TPI in metric? And a few other leadscrew confusions for newbie

[IanParkin]

Actually thinking about the 8-4 it probably means 8tpi but as its 2 start it is in effect 4 tpi
it means 8tpi (threads ) but a equivilent lead of 4tpi

it makes for a stronger thread form more teeth engaged in a nut at any one time

[Neale]

Take a look at this. That's a trapezoidal thread (stronger than a conventional V thread as used in normal nuts and bolts). It's 12mm outside diameter (a bit bigger than your 3/8" but it's the nearest example I found with only a quick look). The important bit, though, is that it has effectively 3 threads running round it. Each thread has a pitch of 6mm (one turn of the rod would make a nut move along 6mm - that's what pitch (well, theoretically you should say "lead" but let's not get too pedantic) means) but is only as deep as a 2mm thread. So this rod has 3 threads, so if you took a rule to it, it would look as if the threads were only 2mm apart, but that's because there are 3 threads each with the 6mm pitch. It's a 3-start 6mm pitch leadscrew. The really important bits of the specification are the diameter - that affects how fast you can spin the leadscrew as they start to whip as they get longer and if they are too thin - and the pitch, which is how far a nut moves for one turn of the screw. You need that for various things when you are setting up the control software.

I don't know exactly what that original leadscrew specification means, but there's nothing magic about the exact values. Something like a 12mm x 6mm pitch 3-start thread would do a decent job for the kind of machine you are talking about. If you could find it, then a 10mm x 4mm pitch 2-start would probably be OK and probably nearer to the original. The Zapp website has a bunch of alternatives that you could look at.

[glynster]

Thank you all for your explanations. So to my understanding then, what I need is a "12mm pitch leadscrew". What I am still a little confused by is whether a 2 start leadscrew equivalent would therefore be labelled as 12mm pitch or 6mm? What I mena is, do they account for the extra distance of travel by the extra thread in their terminology or do you have to account for that yourself?

[Neale]

No, it will be labeled as 12mm, 3-start or 6mm, 2-start or whatever. Not 4mm or 3mm.

Problem is that you should really talk about "lead", which is the amount a nut would move for one turn of the screw, and "pitch", which is the distance between adjacent crests on the thread. But most of the time we tend to say pitch instead of lead, hence the ambiguity.

[glynster]

Excellent, I think I am finally understanding, so to confirm then this product from Moore International is 10mm diameter will advance by 12mm with every turn (approx half an inch)?...

http://www.mooreinternational.co.uk/...0-12screw.html

Does the nut also have to match the 12mm pitch?

[Neale]

Yes, that's a 12mm effective pitch (=lead) 4-start thread. And yes, you need the matching nut. There appears to be a dropdown box to choose Delrin or bronze for that.