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What resin should I use? Answered

I have a 1958 Pfaff 332 sewing machine with a broken knob made of bakelite.  Replacement knobs are not available. It occurred  to me that it might be possible to cast a replacement with a different resin.  I have no experience with casting and don't know what kind of resin would be best..  I saw the antique radio knob replacement ible, and liked the idea of using a flexible mold for detail.  

This knob  engages  a major cam mechanism and should be fairly strong.  The old knob 'handle'  was broken off when I got the machine. The images of complete knobs are from the web. The complete knob is about 1 inch by two inches.  

What  product available in the USA do you recommend? 

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petercd (author)2014-01-30

I would advise against using a casting resin, they set nice and clear for preserving insects etc and might not have the required strength.

I would go for an expoxy resin but reinforce it with fibre strands that run around the mounting hole down into the lever arm.

The yellow kevlar strands that are found in optic cables would be perfect for this.

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mole1 (author)petercd2014-01-30
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petercd (author)mole12014-01-31

Yep, thats an epoxy and should work, saves you having to buy 500ml tin to make one knob, just be aware of the setting time, some of them are kwikset. You could even try the Original Cold Weld Formula Steel Reinforced Epoxysets in 20~25 min as opposed to the 6 min for the kwikweld variety.

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rickharris (author)2014-01-30

If you can make a pattern from the broken part you could use shapelock plastic (pollymorph in the UK)

You will need to rescue the metal centre bush or make/find a new one.

Shapelock can be reshaped by heating but sets at room temp to be quite hard.

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mole1 (author)rickharris2014-01-30

Fresh "Friendly Plastic" is too soft/flexible for this. Old "Friendly Plastic" is shockingly brittle and wouldn't last in this application. Do you know if 3D printing plastic is more rigid and stronger?

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rickharris (author)mole12014-01-30

It is hard and stiff when cooled. It should be plenty strong enough and is fairly cheap to try.

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rickharris (author)rickharris2014-01-30

Note friendly plastic and shapelock/polymorph are 2 different things.

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rickharris (author)rickharris2014-01-30

Personally I would find a suitable chunck of aluminium and set to with a file to shape it as I needed.

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mole1 (author)rickharris2014-01-30

I'm surprised the knob wasn't aluminum in the first place. This is the first generation, I think, of aluminum machine bodies.

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steveastrouk (author)rickharris2014-01-30

The OP has engraved detail on the knob though, so I'd look at a moulded version personally.

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rickharris (author)steveastrouk2014-01-30

gnerally on a sewing machine the graduated settings are approximate and few sewers look at them they adjust until it works OK.

I guess she ciould put the arks on with a permenant marker though,

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mole1 (author)rickharris2014-01-30

That is generally the case with stitch length or width. I don't know yet if it applies here. This knob engages /disengages the cam mechanism and sets the length of a given pattern... adjusting the rate of the feed dogs to the rotation of the cams. It moves a lot of stuff, and was probably broken when someone tried to force it to move the frozen mechanism.

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steveastrouk (author)2014-01-30

You can use a Polyester casting resin, or a (better, stronger ) a polyurethane resin - but you need to "load" the plastic with a filler, like limestone, "microballoon", or chopped fibres - the filler has several effects, it increases the strength very considerably, if you get the right one, it reduces the amount of resin you need, and it reduces shrinkage in the part.

I suspect the part you show may have a metal core that provides necessary strength.

I made a couple of hundred control lever knobs for classic military Landrovers here in the UK using the material. I've had no failures.

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