Basic threading

I have just cut a 1" x 8tpi on the lathe, then pulled out the backplate it was to fit. nowhere near - I used 1" rod - the thread - no problem - the diameter of the backplate thread maximises about 22mm - does this mean a 1"x 8tpi is not 1" at all? - Help! What diameter should I be using - a table I looked at said 7/8. How?

A 1" thread is nominally 1" OD. In practise you might need to skim a thou or two off to get a good fit. But if the thread in the backplate is 22mm maximum then it ain't a 1" thread of any variety. If by "maximise" you mean the ID of the backplate thread then that's not far off correct for a 1" x 8 thread, albeit slightly large. How do you know you cut the external thread to the correct depth?

Andrew

Thanks Andrew - I cut the thread into a 1" rod, which started as 1" - the backplate I ordered from RDG says 1" x 8tpi - am I being stupid - the max ID of the backplate is about 22 - not the inside on the threads - it's even smaller - I just need to know, and you are cementing my doubts that this is a 12 x 8tpi....

RDG woops - Oh, sorry, we have a 7/8 one as well - it might just be wrongly titled - we'll send you another, and check it's right before we do............I no stupid after all.........

Dooooh!

Andrew

First make a gauge that matches your spindle exactly. Use that gauge to check your internal thread.

The critical dimensions are the length and diameter of the cylindrical pilot and the OD and pitch diameter of the thread. You can tolerate a relief groove at the end of the threads as long as the pilot diameter isn't touched.

Save the gauge to use on future projects.

Is your threading tool with a sharp point? That would require a greater depth of thread than a tool made with a flat equal to pitch/8.

22 mm looks like a 7/8" (0.075" ) truncated by 0.009", to prevent crest/root interference.

So inch bar will be oversize.

7/8 x 8tpi sounds like a special.

If it is 7/8" thread, standard BSW is 9 tpi, as used on early Myford ML Series.

Later on, this changed to 7/8 x 12 tpi..

HTH

Howard

Do you have a 4 jaw chuck (with backplate) that you could test the thread out, or are you making the backplate for it ?

After long hours, winding my self made mandrel handle on my new (old) Myford, I found that very light cuts were the answer to not breaking carbide tips, and made up a PTFE model - I cut the inner 18x 2.5 with three taps, then cut an outer 1" x 8 tpi, then parted offf, leaving a 6mm collar - worked a treat - now tramp on to 33 x 3.5 - cut the inner, started the outer thread, and half way through, stupidly realised I was cutting 30mm bar, as I had done for the 1 x 8tpi - Duhhh! But it's been a good learning curve for the Myford - amazing range of gearing with very different setups for the above threads. Thanks for all your help guys. ( and I don't think I'll be parting with the old Clarke 530 - it's a much heavier beast!

Dave, Fife.

When I did a lot of threading practice I found that there were error tolerances in stock material that make the screwing together part easy, but it's kinda wobbly

For a more critical fit I had to oversize my rod slightly so the stock nut was a good snug fit on the thread

edit: If you're doing a really critical job I would do a test piece first so you know exactly what you're dealing with before you dive in and use the irrecoverable expensive bit

Edited By Ady1 on 06/09/2020 11:59:53