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  1. #1
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    Default Change Gears Again

    Sorry I know this is a bit of a pain in the bum topic, and I feel like I should be able to figure it out but I'm stumped. And since it seems to be a slow day ...

    Now that I have a metric machine in the stable I want to get metric screwcutting organised. Has anyone seen a chart like this?

    The part at the top is straightforward imperial threads, with the operator left to fill in the blanks. The coloured numbers on the left I believe are metric thread pitches. They line up in columns with the gearbox letters. And the colours link the pitches with the gearbox numbers at the top.

    Say you wanted a one mm pitch. You find the 1 in the table, see that it lines up with the B, and that it's red. Red corresponds to 5, therefore B5 is the ratio for a 1mm pitch.

    The baffling bit is the column on the far left. I can only imagine it's about the change gears; that the tooth count on the left should somehow produce all the pitches on that row. But I cannot for the life of me make sense of those numbers as change gears.

    Before I go into the numbers, I'd like to know if there are any flaws in my logic so far? Is there some other way this chart can be read?

    Edit: Have reproduced the chart as a pdf and hopefully made it easier to read.
    Last edited by Bryan; 21st Jan 2011 at 05:25 PM. Reason: Added PDF

  2. #2
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    Default

    I reckon they are change gears also. Do you have any gears that match those numbers?
    Do you have a picture of your gear train?
    Cheers.

    Vernon.
    __________________________________________________
    Bite off more than you can chew and then chew like crazy.

  3. #3
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    Vernon, the only gears that came with the lathe were 2 x 40s and a 70 idler. I don't believe there was ever a 127 because it simply wouldn't fit.

  4. #4
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    So the 3 in 43 isnt green then?

    I think the numbers are the driver gear.

    my e.g goes like this
    Take 0.7= 26 c yellow

    1.125 is also C yellow so the only difference is the 25 and 43
    43 is 17 teeth bigger than 26
    17 about 65% of 26
    0.7 + about 65% = 1.157

    Sort of about right

    Of course I've been wrong before, more than once
    Stuart
    Last edited by Stustoys; 21st Jan 2011 at 05:42 PM. Reason: maths

  5. #5
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    Stuart, yes the 3 in 43 is green, and I have no explanation for that. Got a theory?

    I think I can follow your logic. It might be a question of how close is close enough?

    The way I thought about it was this:
    Leadscrew = 8TPI.
    8TPI = B1.
    B1 = 1:1.
    Change gear ratio = metric pitch at B1 / imperial pitch at B1 (in mm).
    Change gear ratio = 2.25 / 3.175 = 0.7087

    I've used a spreadsheet to multiply .7087 by every whole number from 2 to 127 inclusive, and there's not a single whole number among the answers. The closest ones are 24:17.0088 and 127:90.0049. They may be close enough but they don't sound like realistic change gears to me. Something's not right.

  6. #6
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    Some lathes use a 63 tooth gear instead of a 127 tooth for the imperial/metric conversion (given that 63 * 2 is 126 which is close to 127) due to the size.

  7. #7
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    Gavin beat me to it.
    As I understand it, all thread cut on the "other" leadscrew are approximations, some are just closer than others.
    Not theory on the green 3 sorry
    I dont follow your maths really, you could have picked 2.5 instead of 2.25, its B1 also.

    Stuart

  8. #8
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    Quote Originally Posted by Stustoys View Post
    Gavin beat me to it.
    As I understand it, all thread cut on the "other" leadscrew are approximations, some are just closer than others.
    So the pairing would be 63:60? That gives 1.05. Lead of 3.175 / 1.05 = 3.0238. I'm guessing a 1 percent-ish error wouldn't matter a damn for normal purposes. So just replacing my idler with a 63/60 would give me some metric threads. But it doesn't explain the chart. I guess I can live with that if I have to.

    I dont follow your maths really, you could have picked 2.5 instead of 2.25, its B1 also.

    Stuart
    Yes, I see. I don't have the answer yet. Looks like a busy weekend ahead with family commitments, so I don't know when I'll get a clear head for this. Wish I was more competent with maths. Thanks for the input so far.

  9. #9
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    Bryan,
    It would assist greatly if we knew the gear set to give the imperial table.

    If you cannot easily determine them, look at just the gearbox.
    It is easy to mark the input gear, select a ratio (say A1), and observe the leadscrew rotation to determine the gear ratio for that selection.
    Only do it once as the rest of the box is just multipliers or divisions from that value.
    Once you know the gearbox overal ratio,and using the leadscrew value, you can calculate the ratio spindle to gearbox input, and that in turn will tell you the gears required.

    Don't worry about odd ratios, for example my Bantam has a 7:1 in the gearbox somewhere and 49 teeth gears for some metric threads.

    The imperial table is identical to numerous lathes copied off the imperial South Bend 9A.
    My Sheraton 9A with an 8 tpi leadscrew, for example, would produce an identical set of values with a 40 Driver and a 56 Follower gears.

    The reverser yoke seems odd, the two gears engaging on the spindle must be the same size to get equal thread pitch Forward and Reverse.
    I count the teeth from the photo as: 20, 25, 40, 40, 70.
    Is it possible the two 40s should be on the reverser yoke ?

    John.

  10. #10
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    Quote Originally Posted by electrosteam View Post
    The reverser yoke seems odd, the two gears engaging on the spindle must be the same size to get equal thread pitch Forward and Reverse.
    I count the teeth from the photo as: 20, 25, 40, 40, 70.
    Is it possible the two 40s should be on the reverser yoke ?

    John.
    You sure about that John?
    One rotation of the spindle is XX teeth, doesn't matter what size and number of gears you put that through they all turn XX teeth. So only the gear on the spindle and the gear on the input shaft of the gearbox matter.

    Unless you have a double gear like a 127/120(I think thats my double gear, or is it called a compound gear?)

    As the spindle gear looks pretty fixed on Bryans lathe, I assume its the gear on the gearbox input shaft the table it meaning.

    Stuart

  11. #11
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    John, attached to post 1 is a PDF chart which shows all the imperial thread pitches. As a starting point I was trying to simplify matters by assuming QC ratio B1, which is 1:1. I know this because the thread produced is the same pitch as the leadscrew, with change gears of 40:40. If I can get the right conversion ratio at B1, I figure the rest should fall into place. The gears I have, including the tumblers, are in the only positions they can go.

  12. #12
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    Stuart,
    You are correct, all the gears in the photo are only idlers between the spindle and the QC input gear.
    I spent so much time analysing my Sheraton, I just followed that arrangement with a layshaft after the tumber reverse allowing a selection of driver gears.

    Bryan,
    My Sheraton has a QC ration in A1 of 1:2.8 and for B1, 1:1.4.
    The Hercus will be similar, but not necessarily identical.
    Are you really sure B1 is 1:1 ?

    Once you know the actual QC ratio, you can calculate the compound intermediate to produce the metric table (after a few tries because of the approximation).

    John.

  13. #13
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    I thought the way to know what gears I needed would be to understand the thread chart provided on my lathe. That may be true, but I can't understand the chart. The numbers that we think are tooth counts do not work with my existing gears in any combination that produces that chart. So other gears must be needed. If there's a way to calculate those missing gears I'm not smart enough to figure it out.

    However, by once again messing around with spreadsheets I've concluded there are numerous ways to achieve common metric pitches on my lathe. And I can do it by only adding two or three gears. No transposing gears needed. (Transposing gears I understand to mean a keyed pairing of either 127/120 or 63/60.) So I think I'll settle for that.

    Thanks to all respondents.


    Edit: John you posted while I was composing. I think my 1:1 logic is sound. If you want to keep chasing this bunny I'll watch.

  14. #14
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    Bryan.
    I might be confused as to what you are thinking here but.
    The 1:1 ratio of B1 is for the complete gear chain from spindle to leadscrew. Not just the gearbox.

    Stuart

    p.s. what are the tooth counts on the spindle and gearbox input?

  15. #15
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    Stuart,
    Exactly - there is probably a reduction from the spindle to the QC input, then a speed-up within the QC for the A1 position.

    My Sheraton:
    40 Driver to 56 Follower gear reduction 56/40 = 1.4 from spindle to QC input,
    QC position A1 speed-up of 2.8 input to leadscrew,
    result is leadscrew speed is double the spindle speed,
    equals 4 tpi with an 8 tpi leadscrew.

    Once we have the QC ratio, we can calculate what the gear set should be.
    Don't assume that the gears as set in the photo give the correct results until confirmed by Bryan.

    John.

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