mmm, makes sense.
On 21/10/2011 16:51, Randall wrote:
>> My recollection is that for ANY thred you can re-engage the
>> nut on the
>> same point that you were on the dial (eg if you were first
>> engaged on 1
>> - you can always re-engage on 1).
> You got me wondering, so I looked it up. On a lathe with Imperial threads
> on the leadscrew, engaging on 1 works for any thread that has an integer
> number of threads in 2 (or 4 for some lathes) inches. But metric (and BA,
> which is what I was thinking of when I said Whitworth before) threads
> typically don't have an integer number of threads in 4 inches, so the
> standard Imperial threading dial cannot be used when cutting those threads.
> Imagine the lathe stopped, with a long threaded bar between centers. Start
> with the half nuts engaged, the threading dial on '1', and the tool bit
> exactly in the thread groove. Now imagine moving the carriage to the right
> until the dial comes back on '1' and closing the half nuts again. The
> carriage has moved some number of inches (2" on my 10" Atlas). If, for
> example, your bar has 11.5 tpi, then the tool will be exactly 23 threads
> away from the original position and still at a thread groove. (Assuming
> you've taken up the slack correctly and so on.) But if, for example, your
> bar has 25.4 tpi, the tool will now be 50.8 threads away from the starting
> position, and not lined up on the groove.
> The rules are different for a metric leadscrew, of course, but similar
> limitations apply. From what I found on-line, most metric lathes come with
> 3 different gears for the thread dial (and still don't cover BA threads).
> Here's a neat little device that apparently includes the gears for both
> metric& imperial on the same leadscrew
> But it still doesn't appear to cover 4 BA (38.46 tpi)<g>
> -- Randall
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