Split Dies

In this months MEW (Autumn Special 2018 - No. 272), Neil in his "Lathework for Beginners" series describes that Split Dies can be adjusted to get an accurate thread size - without saying how you would measure the difference in diameter - and why you might need to do this?

I have a range of split and unsplit dies, courtesy of my late father, and I have never questioned why the two types were different - assuming that this was a manufacturing option. I can see that by adjusting pressure on the screws in the die holder you could alter the overall diameter of the "screw" but never thought to do anything other than make certain the die was secure in the holder, whilst aware that undue pressure would alter things.

Can someone please explain why there might be a need for different diameter threads (if this is the adjustment), how you would measure the difference and thus adjust the die accordingly, and why there is no equivalent adjustment for Taps (although here I can see a manufacturing difficulty)?

Generally in the home workshop, you adjust the split die to cut a thread that matches the nut you want to screw on there. Start with the screw in the slot in the die so it pushes it open slightly. After one cut, try the nut, or fitting with female thread etc, for fit. If it is tight, rotate the die in the holder so the two screws are pushing it slightly inwards. How much you tighten those screws is a matter of judgment, or "try and fit". It may take several passes to get it "just so".

Very handy when making a male thread that a commercially made nut will fit on to. Many are a bit of a sloppy fit, so you can make it a bit tighter.

If we were working in industry doing aerospace contract work that was going to be stringently inspected against ISO standards etc we might use thread wires that sit in the V of the threads on either side of the bolt and then meausure across the wires with a micrometer. But absolutely no need for all that in the home workshop. "Try and fit" is all we need.

Edited By Hopper on 06/09/2018 11:04:52

Hi Colin,

Its all down to the precision of the fit needed. In the old days there could be quite large variations in thread size for the same size screw. Today there is more standardisation and while split dies are nice to have, are less needed nowadays.

Thanks Hopper - that makes sense. But a useful bit of info, implied in your answer, is that if you are making matching threads, then you should make the male thread first because then, in making the female thread you have this adjustment - that would be absent if you made the female thread first. That is a very useful thing to know.

Sorry - last post completely round the wrong way - I was thinking of the male and female tools - not the resultant machined threads. So best to "Tap" the female thread first, then us the split die to make a tight fit male thread.

Yes, tap your hole first then make the bolt or stud to fit it. The adjustable die gives you a bit of leeway for worn taps too.

It's worth saying that the centre screw on the diestock should best have a cone point so as to assist forcing the split open a little. Many or most purchased tailstock die holders have dog points in all 3 places, and those are pretty useless in the centre position.

The only dies I ever used were/are split type. I always thought the fixed type were just the cheaper D.I.Y ones.

I dont see much sense in them,since you have no control over the thread and they are more difficult to start.

I notice that most of those "Sets of dies" are plain type,and carbon,never seen split ones offered like that,you just buy them as required,usually HSS.

There are two types of fixed ones, cheap die nuts (good for binning) and expensive ones made to accurate tolerances.

Split dies are naturally a bit cheaper to make as being adjustable means tolerances can be a tidge looser. yes, as hopper says, bets to set the die by comparison to known good nut on the thread.

Neil

Strictly speaking a die nut is just that. A nut made out of hardened steel and equipped with cutting edges to clean up damaged or rusted threads. It's a repair tool. It's not to create a thread from scratch.

There was something in a ME probably early 1990s about a visit to one of the tool and ME supplies dealers, one part of the visit noted that they could only get solid button dies, so they were splitting the with a thin grinding disc.

On a different theme; some of my split dies have a grub screw to hold the split open at a fixed position, quite handy.

Ian S C

It strikes me that the dies were probably perfectly good unsplit, but no doubt they sold better once split...

Neil