What's new
What's new

Making a "helicoil"

clarnibass

Aluminum
Joined
Dec 23, 2015
Not exactly a helicoil...

I repair musical instruments and have this one for repairs. The "triplet post" (the bar with three small threaded holes in it) is attached to the (wood) clarinet with two screws (on top, which you can see). The thread is stripped (in the wood). The screws are 1.75mm diameter and 0.6mm pitch. The clearance holes were 1.8mm. I enlarged them slightly to 1.9mm and used longer 1.9mm diameter screws, also 0.6mm pitch. Unfortunately the threads in the clarinet body are ruined enough that it didn't help. One kept turning and the other felt like it's about to strip.

Some problems like this are fixed by a few methods using various types of inserts that in this case won't work well. I think I'll make a metal inserts (likely nickel-silver, maybe brass), threaded and Loctite to the body, with inner threads for the screws to go into (I'll make other screws instead of the non-standard M1.75x0.6mm ones, which look a bit like a "tweener" metal/wood screw shape). A bit like a helicoil in purpose.

The screws only really need to hold the part to the body, it's the flat bottom and the shape of the part and the cutout in the wood that hold it in the correct position for alignment.

The thing is, I've never made a part like this in such a cramped area. If the inserts are too large, they would stick out and get to the hinge tubes of the keys themselves, not allowing them to move (plus sticking out of the wood and looking ugly).
I could make the inserts completely off the instrument, then thread them in. Or I could drill and tap, or just tap, in situ.
I'd probably go for M1.8x0.35mm, or maybe 1-64 or 1-72 for the inner threads (and the screws). Question is then how small can the outer threads be, without compromising the wall thickness too much. I would want it as small as possible in this case.
I guess it could be slightly smaller if I tap the inner thread once installed, but not sure it's not too much of a risk (e.g. depending on wall thickness, might just strip the wood again, even if Loctited in place).
If I drill in place too, maybe it could be even smaller, but drilling this way is also a little tricky, risking the wood stripping, plus harder to keep it exactly perpendicular (though it doesn't have to be very accurate).

So first I'm trying to see what's the smallest thread I could use for the outer diameter, without the part itself being ruined by making it off the instrument (i.e. the part collapsing from the second threading operation, now being too thin). Also whether there's a significant difference to that risk by drilling/turning and tapping the inside or outside first.

Thanks

20210916_115135[1].jpg
 
Can you fill the holes with epoxy like JB Weld and use the same type screws as the original?
Thanks. I could but I don't think it will hold. IME with various types of epoxies (including JB Weld) they don't hold threads well enough for things like this, whether mixed with wood dust/chips or not. I wish it did, but it just never has in similar situations when I tried it.
 
I have needed to fix quite a few inferior screw holes in wooden boats. The standard fix is to drill out the hole a bit to get to good wood and fill it with epoxy as jbace suggested. You could then drill and tap or even cast the threads into the epoxy using the original screw and a release agent.
 
I have used an epoxy from West to repair wood components. I think it was called G flex West system epoxy. I needed to fill a void created by loss of wood as well and filed a piece of scrap to get a filler the same color and texture as the original wood.
 
IMO, the above drill, glue dowel method will be closest to originality, especially if you match the wood. I'd probably make a threaded brass insert with as thin a wall as possible, rough it up or give it a shallow thread, and epoxy it in place. It isn't clear if the original screw is a proper wood screw or a machine screw.
 
drill, glue dowel in hole and redrill after for wood screw.
Good idea, but rather than a dowel with grain running the length of the hole, a plug (or stacked plugs) with grain and wood type matching the original wood would be better. You may have to make a plug cutter—- easy enough to file a few teeth in a properly-sized steel (need not be hardened for just a few plugs) tube. If you want to get fancy put a little alternating set in the teeth.

I also agree that West System epoxy with some ebony sawdust filling the hole would be as strong or stronger than the original wood. I mix it in small batches using 10cc syringes to measure 1-2cc amounts of 1:5 hardener:resin. I am confident any other brand-name epoxy system would work comparably.

Denis
 
It's pretty simple....threads in epoxy are not gonna be as good as threads in metal. But they'll be better than threads in wood.
 
IMO, the above drill, glue dowel method will be closest to originality, especially if you match the wood. I'd probably make a threaded brass insert with as thin a wall as possible, rough it up or give it a shallow thread, and epoxy it in place. It isn't clear if the original screw is a proper wood screw or a machine screw.
I thought I'd Loctite it but epoxy would work the same I guess.
Do you or anyone can say what wall thickness is the thinnest possible for making the part (so it doesn't tear, collapse, etc. while being made)?
I calculated it a bit.
With an M3.5 thread, wall thickness at the thinnest possible area, between the thread, would be 0.48mm with a M1.8 inner thread, and 0.46mm with a 1-64/72TPI thread.
With an M4 thread, it will be 0.67mm and 0.64mm for M1.8 and 1-64/72TPI respectively.
The part will be made of nickel silver.
The thickness of the area the inserts would go into is pretty much exactly 4mm. So tapping the wood to M4 might just barely break to the side a fraction. As far as cosmetics and considering the amount and direction of forces, it's not a problem, but if the part with the M3.5 threads sounds like it wouldn't be a problem to hold it self when it's made, with that wall thickness, I'd rather go with that.

As far as all other options, a lot is the circumstance...
I checked and West System doesn't have a distributor in the country. There might be an equivalent, but it's the weekend, then holiday, everything is closed, and the instrument needs to be ready by Wednesday, or preferably Tuesday.
I have enough time for one attempt with glue, with or without wood dust mixed in. Right now I have JB Weld and Araldite epoxy in stock. Though IME using these and other epoxies for similar parts, it never worked that well.
I have a spare of those "tweener" wood screws that I can turn into a tap before screwing the actual screws in place. The hole will be an approx measurement from the screws with the weird M1.9x0.6mm thread.

A wood insert is a standard repair for many similar problems, but in this case, the screws I have are already a little longer than usual, and I can't really get longer or different ones at all. I'm concerned it wouldn't hold just like it failed before (even with the slightly larger and longer screws I have). this was never taken apart, so apparently it half failing from manufacture.

Both the original screws and the longer screws I used are not exactly machine screws or regular wood screws. They are sort of in between, not tapering as much and as pointy as regular wood screws. The heads are also important, they are as large as they can be, at 2.7mm. I don't want to enlarge the countersink diameters.

As far as originality, it's not that important since all methods will basically look like the original once it's all installed. The only difference would be if using M4 inserts and they break out a fraction, it might be barely visible through all the keys and posts, but very likely not. "Under the hood" it's only important that it will last and hold if I need to remove it in the future.

Thanks again
 
Not exactly a helicoil...

I repair musical instruments and have this one for repairs. The "triplet post" (the bar with three small threaded holes in it) is attached to the (wood) clarinet with two screws (on top, which you can see). The thread is stripped (in the wood). The screws are 1.75mm diameter and 0.6mm pitch. The clearance holes were 1.8mm. I enlarged them slightly to 1.9mm and used longer 1.9mm diameter screws, also 0.6mm pitch. Unfortunately the threads in the clarinet body are ruined enough that it didn't help. One kept turning and the other felt like it's about to strip.

Some problems like this are fixed by a few methods using various types of inserts that in this case won't work well. I think I'll make a metal inserts (likely nickel-silver, maybe brass), threaded and Loctite to the body, with inner threads for the screws to go into (I'll make other screws instead of the non-standard M1.75x0.6mm ones, which look a bit like a "tweener" metal/wood screw shape). A bit like a helicoil in purpose.

The screws only really need to hold the part to the body, it's the flat bottom and the shape of the part and the cutout in the wood that hold it in the correct position for alignment.

The thing is, I've never made a part like this in such a cramped area. If the inserts are too large, they would stick out and get to the hinge tubes of the keys themselves, not allowing them to move (plus sticking out of the wood and looking ugly).
I could make the inserts completely off the instrument, then thread them in. Or I could drill and tap, or just tap, in situ.
I'd probably go for M1.8x0.35mm, or maybe 1-64 or 1-72 for the inner threads (and the screws). Question is then how small can the outer threads be, without compromising the wall thickness too much. I would want it as small as possible in this case.
I guess it could be slightly smaller if I tap the inner thread once installed, but not sure it's not too much of a risk (e.g. depending on wall thickness, might just strip the wood again, even if Loctited in place).
If I drill in place too, maybe it could be even smaller, but drilling this way is also a little tricky, risking the wood stripping, plus harder to keep it exactly perpendicular (though it doesn't have to be very accurate).

So first I'm trying to see what's the smallest thread I could use for the outer diameter, without the part itself being ruined by making it off the instrument (i.e. the part collapsing from the second threading operation, now being too thin). Also whether there's a significant difference to that risk by drilling/turning and tapping the inside or outside first.

Thanks

View attachment 329444

I suspect that the past attempts at using epoxy for thread repairs has to do with the base wood material issues.

I suspect that there is likely an oil in the wood that prevents a proper bond to form. The fine grained wood material further exacerbates getting a good epoxy bond.

I think using an epoxy will be your best bet for a repair but surface preparation will be necessary. I think if you would use an approach similar to gluing or epoxying teak would likely get you to where you need to be.

Now as to the question you actually asked. I would start by drilling and tapping the internal thread first. Then I would insert a screw into the new tapped hole. You could also use one of the low temperature melting alloys to fill the tapped hole.

Next I would turn the insert to desired size and do the external threads last. You could likely get by with a fairly thin wall section by doing it this way.
 
All those ideas are way fancier that jamming toothpicks coated in wood glue in the hole until it is packed tight and re-installing the original screw.
 
Now as to the question you actually asked. I would start by drilling and tapping the internal thread first. Then I would insert a screw into the new tapped hole. You could also use one of the low temperature melting alloys to fill the tapped hole.
Great idea, thanks, I'll try that with M3.5 and hope it works.

Re the epoxy, yes it's tricky and the wood is oily, but in most cases that I've tried it, it really looked like the threads stripped and not the glue separated from the wood.
From the little I read, Araldite can wick better into the wood grain, but JB Weld is a stronger and has a better chance of holding the threads. I think I'll try the latter and if it doesn't work I'll make the inserts.

Re making a new "triplet post", beyond all the issues like not having great material for that, the very accurate slip fit, borderline very light friction fit, with curved sides, exactly matching the wood cutout... the main problem is there isn't really room for those holes, since they can't go through the threaded side holes.
 
I suspect that the past attempts at using epoxy for thread repairs has to do with the base wood material issues.

I suspect that there is likely an oil in the wood that prevents a proper bond to form. The fine grained wood material further exacerbates getting a good epoxy bond.

This is what I was thinking. People who make boats of wood have worked out the details of using various often oily woods, so I'd ask around in that community. They will also know where to get what in Israel.

Another option (to epoxy) is Resorcinol Resin based glues, used for making wooden boats and airplanes.

Resorcinol glue - Wikipedia
 
Re making a new "triplet post"... the main problem is there isn't really room for those holes, since they can't go through the threaded side holes.

They could go right next to the threaded side holes, though....It's not in front of me, so I can't say for sure, but it looks like they might fit

Heck, you might even be able to use the existing triplet post, and just drill and counterbore 2 new holes
 
I like the original idea: an OD threaded/ID threaded bushing. Make it from aluminum or steel, not brass. Tap the wood for the outside thread (a coarser thread is best) and use a thin epoxy to retain it.

You can use the original screw thread for the ID thread.
 
I like the original idea: an OD threaded/ID threaded bushing. Make it from aluminum or steel, not brass. Tap the wood for the outside thread (a coarser thread is best) and use a thin epoxy to retain it.

You can use the original screw thread for the ID thread.

That is the basic principle of a Keen-Sert thread repair, slightly different than a Helicoil.

Nice thing about Keen-Serts is they use standard thread taps for installation.

Personally, what I would try first, is to simply clean out the hole to fit the appropriate size brass tubing (K&S Metals, Hobby Supplies store), rough it up on the outside, and super glue it in the now cleaned out holes. Tap the new threads as required, and the butt end of the tubing should be all but invisible.

That would put the technology level of the repair at almost Cave-man levels, a drill bit spun between fingers to open the hole, and a tap holder and tap to thread it.
 








 
Back
Top