Archive for December 12th, 2009
I needed a brass tube with a lengthwise slit to serve as an electrostatic shield around a ferrite bar antenna. There are many wrong ways to do this, all of which produce terrible results, pose a serious risk of personal injury, or both. I say that with some confidence, having tried some of them over the years.
Here’s one right way: fill the tube with Wood’s Metal, thus turning it into a solid rod, then cut the slit with a slitting saw.
Wood’s Metal is a moderately toxic alloy that melts in hot water, which turns casting into a simple workbench operation. You might not want to cast it in the kitchen, but that’s your call. Clean up the scraps, wash the counter even though you used newspaper, wash your hands, and don’t suck your thumb.
As shown, I just poured the molten metal into the brass tube atop a steel block, broke off whatever seeped out, and remelted the scraps. Turns out I had just barely enough for the job.
My buddy Eks gave me a stack of slitting saws a while ago and I modified a standard Sherline holder to fit them. Turns out there’s just barely enough room for everything within the mill’s working envelope; the saws are a bit over 3 inches in diameter.
So I cut the back of the tubing, making the pictures somewhat disorienting.
The tubing fit neatly into an old V-block (evidently homebrewed by a better machinist than I), held down by ordinary Sherline clamps on perilously long studs screwed into the tooling plate. The saw had just enough reach to clear the rather broad V-block’s shoulder.
The tubing is 0.630 OD with a 15-mil wall and the saw blade is pretty nearly 32 mils thick. I touched off Z=0.331 (630/2 + 32/2) with the blade atop the tubing, then jogged away to Y=+1 and drove down to Z=0 to cut exactly through the middle of the tube.
The V-block is aligned with the front of the table, but I did a bit of nudging to persuade it into final alignment. Of course, the saw wasn’t quite centered on the holder, so a blade or three tinged on the tubing when I did a Y=0 trial pass at low RPM.
For lack of anything smarter, I cut at 500 RPM and fed at 5 inch/min. That’s painfully slow, but correspondingly boring… remember, in machine shop work, boring is good.
I did five passes: one trial at Y=0, three cuts at 5-mil steps, and a cleanup cut. The picture shows the 15-mil pass left a very thin web at the far end. A final 2-mil cut removed that web, leaving only a few burrs. You could do it in one pass, but I wanted to minimize the depth-of-cut into the Wood’s Metal.
Unclamp, discover that the cast metal rod slides right out, touch up the edges with a file, and it’s all good. A lovely slit, perfectly aligned, without bent metal or bloodshed.
As a bonus, I get a nice Wood’s Metal ingot out of the operation. The line along the rod is just barely perceptible with a fingernail; it’s more of a polished line than an actual cut.
Turns out the shield works a bit too well: it cuts out the WWVB signal, too. I think the tubing is too close a fit to the ferrite rod and detunes the winding. More experimentation is in order…