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Old 10th Dec 2017, 7:42 pm   #1
David G4EBT
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Default Knob casting experiments

I've had it in mind for some time to have a dabble at casting replacements for Bakelite knobs, and thought it might be of interest to share what I've been doing.

Two questions arise at the outset - what material to use to create the mould, and what material to use for casting. In this regard, I decided to try 'alginate' which is a harmless, odourless, flexible material which is in powder form and is mixed with water. Bizarrely perhaps, it's much favoured for making moulds of new-born babies hands and feet to create plaster casts for keepsakes.

When used for that purpose, the baby's hands or feet can be wriggled free, and a perfect mould remains. Thus, the material will replicate every detail of say a radio knob with ridges around the perimeter, recessed lettering, and even if the knob sides at sloping 'the wrong way' (Roberts R200 etc), the specimen knob can be easily extracted from the alginate. (I believe an alginate product is more conventionally used for taking dental impressions). The product I've been using is known as 'Alginart' and comes in 450g, 900g or larger packs as desired. a 450G pack for knob casting moulds goes a long way.

Turing to the casting medium, there are of course all sorts of resin products used extensively by model makers and the like, but there are two products I had to hand which I thought I'd give a try - namely, Ronseal 'High Performance Wood-filler', which - among other things contains styrene, and uses a catalyst hardener. It sets very hard in minutes and can be sanded, drilled, or turned on a lathe. The other product is 'Super Steel' - a Plastic Padding product, which despite its name, is not electrically conductive, which could otherwise pose a hazard if used to create a knob for a live chassis radio.

To date, I've only used the wood-filler - the Super Steel might give better results as it's much more runny so will most likely be easier to place in the mould.

An essential requirement is of course that when finished, the knob must be perfectly concentric to the control shaft to which it's fitted, so a means of lowering the specimen knob into the alginate is called for, and a means of inserting a brass ferrule of the desired internal diameter (eg, 5mm, 1/4" etc). I'm sure that there is any number of ways of holding the knob upright, but the method I chose was to create a little stand using materials that I had to hand - namely, M8 screwed steel rod and nuts, plywood, acetate sheet, and for a casting cup, the top off a 300ml aerosol can.

I've attached a few sketches and pics to show the completed stand, and will add some further posts to the thread on some knobs I've made so far.

Hope that's of interest.
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Old 10th Dec 2017, 8:32 pm   #2
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Default Re: Knob casting experiments

A few more pics of progress to date.

The first pic shows the knob specimen in the mould making jig ready to be lowered into the casting cup when filled with alginate.

The second pic shows the freshly mixed alginate in the casting cup.

Third pic shows the casting cup in place with specimen knob lowered into the alginate and left in place till the alginate sets.

Fourth pic shows the alginate mould with the specimen knob removed.

Last pic shows a knob cast using Ronseal wood-filler, with a coffee stirrer stick to pull the cast knob out when set.

Some more pics to follow.
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Old 10th Dec 2017, 8:47 pm   #3
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Default Re: Knob casting experiments

Yet more pics!

First pic: I cast four knobs in the mould before discarding it. (The knobs were for a Unitra Figaro).

To machine the hole for the brass ferrule it must be completely concentric with the knob and some means of holding the knob in the lathe to machine the rear of the knob flat, then to use a 12mm end mill to mill the hole needs to be devised. 'Jam chucks' which are simply wooden chucks made for the job in hand are widely used in woodturning. I made a simply jam chuck and turned a hole in it to the same profile as the knob, then used some hand-wipe tissue to protect the knob when I pushed it into the chuck. I then 'faced off' the rear of the rough casting, and using a 12mm end mill in the tailstock, I rotated the chuck by hand while advancing the tailstock to the required depth for the brass ferrule.

The second pic is a sketch of the jam chuck.

Third pic is drilling the jam chuck with a Forstner bit before using a turning tool to create the internal profile to suit the sloping side of the knob.

Fourth pic shows the jam chuck with the casting in it, being rotated by hand while advancing the 12mm end mill.

The last pic shows a 12mm ferrule turned on my metalworking lathe with a 1/4" hole and the outside roughened up on the lathe to provide a good key for the epoxy cement used to glue the brass ferrule into the knob.
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Old 10th Dec 2017, 8:54 pm   #4
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Default Re: Knob casting experiments

Fantastic stuff David - a great post

I'm starting to think about casting a knob for an old BBC loudspeaker unit that I have a pair of and you've given me plenty to think about. My material of choice for the knob was going to be "Milliput" epoxy putty. It's available in a wide range of colours and I had wondered if by roughly kneading two different shades together a sort of imitation Bakelite might result.

Cheers,
Steve.
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Old 10th Dec 2017, 9:11 pm   #5
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Default Re: Knob casting experiments

Just a couple more pics.

The first pic shows two completed knobs, with the original knob used for the casting mould in the centre. After fitting the brass ferrule, each knob was drilled and tapped 4BA and a 4BA brass grub screw fitted. The knobs were then sprayed with acrylic white primer, and acrylic gloss which was a good match for the original knob as can be seen.

The second pic shows a knob in the mould making jig from a Roberts R200, which has sloping sides, but as stated earlier, despite that, the alginate is sufficiently flexible to enable the knob to be wriggled free from the alginate without damaging the mould. (Leastways, I hope so!)

One thing I've learned is that when lowering the knob into the alginate, 'air-locked' bubbles can form under the lower part of the knob. (That's the front of the knob in its normal use). To prevent that, alginate can be smeared onto the underside of the knob with a finger prior to lowering the knob, to ensure that air isn't trapped when the knob is lowered into the alginate.

Although the wood-filler is quite satisfactory in terms of the finish and durability, I'm inclined to the view that a more runny casting medium will do a better job. I'll try the Super Steel epoxy as and when I get time.

I'm nuts, of course - I don't actually need any knobs just now, and all of this is ludicrously time consuming, but it's all good clean fun, it stops my wife or kids putting me in a home, and it's in my skills set, except I'm now getting high on the fumes from the wood-filler!

Needless to say, I'm not in a position to be able to offer anyone my services as a knob caster!

Hope the thread has been of interest. I've got many other projects at various stages of 'un-completion' , but just went off on a frolic to have a go at this on impulse. Quite why, I don't know, but then I'm not a doctor, so how could I?.
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Old 10th Dec 2017, 9:33 pm   #6
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Default Re: Knob casting experiments

Really nice!

I wish I had one of these setups when I was repairing knobs for the VCM163 some years ago!

Since I had to use a center rod that could be put in place during the curing of the glue/plastic and then removed before the center brass collar was placed back I had to use a piece of silicone and PTFE rod since the glue/plastic didn't stick to that material. I wrote something in a thread somewhere here on the forum about the repairs. Your setup would have made it a lot easier to align the center of the knobs I repaired.
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Old 11th Dec 2017, 9:32 pm   #7
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Default Re: Knob casting experiments

Excellent work David. It's really good to see how this can be achieved and thanks for sharing this with us. My neighbour has a half reasonable metal turning lathe so he'll be able to assist me with the drilling part of the process when I try my hand at casting some knobs.

Regards
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Old 11th Dec 2017, 9:42 pm   #8
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Default Re: Knob casting experiments

That's excellent stuff David. You'll be building a vacuum chamber next to sort those bubbles out. I've not done any casting for years now.

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Old 11th Dec 2017, 9:48 pm   #9
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Default Re: Knob casting experiments

Fantastic achievement David
Inspiration to us all !
Cheers Pete
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Old 15th Dec 2017, 1:10 am   #10
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Default Re: Knob casting experiments

When I was a vacation student at STC, the lab used to use Titanium Dioxide powder to colour Araldite white when using it to encapsulate stuff. When I recently wanted some brown pigment to colour some filler to approximately match something I got a piece of a broken earthenware plant pot and ground it to a very fine dust in my wife's granite pestle and mortar (when she was out!). When mixed the colour resembled old brown Bakekite, and as the pigment is inert earthenware, I would not expect it to fade. At the moment I am using that Ronseal wood filler for the jobs I used to use Plastic Padding type elastic for, as no-one local is stocking any sort of Plastic Padding at present. It behaves like Plastic Padding, differing only in its colour. I haven't tried casting entire knobs, only for strengthening the inside of hollow knobs where the plastic boss for the shaft has broken, for which I have been using Araldite precision. I suppose it would be possible to add pigment to the Ronseal filler and mix it in thoroughly before adding the hardener. I like the ingenious alignment jig: where would we be without studding? Well done! .
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Old 15th Dec 2017, 2:14 am   #11
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Default Re: Knob casting experiments

I was casting prosthetics using a blue silicone for the molds and a 2 part colorable plastic for the fangs, knobs, etc.

I did find that temperature made a massive difference on the curing time, and it was recommended putting the item in a vacuum chamber to pull out all the air bubbles.

Not having one, I used an industrial strength body vibrator to agitate the air bubbles out. It worked great. I would think that a vibrating sander strapped to the board that the mold was on would work quite well too.

Casting is difficult under good conditions. My hat is off to you.
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Old 15th Dec 2017, 9:18 am   #12
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Default Re: Knob casting experiments

Hi

A timely post for me as I am about to embark on a knob casting! I have had a similar jig in mind to hold the original. I'm having second and third thoughts about the casting part though which is good thing I suspect as having them afterwards isn't very useful!

My plan is to use the jig to hold the insert during the casting process too. I think that I will cut definite grooves in the insert for maximum grip!

I hadn't thought about making a new thread for the fixing screw but that does make a lot of sense.

I've gone for commercial moulding and casting compounds as the knob I want to cast has a pointer that protrudes like the spout of a kettle - nightmare!

Cheers
James

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Old 15th Dec 2017, 10:31 am   #13
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Default Re: Knob casting experiments

Hi David,

Very interesting post, I have dabbled in making new knobs in a similar fashion, I used some 2 part silicone rubber moulding compound, which is excellent stuff for getting all the fine detail, and some polyester resin to cast the final part. I've had some good results with it, but I think that I could do better, especially if I built a jig like yours!

One thing I do to reduce the effects of bubbles is to mould the item upside down, so the most visible face of the knob is uppermost, that way the bubbles don't settle on that surface while the silicone goes off, then when casting the final part turn the mould so the face of the knob is facing down, then any bubbles in the resin end up at the back out of the way.

I've got a knob from an Ekco U76 to cast, I've already made the mould just need some resin to do it!

Regards
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Old 15th Dec 2017, 11:22 am   #14
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Default Re: Knob casting experiments

Inspiring work! One small question - how do you create those excellent drawings? It must be quite expedient as you have used them so liberally
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Old 15th Dec 2017, 11:46 am   #15
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Default Re: Knob casting experiments

Really interesting, thanks for sharing. In the past I've passed on some nice radios because they had a missing knob, I simply gave up trying to locate one.
So I've been put off buying sets with missing knobs since, this makes you think again. Though having the skill to make them to this high standard is another matter.
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Old 15th Dec 2017, 7:36 pm   #16
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Default Re: Knob casting experiments

Fascinating... particularly the taking-a-mould and casting parts.

I'm wondering - have you considered using a really-runny-resin [without fillers or stiffeners] for the casting?

Someone I know regularly casts large prosthetics [think horns, hooves, claws] for use in Hollywood movies - she always uses a 'runny' resin so it follows precisely the contours of the mould.

Entrapped air-bubbles are always a problem in this sort of thing: she sticks her resin/hardener mix into a vacuum-chamber and pulls it down to a low pressure for a few minutes to make the air-bubbles expand and rise to the surface.
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Old 15th Dec 2017, 8:19 pm   #17
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Quote:
Originally Posted by Julesomega View Post
Inspiring work! One small question - how do you create those excellent drawings? It must be quite expedient as you have used them so liberally
I never put hand-drawn sketches or circuits on the forum - if I do something that I think is worth sharing on here, I draw a sketch, which doesn't take a great deal longer than doing it freehand and scanning it, and for some things, such as the knob mould casting jobbie, I'd do the drawings for my own use anyway. But in saying that, I wouldn't for a moment want to deter anyone who does put hand-drawn circuits and the like on the forum.

I'm not very far up the 'food chain' when it comes to using CAD packages, so I just use MS 'PAINT' - a simple but versatile drawing package, which for now, comes bundled free with all versions of Windows, albeit Microsoft have said they intend to 'deprecate it' by which they mean 'kill it off'. It's a most odd American use of the term 'deprecate' which - in Brit English - means 'to express disapproval of protest against, or belittle', usually in the context of someone's character. It's an odd thing for MS to say about their own creation, used and enjoyed by millions the world over for decades. Due to howls of protest, MS have said they'll leave it alone for now.

I find it easy to use for everything from drawing sketches, designing PCB artwork and UV masks, component overlays, and cleaning up circuits. As a little example, I've attached a section of a original scruffy datasheet from a Unitra Figaro, and a cleaned up reproduction of the same section that I drew using 'PAINT'. I cleaned up all of the diagrams from the manual for an article published in the BVWS Bulletin as none of the diagrams were fit to be published. If I couldn't be bothered to have cleaned up the drawings, I wouldn't have bothered to submit the article for consideration.

There are countless tutorials on internet for 'PAINT' for anyone who wants to find their way around it.

Hope that's of interest and use.
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Old 16th Dec 2017, 2:04 am   #18
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Default Re: Knob casting experiments

That's very labour-intensive, David! I know MS Paint well, it's fine for some small jobs but fails where you need repeated zoom and pan. It doesn't support component libraries for schematics but I built up a small range of components to paste and copy.

It may be deprecated and depreciated, but its original author went on to create paint dot net (pdn) which evolved from Paint, retaining the same keyboard shortcuts and gives a wonderful expansion of the functions. Particularly useful in the case of schematics and engineering drawings is the availability of 'layers' so that you can have separate drawings for, say, components, connections, text and the like, You can then edit and move things around separately. I have also used it to open photos of both sides of a pcb and reverse and overlay the copper side so that you see it faintly beneath the components and screen print.

Try it, download from www.getpaint.net and ignore the spam "download" offers. You may have to search a bit if you need a version for an earlier Windows
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Old 16th Dec 2017, 8:59 am   #19
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Default Re: Knob casting experiments

Yes, you're quite right about Paint, but over the years, I've found my way around it and find it easy and enjoyable to use. Over time, I've built up a little library of components and often open up more than one page to cut and paste from one to another. I do have paint.net, but haven't found it easy to find my way around it as the controls are quite different from Paint.

Really, it's a topic in its own right and there are lots of freeware programs around for circuit drawing and the like, with some splendid example from tine to time on the forum, of dials which have been created or re-mastered. I also use Photoshop Elements 7 - quite an old version now, from 2008, which I manage to fumble around with to the extent that I need to.

As I said earlier, I operate at a very basic level, and I'm on the novice slopes with CAD!

Given that these latter posts are off topic, maybe they warrant a new thread 'Creating Sketches and Circuits' or some such thing, as I'm sure it's a subject of interest to others, but I'll leave that to the discretion of the moderators. I know there have been threads in the past on circuit drawing, and we see some excellent examples on the forum, with component libraries.

Back on topic, I've just cast a knob for a Portadyne Princess, which I'll hopefully finish off and spray to match the original this weekend. The pics below show it in its raw state, cast in Super Steel two-part epoxy.
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Old 16th Dec 2017, 9:52 am   #20
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Default Re: Knob casting experiments

Stay tuned for the next KNOB-CAST! Sorry.
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