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  1. #1
    Join Date
    Aug 2002
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    Williamstown, Melbourne
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    2

    Default Cutting thread with die problem - need split die?

    Hi, first of all, searching three letter words does not work, so I tried but it's hard to find answers to "tap and die" !

    Second of all, all I'm trying to do is manually cut an M8 thread on an 8mm steel rod using the crappy carbon steel tap and die set I have. I've done heaps of taps but haven't used dies since high school.

    Problem: it is really hard to cut the thread. No matter how hard I clamp the rod it spins in the vice rather than cuts the thread. At first I thought the rod was oversize but vernier confirms it is 8mm. It seems it is trying to cut too much at once and so it too hard the turn. The die I have is solid carbon steel. Although it has a groove down one edge which makes it look like a split die, it is all connected, so isn't really split. Not sure what the point of that groove is then!

    From what I have read/seen, do I need a split die, to thread once in the "open" position and again in the "closed" position? I can't believe a basic M8 die can't cut an 8mm steel rod. I'm not trying to thread chrome or stainless!

    I've added photo of the die, plus photo of the thread ( which took ages) which hopefully shows it is also slightly off centre. And yes I am using cutting compound/lubricant. What am I doing wrong???
    image.jpg
    image.jpg

  2. #2
    Join Date
    Oct 2011
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    Norwood-ish, Adelaide
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    Default

    There's a bit of a trap here. While the thread is nominally 8mm diameter, thread specs say that the OD can be around a tenth of the thread pitch smaller. If you can reduce the end where you want the thread to say 7.9, the die will run on a lot more easily.
    As for the off centre thing, that has come about because the die was not 100% square to the rod. We all do it (or have done it). I will manually thread with a die using my lathe for alignment. Another one that I sometimes do is make the rod slightly longer and reduce that diameter to that of the thread minor diameter. The reduced diameter length then acts as a pilot and keeps the die aligned with the rod.

    It's another practice thing but if when starting out you take a lot of care to ensure the die is square to the rod you shouldn't get that. Having that tight rod/ die fit probably contributed a bit.

    Michael

  3. #3
    Join Date
    Jan 2004
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    Mackay North Qld
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    Default

    The die looks like its from a cheap set and is normally used to clean up damaged threads. You will sometimes see them in a hex form. As Michael stated the diameter spec of the thread is a fraction under the nominal round bar size of 8mm.

    The excess is easily removed by chucking the rod in a drill and gently running up against a spinning bench grinder or linisher if you have one.

    I have cut threads using the non split dies in this way. Your carbon steel dies will last a while if you keep the use to mild steel or brass.

    I can tell you they don't last too long on stainless steel.

    Grahame

  4. #4
    BobL is offline Member: Blue and white apron brigade
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    Perth
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    Default

    Quote Originally Posted by John G View Post
    From what I have read/seen, do I need a split die, to thread once in the "open" position and again in the "closed" position? I can't believe a basic M8 die can't cut an 8mm steel rod.
    I've rarely been able to tap a rod that is the same nominal size as the die. Like MG says they nearly always need a little bit taken off the rod to form a thread. Not even split dies always work without a bit of relief.

  5. #5
    Join Date
    Jan 2011
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    Hi, first of all, searching three letter words does not work, so I tried but it's hard to find answers to "tap and die" !
    If you are referring to using the forum search function then don't bother. The VBulletin software the forum runs on has a pretty hopeless search system. It is better to use Google. Other search engines will also work, but I don't know exactly what terms to use.

    Search Term: tap and die site:metalworkforums.com
    or
    "tap and die" site:metalworkforums.com

    Dean

  6. #6
    Join Date
    Oct 2007
    Location
    Sydney
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    3,112

    Default

    Make sure your die is the correct way around. They should have a chamfer to help start the thread, but some manufacturers only put that on one side. If you try to freehand a die without a chamfer you'll have difficulty starting it.

    As Michael mentioned, a short spigot to guide the die is very useful. Ensure your "8mm" stock is indeed 8 mm by measuring with calipers and that's just not the nominal size. If you have access to a lathe the whole process can be much easier.

    Cheap taps and dies are a lesson in frustration and a complete waste of money for anything other than cleaning threads. Invest in some good quality taps/dies. Don't buy a set, and simply buy what you need as needed.

  7. #7
    Join Date
    Feb 2015
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    Oz
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    HSS dies and good lubrication go a long way to creating a decent thread. I have a set of HSS button dies I bought 20 odd years ago for a few bucks, they are still sharp and cut a good thread. To get the thread straight I use my lathe. If for some reason I find it necessary to cut free hand I grind quite a long taper on the end of the rod and use a small engineers square and check constantly. Once you have the thread straight you need to keep checking because the thread will skew off if not careful.

  8. #8
    Join Date
    Dec 2005
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    South Australia
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    I encountered a similar problem recently and despite what the experts and manufactures will tell you as said above if you reduce the diameter a small amount it make a world of difference

  9. #9
    Join Date
    Oct 2011
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    Sydney, NSW
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    Quote Originally Posted by China View Post
    I encountered a similar problem recently and despite what the experts and manufactures will tell you as said above if you reduce the diameter a small amount it make a world of difference
    I had the same problem the other day cutting a small thread (M3). I gave up on the die (a HSS one from the company that sells the SIEG products - which is generally pretty good) and decided to cut the thread with the lathe.

    Long story short - I had to make the diam less than 3 mm even when threading with the lathe for the nut to fit on.

    Ben.

  10. #10
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    Sydney
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    Ben I believe unless you're dealing with some ultra-precise application, generally speaking the maximum major diameter of metric threads is normally slightly less than the nominal value in any case. Depending on the class of fit you're after, it's possible to reduce it considerably further and still have a thread that meets the required specifications. It will make the thread easier to cut with a die and that clearance allows the fasteners to be screwed together without interference.

    This should be, well I know it is in fact, covered in The Black Book, however I can't give specific examples, as I'm away at the moment. It's a good example of how valuable that book is even for reasonably routine operations as it presents those values in a pretty easy to understand format.

  11. #11
    Join Date
    Sep 2012
    Location
    York, North Yorkshire UK
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    Default

    Quote Originally Posted by John G View Post
    Hi, first of all, searching three letter words does not work, so I tried but it's hard to find answers to "tap and die" !
    Hi John,

    I agree what verybody else has said !

    It may pay you to download a copy of the "PRESTO Counsellor" from www.presto-tools.com, you will have to look around the site a bit to find it.
    If you look at page 30, right at the top of the page it gives you the specifications for metric threads along with recommendations, on page 40 it also discusses dies. I find the book very useful and I think that you might as well.
    Best Regards:
    Baron J.

  12. #12
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    I got the spring at the back to put pressure on the die against the rod.

    20170923_215154.jpg 20170923_215252.jpg

  13. #13
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    Not quite on topic, but on a similar subject. Quite a few years ago, I made a 1" die holder out of some hollow bar, the top section held the die, while the bottom held a guide that was drilled to suit the bar being tapped. There was no chance with the die starting off off square. Unfortunately I don't have it anymore, as a lowlife employee decided he needed it more than I did.
    Can draw it up if anyone needs more details on it.
    Kryn
    To grow old is mandatory, growing up is optional.

  14. #14
    Join Date
    Apr 2008
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    NSW
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    537

    Default

    The thread looks a bit off centre to me. Without a die holder start by putting a slight chamfer on the end of the rod then move the tails stock up with a chuck in it and just press the chuck jaws against the die to keep it square with the rod . Then lock the tail stock apply pressure with the tail stock feed and start turning the die by hand . Keep a steady moderate pressure on the die until it cuts a few threads onto the shaft . Then you should be able to continue without the tail stock pressure . Use cutting oil . This is how I do mine and never had a failure on softer steels .
    The volume of a pizza of thickness 'a' and radius 'z' is given by pi z z a.

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