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 Casting replacement knobs
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 Return to top of page · Post #: 1 · Written at 6:18:47 PM on 12 July 2014.
Maven's avatar
 Location: Canberra, ACT
 Member since 23 August 2012
 Member #: 1208
 Postcount: 584

Anyone have tips on making a plastic or resin cast to replace a missing knob?

I'm thinking of just making a silicone mould and perhaps using kid's moulding compound - the kind that they bake after making a 3 1/2 legged horse and so on.

The plan would be to embed a small knob with the right shaft grip in the mould, before casting.

I've seen some knobs being sold recently as "vintage" that looked like they might be reproductions.

Maven


 
 Return to top of page · Post #: 2 · Written at 9:31:36 PM on 12 July 2014.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 5262

I'll be interested in responses to your question, too. The idea of embedding a small sacrificial knob is a good idea as long as the moulding compound takes to it.


 
 Return to top of page · Post #: 3 · Written at 10:39:44 PM on 12 July 2014.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 5505

I am told there are a few collectors that do make reproduction knobs. One even claims to make them from Bakelite, which I find a little hard to believe given the large and energy-intensive equipment required to make a mixture of Bakelite set correctly.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 4 · Written at 1:28:50 AM on 13 July 2014.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3707

There was some sort of rubbery stuff that once set you could roll off the original knob & then use it to make a few. That from and epoxy resin which was able to be pigment coloured. I think the resin was a 1:3 type one.

One of the now deceased members of our radio club had knob manufacture down to a fine art.

Some of that stuff is nigh impossible to drill, so the hole can be an act if you fail to realise that.

Shrink tube loosely around whatever is used to form a shaft hole with a flat side, will help remove it.

Marc


 
 Return to top of page · Post #: 5 · Written at 9:52:41 AM on 13 July 2014.
Maven's avatar
 Location: Canberra, ACT
 Member since 23 August 2012
 Member #: 1208
 Postcount: 584

I've seen clear casting compounds but they are sold in quantities more than I want and at some tens of dollars a pop. As usual I'm looking for a cheap DIY solution.

I'm thinking of trying one of the silicone caulking compounds, of which I have many half-used tubes lying around, to make the mould. They seem to form a pretty good flexible skin when they dry. Maybe anoint the knob with vaseline before slathering with silicone, so it won't stick.

On a previous job fixing a knob with a broken shaft, I had success with generic epoxy filling a cylindrical dam as mould. The flat-sided shaft used for forming the hole was protected with cling-wrap and came out of the hardened hole quite easily. Plumbers teflon tape would probably also work. I hadn't thought about heat-shrink. It would be a bit thicker so perhaps make a looser fit for the shaft? Then might need wrap the shaft in teflon tape to make a firm grip for the knob.

More difficult to wrap a knurled shaft and maintain the precision. Maybe another job for vaseline? Not sure how that reacts with expoxies.

Maven


 
 Return to top of page · Post #: 6 · Written at 10:13:29 AM on 13 July 2014.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3707

There are hard compounds like "Knead it" and a few others that are used to repair pits in pump shafts etc. So Bunnings etc may not be the place to look.

Some of that stuff is that hard that it has to be ground as it will destroy a carbide cutter: Hence the previous warning.

You do need some clearance for the shaft, an interference fit is not always useful. The idea is to use thin shrink tube and I have a piece if salvaged shaft turned down slightly. I have used dowel.

If you get the right resin you can drill & tap it. That helps with a fully round drilled hole, and a lock screw.

There was an add in HRSA mag re knobs.

Marc


 
 Return to top of page · Post #: 7 · Written at 11:18:58 AM on 13 July 2014.
Scraps's avatar
 Location: Blue Mountains, NSW
 Member since 10 March 2013
 Member #: 1312
 Postcount: 401

I've been experimenting on and off for a while to get this right. As you mentioned the real moulding stuff is prohibitively expensive so I've been working with cheap stuff.

I make the moulds in the small circular takeaway sauce containers you can get at most $2 shops. I use Vaseline on it it if I want to get the mould out but usually just leave it in the container as it prevents any possible distortion of the finished mould.

I found this recipe on the internet but have haven't been able to find the page again. The mix I use is 2 parts pure silicone sealant, like guttering or window sealer, about $5 for a cartridge at the local hardware. One part Shellite, around $10 for a 1 litre bottle and 3 drops of glycerine for every tablespoon or so of the mix, available from the chemist. I also put in one drop of acrylic paint from a kids $2 paint set. This is just so you can see a uniform colour when it's mixed up completely.

I coat the knobs with a thin mixture of Vaseline and Shellite using a fine brush. Straight Vaseline won't give a smooth finish and for some reason silicon spray doesn't work very well. There are commercial mould release agents available but they are quite expensive. Don't ever try using just straight silicon for the mould, it's not flexible enough to get your knob out and even if you do you'll spend the next month trying to get all the silicon off it. I use an old pot shaft in the knob and one of those helping hands things with the clips to suspend it in the mix. It's very hard to get a good mould that doesn't have tiny bubbles in it though.

I've yet to find the perfect (cheap) materials for making and colouring the knobs. A selection of artists pastels has been very good in the past for colouring Araldite for bakelite cabinet repairs and I've tried this but it's hard to get a good shine on it. Artists chalks also work here for colouring as well as arcrylic paint from the same cheap kids set.

Some of the best results I've obtained have been using a two part gloss resin kit called Craft Smart Liquid Gloss which I think was about $15 from the artist supply store and colouring it with red oxide. I have concluded that to get the right colour with a gloss finish is going to require some sort of dry pigment.

To get the hole in the knob I've again used an old pot shaft liberally coated with straight Vaseline as a smooth finish isn't important here. For shallow knobs such as those used on an AWA 429 I use a knob such as Jaycar HK-7705 with the outer skirt cut off leaving just the inner part. I've tried making two part moulds for more intricate knobs but this is a whole new world of pain!

As I mentioned, I certainly haven't perfected this yet. The biggest problem seems to getting the colour consistently right with a good finish. I've put this project aside for now, there's quite a few people out there who do a very good job of reproductions and charge around $15 per knob as long as they already have a mould. I've talked with a few of these people but they're VERY secretive about their methods. One guy showed me his workshop and how he makes them but wouldn't let on as to what products he uses. His moulds were extremely flexible and the resin was almost watery in consistency. The finished knobs from the front were indistinguishable from the real thing but you had to drill your own hole. Excellent results can be obtained if you want to pay big $'s for the right silicon moulding and casting compounds but that would be no fun. I'd be very interested to hear other members experiences in this area.

Cheers,

Warren


 
 Return to top of page · Post #: 8 · Written at 12:33:10 PM on 13 July 2014.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3707

Note I trained & worked as a textile dyer and also worked in polymer chemistry One of the best things that will not, in most cases, adversely effect the reactants are the pigments that are used for colourants in stuff like concrete.

One of the problems the guy previously mentioned had, was air bubbles. Stuff like Araldite does have a heating phase as it reacts, which causes it to loose viscosity. With the slower reacting (and stronger) types, if it has a semi liquid phase there is more chance for the bubbles to rise and as it sets slow, more time to fill the mold & avoid or mix out air.

He did, after we had a chat, abandon Araldite & actually found a better product at an Industrial Supplier

Marc


 
 Return to top of page · Post #: 9 · Written at 5:50:56 PM on 13 July 2014.
Maven's avatar
 Location: Canberra, ACT
 Member since 23 August 2012
 Member #: 1208
 Postcount: 584

Lots of helpful points coming, unlike..

I've talked with a few of these people but they're VERY secretive about their methods

the ones who sound more like the CWA cake competition ladies and their marble cake secrets.

One challenge for me is that I need to replace a knob that was more or less transparent, so I can't use colouring agents to hide any bubbles etc. It will have a gold disc on the top, so probably not too hard to disguise flaws from the casual observer.

The more liquid epoxy compound sounds better, to let bubbles escape. I have never been able to mix Araldite, however slowly, without a lot of bubbles getting whipped into it somehow.

Maven


 
 Return to top of page · Post #: 10 · Written at 6:21:23 PM on 13 July 2014.
Scraps's avatar
 Location: Blue Mountains, NSW
 Member since 10 March 2013
 Member #: 1312
 Postcount: 401

If you need something crystal clear then the Craft Smart Liquid Gloss is perfect. It does form a layer of tiny bubbles on the surface as it cures but these are easily removed with a small blowtorch or hot air gun such as a gas soldering iron. It is very easy to mix and pour. The knobs I've made using this have a mirror like base but a slightly dull surface from the mould. When cured it's as clear as glass.

I'd have a go at successfully making a mould before moving onto trying to cast a knob. I'm still trying to make the perfect mould Sad


 
 Return to top of page · Post #: 11 · Written at 6:24:16 PM on 13 July 2014.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 5262

 
 Return to top of page · Post #: 12 · Written at 1:02:27 AM on 14 July 2014.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3707

It may not help? But if you have a knob to copy, it may be an idea to post a photo. I am wondering if it can be made in one or, two parts with Perspex if its clear?

Saw a former done like that in a radio recently.

Marc


 
 Return to top of page · Post #: 13 · Written at 11:45:40 AM on 14 July 2014.
Maven's avatar
 Location: Canberra, ACT
 Member since 23 August 2012
 Member #: 1208
 Postcount: 584

I'm sending a photo as Marc suggests.

Knob on right is a fairly complex shape on the exterior. I don't have the skills or equipment to machine that complex knurling, hence interest in casting.

Knob on left is even bigger challenge - shown is the current kludged-up replacement for a missing 3.5inch round dial that mates to the tuning gang axle. Transparent disk with pointer index lines (red) over the circular dial printed on the fascia panel. For the kludge I've used a standard Jaycar knob and the bottom of a round plastic food container. The index lines are hook-up wire. Shaft extension is a short length of 1/4inch plastic irrigation tube, split and wrapped in rubber to grip original sunken shaft of tuning gang. For a permanent knob, I'd try shrink-tube to reinforce the plastic shaft extension and make it more rigid.

So what I would like to do ideally is cast two of the smaller knob, then add a 3.5" plastic or perspex skirt to one of them for the tuning dial.

Maven


 
 Return to top of page · Post #: 14 · Written at 12:26:48 PM on 14 July 2014.
Scraps's avatar
 Location: Blue Mountains, NSW
 Member since 10 March 2013
 Member #: 1312
 Postcount: 401

Can't wait to see pictures, the description sounds like something out of MacGyver


 
 Return to top of page · Post #: 15 · Written at 1:15:57 PM on 14 July 2014.
Maven's avatar
 Location: Canberra, ACT
 Member since 23 August 2012
 Member #: 1208
 Postcount: 584

I take "MacGyver" as a compliment!

Pye Knobs


Meanwhile checked some resin prices. Smallest quantity of Craft Smart Liquid Gloss seems to be $30 for 125ml x 2 (resin and hardener).

Bunnings have this Diggers casting and embedding resin product for less than half that, though not sure how much the hardener costs.

Anyone used that? I'll go out and have a sniff around.

Maven


 
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