Tuesday 14 April 2009


Khiraly asked me to explain how I manage to put a thread on stainless steel, so here goes.

Aluminium and brass are fairly easy to thread, but stainless steel is very tough. In order to make it easier you need to use a split die and a holder designed for one.

By tightening the middle pointed screw you can force the die to spread and increase the diameter of the thread a little. That allows you to make a first pass that doesn't cut as deep, so does not require as much force. By loosening the middle screw and tightening the outer ones you can reduce the thread diameter and make a second pass.

Another thing that makes it easier is to use cutting compound to lubricate it. I use Trefolex on Adrian Bowyer's recommendation. It is a sort of green lardy gunk.

To start off you need to align the rod or tube that you are threading orthogonally to the plane of the die. The easiest way to do this is with a lathe. You put the work piece in the headstock chuck and mount the die in a die holder that slides along a bar held in the tailstock.

You then turn the chuck with one hand and the die holder with the other. I use the handy chuck grip that I RepRapped, but a chuck key can be used to turn the chuck in 1/3 turn increments.

You need to go about half a turn forward and then one third of a turn backwards to break the chips off. If you don't it may jam.

When you start you need to feed the die against the workpiece with some force, but once the thread is started it feeds itself.

It is unlikely the chuck will have enough grip for cutting a stainless steel thread from scratch. You may have to file some flats on the stock.

If you don't have a lathe, the next best thing is to put the workpiece in the chuck of a drill press and put the hand die holder flat on the bed. Let the weight of the head press the work into the die and turn the chuck by hand. Once started you can put the work in a vice and spin the die holder.

Using a die to extend the thread on a hex head bolt is much easier because you start on the existing thread and you can hold the head in a chuck or a vice.


  1. I havn't tried this, but a guy I used to work with told me you can use lard as a easily obtainable substitute for cutting compound.

  2. I see, where I made it wrong. I bought a die set, which was not cut in, so did not miss a piece of metal around the periphery. I think the correct one called in english "split" die (assuming from your text).

    And a quick look around the net, I can see only "traditional" die sets, like these:


    I bought something like this:

    I can not imagine why are so many (broken by design) die sets sold everywhere.

    I assume I need to look around for a correct "split" die set. Am I correct?

    Best regards,

  3. Yes I believe the correct term is "split die", again I got this tip from Adrian Bowyer and that is what he called it.

    I have exactly the same tap and die set that you have got. The problem is that it is too cheap, only about £20 IIRC for the whole set.

    I buy the split ones individually and they cost several pounds each. It is the same with the taps. You should really have a set of three for each size: taper, second and bottoming, but the cheap sets only have one. You can get away with the none split die on softer metals, probably even with steel, but not stainless steel. You have no control of the size though. With the split design you can make threads that are deliberately tight or loose.

    So a complete set with split dies and three taps of each size would cost > £200 at the prices I buy them individually.

  4. You can probably buy a "normal", cheap die and use a good hacksaw to make a split die out of it. I've seen some machinists do that, so I think it should work.

    Of course, the cheapest die available will wear down very fast anyway, split or not, but the second cheapest thing is usually good enough to last a reasonable period and still be cheap enough.

  5. Yes you could probably expand it that way, but to close it you need the three screw die holder and ideally the two extra dimples in the die.

    Some die holders have round ended screws, whereas the middle one needs to be a cone shape to work with a split die. I had to make one for the die holder I got for the lathe by turning an M5 grub screw to a point.

    I think all the individual dies I have seen for sale are actually split, whether they say so or not. I think that is the normal design. I have only come across the solid ones in the cheap sets.

    Mine are also smaller, so need a different die holder anyway.

  6. The show goes on...;)

    It is not that easy nophead. Today I was at the biggest Tool distributor here in Hungary (at Budapest).
    You can look at here: www.noniusz.hu
    It is part of hoffmann-group.com, you can look for catalogs and manufacturers at that site (www.hoffmann-group.com).

    They said to me, that only solid taps exists.

    I didnt give up, I have gone into an another huge distributor: www.fairtool.hu (you can choose english language).

    They had only solid taps too, for example they showed me the JBO manufacturers catalog, and there are just solid taps.

    They found in the "EMUGE" catalogue a variable tap where hand holder exists too. But it is not as you describe, it is formed conicale, so when you tap it, it stress the tap (the tap is solid!). So I can make threads with it in ISO tolerance between ISO-4h/6g. (in M5 normal thread).

    Im not an expert, but I was told, that ISO-4h is basically a normal thread, so it does not make more easily the hand taping process (so I cant "pre-cut" with it).

    They was so kind to me, they photocopied the relevant catalog pages for me (page 228, 270). If you are interested I can take photos and upload for reference. I didnt find (yet) the catalog at www.emuge.com .

    Oh, and they are not cheap at all, there are solid taps which costs around 50 euro per piece!

    So Im out of ideas where to buy it.


  7. Odd it seems to be the normal case for them to be split in the UK. It is just the cheap sets from China that are solid. Here are a few places I have bought them from:-

    www.arceurotrade.co.uk (Cheap)

    www.engineering-supplies.com (expensive but huge range)

    www.modelfixings.co.uk (where I got most of my dies and the holder)

    As you see all the pictures show the split variety. I don't know if they ship overseas. You can try and if you get stuck I could buy them for you and post them to you.

  8. Im thinking to M4, M5, M6 and M8 dies, which should cover every possible need. (hmm, maybe M8 is not needed)

    Do these size fits into the same die holder?

    By overseas you mean outside of UK? (ie into EU)

    I will write them an email to ask details.

    It is strange however .... (so many multinational manufacturers with only solid dies)

    Thank you for your support!


  9. The outside diameter of the die is specified in most of the listings. I think up to M6 is 13/16" but M8 is 1".

    Yes I meant outside the UK. Most companies seem to ship worldwide these days with the notable exception of McMaster Carr.

    I don't know why there are so many solid ones. I have some very old ones that I inherited from my dad and they are all split. I never knew what the split was for until recently though.

  10. Did the dies offered by those Hungarian suppliers have a wide notch on the outside? If so, that's where you're supposed to cut them if you want a split die. Cheap solid dies don't have this notch, but many good ones do.

  11. Enleth,
    Ah yes you have solved the mystery. My cheap set of dies have one dimple and a slot, and the holder has one pointed screw. If you cut through the slot then you could close it with the screw in the dimple and open it with the screw in the slot.

    So I didn't need to buy new dies, I could have used the ones I had!

  12. To be absolutely, pedanticly correct, "tapping" is cutting female threads in a hole, and "threading" or "screwing" refers to cutting male threads on a rod. Hence the term, "threaded rod" for the long threaded metal rods used in RepRap.

  13. Thanks for that correction. I have fixed the post, including the title!

  14. I have managed to tread M8 with a closed die. I now know why it was so hard to do. I didn't apply cutting compound, I had no lathe, drill-press or a strong vice (only a PCB vice).

    What I did was clamp the end of the bar with a plumber wrench and tighten it VERY tightly and put tie-ribbons around them (several, otherwise they couldn't handle the tension).

    Then I stood on the wrench and I started on a slightly filed end of a rod. After doing this the rod was pretty hot and I was very tired. The next day I had very sour muscles :S

    One thing I noticed: All the rods I threaded were quite magnetic. A 1 meter bar could hold half of its weight (a just tilted the one side and the other could 'hang').

    If I just apply a few of more your tips then I think I will be just fine the next time.

    I did turn back a bit after turning forward. Did something 'right' after all ;)

  15. Wow - it takes someone really desperate to thread steel by hand. And being magnetic, it probably still wasn't even close to being as hard as 304 grade stainless, for which I had to buy a separate set of cobalt steel cutting tools and literally soak them with cutting oil when machining it.

  16. whats that thing called, the one used to make sure the split die is at 0 angle with the horinzontal?

  17. Can you tell me what is that thing called, the one that is used to make sure that the split die is at 0 angle with the horizontal?

    I am a university student studying mechatronics and we have a course of workshop technology.
    Looking forward for a quick reply.
    Thankz in advance

  18. Sorry, but that's not it, we were using something that had colored water in it and we checked its level using a bubble,i just don't know what it is called.