A couple of years or so ago, I spent a lot of time and effort in knob casting exeriments, and soon discovered that it's one thing casting the knobs - it's quite another devising a method of mounting them which is robust and is absolutely concentric, so that when mounted on the control shaft, there's no sign of eccentricity. If the mounting hole in the knob is just 1mm off centre (not a large error), that will give 2mm of eccentrity as the knob is rotated. Also, the hole must be perfectly vertical or the knob will be wonky. By no means an easy task if relying on a pillar drill say.
Based on Tony's experience, I used alginate to make the moulds. It's cheap, safe, flexible and not unpleasant to use. (For reasons that elude me, a popular use is to make moulds to cast replicas of new-born babies feet). The only downside - which Tony mentioned - is that when you've made your mould, you need to use it right away as alginate shrinks as it dries out.
To ensure that the means of fixing was robust, I made brass 'ferrules' on my lathe, 12mm O.D. drilled either 6mm or 1/4", roughened the outside to give them a good key to fix into the knobs with epoxy cement, then to drill and tap the side 4BA for a grub screw. I made a 'knob casting jig' to mount the original knob on a shaft to lower the knob down into the alginate to make the mould. The jig ensured that the knob was perfectly aligned and not wonky in any way as it could be if lowered into the alginate by hand. So far so good.
My intention was to then fix the brass ferrule onto the shaft that the original knob was on, to fill the mould with casting medium and to lower the ferrule into the casting medium, figuring that in theory at least, the ferrule would be exactly central. But the difference between theory and practice in practice, is greater than the difference in theory. This technique did not as I'd hoped, result in the ferrule being exactly concentric.
I decided that the only way of ensuring that the ferrule could be perfectly concentric would be to mount the knob is what's known as a 'jam chuck'. That's a piece of wood mounted on a woodturning lathe, with a hole the same shape as the knob, with the knob pushed into the hole protected from damage by a piece of kitchen roll. I then mounted a 12mm 'end mill' in the tailstock (rather than a drill as an end mill creates a hole with a flat bottom). I rotated the chuck by hand while gently advancing the end mill into the knob. Then I slid the ferrule onto a 1/4" or 6mm diam rod mounted in the tailstock to make sure it was exactly concentric with the knob, coated the ferrule with epoxy, and advanced it into the knob allowing the epoxy to set.
The knobs I made were perfectly concentric.
It's ludicrously time-consuming of course, but that's irrelevant in hobbies, which are about the 'low tech/high skill' enjoyable use of leisure time.
I'm sure there will be those who don't have the luxury of a woodturning or metal turning lathes, who may have devised a means of accurately centring the mounting hole, or may perhaps accept the limitations of slightly wonky knobs as the best that can be done with their limited facilities. If the end result is knobs that are presentable and functional, then it's 'mission accomplished'.
As to casting materials, they're really quite expensive - even more so if there's a desire to colour the resin to get a good match with original knobs. I used materials that I had to hand - namely, Ronseal two-part wood-filler, which sets rock hard, and Plastic Padding 'Super Steel, (which - despite its name is not electrically conductive, which could be a consideration on knobs for live chassis sets.
I covered these experiments in the thread below and I've attached some pics of knobs that I cast.
https://www.vintage-radio.net/forum/...d.php?t=142098
Pic 1: Four Unitra 'Figaro Special' knobs cast in Ronseal Woodfiller.
Pic 2: An original Figaro Special knob in the centre, with two finished cast knobs either side.
Pic 3: A Portadine 'Princess' knob just cast using 'Super Steel' as the casting medium.
Pic 4: Two 'Princess' knobs, ether side of the original.
Not related to 'casting' but just to show that some shapes of knobs lend themselves to be turned in wood by anyone with a woodturning lathe and moderate turning skills. Pic 5 shows a small and a large Ekco A22 brown Bakelite knob on the left, alongside two that I turned in beech and sprayed with 'ebonising lacquer'. I did it as an exercise to see if passable results could be obtained. They could just as soon have been sprayed brown but I didn't have brown paint to hand. Someone had said it wouldn't be possible to make the smaller one on a lathe as it has a tab on it, but I didn't know that until after I'd made it. See:
https://www.vintage-radio.net/forum/...d.php?t=140529
Hope that's of interest.