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Thread: Why doesn't a thread dial work with metric?

  1. #1
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    Default Why doesn't a thread dial work with metric?

    I have tried to figure out why a thread dial would not work with metric threads.

    But I have not come up with an answer.

    So would someone care to explain why it doesn't work the same as imperial?
    How to become a millionaire: Start out with 10 million and take up machining as a hobby!

  2. #2
    PeteF Guest

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    The thread dial is simply a way of indicating the position of the leadscrew thread so you can pick up the same relative position each time. Depending on the thread you're cutting it may be an identical position on the leadscrew thread, or some fraction or multiple of it. Assuming you're talking about cutting the metric threads using an imperial leadscrew, the same position of the thread you're cutting (ie the start) won't coincide with a consistent position of the leadscrew due to the metric conversion change gear used on the leadscrew.

    Not sure if that explanation makes sense, but the best I can do only after one coffee in the morning

    Pete

    Edit: On the other hand if you're talking about cutting metric threads on a metric lathe, it works exactly the same as an imperial thread/lathe, however due to the ratios involved the thread dial may have multiple gears on it to get the appropriate readings
    Last edited by PeteF; 01-14-2011 at 06:37 PM.

  3. #3
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    Default

    You pretty much nailed it Pete.

  4. #4
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    Default

    Quote Originally Posted by Black Forest
    I have tried to figure out why a thread dial would not work with metric threads.
    It does... But with cutting metric threads with an imperial lead screw you need a Metradial thread dial indicator.. http://www.dgrdesigns.co.uk/metradial.html

  5. #5
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    Default

    I am talking about a lathe with a metric lead screw. And cutting metric threads.
    How to become a millionaire: Start out with 10 million and take up machining as a hobby!

  6. #6
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    Default Metric screwing

    It is quite possible and in some cases quite feasible to cut metric threads with a metric lead-screw but a different approach is required as would be the case if cutting "inch" threads on an "inch" lead-screw.

    Inch threads are easy as all that is needed is the tpi of both the lead-screw and the job. The threading dial teeth number is usually 4"/lead of lead-screw.

    With metric threads there are very few "numbers of threads per metre or mm" as metric thread are expressed as their pitch in mm.

    Metric thread screw-cutting requires a number of gears with differing numbers of teeth.

    I have no real problems with it as my single gear (36 teeth - need to check) and my 3mm pitch lead-screw work very well.

    It is all too easy to make a mistake with working how and where/when to "drop in" with the half-nuts.

    I think I recall Nick saying that even he finds it easier to just leave the half-nuts engaged and just reverse the lathe as he also said as I recall do most/many others in Germany - and by extension, I'd add Europe or any other metric countries.

    I leave my half-nuts engaged for metric (easier) and as my lathe is metric I (have to) leave my half-nuts engaged when cutting inch threads.

    Its easy enough - and there are no "cock-ups" either way.

    This may assist.


  7. #7
    PeteF Guest

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    Quote Originally Posted by .RC.
    It does... But with cutting metric threads with an imperial lead screw you need a Metradial thread dial indicator.. http://www.dgrdesigns.co.uk/metradial.html
    RC, I hadn't seen that before but it's interesting you should post that as I was thinking about adding above that it should theoretically be possible to add a 1.27:1 ratio in a dial thread indicator to enable threads to be picked up, however I thought my post was probably confusing enough as it was

    Have you used this conversion thread indicator or know how much it is? I don't currently have metric transposing gears for my lathe so can't cut metric threads, but it will be one of the first thing I'll make when my mill (finally ) gets here. As it turns out I'll make a swing up tool holder at the same time (making the thread dial superfluous), but the device you posted is still an interesting concept.

    Pete

  8. #8
    PeteF Guest

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    Quote Originally Posted by Black Forest
    I am talking about a lathe with a metric lead screw. And cutting metric threads.
    I don't understand your question then. The drawing Tiffie posted above is a metric thread chasing dial and has the multiple gears as I mentioned.

    Pete

  9. #9
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    Default

    Simple explanation is Imperial threads are always a fraction of the leadscrew.
    16 tpi on an 8 tpi screw is 1/2
    20 tpi is 20/8 = 10/4 = 5/2

    You choose gears that have these ratios and the dial always fits as it working on a ratio of 8tpi

    Metric threads are always worked out as a decimal of the leadscrew
    2mm pitch on a 6mm leadscrew is 0.33333
    1.75 pitch on a 6mm screw is 0.29166

    Because of these weird numbers to do the whole range you need a translation gear on the threading dial and often because of this a new dial.

    Most metric threading dials have a series of gears, usually 3 and at least two interchangeable dials so that these 'random' ratios can be handled.

    I don't know how well I've explained this but imperial is fractions, metric is decimal.
    .

    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.




  10. #10
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    Default Qcgb

    Quote Originally Posted by PeteF
    RC, I hadn't seen that before but it's interesting you should post that as I was thinking about adding above that it should theoretically be possible to add a 1.27:1 ratio in a dial thread indicator to enable threads to be picked up, however I thought my post was probably confusing enough as it was

    Have you used this conversion thread indicator or know how much it is? I don't currently have metric transposing gears for my lathe so can't cut metric threads, but it will be one of the first thing I'll make when my mill (finally ) gets here. As it turns out I'll make a swing up tool holder at the same time (making the thread dial superfluous), but the device you posted is still an interesting concept.

    Pete
    Pete.

    The"swing up" tool holder only works if the half-nuts are left engaged and the lathe reversed. It removes the need to withdraw and then re-position the cross-slide.

    Once the half-nuts are engaged (and left engaged) the threading dial is not required - as is the case when reversing the lathe for screw-cutting anyway.

    I don't know how your lathe is set up, but its surprising just how close some metric threads are to imperial.

    For example:
    a metric screw with pitch of say 0.5mm is 0.5/25.4 = 0.1985" lead which when inverted for tpi = 50.7999 say 50.8 tpi which may well get you out of strife.

    If you have a "Norton"-style Quick Change Gear Box (QCGB) you can do even better by also changing the change gears in the gear-train.

    But a 63 or preferably a 127 gear is even better.

    As soon as you insert that 127 gear in your gear-train, the "TPI" numbers on your QCGB become redundant.

    If you don't have a QCGB is quite easy to set the gears in the gear train for cutting metric threads.

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