Thursday, September 20, 2012

Hundred year old switch

An electrician friend brought me an interesting repair job. I met Anne when she basically re-wired our entire accelerator project. Between her and one apprentice they ran something like three hundred different circuits to supply everything from 1200 Amp 480 down to dozens of separate 110 circuits and a couple of miles of conduit. All said and done three quarters of a Megawatt in power distribution in one room.

One day she brings me an antique electrical switch out of her old Victorian house. The thing is ancient and broken which is the reason for her visit. My requirement for a job like this is it must me interesting. This had all the elements of intriguing little diversion. Some forensic analysis, some tooling fabrication and some challenge.
This is what I had to work with. In this picture I had already removed the fried contacts from the switch. To get at the screws some ceramic or hardened insulator had to be excavated from the screw recesses. One was broken and the other was arced into submission. First step was to measure it up and make a decent drawing. I had to use details from both contacts to fill in all the features. If any more had been missing it would have made the job even more challenging.
True to form nothing starts out perfectly. After measuring the contact parts and making a nice drawing I was anxious to get started cutting the material. Anne makes these great oatmeal pecan chocolate chip cookies that I was thinking about when I got started. First stop was the precision nine inch Diacro shear. We save this shear for all the precision cuts on thin materials that we want to be burr free. I have cut one and a half thou tungsten foil on this shear perfectly. Its only problem is it lacks a decent back gage. I have had an idea in the back of my mind but no time to execute it. We guess what, this was the time. It was pretty easy to do since I had ninety percent of what was needed already squirreled away.
Not super high tech but it works just like I wanted. The switch contact blanks needed to be sheared accurately and they were only .475 x .700. The micrometer gives me easy adjustability to dial in the exact size. It was a small diversion and worth it for this job.Previous to this we would use the calipers to measure up to the blade to gage the cut. For blanks this small it really was a job for a backgage.
Ok second problem. The Dykem I had around the was pretty ancient. In fact it was so bad I had to dispose of it in our hazardous waste collection. The red cherry flavor was in better shape but not dark enough for my old eyes and the really light layout lines I wanted to use on this material. There were quite a few bends in thin material with no real bend radius. These bends combined with deep layout lines spells cracking. Good old Sharpie came to the rescue. I don't know why these guys don't make layout fluid. Its better than sliced bread and always in my pocket. Maybe I'll see if I can buy a pint of Sharpie juice to use as layout ink.Sanford, your missing a business opportunity here.
All the layout was done with small calipers. The blanks were so small using a height gage for the layout wouldn't work. I only needed two mirror image parts so speed was king. For little fussy things like these contacts always make a few extra blanks. Material used was .015 thick Be/Cu half hard condition.This material forms well and is commonly used for RF finger stock,  electrical contacts and springs.
Making the tiny notches with a hand operated Diacro corner notcher. The material is pretty hard and was thick enough to get a nice clean cut.
Final notching was done on the milling machine. The parts were stacked and clamped with my smallest parallel clamp which was a handy handle also in this case. The slot was just a shade over 1/8 inch wide. In this example I just milled to the scribe lines. If I had to make a large number of these without a die I might wire EDM cut them to preserve the heat treat and mechanical qualities. You could stack these pretty high in a WEDM and get a perfect profile.
The one hole in the contact was an odd punch size so I had to be drilled. Not my first choice on thin sheet grabium but I had no punches even close. I ended up center drilling them through first to give a solid starting point for the #32 (.116) drill. The large headed tack is an anti spin device and a hold down. If the part spun the tack would catch it and if the drill caught on retraction like they sometimes do, the oversize head would hold the part down to the backup wood and not bend it and piss me off.
All that just to get to the flat blanks for the contacts.Now for the fun part, the forming. I took a look at the small Diacro box and pan brake and decided it was not up to this job. On thin materials with sharp crisp bends your clamping and followers have to be positioned very tightly together. The Diacro is a great precision brake but not tight enough for this job. There were several bends close together in a back to back joggle and to further complicate the forming the two contacts were mirror images.
In this shot you can see the joggle and the small rounded return on the left hand contact. I made up a tapered Delrin caulking tool to help with the bend forming.
Here is my bending brake for precision parts in thin material. The smaller vise has hard jaws that are surface ground flush to the top surface of the vise with no steps or offsets. I stoned the inside of the hard jaw so it wouldn't act like a shear on the thin sheet. You can see its easy to work off either side of the small vise to hang already formed returns over to access the remaining bends. This whole mess is clamped in the Kurt vise on the milling machine.
  Starting the first bend. The Delrin caulking tool reaches right to the surface of the vise. This gives me a nice tight crisp initial crease on the bend. Additional angle is easy after the first crease sets the bend position.
You can see in this picture its easy to hang the part off the edge of the forming vise to clear the next bend. Measuring the small angles is the tricky part. All the bends here were ninety and forty fives nominally.
The last bend which is the lead in for the moving parts of the switch to engage the stationary contact. I think I lucked out a little on the bend sequence. Many times it pays to make a paper cutout to check the bend order so you don't back yourself into a corner.
All the bends are in. Now you can see what the old contacts might have looked like a hundred years ago. Next up were a few locating dimples for the mounting screw and a spherical dimple for the contact face.
A little hard to get a good picture. The dimple former is made from a 1/8 diameter dowel pin and ground to shape on a diamond toolbit sharpener. Final radiusing was done with a hand held diamond hone stick. This was held in a drill chuck in the mill and indented into a Delrin backer under the part. Delrin is hard enough to get crisp detail, be not so hard that the material get sheared or needs large amounts of pressure to do the forming.
The contacts installed in the switch. You can just see the the spherical indentations in the flats. This was also formed with a dowel pin shaped on the diamond grinder on a Delrin plastic backer. I didn't get a shot of the index bumps you see in the original but trust me they are there. A pretty fun job actually and I got the shear tuned up the way I wanted in the process.

Looking forward to those oatmeal pecan chocolate chip cookies Anne. Thanks for the fun forming job.

9-25-12 Update,
 Score!!!

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