Isn't it annoying when you see an abbreviation you don't know [OMT], is said OMT an imperial or metric thing?

Tony

If imperial likely to be 40 TPI

Tony

Looks a GB make so 40 TPI?

Tony

Beaten to it, but 5/16" x 40tpi is a standard ME thread.

My guess is that it’s a special … screw-cut to the 4BA pitch

Have a look here: **LINK**

http://pages.cs.wisc.edu/~bolo/workshop/thread.html

MichaelG.

Measure over 10 threads not one then any error will be less. You could also start the screw into a hole and measure how much it protrudes, then screw it in 10 turns and measure again, Divide result by 10 to get pitch.

.

That said … I believe that ME threads are only ‘standard’ amongst Model Engineers

MichaelG.

.

I left that for Gerry to spot for himself … but knowing that it’s an OMT microscope, I dared to presume that they would have been using BA wherever convenient.

MichaelG.

Dont you just hate non standard threads……..

My neighbour came round a couple of nights a go with a special bolt that he said is for holding a bed frame together.

His daughter had lost one and could I make him a replacement, he handed me one as a sample and it looked fairly straight forward so I said yes I will do it as a favour (It was dark and I didn't have my glasses on !!)

Every dimension on the special bolt was imperial, apart from the threaded end which at first I assumed must be 5/16" Whitworth. It is a good job I double checked because it was M8 x 1.5 which is very unusual.

I must have thousands of taps and dies and Coventry chasers, in all sorts of obscure sizes, but you guessed it no M8 x 1.5 was to be found.

So that 10 minute job turned out to be 1½ hours by the time I had set up the lathe to screw cut the thread, and having nothing to check it against I am keeping my fingers crossed it will fit the tapped hole in the bed frame.

Moral of this story is……..Dont let the neighbours know you have a workshop.

Phil