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View Full Version : TPI in metric? And a few other leadscrew confusions for newbie

glynster
14-02-2015, 08:50 PM
Is there a metric equivalent of TPI (Turns per inch) in relation to leadscrews?

Apologies if this has been done to death, I dont even know what proper terminology to search for. I am building a solsylva machine - mostly to learn some basics of CNC machine building - and the plans are in imperial measures. I have converted all bolts and screw sizes to metric to make sourcing components easier and cheaper. However the plans call for a "36inch x 3/8-8-4 Start 2 Turn Leadscrew"

I simply cannot find such a thing anywhere. I figured I could use a 10mm leadscrew instead of a 3/8" as they are near enough equivalent. But what is the metric equivalent of 2 TPI. Would that simply be 12mm or 13mm per turn? If so what terminology is used to describe such a thing and what do the "8-4" stand for in my above example in bold?
I find the terminology of leadscrews extremely confusing. Any help would be appreciated.

Thanks

IanParkin
14-02-2015, 09:54 PM
TPI stands for Teeth/Threads Per Inch so on a imperial lathe you would have a 4TPI leadscrew perhaps so meaning that one turn would advance the cut by 1/4 inch
Bolts are specified in tpi so an imperial bolt would be half inch by 12tpi

Metric bolts and leadscrews are measured by pitch so in similar amounts to above the lathe leadscrew would be 6mm pitch and the bolt would be 12mm x 2mm pitch

You can also have 2 start or 3...4 start leadscrews or bolts
this means on a 2 start that the pitch is doubled so the carriage will advance twice as far but there are 2 thread running concurrently..you can see the helix angle is steeper on a multistart thread

IanParkin
14-02-2015, 10:01 PM
Just to add that in your example that you have found 2tpi in metric would have a 13mm pitch in single start making a very steep helix angle you could have a 2 start it will still have a 13mm pitch but to look at it it would appear to have a pitch of 6.5mm ( cos of 2 threads)

IanParkin
14-02-2015, 10:06 PM
Actually thinking about the 8-4 it probably means 8tpi but as its 2 start it is in effect 4 tpi

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

Neale
14-02-2015, 11:54 PM
Take a look at this (http://www.zappautomation.co.uk/mechanical-products/trapezoidal-leadscrews-and-nuts/trapezoidal-leadscrews-270/kre-273/right-hand/kre-12-b-r-tr-12x-6-p3-trapezoidal-screws.html). 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
15-02-2015, 01:19 PM
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
15-02-2015, 03:07 PM
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
15-02-2015, 03:45 PM
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/category-32/SPEEDYSTANDT10-12screw.html

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

Neale
15-02-2015, 04:16 PM
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.