Plugged and Re-tapped Holes 1

[drawoh]

Our inspector has rejected some parts with 8-32UNC helical thread inserts located 1.4 to 1.7mm out of position. He has just asked me if these holes can be plugged an re-tapped. The material is aluminium 6061-T6.

Does anybody know how strong holes are when they are plugged and re-tapped?

--
JHG

 

[fegenbush]

I suppose it depends on the service, but I have heard that the permanent fix for stripping threads in a high pressure oil pump is to drill out the threads and press fit with red Permatex adhesive a threaded steel insert. This sounds similar to a plug and re-tap. I don't know what your application is, but that oil pump sees 3,600 psi on the high end, so the fix is pretty strong.

 

[MintJulep]

Plugged how?

If welded you lose the T6 temper, likely going down to T1. Ultimate goes from 42K psi to maybe 16K psi.

Could you drill out and tap a much larger hole encompassing the right and wrong locations, then install a threaded "blank" and finally redrill and tap your 8-32 hole in the right place?

 

[drawoh]

Thank you fegenbush and MintJulep,

I am looking into installing Keenserts, those screw-in thread inserts with the opposite side hammer-in pins. These are big enough the cover the existing holes completely. The only problem I have is that the local vendor has the worst telephone answering system I have ever encountered.

--
JHG

 

[MintJulep]

Yes, Keenserts are often used to fix this sort of thing.

Since the OD thread is much bigger than the original thread it will be correspondingly much stronger than the original.

 

[ctopher]

I have plugged holes with larger plugs then re-tapped, both alum and steel.
I freeze the plug, heat the part, then plug the hole. You can also use a threaded plug and tack weld it.

Chris, CSWA
SolidWorks '15
SolidWorks Legion

 

[racookpe1978]

Yes - But here - if the nbr 10 tapped hole (.188 nominal dia) is 1.4 mm "off" position, then the "new" nbr tapped hole in the aluminum part will be drilled and tapped into the helicoil , the helicoil wall, the new inserted steel (??) piece that is in the old hole, AND the far side of the ID of the helicoil.

I don't think it will work.

Drill a BIGGER hole that cuts out the bad hole completely. Helicoil THAT hole with a 1 inch plug of aluminum stock the right thickness. Then drill and tap the new nbr 10 hole where it is supposed to be in the big Al plug.

 

[desertfox]

hi drawoh

If the holes are only 1.4-1.7mm out is there not sufficient clearance in the mating part to cope with this?
Also why not consider elongating or making the holes in the mating part bigger if they are not large enough now.

 

[drawoh]

racookpe1978,

The offending holes are 8-32UNC. The keensert of interest has a 5/16-18UNC outside thread. This is a semi-production part going into an aircraft. Structurally, it is rigid enough to keep optics aligned, so it is grossly over-designed from the point of view of outright fracture. Still, MS27039 pan head screws into tapped holes in solid material are good.

--
JHG

 

[tbuelna]

Since you are considering using a heavy-duty keensert to fix this problem, and it's going into an aircraft application, does the internal 8-32UNC thread need to be self-locking? If not you could drill/tap the 8-32UNC threads in a repair plug. But if the threads need to be self-locking you will need to use some type of thread insert.

Also, since this application is for a "semi-production part going into an aircraft", you will likely need to get MRB approval for this type of rework.