Tuesday, July 05, 2005
Building a frame
I've been spending my recent time writing communications infrastructure for a PIC-based network of devices. That's gone pretty well and amongst other things, we now have a working distributed controller for a motor. That means I really need to make some progress building this contraption so I have a use for the new motor controllers.

I have recently realised that Vik's contributions to the reprap project parallel some of my intentions, and involve building a working prototyping machine to produce reprap parts. That makes this project rather superfluous, especially now that some of the experimental ideas (such as threaded rods and nuts for producing movement) are being used there too. That makes it less useful for me to experiment with the same ideas, but I'm going to continue anyway as there are still some slightly different ideas, and I still want a machine for myself.

With that in mind, this evening I brazed together the top part of the frame for my concept:

It was my first attempt at brazing and it turned out to be easier than I expected (certainly easier than welding). I was using an oxy-acetylene welder at a fairly low heat, but a small/cheap gas torch should work just as well.

The trick is to evenly heat the surrounding pipe until it has a bit of a red glow. This is still well below the melting point of the copper. Then just touch the brazing rod onto the surface and it flows quite easily and nicely into the gaps. The result seems really quite strong.

I used duct tape to hold the parts in place while I welded. Copper conducts heat very quickly, so I put a wet towel over parts with the duct tape to prevent it from burning. For the first joint, I taped the tube to the side of a square brick to provide a good right-angle. From there on the angles pretty much just work out provided the tube lengths and cuts are fairly accurate. The only thing to be careful of is ensuring the frame sits flat and isn't twisted at all.

If anybody repeats this, take some care cutting the pipes at 45 degree angles. The closer you are the easier the joint will be to make. I found that using a large protractor I could get the angle to within 2 degrees or less, and the joints are quite close. I just cut the angles by eye, which I found I could reliably do to within about 10 degrees after a few. After that, I used a grinder to fine tune the angle. A regular old metal file works fine too, but it's a bit slower. Spend the time to get the angles right in the first place and make sure the ends of a pipe slope in the same plane.

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