Repairing threads in aluminum?

[mwkeefer]

Hello all,

I have in the past filled threaded holes which had been stripped with jbweld, drilled them and then tapped them for threads but I have never done this in a high temperature - IE: hub motor type of application.

I have a few hubs here (geared internally reduced) which I have opened and worked on several times to get the setups I wanted:

1.) Measured location where threaded portion of axle passes through dropout on each side with all attachments (disc, cluster, etc) in place and then filled those areas with jbweld after coating the axle threads with chapstick - this gives me a proper bottom out in the dropout with full axle contact (sort of) instead of having to file down the threads there (which works too)

2.) Rewired internally - arrived in wye configuration and with 18g or some crud like that... Added Delta wires out and replaced existing phase lines with 6 x 14g teflon silver wire but only coming from the internal PCB to about 3" past the axle end so maybe 6.5" in length each section... I peak at 70A but being geared this only occours when launching from dead stop under full throttle or as I shift into delta mode.

3.) Applied high temp epoxy to all the magnets within the hub, the magnets these things ship with will loose their magnet far before the epoxy fails but adding delta increases my maximum inrunner RPM to a level of 1.73 x the wye mode (and yes I suffer from the same recirc current issues as this motor wasn't wound right anyway).

4.) Found and replaced seals for what were spring coils of some sort - these are for retaining oil bath for the all steel gearset within.

5.) Replaced all internal bearings with high quality and temperature ceramics (who needs a press when you have a full socket set)

6.) Removed the disc thread on threads and part of the lip (about 3.5mm total) then drilled the 6 iso standard holes to mount a real disc to the left side hub cover instead of the threaded disc adapters which seem to be configured for 40mm instead of the std 42mm mounting holes.

So after all this and a few other additions (internal temp probe... tiny tiny uber thin wire) I found that a few of the bolts no longer threaded properly... the bolts (allen heads) are fine so the holes must have stripped at some point...

The question is - what is the best way (short of weld filling them and retapping) to repair these to accept the same diameter and thread they already take (I know I could just retap them all to a SAE std a bit larger and it would likely work fine).

Thanks to all in advance!

-Mike

 

[AussieJester]

Instead of JBWeld use Devcon makes JBWeld look like plasticine.

KiM

 

[Tiberius]

Helicoils

There is a standard system of thread repair. You get a kit that enables you to drill out to a larger diameter, tap with the original tpi, and then wind in a coil insert to bring it back to the original diameter. The kit is expensive for just one or two holes but it works very well.

Nick

 

[rkosiorek]

helicoil inserts are the common method. but they are generally single use. ie. repair the thread, bolt it together and never take it apart again. if you do, you have to replace the helicoil insert. the helicoils will often back out together with the screw when you remove it. but they are good where you don't have much space for the repair. for example an M5 helicoil insert needs enough meat for a thread approx 6mm in diameter for instalation. helicoils need special taps and drills for instalation, sold only as a kit. expect $30 to $50USD for a kit to repair 10 of your M5 holes
View attachment 3

another choice are thread inserts with locking tabs. commonly called "Keenserts" these have little keys that lock them into place. now you could try to drive these tabs into place with a hammer but you need a special instalation tool for best results. uses standard taps and drills for instalation. to repair an M5 thread you drill 6.9mm and tap M8-1.25. a kit with 8 inserts and a tool to drive down the keys will be $50 to $60 for carbon steel and about double for stainless. but once you have the tool you can buy the inserts alone for a couple of bucks each. these are permanent. once you drive those tabs in the only way to get them out is either rip them out or EDM to burn them out.


then there are self adhesive threaded inserts. these are the same as Keenserts except that they don't have those locking keys. the adhesive is similar to locktite. you can get them as "thinwall", "medium wall" and "thick wall". for M5 the most common is the "medium". you would drill 6.9mm and tap M8-1.25. then screw them down. you can get them for a semi-permanent instalation


or other ones that are slightly easier to remove and install because they have a screw slot in the top.


in practice that locking compound or adhesive is pretty tough stuff. these things are very difficult to remove once the adhesive has had a day to cure. these would be my choice. a pack of 5 in stainless would cost $10 to $15USD.

when considering your purchase remember whether you also need to buy the correct drills and taps. for helicoils the taps are custom sized and specific for that size of insert. you cannot use them for anything else. they are also rather cheaply made and will only last for a dozen holes or so before you need to replace them. good thing that they come included in the kit with roughly the number of inserts they are good for. the other threaded inserts use standard size drills and taps that you can use elsewhere.

rick

 

[Dave Sloan]

Best / easiest way to remake a thread - been there - done that - is to make a plastic thread bolt using a tap and die set - Coat the hole with Devcon or your favorite / strongest bond material - thread the plastic insert then let dry. Once dry either drill, tap or melt the plastic material out and your left with a perfect threaded hole that is probably stronger then the original. Just make sure that the hole is clean before starting. I used this method on a cheap stripped brushless hub motor that I got from VPower where the thread that fastens the cover on was stripped. = Hope this helps

 

[John500]

I'm very surprised that one could consider a Heli-coil a single use fix. In the marine industry we drill out and Heli-coil all holes in cast iron pumps. This allows us to rebuild the pumps for many years without problems of broken capscrews. We also Heli-coil plastic flanges for added strength. We have $1000 tied up in Heli-coils, been using them for 40+ years. We have used them in aircraft heads to hold the exhaust manifold studs. This is a permanent solution to blowing gaskets and pulled out studs.

 

[spinningmagnets]

I was a mechanic on a Navy submarine, and the hydraulic valve bodies are aluminum. They are attached to aluminum subplates with aircraft-quality bolts that thread into heli-coils that are inserted when the subplate is a new product. Based on my Navy job, I got a job with Sargent Hydraulics (They made the submarine valves) in Southern California for 3 years (now in Arizona).

Drill and tap the aluminum hole to insert a Heli-coil, seat it with a smear of JB-Weld, and it should be a solid set of threads for years. Certainly stronger than the original aluminum threads. JB a helicoil on some junk aluminum, let it set a day, then use a torque wrench to see what its "break-away" torque is.

You don't show a picture of the original stripped part. Depending on where it is located, you may have the option of using a threaded blind insert, common in the furniture industry. (also seated with JB)

 

[mwkeefer]

The stripped bolts in question hold the oil side (chain side) of a geared hub motor cover in place... They are not a big deal and for temporary measure I broke out the lip balm and jbweld (coated the bolt with chapstick then injected some jbweld into the broken threads and turned the bolts in slowly - when I backed them out about 2 hours later, the threads stayed and hold the covers on again!)...

I just wanted a better solution for permanent repairs as this is they type of issue I have alot with one thing or another so thanks to all for the info and solutions... the helicoils seem to be the way of professionals and since it seems they are used in subs as a (I guess) mannor of maintaining serviceability of the part in the field (so when the threads get stripped because as the mechanic was torquing the intake a huge wave comes on and too much torque is applied... - no problem, pull / drill the old threads and sink a new helicoil).

And thanks to all for the inspiration for my new frame enclosure covers, or well atleast a new fastening system = )

-Mike

 

[rkosiorek]

in my defense i was taught in a certified aircraft context that use of a helicoil is single use because the helicoil deforms when the bolt is torqued. so you will only get an accurate torque reading the first time it is used. when you rebuild you must remove the old helicoil and reinstall a new one. most spects would not allow use of a threadlocker or epoxy when installing a helicoil in aluminium. again becauce of fears of inaccurate torque readings.

then again the standards used for commercial aircraft are incredibly nitpicking. everybody is ready to sue your butt over the smallest issue. gotta eliminate even the faintest wiff of anything ever going wrong. so my excuse is - for this application anyway - i got a bad education that maybe makes me a little too fussy.

rick

 

[amberwolf]

FWIW, in the aircraft avionics and cockpit displays Honeywell CFSG makes for A320/A340/A350, DC-9x, MD-8x, MD-11, 737-xxx, 747-xxx and 777 prototypes (all things I was involved with testing/etc on the factory floor), helicoils were used in most of the aluminum unit chassis to create threads, rather than tapping threads directly into the aluminum, for holding the covers on and the chassis bits together. They weren't one-use in those applications, but I suspect that's a lot different from anything structural or airframe-related. AFAIK they were only ever replaced if a screw installed in one was damaged, in case the damage was caused by an improperly-installed helicoil (which happened, especially when they first moved major pre-assembly of chassis/wiring down to Mexico after NAFTA).

THe lack of actual fussiness in all stages of the products from before I saw them to after they reached airframe manufacturers and airlines is what kept me from ever re-applying at Honeywell after the layoffs ended and they began re-hiring. I don't think I could take that stress again, knowing that I and maybe a handful of others are the only ones standing between a stupid failure and dead people.