Metric Bantam lathe and 19tpi threads

[Deleted AccountParticipant @deletedaccount69088]

Posted by Mike Lunch on 01/07/2019 10:16:22:

"may I ask if you could be kind enough as you have a metric Bantam like me to measure the leadscrew whether it is 4tpi or 4 in 24mm (6mm pitch metric)."

Dear Pence,

I have just measured mine, and yes it is 6mm pitch.

Regards, Mike

Edited By Mike Lunch on 01/07/2019 10:18:25

okay thanks for that – looks like I have a 4tpi thread on a metric machine, not sure what kind of threads that will cut! Looks like I will have to hunt down a metric leadscrew.

[Nigel Graham 2Participant @nigelgraham2]

I've investigated using a spread-sheet (MS 'Excel&#39 to calculate change-wheel combinations.

'

It proved an interesting exercise, showing some threads apparently awkward for the equipment available can be solved with pitch-errors within tolerance or feasible by part-cutting then die-finishing, at least over short distances – more common than long threads. (E.g. studs, pipe-fittings.)

Some unlikely threads even proved numerically possible, though perhaps not to aerospace standard, on my EW 2-1/2" lathe, limited by its 8tpi lead-screw and change-wheels of 25 to 65T X 5T increments.

Using a spread-sheet, including such tools as absolute cell references, allows you readily to model the effect of changing just one variable so as to minimise the errors, and show them singly and cumulatively.

I'm a bit rusty on Excel now, but it may be possible to plot one " side " of the true and calculated threads at a readable scale, as two superimposed traces on a Cartesian graph, to show where the inaccuracy starts to exceed tolerance. NB: this is not a design-drawing, but gives simply a series of sharp triangles on the thread's axial plane.

(I once used a manually-drawn version of such a graph to establish using a metric taper-tap to "pilot-tap" a hefty BSF thread in heavy steel plate with existing but slightly undersize holes!).

[Deleted AccountParticipant @deletedaccount69088]

Posted by Nigel Graham 2 on 07/07/2019 23:11:35:

I've investigated using a spread-sheet (MS 'Excel' to calculate change-wheel combinations.

'

it is possible to cut most threads with a wrong 4tpi leadscrew on a metric Bantam using the 127/120 gears to correct only one looses the modular threads as far as I can see according to boxthreadp.exe. Maybe something can be done with some wacky gear combinations for the modular bit but it is not ideal.

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