge drill. It's right at 0.5 mm.

Here is my extruder barrel after drilled the extrusion orifice and cleaned it up a bit more. The orifice is there, but too small to photograph well. I put a drop of coolant on the end of the extruder before I drilled. I had drilled a test hole before through that thickness of copper without incident. This time, however, the drill bit binded when it went through the copper end piece and snapped. While I was doing this by hand I don't know that it wouldn't have happened in a drill press just as easily.

Adrian, Simon and Vik, you've done this before. What am I missing in the drilling technique. I stayed extremely steady and very, very light on the pressure on the bit. Indeed, I wasn't even letting a very small fraction of the weight of the Dremel onto the bit. Suggestions?

In any case, I now have a braised, low thermal intertia extruder barrel with a 0.5 mm orifice. If I can sort out the drill bit breakage problem this could be a nice way for people without lathes to DIY this fiddly bit of the Mk II.

After welcoming in the New Year I decided to go ahead and braise the support collar onto the extrustion barrel so that it would be ready for spraying with BBQ paint as per Vik Ollivier's prescription in the morning.

Given that I flared the extrusion barrel after slipping the support collar onto it there was no need for an alligator clip to hold things together this time. I did use the alligator clip, however, as a heat sink to prevent the PTFE block in which the barrel is mounted from overheating. I monitored temperatures with my IR thermometer during the braising operation. Neither the PTFE nor the top of the extruder barrel ever got anywhere near the temperatures needed to cause problems with degradation of the PTFE. Mind, this time I knew what setting on the torch to use so the job got done in a less than a minute.

Here you see the PTFE block pushed back to its permanent position. I've cleaned the support collar as well.

Braizing? I know just enough to be dangerous......

Was the torch straight propane? or did it have an o2 tank as well?

...(I've been putting off a trip to the hardware store for other stuff... looks like I put it off just long enough!)
Cool! For small parts, brazing can be done with straight propane. Very easy. :)
Straight propane. No hassle whatsoever. Not having to use flux makes it a LOT easier and cleaner.
The trick in using fine drills without busting the bit is never to drill all the way through. Drill part way then finish off from the far side with a larger drill. This also gives a shorter channel, and so a lower extrusion resistance.

Vik :v)
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