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Old 10th Aug 2021, 1:11 pm   #1
vinrads
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Default Making /casting knobs

I have made knobs in the past but the are made of what I would call soft plastic ,I need to make some like Bakelite ones so will take a 4 ba thread, any ideas. I have made the mould. Mick.
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Old 10th Aug 2021, 5:31 pm   #2
McMurdo
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Default Re: Making /casting knobs

I'm always interested in new ways to make knobs. What did you use to make the ones that were too soft for you?
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Old 10th Aug 2021, 5:55 pm   #3
Superscope
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Default Re: Making /casting knobs

The solution I use, is to machine a Brass Thread (Nut) or Bolt out of
a Brass Rod and imbed this into my Molding.

My Resin of choice is Jesmonite.
There are pigments available to make it almost any colour you want.

It's expensive, but gives very good results.


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Old 10th Aug 2021, 5:57 pm   #4
Lloyd 1985
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Default Re: Making /casting knobs

Iíve made some for my A22 using black fire retardant potting compound, they come out almost as hard as Bakelite, and they look like Bakelite too! this is the stuff I used Hope itís ok to post a link to it! Itís not cheap, but so far, itís turned out pretty good, the knobs donít seem to have suffered any shrinkage. It does take a while to get fully hardened, and if you mix it slightly wrong like I did, then it takes even longer, with the stuff feeling like soft plastic for a long while.

Hereís a link to my A22 restoration, page 3 shows the making of the knobs! All very experimental...

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Old 10th Aug 2021, 6:12 pm   #5
David G4EBT
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Default Re: Making /casting knobs

I did quite a lot of knob casting experiments back in 2017.

For casting light coloured knobs I used Ronseal two part epoxy wood-filler which is cream in colour and is extremely hard when set (in a matter of minutes).

https://www.diy.com/departments/rons...E&gclsrc=aw.ds

For casting dark coloured knobs I used Plastic Padding ''Super Steel' two-part epoxy, which also sets rock hard in five minutes.

https://www.amazon.co.uk/Plastic-Pad...38829816&psc=1

(I should say that I had the above products to hand as I use them for other purposes).

For making the casting moulds I used 'Alginart' chromate alginate, the most common use for which seems to be casting impressions of new-born babies feet and hands, which - for reasons that elude me, seems to be a rather surreal and popular pastime.

https://www.amazon.co.uk/450g-Algina.../dp/B00FVBFXK0

Alginate will make an excellent impression of a knob, but after an hour or so, as it begins to dry out it shrinks so the mould is really a one-trip affair, and if more of the same type of knob is required, they need to be made right away or another mould made later.

The big pitfall with DIY knobs is how to drill an exactly concentric hole for the control shaft and I've seen no examples on youtube or elsewhere of anyone achieving that by say using a pillar drill. If the hole is just 1mm off centre, when rotated the knob will have a 2mm swing from side to side, (even worse if it isn't exactly perpendicular to the control shaft as it will look wobbly. That might be acceptable to some, but I couldn't live with it.

Ideally, the knob needs a brass bush rather than expecting to be able to drill and tape the casting material and expect to e able to tighten the grub screw onto a shaft in a cast knob. The problem of centring the bush in the knob still exists, so I designed a jig which I felt would hold the bush exactly central in the mould when I poured the casting medium in. Except that it didn't.

The only successful way I found of exactly centring the bush was by casting a solid knob, putting the knob in the headstock of my woodturning lathe in a 'jam chuck' purpose made for the particular knob. then using a 12mm end mill in the tailstock, turning the headstock chuck by hand to mill out the knob into which I then superglued the bush (turned from 12mm brass drilled 6.4mm for the shaft). Finally, I drilled and tapped the knob and bush 4BA for the grub screw.

I posted a thread on my rather mixed efforts at this link:

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

In terms of time and cost, none of this makes economic sense, but then I/we don't engage in hobbies to either save money or to make money, but for enjoyment, relaxation and a sense of fulfilment using scarce leisure time. And if a knob is uncommon or rare, the likelihood of one turning up is slim to non-existent, so it's either 'make one, or do without'.

Some knobs lend themselves to being made from wood - I posted a thread of making wooden knobs for a 'Bestone' radio small and large replica knobs for an Ekco A22. Partly because a chum said it wouldn't be possible on a lathe because the small knob has a 'tab'.

https://www.vintage-radio.net/forum/...d.php?t=140529

I'm not sure any of this convoluted long-winded answer to a simple question is of practical use to anyone, and it's no use at all for those who don't have lathes etc, but it might well be that others on the forum have achieved satisfactory results in a simpler way with much less faff. As the saying goes: 'Use what talents you possess - the woods would be very silent if the only birds that sang there where those that sang the best'.

I've attached some pics of my knob casting efforts which might be of interest.

Pic 1: A sketch of a 'jam chuck' made to hold the knob centrally on the lathe while drilled for a bush.
Pic 2: One of those manky poorly designed Bush Bakelite knobs, alongside a replica I cast with a brass bush fitted.
Pic 3: The top view of the old and new knob with the brass disc still on the old knob.
Pic 4: Two cast knobs I made for a Unitra 'Figaro Special' with the original in the centre which was used as a pattern for the mould.
Pic 5: An original Portadyne Princess knob in the centre, with two cast replica knobs I made by using the original one to create the mould.
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Old 10th Aug 2021, 6:53 pm   #6
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Default Re: Making /casting knobs

Although I am not an expert by any means, I have been experimenting with 3D CAD and 3D printing. My experience so far suggests that it would be an ideal method to produce knobs etc.
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Old 10th Aug 2021, 11:03 pm   #7
McMurdo
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Default Re: Making /casting knobs

I've used the Ronseal buff wood filler for moulding but AFAIK it's a polyester rather than epoxy resin, ie more akin to car body filler. I last used it to make a replacement organ key that had snapped and been lost and the end-cap off an extruded aluminium enclosure. Fiddly!
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Old 11th Aug 2021, 8:27 am   #8
vinrads
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Default Re: Making /casting knobs

Quote:
Originally Posted by McMurdo View Post
I'm always interested in new ways to make knobs. What did you use to make the ones that were too soft for you?
Hi Kevin I was using Polycraft SG2000 two part casting resin, Mick.
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Old 11th Aug 2021, 8:35 am   #9
vinrads
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Default Re: Making /casting knobs

Some of the knobs I have made using the Polycraft casting resin , remarkable how it picks up the detail ,it almost looks like there is a screw in on of them the small on in the centre is the original .As David has outlined the most difficult part is getting the hole central, and deep enough ,I just fancied casting them with a harder finish . Mick.
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Old 11th Aug 2021, 1:11 pm   #10
frankmcvey
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Default Re: Making /casting knobs

Jon, who goes under the forum handle "Ti Pwun" has done quite a bit towards developing printing techniques to reproduce Roberts grilles and various types of control knobs and other plastic fitments; on his profile page he invites queries regarding supplying these with a link to his web page. Unfortunately the web page link does not work and he hasn't posted since January. Might be worth messaging him, though.
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Old 18th Aug 2021, 12:46 pm   #11
vinrads
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Default Re: Making /casting knobs

I have just received my Polycraft fr4 potting compound , I see the mix ratio is 9.75 to 1, I may need some new scales , Mick.
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