Required diameter for cutting male metric threads

If I want to cut a M4 male thread using a die, what diameter should I start with? Would this be 4.0mm or should I allow for any increase in outside diameter due to extrusion when cutting the thread, and so use a smaller diameter than 4.0mm?

Thanks,

Gareth

3.9 mm should do it.

Tony

4.0mm works for me every time.

4mm should do it too, as you're cutting it - not rolling it. Or at least not > 4mm.

But let that be our dirty little secret. Shoot me down....?

Cheers, Colin

If we want to be pedantic for a normal use M4 screw thread the OD should be between 3.838 and 3.978mm.

www.amesweb.info/Screws/IsoMetricScrewThread.aspx

4.0mm will work just fine.

Neil

Neil to be really pedantic that is the finished size and not what you start with, the finished size will be determined by how much the die is expanded or contracted by the screws (assuming a split die).

I'll just go by the sizes at the bottom of your link which give the "Basic" OD as 4.0mm

Ah, assumed it was cut on lathe because of Colin's comment.

I generally take off a light skim by eye if using a die.

N.

Colin? who's Colin??

I did about 30 studs the other day M1.6 M2, M2.5 and M3 all cut straight onto nominal 1/16" 2, 2.5 and 3mm dia stock if I had skimmed them all both ends the job would have taken ages. All cut with a tailstock die holder under power. No wonder it take you a lot longer than me to finish an engine

Also most stock is a fraction under nominal size so probably within your limits anyway.

Edited By JasonB on 10/01/2018 13:24:45

Guilty. And I mentioned 'cutting', because of use of a die. Yes, there might be a wee bit of displacement going on too - but it's a cutting tool, right?

Cheers, Colin

Depends if you want truncated threads?

(to Jason and Gareth)

Jason: Bijezuz - where do you find metric stock? All I can find here is inch-based, even though I'm in a metric country (Canada). Ok, plywood is metric thickness, inch dimensions (in feet, of course) but we are not talking about Plywood. I can see why Muzzer moved back to the U.K.

ALL of my threading requires turning inch stock to metric dimensions, maybe that's why it takes me so long to finish an engine?

BTW - I'm jealous; those of you in truly metric countries are lucky; those of you in truly metric countries who choose to use BA I don't understand!

Gareth - as Jason points out, use a tailstock die holder, nominal diameter (or slightly less if turning the stock first) and just go for it, as the kids say. I use hand power to thread (turn chuck by hand, power off of course), and I do have some odd-ball size dies, in which case I use them in a hand-held die stock, but rest the die stock against a drill chuck in the tailstock, which really helps with the alignment, and "forcing" the starting cut by keeping pressure on the die stock. One arm of the die stock rides on the lathe compound rest, so left hand turning chuck, right hand advancing the tailstock ram - works wonderfully.

John.

Edited By John Alexander Stewart on 10/01/2018 16:12:36

4mm would be high on my list to begin with. Maybe I,m missing something here ?.

Colin has raised his hand

I probably wouldn't bother below M5 TBH. Makes a big difference if you die cut M6 or M8 in stainless or tough steel.

Haha yes it was very frustrating. Not least because in Vancouver I was developing products all day that used metric fasteners and dimensions but when it came to getting my hands on metric stuff for my own use it was a major challenge, almost impossible. The UK is a funny place - although we are nominally metric, everything is in miles and pints - and kg and tonnes. At least we drink proper sized pints over here - and have proper pubs! But I really miss the camping, skiing, kayaking, hiking.....

Murray

Thanks everyone for your input.

Regards,

Gareth