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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 8th Jun 2010, 12:05 pm   #21
Radio_Dave
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Default Re: Copying a control knob by moulding

As I mentioned earlier in this thread, the cost of buying the required materials can be prohibitive, especially if you're only making one knob. There's also the added complication of making a fixing for it.

It's therefore, often, easier and cheaper to have them made for you. I have absolutely no connection with Wim Jaegers, apart from a thoroughly satisfied customer. He makes really excellent reproduction knobs and he advertises on Malcolm Bennetts website here (about 3/4 way down the page) http://www.valve.demon.co.uk/Wanted.htm

David
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Old 8th Jun 2010, 12:33 pm   #22
Aerodyne
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Default Re: Copying a control knob by moulding

A couple of points from the messages on this thread:
1. Certainly it is expensive to purchase the materials for a 'one-off' and if it is feasable and the price is no deterrent, having one or two made to order would seem to be the optimum method for the single occasion. It might be that the maker would require an original knob to copy, though. This could be a problem. Enquiries to David's source would help determine the facts.
2. Resin castings will shrink, but there are some resins formulated for minimum shrinkage so it pays to check manufacturer's literature, available from most of the retail sources on the web.
-Tony
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Old 29th Jun 2010, 8:23 am   #23
TriMan66
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Default Re: Copying a control knob by moulding

I have just made my first mould of a knob using RTV silicone. So far it all looks very good.
I'm just working through the suggestions for colouring the resin to get the correct colour matching.
My question is to do with the grub screw. In the first message of this thread, it mentions that you can tap the resin directly with a thread for the grub screw. Is it not necessary to use a brass collar as found in most knobs. Is the resin strong enough without this brass collar. How does it wear with time after several removal/replacement cycles.

Craig
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Old 29th Jun 2010, 4:53 pm   #24
Aerodyne
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Default Re: Copying a control knob by moulding

I've 'got away' with tapping the resin. After all its a solid lump, not hollow as many control knobs are. I do agree that a brass insert would be preferable; it justs adds to the complexity. It would need casting in situ rather than retro-fitting, so some support for it would have to be created to secure it in place until the resin sets.
I don't tend to take home-made knobs on and off very often!
Another possibility is to cast-in a suitable (say, 4BA) nut or, as I have done successfully on occasions, grind a slot in the back of the finished casting to take a nut. I used a Dremel drill and slotting tool for this. THe grub screw hole should be drilled first, I consider. The nut spreads the loading pressure from the grub screw but accuracy of 'slotting' is essential or the resin might crack when the knob is in use.
-Tony
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Old 11th Apr 2013, 10:58 pm   #25
mintras
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Default Re: Copying a control knob by moulding

I thought I would try making some Murphy knobs so I got some white rtv silicone from Fred Aldus 25 kg and a casting kit for another 25. This seems a lot but I am using all the material on other projects.

The Murphy A124 knob came out grand.

I used a spray can plastic top and stuck a cleaned original knob into it pressing the base in place with double sided tape.

Mix the RTV silicone (3% setting agent 97% white stuff), I did spray some silicon oil over the original first but I don't think it needed it. then just pour it in.

8hrs later I had a perfect mold.

I mixed the other material with 50% stone power included in the kit, with some black colour. The result would be very hard to distinguish from an original, after a little polishing.

Used the same mold again for a second knob in dark brown.

This looks and feels like the original.

This was all too easy.

Now All I need is a source for brass inserts, I suppose I could butcher some existing knobs, but I would like to find a source.
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Old 12th Apr 2013, 9:49 am   #26
Radio Wrangler
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Default Re: Copying a control knob by moulding

That sounds very good, but for brass inserts, this is where you need a little lathe. You don't even need a screwcutting facility for this job so one of the little 'toy' jobs would be fine.

Things like a bakelite knob with the inner collar cracked, you can just turn out the broken collar section, machine up a big brass piece to fill the space, araldite it in, then drill and tap the grub screw hole straight down the original hole.

David
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