Which plastic to bond to brass.

Hi everyone,

Are there any plastics gurus on the forum?

I'm looking for a plastic that is machineable, gives a little and can be bonded to brass.

I am trying to make some plastic internal sleeves for those brass volume and tone knobs I made. The plastic will be 10mm diameter bonded into a blind hole in the brass and then rotary broached to take an 18 tooth serrated spline (potentiometer shaft).

The original brass knobs were broached directly into the brass but the tooling to do that is very expensive and differences in tolerance and shape of splines across the many potentiometer manufacturers is surprising, hence the need for the plastic to have some 'give' in it, hopefully one size will fit all.

Thanks, Ed.

Are you planning to secure the knob with a setscrew? If so you might want to rethink your approach as a setscrew holding in just a few threads in plastic with not be robust. It's more usual for knobs to be plastic and their inserts to be brass. The setscrew threads are usually in the brass.

If you still want to make the inserts in plastic probably PC/ABS alloy or plain ABS or PC (polycarbonate) would be my recommendation. These plastics are tough and gluable. Nylon or PPS would also work well mechanically but are not so gluable. If you serrate both the knob ID and the insert OD with grooves radially and axially and use epoxy to bond the parts, the joint should be strong enough. The serration grooves can be hand cut with a dental burr or engraver bit in a Dremel tool.

If you do use a setscrew through threads in the plastic I suggest you plan for 4 to 5 threads minimum, and don't overtighten the setscrews. JD

Hi, thanks Jeff,

There are no setscrews. The knobs will be just a push fit as per the original and the potentiometer shaft has a split through it's splined length, providing a compression type fitting. The spline will be cut into the ID of the plastic sleeve.

I've seen examples of the plastic knobs with the brass insert but those inserts are two-part and fixed with a screw under a plastic cap.

I'll try to get a sketch sorted for later.

Regards,
Ed.

I dont know how many you want to make but i'd buy some cheap knobs with the correct spline tun off the outside and bond into your nice brass knobs

I would use loctite 330 my glue of choice for differing awkward materials

I'd make the plastic sleeves a press fit.

Neil

Jason Udall - your statement

"This leaves acetal.
Acrylic
Polycarbonate.
These all glue well"

Is not true for Acetal. Acetal is almost impossible to glue or paint just FYI.

You missed out the styrenics like ABS and PC/ABS blends in the gluable ones.

I didn't mention plain PS and HIPS as the OP said a light press fit on the pot shaft was involved - ABS or PC would deal better with that than plain PS or HIPS. I didn't recommend acrylics as there is a lot of rubbish pseudo acrylic that are not true PMMA, or weird blends, in plastics stores. Pure acrylic/ PMMA would be OK for OP's application but getting the real thing is not so sure these days - true PC or PC/ABS or ABS is much easier to get. JD

I would also like a bit more proof that Polycarbonate can be glued well. It might depend of the type of glue - I dunno.

Btw. many of the 'non-gluable' plastics (PP, PE. Nylon) can be glued quite well, when they receive the correct treatment before.

Kind regards, Hans-R.

Couldn't you just fill the holes with full strength araldite? Then bore for the pot shafts or cut spline or whatever.

Hi Ed,

We have to agree with Jeff that p.acetal is `almost impossible’ to glue or paint. I would add `successfully’.

This applies to various crystalline materials (polymers) such as polypropylene, polyethylene, nylon, polyacetal, etc.

That said, there are situations where these very materials are regularly painted. For example, automotive bumper bars injection moulded in polypropylene are etch-primed before given a top coat.

I’m unclear about p.acetal but I have in the past used an oxidising primer called ThreeBond 1797. (See photograph and link below.)

The label shows it to be for cyanoacrylate adhesives, and I’ve used it to prime polypropylene with some success. The bottle holds 100ml, but how much it cost I can’t recall.

**LINK**

Of the amorphous plastics (polymers) which can be glued with less difficulty, rigid PVC, and ABS should be fine, keeping in mind that ABS can exhibit a lower value of friction.

On balance, p.acetal would be my choice since it machines easily; will hold its shape quite well (i.e. low creep); correctly primed, it can be glued.

I would however, be tempted to introduce grooves into the OD. These would be of the spline type to resist torque, and circular grooves to resist pull-out.

Polyacetal is however, used for bearings and is rather slippery. Since the task seems critical, maybe you should do a few tests before attending to the real thing.

On a final note (or two), acrylics and polycarbonate (and others especially amorphous polymers) are prone to ESC (environmental stress cracking) so watch out for chemical agents which can introduce this hazard.

I’m also very conscious of the notch sensitivity of materials. Far too many parts fail due to notching (lack of radiusing. Good luck!

Regards,

Sam (aka Dennis)

Edited By Sam Stones on 04/05/2016 01:31:14

I couldn't resist taking a gander at what industry are doing wrt to insert design. See here -

https://www.google.com.au/search?q=metal+inserts+for+plastic&rlz=1C1GGGE___AU555AU555&espv=2&biw=1001&bih=885&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwia9umzlr_MAhVkNqYKHZxpDbwQsAQIJw&dpr=1

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Sam,

Unfortunately this ^^^ seems to be xactly the opposite of Ed's requirement .... He wants plastic inserts for his brass knobs.

MichaelG.

Blimey, he's only making a volume knob!

In practice a simple cylinder of any relatively easily deformable plastic will work excellently, bored slightly undersize for the spindle. If it's a firm fit in the knob, inserting the pot spindle will deform it enough to create 'splines' and ensure a firm grip inside the knob..

Neil

If you leave the outside of you plastic liner rough and likewise the hole in the brass, I would think that any none hardening glue would do.

There is no reason why you can't "machine" internal striations in your brass insert. Use the thread turning set up on your lathe with a suitable stop and a weight on a rope around the chuck as a division head and the tailstock to push a suitable tool into the bore to make a crude spline. This is not for NASA, use a bit of silicone goo to stop the knob falling off.

Frank

Ed,

I suggest a slight re-think of your proposed process, which might make more appropriate use of materials.

• Obtain a spare splined shaft, and fill the slot with a packer and apply a mould-release agent
• Form a mouldable epoxy material [such as Milliput] around this, to make a 'lump with a splined hole'
• Turn the lump to size
• Fit the insert with your preferred grade of 'anaerobic retainer'

[some obvious steps have been omitted for the avoidance of tedium]

MichaelG.

Edited By Michael Gilligan on 04/05/2016 08:54:45

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Neil [and Frank]

It might be worth looking at what Ed has already done, before getting into "only" territory.

**LINK**

MichaelG.

Quite right there young Michael,

I think it's a shame and a sin. To go on and on as I have done, And after Ed paid to come in.

With apologies to Stanley Holloway.

It is hard to shake off a life in the plastics industry.

Sam

Thanks everyone for all the interest in this, all very good info that gets the brain working

...just a volume knob indeed!

If I can, I'll make quite few sets of 3 of these knobs as the owners of the vintage guitars that they fit onto are always after replacements. I might have to watch our for patent and copyright etc, but if I change the design to incorporate a plastic inner sleeve I could probably get away with calling them "reproduction". But that's a different subject for some other time.

I could do each serration by hand directly into the brass and would be a job for the QCTP broaching tool, but it would take forever on numerous sets and is what spurred me into making the rotary broaching tool. Thing is I can't get consistency in the broaching bits. They are either too big, too small or for some reason won't harden properly (Silver steel) to cut brass. There's also the inconsistency of pot' shafts having different shaped splines, some are pointed, others have squared points and roots, so the plastic approach I'm hoping will allow some flexibility ...er literally.

I have some Araldite and Superglue-Industrial-strength which I can try on some of the suggested processes, maybe even some "No more nails" would work. I like the push fit idea as it saves on the cost of glue and having to add extra process time into roughing up the mating surfaces and gluing etc.

Thanks again,
Ed.

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I think with that "refurbished" would be my take upon it.

Nick