2 start thread

Hello all, cor ain't it 'ot!

I have recently found myself repairing an exhibition stand, where the flat panels have 16 mm rods each sides to hold them together. I discovered that the thread I thought was M8 in each rod was actually 8 mm OD, but was two start. So the thread pitch is actually 2.5 mm not 1.25 mm.

Which led me to wonder how such things are specified. The thread pitch measured with a thread gauge is 1.25 mm, but would I be right in supposing that, if I needed to specify such a thing to a professional turner I ought to call it "M8 x 2.5 pitch 2 start" (assumed symmetrical)?

Look forward to hearing what the forum thinks?

Thanks as always, Simon

You are right, thread diameter x pitch (or lead) of thread and number of starts. I have used multi-start threads in designs over the years but most have been buttress or acme and in larger sizes.

On my pen making have up to 5 starts, and that's just the way to specify it.

Regards

Richard.

You're nearly right. The pitch is still 1.25mm, but the LEAD is 2.5mm If you specify 'Thread 8mm, 2.5mm pitch, 2-start you'll' either get a 5mm lead thread or (more likely) an irate phonecall asking what it is you really want.

So ask for 'Thread, 8mm OD, 2.5mm lead, 2-start'

Yes I agree with Pete, pitch is distance from one peak to the next, lead is distance travelled in one revolution.