Thread Cutting

You’d be unlikely to make a start on the thread with a taper tap Tony if you’ve only got a few mm’s to work with. You’re more likely to be successful with the bottoming tap, used last usually. I’ll have a look in my tap and die set for the size you want and I am willing to lend you them via the post if you want to have a go at the repair. Stand by.

 

It's important to drill out the hole to the correct size first. There are set drill bit sizes for tap sizes if my memory serves me well. I may be talking bollocks though as I've only ever done one.

 

Quite right Andy, if Tony is tapping a thread M3x0.5 he’ll need to drill a hole 2mm.
Tony, I have an M3x0.5mm, but it’s got a bit of a taper, see pic.

 

File it down?
Not yours @Yorkshireman but get one and do it?

 

A "set"of taps is a taper, a 2nd and a plug tap but you can use just the taper and plug. Taps are really not that expensive so in soft alloy you can get away with a cheap set. If you're drilling out rivets I'm guessing it"s not a blind hole you're dealing with which should make it easier and use plenty of lube.

 

He could grind the bottom off that one if he wants to borrow it, I’m unlikely to ever need it as it is. Depends how much he’s got to play with, tapping into a blind hole in alloy needs care or you’ll end up bottoming out the tap and stripping the threads you’ve just cut.

 

I know them as taper, intermediate and bottom, but I get what you mean. Just realised you’re right, if he’s drilling out a rivet it won’t be a blind hole.

 

It would be easier with a T handle to get the thread started square. If you end up buying a cheap set and it doesn’t come with one the offer of borrowing mine still stands.

 

Looks like you have two options there Tony, drill all the way through and tap a new thread or there looks to room on that shoulder to take a small nut and use a small nut and bolt to hold the front plate on.

 

You could grip it with your teeth but you could end up chipping them. Then again, @Yorkshireman is a very accommodating bloke, he might lend you his teeth through the post.

 

I've just found this thread. TonyG you will find it very difficult to get the thread to run square if you do not use a T handle so that you can put pressure on the tap from both sides equally. You could use a small socket and T handle if you have that rather than a specific T handle for taps.
I agree that you should tap all the way through the aluminium.
I would also advise against buying a cheap set of taps and dies to cut this thread, the cheap sets are OK for cleaning up damaged or corroded threads but a decent quality tap is better for cutting new threads whether in ally or steel. You have a tiny thread here and it's not deep so you need to have the thread as good as you can get it.
My advice would be to use the taper tap to start the thread, you will be lucky to start it with the plug or bottoming tap.

 

I bought a full set of "cheap" taps, must be 20 years ago now, which have served we well many times and I still have most of them. Thread cutting in any sort of light alloy is easy, just make sure you cut it square as mentioned above. I had occasion to drill and tap a tapered thread in the cast iron exhaust manifold of the car so for that I did buy one quality tap which cost me more than the whole cheap set but the consequences of snapping a tap in the manifold were dire so, in that instance, the cost was justified. JMO

 

what would allen millyard do.....
mmmmmmm

 

Make a new tap from scratch