On 2008-12-27, Michael Koblic wrote:
[ ... ]
I am still trying to get my head around the fact that a lathe cannot cut
both SAE and metric screws. Why? The screwcutting is a function of matching
the carriage travel to the number of spindle turns. As this is achieved by
gears, should one not be able to achieve a more or less infinite combination
to match both systems?
One can use certain gear ratios (100:127 is the ideal, smaller
ones come close) to provide a conversion between metric and imperial
threads.
*But* -- during most threading, it is common to use half-nuts
in the apron -- engage them to start cutting the thread, disengage and
quickly crank the carriage back with the large handwheel, then re-engage
to cut the next pass of the thread.
However -- there are multiple points at which the half-nuts can
be closed, and only some of those will retrace the previous cut (which
you want to do -- only deeper). For this, there is usually a "threading
dial", usually on the right side of the carriage, though I have seen
them built into the apron and flush with the top of the carriage.
Anyway -- the dial tells you when to close the half nuts to
retrace the same thread path.
On imperial lathes with an imperal leadscrew, this works only
with imperial threads.
On metric lathes with a metric leadscrew the threading dial
needs to have a selection of (typically) four different gears to engage
the leadscrew to deal with the different ratios which metric threads
require.
In either case, you *can't* use a threading dial for an imperal
leadscrew to cut metric threads -- even with the 100:127 transposing
gears.
And -- you can't use a threading dial for a metric leadscrew to
cut imperial threads even with the 100:127 transposing gears.
So -- the only way to cut threads on the other system (unless
you are cutting a fine enough and shallow enough thread so you can cut
it in one pass) is to halt the spindle before you reach the end of the
thread, perhaps use a hand crank locked into the spindle, to crank the
rest of the way to the end of the thread, back the tool out of the
thread, crank in reverse to the starting end, crank the tool back in
with the extra depth for the next pass, again use the power for a long
thread but being sure to stop it before you reach the end, and crank
again and again.
So -- you *can* cut metric threads on an imperial lathe or vice
versa with the transposing gears -- but is is a royal plain. So modify
the statement to read "You can't *conveniently* cut X threads on a Y
lathe" and it becomes true.
The only exception to this is on CNC lathes, where there is no
direct gear connection between the leadscrew and the spindle. Instead,
the spindle has an encoder which gives one index pulse per revolution,
plus a lot of incremental pulses ever few degrees. The computer handles
turning the leadscrew the right amount for each incremental pulse to cut
the threads. The computer then backs the tool out at the end of the
thread, and reverses rapidly to reach the starting point, and then waits
for the next index pulse to start the same thread pass again -- deeper.
So -- it is a mater of telling the computer whether you want to work in
mm or inches. (Oh yes -- and learning to program the computer. :-)
I hope that this helps. It would have been easier if you had
already had the practice of cutting threads on a manual lathe of one
system or the other, so you would know what you would be missing.
Enjoy,
DoN.
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