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Gazing balls and threaded inserts - tips

Original poster: FIFTYGUY@xxxxxxx

Just wanted to pass on my recent experience with gazing balls and the threaded inserts. I'm putting together a large-ish Marx generator, and I've been needing a bunch of metal balls to make the spark gaps and to terminate the charger electrodes. I bought 12 of the 4" stainless balls and two of the 6" ones from Krupps.com. Cheap, and quality to match - the seam is fairly obvious, and the pre-drilled single holes they come with have random orientation with respect to said seam. But I've been waiting a while for such things to become cheap enough that I didn't mind buying them - they're much less expensive than the spherical float balls listed in McMaster, for example. However, unlike industrial floats, the gazing balls don't come with threaded inserts. So I took the advice I heard here and ordered some threaded inserts. McMaster #95195A143, "Zinc-Plated Steel Open-End Knurled Rivet Nuts", to be exact. They have a 1/4"-20 TPI internal thread, and are suitable for material thicknesses of .027"-.165". Cost $8.32 for a pack of 25. Now, I've never used this type of insert before, so here's what I found out: Dunno why so much commotion here about drilling stainless, even the work-hardened stainless these things are made of. Didn't make any difference whether the supplied hole went through the seam or not. The main headache was simply the fact that they are made out of thin sheet metal and the relatively large twist drill tended to pull through. In retrospect, I think a step drill ("Unibit") would probably have worked better. A suitable pointed burr with a correct body diameter equal to the desired final hole diameter might work quite well. Grinding would probably be the best. But I did it the worst way, with a high-speed steel twist drill bit I sharpened myself, and it worked well enough. As for holding the thing in place, I simply put it on the ground sitting on a leather glove, and held it while wearing the other glove. Clean, dry leather on smooth metal has a tremendous amount of friction! If you wanted to be safe and sane about it, I suppose you could work up a fixture using some sort of mounting adhesive (wax?) or a hold-down plate with a circular cut-out (like they use for drilling bowling balls). I just used a battery drill. If I thought it was worth the trouble I could have chucked them up in the lathe, but again it was no big deal... Doing it quick and dirty, about half the time the holes ended up perfect. The other half, where the drill caught and "screwed" itself in, the edges were a little wavy. A little careful prying, and a little careful tapping with a small hammer, and everything was OK. Regarding the hole size, with these particular inserts the catalog specs a 25/64" hole. Well, that resulted in a hole that I felt was way too sloppy. So I drilled with a 3/8" bit after the first one. I was after a bit of a press or "tap it in" fit, so the knurling on the insert would catch and help prevent it from rotating during installation. But the annoying thing about these inserts is that the end that you insert into the drilled hole, is a few thou's *larger* than the knurled body by the head! So they still start out loose, no matter what. But using a 3/8" drill, followed by slight filing to fit, is definitely the way to go. Make sure you remove any ridges or chips stuck to the hole edges that would prevent the thing from seating flat. I also used red Locktite ("262 High Strength Threadlocker", for the purists) on the knurled section of the insert. I found that a liberal application that put some Locktite on the top surface of the insert against the installation tool helped keep the insert from turning, and presented no problems at all with freeing the installation tool. Speaking of installation tools, I didn't buy the correct tool, nor did I have a drill bushing of the correct size handy (as had been suggested here). I *did* use a high-strength 1/4"-20 bolt (a black-oxide finished, 1" long socket-head cap screw). It's lasted fine for over a dozen inserts with no sign of wear or galling. I believe my liberal application of copper-based anti-seize compound on the bolt threads helped this tremendously! As for a bushing surface, I simply used a single 1/4" Grade 8 flat washer. I clamped a small pair of Vise-Grips onto the washer to keep it from spinning, and kept my eye on the insert to make sure it didn't turn with the bolt. I found even if the insert *did* turn, I could "outpace" it if I worked the hex (Allen) wrench carefully. After a coupla turns, the rivet body of the insert starts to collapse and things go much easier. The insert doesn't grab fully until the last turn, and when it has completely "riveted" the abrupt increase in torque is obvious. Really is a piece of cake after you've done a couple of them, and I'm glad I've learned how to use this type of insert. I can think of many applications now where they'd be handy!

-Phil LaBudde