Ever aquired an old TV or radio with one or two small knobs missing?
Sure, we all have!
...But finding a replacement that matches the existing knobs can be difficult...

Nowadays, there's a variety of easy-to-use flexible molding products on the market (some of which are readily available at various craft stores) and it occurs to me that this sort of thing might be well-suited for making molds of radio/TV control knobs. Has anyone out there ever tried casting knobs this way..? If so, what did you use for making the mold and for casting the knobs? Is casting resin strong enough to handle the torsion/twisting force on a control knob, or do you need to reinforce it with metal tubing or something?

I realize there are some additional complications to doing something like this, but thought I'd throw this out to see if anyone else has tried it before.

There are a couple of products you can use. One is MarineTex which is a fiberglas resin-like liquid. The other is a gunsmith product from Brownells called AcraGlas and is used to glass-bed rifle stocks. Each one will harden into a near indestructible form.

What I've done in the past in my moments of Wiley E. Coyote School of Home Repair is to take the part I need to replace, and press it into modeling clay. Then I pour the casting resin into the shape and let it harden. Afterwards you'll need to clean it up some with a needle file, but that's just craftsmanship.

Be especially careful as they will stick to almost anything or any surface. AcraGlas comes with a release agent used to coat the rifle barrel or else when you bed the barrel in the stock it will be there forever if you don't use release agent. Read the directions carefully.

AcraGlas can be supplemented with fibreglas floss or aluminum shavings for strength also. Wiley E. Coyote lives!!!

You can make a "negative" mold easily with RTV silicone. It's flexible, so it's easy to remove the original part and any subsequent castings. If the part has a lot of detail, or is an odd shape, you may need to support the RTV mold with a plaster shell.

Check out some Theatre Prop Building books from your local library, lots of good casting info to be found there.

Did this 6 or 8 years ago. The right knob is a casting of the left knob. I made the mold out of plaster, the new knob out of JB-Weld. If the knob you're reproducing has a shaft (as this one did), find a knob that fits the radio but otherwise is worthless, file or grind the body of the knob down so that it's just the shaft, plus a thin flat stub of the knob, and then press it into the liquid JB, nicely centered and perpendicular, before it hardens. Then you have a nice shaft with the "D" hole and metal spring already done. Cure overnight, tap the plaster with a hammer, it breaks off the new knob cleanly. Cured JB-Weld is rock.

Paint to match.

Clever idea with using RTV for the casting. I suspect that would work better...

I think of those many knob collections I have seen and I think every knob ever made is in one of those collections. I think it is very hard to match the original in many cases. If I needed a rare knob I would keep checking the ebay knobs and also any other source that may be available.

Oh, and if you're trying to replace a knob that is stripped, RadioShack has a variety of equipment knbs that are pretty cheap. You can buy a pack and just cannibalize the center shaft stub out of it to use with any of the methods above. They have ones that accomodate both D-shaft and set-screw type shafts. Wiley E. Coyote School of Audio Engineering.

Did this 6 or 8 years ago. The right knob is a casting of the left knob. I made the mold out of plaster, the new knob out of JB-Weld. <SNIP> Clever idea with using RTV for the casting. I suspect that would work better...That JBWeld knob looks great! :thmbsp:

There are really two advantages to using the RTV mold: 1. You can easily get your casting out and reuse the mold. 2. You don't have to clean your casting, RTV releases without an agent from most casting materials.

There are a bunch of threads over on the antique radio forums about this. Heres one:

http://antiqueradios.com/forums/viewtopic.php?t=39730

If you search around over there, you will find a bunch more. I havent actually tried it myself.

John