Tapping M2 thread in plastic - which?

I'm repairing something that contains a sandwich of 2mm thick alloy plates (unknown spec.), which was originally held together by M2 brass countersunk screws, tapped into one plate. Moisture exposure has caused the alloy plates to develop surface corrosion, looking like filiform corrosion, although the alloy was not coated in any way. Not surprisingly, galvanic corrosion caused the threads to seize and then be damaged or destroyed when the brass screws were removed.

I considered Helicoil-type inserts, but assume that these might also cause galvanic corrosion, as would the use of metallic screws (perhaps Zn-plated would be OK), so we'd be back where we started pretty soon.

Plan B (which seems better) is to make some flanged, square-section, tapped plastic inserts, locating in square-section holes (anti-rotation), and use nylon screws, to avoid dissimilar metal contact. The square would be about 4mm AF. There is no access from the back of the assembly, when it's fixed in place, so conventional nuts are a no-go. Please can anyone suggest a suitable plastic, other than perspex, which can be turned and milled, and into which a M2 (coarse) thread could be tapped successfully?

I would have a peek at PEEK

MichaelG.

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https://dielectricmfg.com/knowledge-base/peek/

https://www.curbellplastics.com/Research-Solutions/Technical-Resources/Technical-Resources/PEEK-Data-Sheet

Edited By Michael Gilligan on 31/07/2022 00:53:50

No experience of using them myself, but might titanium screws be a possibility?

https://www.corrosionpedia.com/5-things-to-know-and-understand-about-titanium-corrosion/2/6657

Bill

Or make metal inserts and assemble with anti-seize designed for use on aluminium car heads and blocks. Even blue Loctite does a pretty good job of stopping corrosion of steel fasteners into ally.

Zinc plated screws still seem to corrode into ally threaded holes, as evidenced by a million motorcycle engine cases held together by zinc plated screws assembled dry at the factory.

The insert shape you propose is amenable to making a single one and using it to create moulds in some putty or plasticine type substance. Would it be daft to suggest making the plugs out of JB Weld and tapping those?

pgk

Corian. Mills beautifully, can certainly take M3 tapped holes. People making kitchen worktops discard loads of it as scrap and could probably give you a lifetime's supply for small jobs.

I agree on using PEEK. Machines and taps well with a strong thread.

The only drawback is its price! (having spent a while machining PEEK at work, I've got a healthy stock!)

Jim

If you are going to be using plastic screws why can't you tap the rear ali plate larger say M3, clearance through the others and just have the plastic screw go into that. A lot easier than making square holes and nuts.

How do you think aeroplanes are made ? Plastic screws to hold the wings on?? Careful selection of the galvanic reaction between dissimilar metals is the solution. Many years ago, the norm was steel screws cadmium plated and passivated. I do not know what is used these days, but the answer I suspect is well known in the aircraft industry.

Bob

You can use stainless steel steel helicoil inserts and stainless steel screws which have been silver plated to prevent spalling. I have made parts for RR aerospace with this spec. The screws were silver plated to spec specified to prevent the similar materials picking up with no lubrication etc on assembly.

David

where i work its mainly stainless screws and nearly all tapped holes in ally are inserted. the inserts are usually coated with something before fitting. don't see much corrosion.

Stainless in aluminium is prone to glavanic corroson unless protected.

Using Duralac (barium chromate paste, I think) or Tef-Gel on assembly will prevent this.It is standard practice in the marine rigging world.

Either of these would be close to a lifetime's supply!

Duralac

Tef-Gel

(I use Duralac, but it has to be said, it can be quite messy.

I think Tef-Gel originates in Australia, so may be more readily available to a 'Kiwi'  )

Edited By Andy_G on 31/07/2022 13:15:27

All of the Helicoils we fitted to aluminium aircraft parts, (thousands) were cadmium plated stainless steel. They were fitted wet with compound, or occasionally wet epoxy primer to reduce water penetration. Titanium screws in aluminium would cause the aluminium to corrode even faster, check out the electro chemical series charts. Duralac would be easy to get hold of both for fitting any bush or insert type and also when the screws are fitted.

I would consider 3D printing the insert using PETG or PLA filament. Than you can use metal screws that "expands" the insert for more holding grip.

I do not know what your sandwich structure is used for, butI would simply use stainless steel screws. I know all of the theories about aluminium and stainless steel being subject to galvanic action, but it is a fact that nearly all of those aluminium framed glass roofs and facades that litter shopping centres, office blocks etc use stainless steel fixings to secure the corner joints. These structures are of course exposed to the elements, but despite the theories broadcast on here none have yet fallen down due to the dissimilar metals problem. Indeed I have an aluminium pedestrian door on the back of my garage. So today, out of curiosity, I removed some of the screws used in its construction about 27 years ago. Guess what? Not a sign of corrosion or deterioration.

Thanks everyone for the thoughts and advice - keep 'em coming!

This is one of those jobs that gets worse, the more you think about it. I may now be at the stage of over-thinking, but better safe than sorry. Unfortunately, I'm stuck with a countersunk screw head, which can't be of larger diameter than a M2 csk head. The M2 female threads are gone, so some sort of insert is needed. The thread will be 2mm max in length, so 5 threads engaged max for M2. M3 would be 4 threads, but the csk head is deeper, and it sits in 1mm thick plate, so engagement would be less. Also, the Italian who made the thing fettled the threaded holes with quite deep countersinking, so the original threads were a skin-of-the-teeth variety. A self-tapper into a plastic insert is attractive, but there's the risk of dissimilar metal corrosion at the head end. Thus, I feel the safest option is a plastic M2 csk screw. In NZ, Helicoil or Recoil kits seem amusingly to cost (in real terms) about double the UK price, so, even though irritating to make, plastic inserts have a huge cost advantage.

I'm aware that Zn-plated , Cd-plated, and stainless fasteners are used a lot in alloy threads, often without any precautionary addition of a chromate treatment, such as Duralac (of which I've had a tube for years, for similar awkward applications). Also, Helicoil-type inserts are used a lot, but I believe these should ideally not be the standard, non-coated type. Perehaps, as suggested, a good dose of Duralac would be OK.

Murphy's law will operate. If I were to take no precautions, and use brass screws in alloy threads, as originally manufactured, there will never again be any corrosion. But, if I'm aware of this, and take the chance, all hell will break loose. Sometimes it's best to accept that you just can't win, and try to design yourself out of the problem...

I think pgk,s suggestion of using JB Weld (oriqinal) is a good one. You could open up the threaded holes to say 4mm fill with JB and when cured drill/tap M2 again. You could then use metal screws, I think JB great stuff I have turned, milled, filed, drilled and tapped it. If you did go with it leave it for a few days to fully cure before drilling/tapping.

You could try gently swaging the holes smaller using a small punch and then re-cutting a M2 thread. Or try one of the low temperature Aluminium solders to fill the hole.