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Making a "helicoil"

There is a product, powdered grenadilla wood, that they sell to be used to make a filler with cyanoacrylate glue. It's made for musical instruments, and clarinets are made from grenadilla, so maybe...

Grenadilla Filler.

This gives you a fill that is somewhat the consistency of grenadilla.

Putting a chunk of metal into the wood might have some musician's balk. But if that's not a problem:

If your hand is steady, perhaps drill out and tap for a brass bolt. Super glue in. Drill and tap for small bolts that hold the triple bar on. In any case, even if you have appropriate inserts, I'd use super glue.
 
Thanks everyone. I'm going to go with the smaller M3.5 metal inserts and hope it works, using the great method suggested of putting a screw inside while tapping the outside.

I have grenadilla wood powder (and make it when I don't), but as mentioned, I had threads fail when I tried this (both with super glue and epoxy, both with and without wood powder). Since it was almost always the threads stripping rather than the glue failing, I just don't trust this, at least not with any glue I have or can get in a reasonable amount of time.

By coincidence someone just told me about those thin brass tubes available in the US because of some other repair. Unfortunately I looked into this and it's very difficult to find here. Last time I tried I gave up after a couple of days wasting a lot of time.
 
I think part of your problem is that Metric Coarse is a rather fine thread for diameter, BSW or UNC would be better in wood, just as they are better in cast iron & aluminium.
 
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I have had to make all kinds of odd ball thread repair inserts. Sometimes from scratch when I can't find a bolt or all thread the correct size to tap. Grade 5 bolts or metric 8.8 works with a quality tap.
 
David_M that's a very good idea too. If I wasn't half way into making it the other way I would probably do that. Because there isn't enough space I would just have to make the "sex bolt" (I didn't know that's a thing) a little different, with the bottom pressing against the countersink, without a head. Shouldn't be a problem with M1.8 threads in the 2.7mm countersink ID.

Other models don't have this type of post, and this one isn't that common, but if I get another one with the threads stripped (and seeing how it's made it would'nt surprise me if they would be...) then I will definitely remember this option.
 
I'm fixing a guitar right now, with a similar problem. The Fender stratocaster has a "Tremolo" apparatus that allows one to vary string pitch (which is more correctly "vibrato") and the plate that holds it all together is mounted into the solid wood body using 6 wood screws. The most critical are the two most outboard screws (newer designs have only two fasteners, mounted using metal anchors) and the screw on my treble side is loose. I plan to drill out the hole and glue a hardwood dowel into the hole, then drill and re-install the screw.

Would it be better to just drill out a plug of wood around the oversize holes and glue in a solid piece of grenadillo? By "better", I mean that you'd be using the original material.

A picture of my setup from the web (my guitar is cleaner!)

tremolo.jpg
 
I'm fixing a guitar right now, with a similar problem. The Fender stratocaster has a "Tremolo" apparatus that allows one to vary string pitch (which is more correctly "vibrato") and the plate that holds it all together is mounted into the solid wood body using 6 wood screws. The most critical are the two most outboard screws (newer designs have only two fasteners, mounted using metal anchors) and the screw on my treble side is loose. I plan to drill out the hole and glue a hardwood dowel into the hole, then drill and re-install the screw.

Would it be better to just drill out a plug of wood around the oversize holes and glue in a solid piece of grenadillo? By "better", I mean that you'd be using the original material.

A picture of my setup from the web (my guitar is cleaner!)

View attachment 329709

I suppose it is true that you feel duty-bound to make a fix that keeps the guitar “stock.” The reason I ask is that boring out holes and plugging with plugs of identical wood in species and thickness would seem to comply with the stock requirement. At the same time it seems to be design proven to fail as evidenced by your guitar and the design change subsequently made by the manufacturer.

Options, depending on your tolerance for deviation from stock might be, in rank for deviation:

1). Bore and plug with same thickness, species, and grain orientation. (Most “true” to the original design.)
2). Bore and plug with plugs that are shaped like a rivet so that the portion seen is say 1/4” diameter matching the bored hole, but on the inside of the body the plug widens to maybe 3/8” and results in a plug that is maybe twice the thickness of the original wood. The plug heads could have angled sides that converge so the plug looks like a truncated cone from inside the body. This provides a lot more holding power than the original.
3). Bore and plug with the same thickness and species. Glue in a backing plate of the original Spey of wood or a plate of thin Baltic birch plywood. Provides considerably more screw holding power especially if the plywood is “ruggedized” by soaking in thin cyano glue and kicked off prior to gluing into the body.
4). Bore and plug holes 2,3,4,5 with original specy wood. For holes 1 and 6, make metal fastener backers that receive custom screws whose visible heads are dead ringers for the original screws but are actually machine screws that thread securely into the metal backer hardware. I like this method the best as it is likely to be the most durable and respects the aesthetic of the original design but recognizes and deals with its limitation.

Sounds like a satisfying project you are committed to doing “right.”

Denis

Added: If you hate the idea of Baltic birch backer plywood, my grandson and I recently made a small piece of
custom plywood by using a sharp blade on the table saw and sawing .020” thick plies from yellow cedar and then glued up a few plies to make an incredibly stiff and tough plywood. It was very easy to do. Same thing would work with your rosewood.
 
Tapered plugs that are larger towards the back would be especially tricky on a solid body electric. I know as soon as I say impossible, some machinist will take it as a challenge to show it can be done.
 
Tapered plugs that are larger towards the back would be especially tricky on a solid body electric. I know as soon as I say impossible, some machinist will take it as a challenge to show it can be done.

Do in two or three pieces that are glued together during installation, modeled on an expanding-wedge anchor for masonry.
 
I have had great success epoxying "hard to glue" or "impossible to glue" items by creating a mechanical lock for the epoxy. I have even secured Delrin bushings into steel sockets using JB Weld this way.

What I do is create grooves and dimples at random angles on both the OD of the insert and the ID of the hole. I usually use a jeweler's file to create shallow random grooves on the OD and anything from a sharp graver to a bit on a Dremel tool for the ID, depending on the size of the hole.

This has worked with Delrin and other plastics that don't usually work well with epoxy as well as oily woods such as Brazilian Rosewood etc.

If you can get this to work for you you only need straight-walled inserts similar to round electronic spacers.

I recently used this method to epoxy (with JB Weld) a bunch of 10-32 aluminum spacers into Birch plywood while building some jigs. None have spun or pulled out in use.

PS: For best results, thread the spacers with the stock oversize and then hold them with a threaded mandrel to turn the OD.
 
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.

Perhaps use Araldite first to seal the grain, then use JB Weld for the actual repair. As I recall, Araldite will flow better if heated somewhat. JB Weld should bond just fine to fully cured and roughened Araldite.

As for your particular format screws, I wonder if you could coat them with mold release and wind some wire around them and actually bond the wire into the hole as a sort of round (rather than diamond) Heli-coil. You would want to back out the screws when the epoxy is firm but not fully cured - just in case. JB weld is strong but IMO without metal reinforcement won't make very durable threads.
 
A old school woodworker craftsman that I'm friends with swears by Superglue and baking soda mixed together. Bonds well to the wood and machines nicely according to him. Never had a chance to use it myself.

Mr Bridgeport
 
I like your idea of making a nickel silver insert threaded inside and out. Make the OD threads similar to the Rockler inserts shown in another post. You may need to make the insert extra long for a screwdriver slot. Drive the insert into the wood to form female threads. Practice on scrap to see what size hole works best. Drill the clarinet body and drive it into hole. Back it out and machine off the extra length. Reinstall the insert using CA adhesive or Loctite to secure the insert. Use the 1.9mm screw in the ID of the insert for purchase for a screwdriver.

An alternative wuld be to drill and tap an oversize hole and fill with Devcon aluminum putty from McMaster Carr. Then drill and tap for your desired screw.
 
A old school woodworker craftsman that I'm friends with swears by Superglue and baking soda mixed together. Bonds well to the wood and machines nicely according to him. Never had a chance to use it myself.

Mr Bridgeport

I've used that method to repair plastic parts such as automotive interior trim. Often the hooked mounting tabs break over the years as panels are removed and reinstalled. I usually use a metal "backbone" such as part of a miniature paperclip and then build up with super glue and baking soda before filing to shape.

I've long been a believer in using metal reinforcement with adhesive repairs. I have yet to find any glue, epoxy, or Loctite product that has the strength and wear resistance of actual metal, even when metal filled.

PS: One trick I've used when I need threaded inserts in a hurry is to cut the head and unthreaded portion from rivnuts and either epoxy or solder the remainder in as an insert depending on material. The rivnuts, especially the aluminum ones, tend to have a slimmer OD than most other inserts.
 
Denis, Thanks for the nice ideas. It turns oujt the two outer screws are key. In newer designs, the screws are all gone and the two outer screws are replaced by bolts sunk into metal anchors.

Some of the Fender guitars (e.g. early models, such as a 1954 stratocaster) are worth a lot (more than $100,000). On those, I'd not touch any mods with a ten foot pole. But this '86 Japanese strat, which is probably as good as any American strat from 1986, is worth <$1000.

I plan to use a maple plug, glued into a hole with fairly close tolerances (not too tight, the glue causes swelling). Then I'll drill a pilot hole very carefully. Should work.

In some cases, guys have had to replace all of the wood around the screws. These bodies are basswood. If I did that I'd use basswood.

Properly adjusted, this design shouldn't be an issue. There are alternate tremolo designs (Vega Trem) that screw a mounting plate down to the body, and have the trem plate (that moves and has the string saddles on it) pivot on the fixed mounting plate. A good idea, but it's $280.
 
make a bushing out of iron wood thread it in and tap a hole in it before or after installing it
 
From the title: "Making a 'helicoil'".

But we are talking about anything but an actual helicoil. So, why not actually make a helicoil.

Getting or making hardened wire with a 60 degree parallelogram cross section may be difficult. But you could use round, music wire of an appropriate diameter that matches the thread's pitch. Then make an installation tool and a matching tap. Install it with whatever glue or epoxy the above posts have settled upon.
 
From the title: "Making a 'helicoil'".

But we are talking about anything but an actual helicoil. So, why not actually make a helicoil.

Getting or making hardened wire with a 60 degree parallelogram cross section may be difficult. But you could use round, music wire of an appropriate diameter that matches the thread's pitch. Then make an installation tool and a matching tap. Install it with whatever glue or epoxy the above posts have settled upon.

The basic problem with that, as I see it, is that you can do so much better, with a lot less work!
 








 
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