Whats the diameter size to correct out of position hole (up to .016) using an oversized Helicoil

[Jonathan DUmoulin]

I need to correct out of position on hole already threaded and with helicoil installed.
to do so, i need to machine 0.112-40 UNC-3B holes oversize sufficient to correct the true position
Once i have establish my nominal position of the hole to rework with CNC
to what diameter do I drill for tapping to ensure oversized helicoil installation?
PS:
I would re-thread for oversized Helicoil PN3885-04CN or equivalent.
Using only 1.5 diameter length insert
Reference tap 56187-04-2.
Thanks for your help/suggestions

 

[rb1957]

ok, you have two misaligned holes. how much misaligned ??

Correcting the helicoil hole is harder than correcting the other hole (the one that doesn't have a helicoil in). But there's probably some reason why you can't do this.

I would want to remove all the helicoil thread an tap a new thread (in "virgin" material). But you need to move the centre of the hole as well.

My suggestion is a freeze fit plug, centered on the desired hole center, big enough to take out the helicoil thread and at least 0.063" more than the new helicoil thread needs. This is a minimum based on about zero thought. The issue is "is this enough wall for a helicoil to work properly?" … probably not. And there are other issues, like mechanical retention.

another day in paradise, or is paradise one day closer ?

 

[BrianE22]

There are solid thread inserts, available with thick walls, that might give you more ability to move the thread:

https://www.mcmaster.com/threaded-i...steel-easy-to-install-thread-locking-inserts/

 

[Jonathan DUmoulin]

it is a misalignment is of .016'' on the diameter, meaning a centerline shift of approx .008''
I cannot simply plug the hole an re-thread to nominal value unfortunetaly
Standard oversize helicoil installation tool does not require to redrill the hole in case worned thread.
but this is not my case, i need to re-locate a misaligned hole, shift the centerline of the hole and re-drill to accept oversze helicoil.
I cannot not find any information on how to correct a misalignement using an oversize helicoil
Does anybody know to what diameter should i redrill to nominal position to correct the misalgnement with the
oversize helicoil?
Thank you

 

[rb1957]

ok, so you've a very small fastener (0.112" dia) with a 0.016" mis-alignment.

I think you appreciate how difficult it is to "drag" a tapped hole.

I can think of only two solutions …
1) plug the hole and re-drill, which you say you can't do.
2) drag the plain hole in the other part being bolted together (something you may have already dismissed).

the 3rd option (which is really awful, and not recommended) would be the machine the shank (down to 0.08" ?) to fit.
The idea being to position the fastener to suit the helicoil, so you need to machine 0.016" off the shank of the fastener,
the fastener would fit in the helicoil and be off-center in the plain hole.
like I said … not recommended (but possible).

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

J'DU...

Be wary of hole edge margin... 1.5-D EM for the final hole that is reinforced with a threaded insert should be a 'no-brainer. However, closer EM spacing to edges can be structurally un-sound... and could need 'stress' to validate the install. I have seen tapped holes by design, sooooooo close to edges that repair is virtually impossible.

There are commercial, NASxx and MIL- thin/thick-wall threaded inserts and helical coils [including repair 'twin-serts'] available that might work.

Assuming EM is NO problem...

As rb'57 indicated, it may also be practical to 'pull the non-threaded hole' into alignment, over-size the plain-hole by 0.032-to-0.125-inch, and install a thin-wall bushing [plain or with a thin-flat-flange or thin-countersunk flange... or double-flush plug... on-line with the threaded hole.

NOTE. When 'pulling' hole centers, rigid tooling and sharp/fresh drill/reamer/cutter-bits are essential.

NOTE. There are various options for 'thin-wall bushes'... so that is a separate topic.

LASTLY... IF the material of either part is 'suitable'... such as low-strength but otherwise 'sound' aluminum or magnesium castings... welding-one-or-both holes 'closed' and re-drilling/tapping has worked for me on a few/rare occasions.



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[verymadmac]

you don't give any of the secondary detail to really allow us to offer good solutions. These being things like the cost to scrap the part, what it mounts to, is it cheap just to change mating part, how much space ED etc to play with.

Potentially one could drill & tap the hole for a left hand thread bolt / screw, lok-tight the fastener and then drill & tap that.

 

[Jonathan DUmoulin]

Unfortunetaly i have no option it is the repair disposition from an NCR(non confirming report) from my customer.
Meaning correct position with oversize Helicoil.
Thanks all for your help.
FYI Digging into the oversize Tap diameter dimensions (Which i could not find intially), we have a way fwd.
Cordially

 

[Rogue909]

I'm assuming this is being performed with a drill as opposed to a mill. (With a mill this should be as simple as milling out the hole preferentially, correct?)

Is it possible to fill the hole (to prevent the bit from wandering) then re-drill in a manner that drills out the original hole and filler material?

 

[Jonathan DUmoulin]

We will be using an endmill, tested first to see if the correction works on testing pieces.

 

[mfgenggear]

first step if a detail that has not been assembled
draw it out with cad , print out a Mylar template , very milling for the new insert will remove the old hole
do not drill, repeat do not drill , mill out the new hole for threading.
thread the helicoil thread
install the new insert.
done

 

[rb1957]

"Unfortunately i have no option" … when it comes to repairs there is always an option … "scrap"

Quite possible someone upstream of you accepted the NCR/RNC as "ok, yeah, we'll drag the hole" and now you're the poor designer who's been told "drag the hole".

We've given some insights into the complexity of this beguilingly simple sentence, so you can go to your boss with some work done … "here are a couple ways we can do it, none work (well)"

another day in paradise, or is paradise one day closer ?