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Thread: Metric thread dimensions :-?

  1. #1
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    Question Metric thread dimensions :-?

    Can someone clarify the dimensioning of metric thread for me please?
    From what I can find of the spec, everywhere states that the Major Diameter/OD=Nominal Diameter. For everything I can measure, this isn't true as it's slightly less. I've found mention of tolerances which would seem to indicate that it's actually at least Major Diameter = Nominal +0/-x but I've not found a good list of dimensions for basic threads - a list of "If you want to cut an M6, first turn it down to 5.8mm" sort of thing.

    There are plenty of charts for tapping drill sizes but I've not found one for what size you should turn a rod down to before running a die down it. Is it as simple as just nominal diameter? ie 6mm for M6.

    I also had trouble with a mismatched tap and die. Had to make an M7x1 thread to replace an M6 that had been stripped (M8 would have left no material to hold the thread) and I only had M7 in an old really cheap set. After it didn't work, I found that the die was closer to M6.5 (OD of the thread it cut) than 7 and I think the tap was closer to M7.2 (OD of the thread on the tap). Managed to single-point a male thread to go with the tapped thread by starting at an OD of 7.2. Both are clearly marked M7x1.0 but the results of both don't fit together. Obviously I won't plan to use them again but am I measuring these correctly or have I missed the point?

    While I'm at it, could someone speculate how I managed to cut a reverse-tapered thread?! Turned down some aluminium to 6mm for an M6 thread and it was pretty much parallel - few microns of taper for sure, but we're only talking about a 30mm length here. Single-pointed it with an ER16 1.0 insert. Result was that the end farthest away from the spindle was wider by enough that by the time the thread was deep enough to accept a nut (yes, I know I should be using pitch wires....I'm getting there!) it was then quite a loose and sloppy fit further up the thread. Turned out to be good enough, thankfully, but I'm curious as to what went wrong so I can avoid the same mistake again.

    Many thanks for the input guys.
    Gareth

  2. #2
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    Most bolt threads whether metric or imperial measure less than the nominal OD/major diameter.

    For general purpose work, if you are cutting external threads with a die make your stock 1% or 2% undersize, this applies to metric and imperial threads.

    With your reverse taper thread, most probably the cutting forces from the insert were deflecting the stock at its loose end, i.e. it wasn't cutting deep enough at the start. So by the time you get a nut on at the start you have overcut the rest of the thread. Best solution if deflection is going to be a problem is to partially single point the thread and finish off with a die.

  3. #3
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    https://en.wikipedia.org/wiki/ISO_metric_screw_thread

    Thread crest truncation is sqrt(3/256)*pitch or ~11% of pitch. So 22% of the pitch reduced from diameter.
    M6x1 thread should then have 6mm-22%*1mm =5.78mm major. Many of the store bought bolts that I have actually larger major diameter than this.

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    Thanks Bob. That would seem to add up. The part was threaded both ends and held in a collet from the end threaded first - partly by the shoulder and partly on the thread - so I guess some movement was inevitable. Surprised I didn't seem to get any just turning it down considering that the DoC was 1mm (2mm off diameter) and the part only started at 10mm. Looks like I'll have to invest some time making some female-threaded mandrels to get round this sort of thing. Irritatingly, I originally meant to try out the die holder I'd finally got skimmed to fit my dies (it was fractionally too small) as M12 was too much for it and this would have been perfect....but I was determined that 'just a little more' would get me a perfect single-pointed thread

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    Thanks MattiJ. That makes sense and is one of the formulae I'd shoved into Excel to run up my own chart. The information published online seems to be quite contradictory and finding a definitive standard seems to be impossible. As an example of what I mean:

    Maryland Metrics' metric thread data showing that Major diameter = nominal diameter and that it's measured to the flatted crest: http://mdmetric.com/tech/thddat2.htm
    Maryland Metrics' extended metric thread data showing that Major diameter < nominal diameter and giving it a range: http://mdmetric.com/tech/M-thead%20600.htm

    The latter specifies a class of 6g - hence why I was theorising that the major diameter may be defined as nominal minus a tolerance for a class....but I'm getting in above my head by now and just further confusing myself.

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    To add a bit more, threads are undersized a bit from nominal because the rounding off the points does not reduce the strength and actually increases the strength by rounding the stress riser at the root. The place where fit is important (pitch diam.) is not affected. Exception is tapered pipe threads where points help seal fluids. Try grinding a high speed threading tool. The last step is to take a prescribed amount off tip. The tip is the first thing to wear off and to maintain it you would be constantly resharpening it. When cutting small threads in the lathe I will use a die in the tail stock to get around the tool bit deflection issue. Stop short and finish by hand turning if worried about crashing. If the same size tap and die don't produce acceptable threads, replace them with quality tools.

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    That's why so many of us Colonists resist the metric system--Too complicated.

    Fortunately for me, I've only had to drill and tap a few metrics. I look on the old Craftsman tap and it tells me what drill to use in English. I drill and tap and it works. Yeah! That's all I want to know about it.

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    All this agonizing over the starting OD dimension is really pointless except perhaps in the most finicky of applications.
    Machinery's Handbook shows an OD range for M6x1 threads of 5.974 to 5.794 mm. That's anywhere from .001" to
    .008" undersize. Variations in material diameter can easily account for half of that range or more. In the world of
    working machinists the reality is that most will grab a piece of nominal OD 6mm rod and either single point a thread
    or run a die down it--by the time they've dusted the OD of the thread with some fine grit emery tape and de-
    burred it with a wire wheel they'll have a perfectly satisfactory finished product...
    Keith
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    Just one project too many--that's what finally got him...

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    Snaps from "Mechanical and Metal Trades Handbook"... (the English translation of a German reference book). ISBN 976-3-8085-1914-1. Sorry they are a bit grainy. The board here limits maximum permissible image sizes, so I had to downgrade them a bit.



    Last edited by Euph0ny; 01-12-2018 at 10:09 AM.

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    Quote Originally Posted by deltap View Post
    To add a bit more, threads are undersized a bit from nominal because the rounding off the points does not reduce the strength and actually increases the strength by rounding the stress riser at the root. The place where fit is important (pitch diam.) is not affected.
    I've bought myself some pitch wires as I've reluctantly accepted their necessity. I'm just struggling with the start point and specifically its definition. I've cut some acceptable (in that they work) threads and I've finished some off with a die....but I want to get my head around how they should be sized rather than 'ah that'll do' and I'm struggling as I'm either missing something or just confused by conflicting information.

    Quote Originally Posted by deltap View Post
    Try grinding a high speed threading tool.
    I'm going to be difficult and obstinate in my use of carbide inserts I'm afraid I had to relegate the grinder to the bottom shelf in order to get the lathe in....not that I'd got exactly what you'd call skilled with it yet anyway. :-D

    Quote Originally Posted by deltap View Post
    If the same size tap and die don't produce acceptable threads, replace them with quality tools.
    Definitely with you on that. Have a reasonable set of Sealey tap and dies (and a nice set of Thurmer spiral flute taps) but neither has this I'll-never-need-it-again M7 size and how bad could it be in plastic?! It never occurred to me the "matching" die wouldn't match and once I'd tapped the plastic, I was committed to the size - no room to go larger.

    Quote Originally Posted by CCWKen View Post
    That's why so many of us Colonists resist the metric system--Too complicated.
    I hate the sheer variety in imperial units - 12 inches in a foot, three feet in a yard, 1760 yards in a mile (seriously?! Yeah, I had to look that up) - and yet I'm conditioned to miles, mph, mpg, height of people in feet and weight of people in stone. Beginning to come round to your way of thinking on the threading too....although part of that's just that I want a cheap simple thread dial!

    Quote Originally Posted by LKeithR View Post
    All this agonizing over the starting OD dimension is really pointless except perhaps in the most finicky of applications.
    Machinery's Handbook shows an OD range for M6x1 threads of 5.974 to 5.794 mm. That's anywhere from .001" to
    .008" undersize. Variations in material diameter can easily account for half of that range or more. In the world of
    working machinists the reality is that most will grab a piece of nominal OD 6mm rod and either single point a thread
    or run a die down it--by the time they've dusted the OD of the thread with some fine grit emery tape and de-
    burred it with a wire wheel they'll have a perfectly satisfactory finished product...
    Ah, pointlessly finicky is where I excel! A digital micrometer was a fatel purchase for me....now there are three decimal places that can be 'wrong'!
    I'm not good with 'the art of' something, I need to understand how it works to get on with it. Most of the pieces I've had call to work on have required a different thread at one end to that at the other so it has to be turned down to a 'starting' point.

    Quote Originally Posted by Euph0ny View Post
    Snaps from "Mechanical and Metal Trades Handbook"... (the English translation of a German reference book). ISBN 976-3-8085-1914-1. Sorry they are a bit grainy. The board here limits maximum permissible image sizes, so I had to downgrade them a bit.
    Thanks for those. That pretty much confirms where I'd got to...just wasn't sure if I was right. So if I'm reading it correctly, the thread form is defined such that DMajor=Nominal diameter - tolerance and it is measured to the flat of the crest, not the point that would be there if it wasn't flatted. That about right?

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