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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 19th May 2005, 9:40 am   #1
Gavtech
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Default Remoulding a missing knob

Some of you may be interested to read an account, written by Malcolm Burrell, of a vintage repair to a Ferguson 352U Table Radio Receiver which includes a description of the process of moulding a missing wavechange knob.

Sadly not available on the web {AFAIK} .
It can be found in this months copy of 'Television and Consumer Electronics' ... Just out { June 20005 Issue }

Gavin
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Old 19th May 2005, 9:55 am   #2
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Default Re: Remoulding a missing knob

Yes, very interesting, and it looks reasonable.

Basically, he covered the knob to be replicated with Vaseline and a layer of tissue paper, and then encased it in Polyfilla. He did twice, once for the back and then again for the front.

Then, the 2 half-moulds were covered with Vaseline, filled with polyester filler, and brought together. The polyester filler was then painted brown.

It doesn't look perfect, and has a slightly "wobbly" look to it. I suspect it took ages to make too. But I'm still impressed.

Television seems to have more and more vintage radio articles (of varying quality) every month. I suspect the "throwaway" TVs that predominate today mean that fewer people are interested in reading articles about servicing modern equipment.

Having said that, they still seem to accept fault reports re. TVs, VCRs and DVDs, and I've had a handful published over the last few years. They don't pay you a fortune for them, but it's all useful extra pocket money.

Nick.
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Old 19th May 2005, 10:30 am   #3
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Default Re: Remoulding a missing knob

Hi Nick

That's a good synopsis of the process!

I agree the result looks a bit 'wobbly' ... but I'm sure it would pass a cursory glance. Much better than a gaping hole or an inappropriate knob.

Can I direct you to near the end of the 'Belts again' Thread . I asked you a question there a while back which I think you must have missed.
I'd reproduce it here but I've already had a smacked botty for going off-topic . I probably should have sent you a PM...
Still learning

Regards

Gavin
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Old 21st May 2005, 11:01 am   #4
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Default Re: Remoulding a missing knob

A good moulding material is AFAIK latex. Its fairly cheap and takes a pretty good impression of the item to be copied. RTV flexible rubber is the best but is not cheap.

The knob should be covered with a release agent (Vaseline thinned with white spirit) and then covered as far as the back of the knob, or as far as its maximum diameter if its an awkward shape. When the mould has set apply more release agent to the knob and mould and cover it completely in latex. When that has set remove the knob and you now have a mould in two halves ready to make a new knob.

At all stages its important to keep air bubbles to a minimum. Again an ideal material is polyester resin but its very costly. There are however clear casting resins to which a pigment can be added.

Joe
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Old 14th Dec 2006, 9:05 pm   #5
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Default Re: Remoulding a missing knob

hi all
just read through a few threads here trying to solve a problem and this thread caught my eye.
i'm a modelmaker and i make and use rtv silicone rubber moulds all the time and i use many litres of resin a month. if any one wants more detail on making moulds i can help.
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Old 15th Dec 2006, 12:05 am   #6
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Default Re: Remoulding a missing knob

It would be very helpful if you could advise of a suitable casting resin to imitate bakelite. I am sure there must be a material out there somewehere.
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Old 15th Dec 2006, 3:08 am   #7
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Default Re: Remoulding a missing knob

Quote:
Originally Posted by PJL View Post
It would be very helpful if you could advise of a suitable casting resin to imitate bakelite. I am sure there must be a material out there somewehere.
the best material to use to imitate bakelite is bakelite.
you can grind bakelite into something like sawdust and mix the dust with the resin but the resin might alter the colour a few shades lighter so you need to add a dark dye aswell. also you can just dye the resin and hope it matches with whatever machine it came off.
it's mostly trail and error because different brands of resin set in different colours normaly light browns. here in ireland 2lts of resin costs €40 but you could make at least 200 medium sized knobs with that much resin.
hope that helps
Luke
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Old 15th Dec 2006, 9:49 am   #8
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Default Re: Remoulding a missing knob

Quote:
Originally Posted by threenero View Post
you can grind bakelite into something like sawdust and mix the dust with the resin
I wouldn't recommend this as it was not unknown for asbestos to be added to the bakelite

David
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Old 15th Dec 2006, 6:27 pm   #9
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Default Re: Remoulding a missing knob

well resin and silicone fumes aren't to healthy either thats why i have a heavy duty mask and a box full of latex gloves and when the resin is sanded the dust is like talcum powder so the mask is used alot. if you put the bakelite in a plastic bag and whack it with a hammer for a while you can get it small enough to use and keep the airborne particles in the bag.
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Old 15th Dec 2006, 8:16 pm   #10
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Default Re: Remoulding a missing knob

RTV is expensive for a 'one-off' operation. There are cheaper alternatives, take a look on craft supply web sites for these. I entirely endorse David's warning . Also economical to use is resin as used for fibreglass repairs. It works pretty well - its natural gloss can be tempered by adding just a little talcum powder or very fine sawdust, such as that produced when sawing MDF with a fine bladed saw (too much of any additive causes weakness) - or you can matt down by hand with fine wet/dry paper to suit your needs. Colour the resin with very small amounts of modelling paint, or use aerosol paint by spraying into the aerosol lid and adding the liquid paint to the resin - you've got to be sharpish before it dries!
Hammering Bakelite in a bag doesn't sound like a barrel of laughs. but I suppose, each to his own!
-Tony
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Old 16th Dec 2006, 4:15 am   #11
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Default Re: Remoulding a missing knob

rtv is about €30 per kg but some art supply shops are selling it in smaller proportions. mdf dust is almost as bad as asbestos anytime you work around dust of any sort you need a mask. a cheaper mould making material is laytex but you need to paint on a thin layer and let it set then another layer and so on for a good strong mould you need at least 10 to 15 layers and that takes a long time. so the quicker you want to make something the more you have to spend. the resin for fiberglass can take weeks to set when it is in one thick shape. it all just depends on how quick you want your part. there is another casting plastic that you can buy it comes in small beads and need to be melted in an oven to become liquid but it needs to be melted while in the mould so you need a metal mould so thats usually way too expensive for small runs of any part.
you can buy toy casting sets that use the castin resin and candel making sets that use the latex.

heres an excelent site http://alumilite.com/index.php?page=show_how&id=68 it has lots of how to's and they also sell casting supplies. everthing you need is there. they also have a forum so you can ask more advice if you get stuck.
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Old 16th Dec 2006, 5:52 pm   #12
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Default Re: Remoulding a missing knob

Luke (threenero) said
Quote:
the resin for fiberglass can take weeks to set when it is in one thick shape.
The liquid resin I've used from car accessory suppliers always set in 10 minutes, regardless of thickness - which is logical, as 'setting' is actually a curing reaction which is independent of, say, air contact, unlike 'drying'. That's been my experience, at least. In fact, air contact seems to slow the process. Perhaps some resins act differently in some way?
I can't agree that MDF dust is almost as bad as Asbestos, but you do have a good point, Luke - it certainly is nasty stuff and always, a mask should be worn when working with it. I should have said so!
Two useful moulding methods are: 1, Gelflex (hot melt rubber) and 2, Alginate (cold working). I've used both with success. Gelflex is tough and can be used repeatedly, then remelted for further use. Problems are the danger of overheating, which ruins the stuff (and creates the mother of all stenches), also air bubbles trapped in the mould at the interface between the mould and the object being moulded. These are hard to prevent completely but as they leave a tiny 'proud' bubble in the final casting, these can usually be removed without too much difficulty. Alginate is weaker but a given mould can produce several copies before it fails. Both methods will cause slight shrinkage of the final casting but this is generally matterless.
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Old 18th Dec 2006, 6:12 pm   #13
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Default Re: Remoulding a missing knob

Quote:
Originally Posted by threenero View Post
rtv is about €30 per kg but some art supply shops are selling it in smaller proportions. mdf dust is almost as bad as asbestos anytime you work around dust of any sort you need a mask. a cheaper mould making material is laytex but you need to paint on a thin layer and let it set then another layer and so on for a good strong mould you need at least 10 to 15 layers and that takes a long time. so the quicker you want to make something the more you have to spend. the resin for fiberglass can take weeks to set when it is in one thick shape. it all just depends on how quick you want your part. there is another casting plastic that you can buy it comes in small beads and need to be melted in an oven to become liquid but it needs to be melted while in the mould so you need a metal mould so thats usually way too expensive for small runs of any part.
you can buy toy casting sets that use the castin resin and candel making sets that use the latex.

heres an excelent site http://alumilite.com/index.php?page=show_how&id=68 it has lots of how to's and they also sell casting supplies. everthing you need is there. they also have a forum so you can ask more advice if you get stuck.
Hello All,
UK supplier can be found here http://www.ema-models.co.uk/
Have not ordered from them yet but may give them a try in the new year Paul.
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Old 18th Dec 2006, 8:41 pm   #14
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Default Re: Remoulding a missing knob

EMA are good, but there are others and I'd recommend shopping around before ordering, to compare prices. One example among many is www.trylon.co.uk
who supply schools, much as do EMA. I ordered from both during my years teaching and was satisfied with their service, but for specific moulding materials, Google a search for craft supplies or Art and craft supplies. There's a few out there!
-Tony
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Old 22nd Dec 2006, 8:12 pm   #15
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Default Re: Remoulding a missing knob

I once heard Gerry say re - knobs: If We Haven't Got 'Em, We Make 'Em.

How does he do it?
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Old 23rd Dec 2006, 6:31 pm   #16
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Default Re: Remoulding a missing knob

I swear by Alumilite. Nice thing about it is you can dye it. And it will register all the details of the original.

Example--my prototype Eveready 768 B battery. The original top was dark brown wax. Made a mold with some Silicone RTV compound that I bought on eBay. For about the same price as Alumilite's RTV, I got twice the material, and it cured in half the time. Then I added a couple drops of yellow, red and a small drop of black into the clear resin, mixed it with the other part, poured and catalyzed.

Finished cover with resin top:
http://users.mw.net/%7Ebmorris/finished7681.jpg
http://users.mw.net/%7Ebmorris/finished7682.jpg
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Old 23rd Jan 2007, 6:54 pm   #17
Ben_Dijkman
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Default Re: Remoulding a missing knob

Marrrvellous.!
Verry nice to see these verry good improvements.
B.R.
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