Copying Control Knobs

[Aerodyne]

I've put together a quick slide show about the simplest and cheapest possible way to copy a control knob. Needless to say there are other ways but all are both more expensive and more complex, especially when the knob to be copied is a complex one requiring a two-part mould. RTV silicone is superb stuff but very expensive for just the odd one or two copies. Casting resin can be modified by adding fillers: its glossiness can be a little too obvious but with a powder filler this can be softened. Colourant in powder form can be added at the time of casting - this is preferable to painting the cast afterward, for obvious reasons.
I've also used Gelflex moulding remeltable moulding compound but this is tricky to handle, hard to get to the correct heat level without burning and it is all too easy to create bubbles in the mix, which creates blisters - negative voids - on subsequent casts. Industry avoid air bubbles by use of a vacuum chamber when casting, not something we all have standing by at home.
Hope you all find something of use in the video.
-Tony

http://www.youtube.com/watch?v=SIdYBPQsjxk

[Mark R]

Tony hope you don't mind but I have given this video a sticky position in the cabinets and components section as it is hugely important.

cheers Mark

[pwdrive]

I agree, excellent post Tony.

Lawrence.

[Paula A]

Excellent.. very informative.

[Robert Darwent]

Thanks Tony for putting this really excellent little video together, at short notice too. It is just what I was looking for and I will be sure to give your alginate method a try.

Best wishes and regards

[Radio Fixer]

I'll add thanks to the list Tony.

I did look into making some knobs myself and it wasnt the expense of the RTV materials but there short shelf life. Had the stuff lasted for years it would have been OK... but this is not the case. Also, Im told that there is a lot of experience needed to get really good results. So I went to an expert: a Dutch guy called Wim Jaegers who already had moulds for the knobs I needed. And what knobs he came up with: flawless, perfect colour match and even the satin finish was spot on. A discerning friend said "... they even feel like Bakelite". He included brass inserts for the screws which to me are essential as the plastic is soft and doesnt take a great thread.

I was delighted but after a year or two a little disappoinment crept in: the knobs were changing colour. Simply getting darker. I asked Alumilite (http://www.alumilite.com/), one of the most respected suppliers of materials, and they said it was just going to happen and there was nothing that could be done about it.

I have thought about asking Wim if he could make another set of knobs in 'snow white' so eventually they would age to the right off white colour... I'm not sure how white you can get with these materials so maybe it still wont work. Painitng is an option but one knob has lettering that is filled on the front. This is shallow and so may not fill nicely after painting. Also from repairing and spray painting electric clocks I know the paint colours fade.

cheers Gary

[Aerodyne]

There's always going to be problems of cost versus quality with the making of repro knobs. Some slight shrinkage is practically unavoidable, too. Wim Jaegers uses RTV mouldings and 'proper' casting resins. Plus he has access to a lathe, so the knobs he makes, with a filler included to give the right 'feel' are always going to be better than ones made using the simple method I outlined. But expensive to make and therefore to buy.
I've used other moulding compounds in the past, notably Gelflex, but there are problems with air entrapment - as in truth there always is, no matter what moulding and casting method you choose, which is why industrially they use a vacuum chamber for degassing before the compounds set.
The beauty of making knobs the way I've suggested, using Alginate, is the speed and low cost. One poured the mould is ready in minutes, rather than hours. Casting using resin - with a filler such as a small amount of dry powder plaster (I do mean a SMALL amount) to give a better feel to the finished control - will give you an at-a-glance accurate control knob.
I do not suggest by any means that the Alginate method is the best, but it seems to me to be the cheapest and the moulds are very accurate and detailed.
Loads of other ways: a search on U Tube will bring up a host of examples and some of the supplying firms (the American ones at least) offer explanatory videos.
You pays yer money and takes yer choice, folks!
Gary, it must be rather galling to have paid out for good control knobs only to find they fade. I don't see why that should be inevitable: quality colours should be lightfast, surely.
Mark, sure! I'm pleased you think it important enough.
Finally: in the past I've drilled and fitted brass tubing into cast knobs to take the grub screw thread, also used a mini-tool to clear a slot in the back of a casting to take a 4BA nut. These were when the repro knob was of a type where there was too little thickness of resin to take a secure tapping. The more you get into it the more avenues open up to you. As the old saying goes, there's more than one way to skin a cat - not that I have any personal experience of doing so, you understand. I like cats, generally!
I'm very pleased at the positive response to this quick summary video. My thanks to Lawrence, Paula and Robert for their replies.
-Tony

[Radio Fixer]

Thanks for your comments Tony.

Actually I think Wim's knobs were not expensive for something so good and the radio was worth it. He charged me 10 Euro a knob but its 2 or 3 years ago so that might have changed. Also, apart from me buying a lot of RTV that would have gone off before I used it I had no original knobs to cast from and no one with a set was prepared to lend them to me. It was just lucky that he already had the moulds.

From your experience, I have none, do you think it possible to get another set made in 'snow' white that hopefully, with time, would darken to the right colour? I ask as I dont know how white the natural materials are... maybe how they started is as light as can be got?

Its not easy asking Wim as his English is not great...

Does the alginate keep? If so I might try that on a project

thanks Gary

[Aerodyne]

Alginate is a water-based compound. The chemistry is complex but it is derived from seaweed, I believe. No, the moulds will not keep more than a few days without shrinking, distorting and drying out so the moulding and casting should be done as a piece rather than separately, where possible. 'Clear' resin for casting is likely to have a natural yellowy-gold tint. I do not know how great the tendency for such resin to degrade colour-wise is but I suspect it may be more stable than Urea or Bakelite in that respect. The type of colouring matter is more of a problem - most colours will change slightly due to age and worse when subjected to UV light. The powder colours supplied by most craft sources may be the most stable, I believe.
I do not know how Wim tints his castings, though I did once come across a video showing his process (some time ago now, so I didn't keep a track of it). If you could find it you might have a better idea of the process and what best to do.
-Tony

[Radio Fixer]

Tony I was actually asking about the shelf life of the Alginate components? Do they keep? I'm getting fed up with stuff that after a year is dodgy to use... glues, decal paper and so on. A year whizzes by to me. Its embarassing posting off for two sheets only of decal material.

thanks Gary