How to fix a thread that is too loose?

[Sebastian KowalParticipant @sebastiankowal44276]

At the beginning I will add that I use google translator, so sorry for any gmamatic errors. I am young and I am gaining my first professional experience in machining. I made a mistake and made a few details on the lathe with the used tool. The problem is that the internal thread (metric M2.5) is too loose, the intransitive thread gauge is screwed in without much resistance. Is there any possibility of repairing this thread? I will add that the material is brass. Thank you in advance

[Sebastian KowalParticipant @sebastiankowal44276]

[DC31kParticipant @dc31k]

The standard way to repair an internal thread is a helicoil:

https://www.stanleyengineeredfastening.com/brands/optia/heli-coil

Helicoil is a brand name, so you can also look for 'wire thread insert'.

[HOWARDTParticipant @howardt]

The problem occurs if the drilled hole is oversize. Unless you buy quality named branded drills a fair proportion are not ground very well and cut oversize. With small tapped holes I always use a smaller size drill than recommended them open them up to the correct size. Doing it this way uses the edges of the drill used to open up rather than the point so getting over the point grind error.

[Paul LousickParticipant @paullousick59116]

Hi Sebastian, More information, photo of what you are making would help with a suitable solution to your problem. (some holes that I have drilled/tapped oversize or out of position have been remedied by silver soldering, filling with JB Weld or making an oversize screw to suit the hole)

DK, Do they make M2.5 helicoils ?

[Bill PhinnParticipant @billphinn90025]

Posted by Paul Lousick on 02/11/2022 22:07:27:

DK, Do they make M2.5 helicoils ?

Yes, and M2:

https://media.boellhoff.com/files/pdf12/helicoil-kits-repair-solutions-en-0180.pdf

[HopperParticipant @hopper]

So you are using a tap to cut the thread? In that case, size of the hole will not make the thread loose. Your tap must be incorrect. Use a different tap.

Be sure when tapping the thread to back the tap out of the hole regularly, using a back and forwards motion, to keep the tap clear of brass chips. And use the tailstock centre to hold your tap wrench square to the job.

As far as the existing thread goes, there is no way to make it smaller. Possibly you could make the external male thread larger to fit the loose internal thread. Do this with a split die and spread the die open as far as you can to cut the thread then try it for fit.

[not done it yetParticipant @notdoneityet]

Apart from the above comments re tapping drills an tap sizes, there is also the risk that the fixings may be undersized, as well.

For this particular instance, one solution could be to drill deeper, increase the thread sufficiently and use longer screws. Whether appropriate, or not, depends to some extent on the items to be fixed together.

Practising/testing, on some scrap material, to measure the real diameter of the drilled hole and comparison with different supply of screws may determine where the fault lies.

I check hole sizes, if necessary, by using drill shanks to find the real size and then checking the shank with a micrometer. Assuming drills actually make the hole size indicated is not good practice – especially with such fine pitch screws. But beware while tapping with such small diameters (particularly in brass) that the tap does not jam and break.

[BOB BLACKSHAW 1Participant @bobblackshaw1]

If its a test piece and you want get through inspection, use a small ball-bearing and tap the top of the thread to close the first twirl of the thread, then try the no go gauge.

Bob

Edited By BOB BLACKSHAW on 03/11/2022 08:08:43

[bernard towersParticipant @bernardtowers37738]

silver solder the hole up and start again

Edited By bernard towers on 03/11/2022 08:18:51

[Robert Atkinson 2Participant @robertatkinson2]

You can get 2.6mm screws. You could try using those in place of 2.5mm. Pitch is the same. Don't retap the holes.

They seem to be used on model helicoptors. I found out about them when fixing a Rubidium atomic frequency standard. It had loose screws, obviously from new, and measuring as 2.5mm. Checking an older version of the manual showed they should be 2.6mm.

Robert.

[Paul LousickParticipant @paullousick59116]

Not an elegant look but deforming the hole with a series of small dimples around the hole with a centre punch will tighten the fit with the screw. (OK for lightly loaded applications)

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