Author Topic: Repairing a small tapped hole.  (Read 1985 times)

Offline Gas_mantle

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Repairing a small tapped hole.
« on: July 27, 2017, 06:37:24 PM »
Hi,

I've made a bit of a hash of tapping 2 holes in the side of an aluminium cylinder, they are 6BA and too shallow to start getting any decent thread cut so I've thought about milling out the holes to maybe about 4mm dia and making 2 6BA threaded rings to Loctite in place.

Is something like Loctite 638 likely to be suitable if I run the engine on steam occasionally or can anyone suggest a better way of tackling the problem.

The holes are to secure cylinder support bars so they will be subject to some stress.



Thanks
Peter

Offline peatoluser

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Re: Repairing a small tapped hole.
« Reply #1 on: July 27, 2017, 07:32:24 PM »
could you , instead of opening out to 4mm, drill and tap 2BA or 4BA. Trick is not to put the taper tap right through leaving a slightly tapered threaded hole. then you can use a piece of threaded rod or screw to blank the hole off, the tapered thread helping to lock the thread. use loctite as well by all means. mill everything flat and start again.?

peter

Offline Gas_mantle

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Re: Repairing a small tapped hole.
« Reply #2 on: July 27, 2017, 11:50:46 PM »
Thanks Peter, but the holes really are too shallow so opening them out to take a larger (coarser) thread isn't really an option neither is milling flat and restarting.

I think unless anyone can suggest anything better I'll make round threaded plugs and either use Loctite 638 or maybe JB weld to seal them in. To be honest it might actually turn out to be a blessing in disguise as I never seem to have much luck with cutting fine threads in aluminium so if I can have steel inserts then that may be a better arrangement.  :)

Peter

Offline steam guy willy

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Re: Repairing a small tapped hole.
« Reply #3 on: July 28, 2017, 01:37:13 AM »
could you tap the holes in the part you are attaching, then clamp that part together in the correct place ,then put a bottoming tap back in and use that to cut into the hole you need to tap, if the hole is a clearance hole ,make a new part, line it up then tap them together, then take off the top part and drill out to clearance size. After taping the top part you could use some grease to prevent any undue pressure on it I could make a drawing if this is not so clear.You should use a taper tap first and put it right through the top part then grind off the bottoming tap so it is dead square then put that through the top part and it should cut the cylinder hole .....if this does not work there is enough room to put in 3 more holes to attach the bars to.
« Last Edit: July 28, 2017, 01:51:36 AM by steam guy willy »

Offline Jo

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Re: Repairing a small tapped hole.
« Reply #4 on: July 28, 2017, 06:37:59 AM »
Hi Peter,

Is there any reason to take the bars off again afterwards? If not make sure both the bars and slot are really clean and fix them in place with the 638.

You could add a an over sized bolt in both of the holes with an oversized 6BA thread to make use of whatever there is in the bottom of the holes but the 638 should be adequate.

Jo
Usus est optimum magister

Offline Stuart

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Re: Repairing a small tapped hole.
« Reply #5 on: July 28, 2017, 06:49:23 AM »
Peter
Jo is correct 638 will do the job

I use Brake cleaner to clean the parts sold in a spray can from halfrauds

Caution use in a very well ventilated location or you will get flying elephants around your head and itís very flammable like most cleaners

But itís very good and cheaper that the loctite cleaner/primer

Take care and be safe
Stuart
My aim is for a accurate part with a good finish

Offline Jim Nic

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Re: Repairing a small tapped hole.
« Reply #6 on: July 28, 2017, 11:37:22 AM »
Could you not just move the holes towards the ends of the cylinder a bit and make new ones to have another attempt?  You could then make new bars which would hide the evidence  ;D

Having looked again at your photo it appears there is very little metal on the cylinder wall to work with so maybe well applied Loctite as suggested by Jo and Stuart is the way to go.
Jim
« Last Edit: July 28, 2017, 11:47:36 AM by ImIndoors »
The person who never made a mistake never made anything.

Offline Gas_mantle

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Re: Repairing a small tapped hole.
« Reply #7 on: July 28, 2017, 12:35:05 PM »
Thanks everyone for the help, it sounds like 638 is the way to go   :)

It's my own fault that the holes are shallow as I decided to make the cylinder bores larger than the plans so now the walls are only about 3mm thick. The set of taps I have are 2 in each size so I assumed one is a plug having looked again I think its a 2nd cut (I've never used them till now) The slight taper in a shallow hole means I can't get a proper thread started before it bottoms out.

There won't be any need to remove the bars once they are in place so I make a couple of threaded plugs and Loctite them.

Peter


Offline steam guy willy

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Re: Repairing a small tapped hole.
« Reply #8 on: July 28, 2017, 03:19:39 PM »
With a 6BA thread which has 47 TPI you should get 5.64 threads to hold the screw which should be ample to retain the bar......if my maths is correct !! willy.......

Offline Florian Eberhard

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Re: Repairing a small tapped hole.
« Reply #9 on: July 28, 2017, 05:41:01 PM »
Hi Peter

I have another (rather unconventional and also more complicated) Idea:
You could machine a dovetail into those slots and make new bars with a dovetail head which fits the dovetail in the cylinder with a slight push-fit. That should also give a quite good connection betwen your parts.

Florian

Offline Zephyrin

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Re: Repairing a small tapped hole.
« Reply #10 on: July 28, 2017, 05:56:41 PM »
You may tap for a larger dia, but with a finer thread, which would give a better strength on a very short length. But you have to do yourself the screws, non available otherwise...
In theory, the length for a taping does not need to be longer than a nut...but I usually tap twice that long !!

Offline Gas_mantle

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Re: Repairing a small tapped hole.
« Reply #11 on: July 29, 2017, 02:48:42 PM »
Thanks everyone who offered advice  :)

I've now managed to solve the issue by making threaded rings and Loctite 638 them in place, I'd much rather have done the job right in the first place but the repair is solid and won't be seen again once the engine is assembled. An extra line of 638 along the channels once I do the final assembly sounds like a good idea too.

I take the point about getting enough threads cut in the shallow holes (the holes are deeper than a nut would be) but the tap I used has too much of a taper to cut full threads before bottoming.

Florian - the idea of dovetails did cross my mind, it would be a better solution all round but I don't really have the tools to do it  :(

Peter.

Offline Stuart

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Re: Repairing a small tapped hole.
« Reply #12 on: July 29, 2017, 02:59:14 PM »
Peter

A dodge I use it involves a bit of tool mistreatment is to take your plug tap( bottoming )
And grind the end flat i.e. No taper lead in helps you get the extra thread and a half in blind holes , but now it's a special purpose tap  :facepalm:
My aim is for a accurate part with a good finish

Offline Gas_mantle

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Re: Repairing a small tapped hole.
« Reply #13 on: July 29, 2017, 03:27:55 PM »
Thanks Stuart, I realise now what the problem was and I'll just have to put it down to my inexperience.

The tap was one of a set I had just bought and none had yet been used, I assumed that the blunt one of the 2 taps was a plug but it's a 2nd cut - I've had a look through some loose taps a fellow forum member gave me some time ago and sure enough I do have a proper plug tap. I've used that on the remaining holes on the other side and it is fine in a 3mm deep hole  :)

A daft mistake I know but I'll just have to take it on the chin as part of learning  :facepalm:

Peter

Offline Stuart

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Re: Repairing a small tapped hole.
« Reply #14 on: July 29, 2017, 03:54:18 PM »
Ok Peter

I still have senior moments and make mistakes, but I always say those that do not make mistakes do not make anything

At this time I am between major projects the company are out of stock of one casting ( the big bit ) so been on a workshop clean tidy , machine maintaining spree

So now I am doing a small job a 3in wier type boiler feed pump a lot of 10ba , my normal small is 12ba so should be ok 16ba is a bit tasking on the lamps now I am 70  :old: ( yes I look like that in stick)

Good luck with the rest of the build and let your troubles be repairable  :stir:
My aim is for a accurate part with a good finish