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Old 9th Jul 2022, 11:45 pm   #1
greg_simons
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Default Help moulding some knobs

Need a little help with some brand names etc of the silicone rubber and epoxy required to mould some knobs for my portarama including how to colour the epoxy the right shade, first time I've had to get involved in this aspect of restoration and any tips would be welcome.
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Old 10th Jul 2022, 11:00 am   #2
Gridiron
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Default Re: Help moulding some knobs

I have used Mouldcraft polyurethane resin together with various types of silicon rubber, including the food grade rubber, all on available on eBay, with good results although it depends on the shape of the item you are reproducing. I was never able to make good repro knobs for an Emerson midget without bits missing on the rim because it was undercut. Air bubbles can be a problem, Clive Mason, who used to make a lot of repro parts although not nowadays, used a vacuum pump to reduce these.
I have no connection with the above mentioned supplier.
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Old 10th Jul 2022, 1:56 pm   #3
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Default Re: Help moulding some knobs

Thanks for that, it's a starting point, know Clive very well, made quite a few round ecko cabinets, in some interesting colours!.
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Old 10th Jul 2022, 8:06 pm   #4
David G4EBT
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Default Re: Help moulding some knobs

Back in 2017, I experimented with various mediums and methods of producing replica knobs.

The forum link to the thread is here:

https://www.vintage-radio.net/forum/...d.php?t=142098

Doubtless other will had had their own approach and success stories to share using various mould materials and casting media.

I'm not holding myself out to be an expert - I'm just sharing my own experiences, successes and pitfalls, which I hope might be of help.

For the moulds, I used 'chromatic alginate' which is quite cheap - the only downside being that once you've made the mould, it won't keep, so you have to do your knob casting right away. (Chromatic alginate is widely used for hand and feet casting on new-born babies and the like, to cast hands and feet as mementos).

https://www.craftmill.co.uk/chromati...yABEgJHh_D_BwE

Whatever materials and techniques are used, the knob casting is the easy bit.

The more challenging task is fitting a brass ferrule into the knob which is perfectly perpendicular and exactly central.

Initially, I made a jig and with the mould place exactly central, I fitted a ferrule to a 6.4mm diameter rod, and lowered it down into the still soft moulding material, only to find that however much care I took, the ferrule wasn't central. It's remarkably how the human eye can discern quite small inaccuracies. 0.5mm off-centre in a 25mm diameter knob is only 2%, but of course, when the knob is rotated, the amount of eccentricity from side to side is 1mm, which is 4% and is depressingly easy to spot, especially if you are 'picky' which I admit to being.

Furthermore, if the ferrule is only very slightly out of perpendicular, when rotated, not only will the eccentricity be noticeable, the knob will also look wonky. Of course, it's only when the knob is rotated that such discrepancies become apparent - if the knob is fitted and left alone it won't be noticeable. Also, if the knob is just for a wave-change switch say from MW to LW, it doesn't have to rotate anyway.

I guess most people will, at best, have just a pillar drill to drill the knob for the ferrule, and if that's so, the more care in drilling the hole for the ferrule and getting the drill table exactly level, the better the chance of success when the ferrule is glued into the hole with 2-part epoxy, having roughened the ferrule with a file or sandpaper to ensure good adhesion.

As I have both a small metalworking lathe and a woodturning lathe, I was able to fit a drill into the tailstock and hold the knob in a 'jam-chuck' in the headstock to drill the knob exactly central. If the knob is quite shallow, it's best to only drill part way into the knob with a 6.4mm (or 6mm depending on the control spindle on the equipment that the knob is for), twist drill, then to remove the twist drill and finish off with an 'end mill' which has a flat bottom so the risk of breaking through the knob is much less.

For anyone who hasn't seen an end mill, there's a picture of a 6mm one here:

https://www.ewequipment.co.uk/produc...yABEgJCDvD_BwE

I don't want to make it sound like a bigger task than it is, and if the first efforts aren't successful, we can just keep trying till we're happy with the end result.

As Winston Churchill put it:

'Success consists of going from failure to failure with no loss of enthusiasm'.

It's also said (attributed to Einstein with no proof), that:

'The definition of insanity is doing the same thing over and over again and expecting a different result'.

Which is why - after several failed attempts - I abandoned my little knob casting jig and resorted to using a lathe.

I hope these wordy notes, and the original thread are helpful.

Every good wish in your efforts.
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Old 10th Jul 2022, 10:11 pm   #5
greg_simons
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Default Re: Help moulding some knobs

Hi David,
Yes indeed, I shall peruse that thread with interest, regret i have neither pillar drill or lathe but I'll find a solution.
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Old 15th Jul 2022, 3:39 am   #6
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Default Re: Help moulding some knobs

I ordered a casting kit off the internet. About $600.00 USD for the huge kit. I was involved in a prosthetics production project at the time. This kit was top of the line too.

It came with alginate- Same stuff the dentist uses to start a tooth cast for a crown.

I then used the way too expensive 2 part plastic to make the final casting.
It worked quite well in the end, but took experimentation to get it right.

Wrapping the alginate with plaster bandages will extend it's life, but is only good for a very limited amount of good castings.

My very artistic friend who was making mass castings used a 2 part plastic available from Michaels, A local craft chain. He found that Ceram Coat, a paint, would work fantastic to color the plastic. As an experiment, he made me a repro Philco knob that is virtually indistinguishable from a factory made one. It even had the knob spring hole exact dimensionally!

It is an art, no doubt, to get everything to come out perfect.

Getting the color to come out to match exact colors of knobs was an art I was unable to master in the end. I was frustrating, to say the least, but the big castings I needed to make came out quite well. Not all of us are artistic craftsman like my friend is.

I did build a vibrator table, (not having a vacuum chamber), using an old commercial massage vibrator that did a great job to remove almost all of the air bubbles from the castings I did. It would likely work well for 2 part epoxy castings also.
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