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What's the teminology?

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Simon Williams 322/07/2018 14:38:15
491 forum posts
78 photos

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

HOWARDT22/07/2018 15:20:36
535 forum posts
15 photos

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.

richardandtracy22/07/2018 15:24:08
avatar
938 forum posts
10 photos

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

Regards

Richard.

Pete Rimmer22/07/2018 20:27:55
679 forum posts
47 photos
Posted by Simon Williams 3 on 22/07/2018 14:38:15:

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'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'

HOWARDT22/07/2018 21:23:22
535 forum posts
15 photos

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

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