I discovered that a couple of techs had little stops or V-shaped blocks, but for the most part, these devices were kind of sloppy holding fixtures. I was pretty sure that with a little ingenuity we could do better.
I knew we wanted to hold the molds rigidly, and ideally, to bolt them down. The problems were that (1) each mold is a little different, and (2) we’d have to embed a nut in the mold. Anything with a nut in it would need a holding fixture to keep the nuts flat and square—otherwise, they’d break the mold when we tightened them. And the nut would have to be submerged below the actual surface of the mold so we could flatten the mold’s bottom without hitting the nut.
Basically, we needed an adjustable holding fixture that would allow us to drop the nuts into the wet plaster cast and hold them in the correct alignment. It would have to be easy to make because we’d need a lot of them.
What we came up with was a flat piece of band stock that was 1.5 in. by 14 in. long (long enough to span a bio-foam box), with slots milled down the center for adjusting the spacing of two nuts. We used ¼-20 zinc T-nuts (they’re cheap and really bite into the plaster) with small plastic standoffs through which the bolts could pass to allow us to actually submerge the T-nuts.
How to Do It
Pass a ¼-20 zinc bolt through the metal plate, drop a plastic standoff onto the bolt, then secure the T-nut to the bolt. When you snug the T-nut down, it pinches the standoff and the plate together and holds everything tight (figure 1). Set one nut dead center into the deepest part of the heel and one in the met heads, then snug them down.
Prepare the mold as usual, then pour wet plaster into it. While the plaster slurry is still wet, drop the jig (nut-side down) into it and let it harden (figure 2). Once it hardens, spin out the bolts and use pliers to pull out the standoffs.
Next, flatten the bottom of the mold; you’ll hold on to the nuts with another jig. Take a 24-in.-long piece of ½-in. by 1½-in. aluminum and mill two slots into it. Match these slots to those in the pouring jig exactly so you can line them up. Then mill the holding jig down so you can slide it into the pipe mandrels you use to hold other molds. To give yourself more options, flatten the jig’s sides so the jig can be held in a vice, and drill mounting holes in case you decide to mount it permanently to a bench. Buy some ¼-20 knobs that you can tighten by hand to allow for quick connection of the mold to the jig (figure 3).
This process is fast, simple, and highly effective. With a small investment in time and tooling, you can turn one of the more difficult, time-consuming tasks in your production line into a positive revenue generator that meshes well with the rest of your production, and you have a cool new tool!
This article was originally published in the September 2009 issue of the O&P Edge © 2009 O&P Edge