Re: Threading under EMC

[Chris Stratton <stratton-at-mdc.net>]

> I know a lot of neat tips and
> tricks for machining but rigid tapping without spindle encoding would
> wrack my brain trying to match speed to feed as a time function.   Ill
> try to spend some time thinking about this for a few days, depending on
> what
> you guys may need, or already know.
> Thanks ahead for not calling me just crazy, :)
> Phil Smith

Phil, I really don't think rigid tapping without an encoder is
possible - just too many variables.  What I would like to see is rigid
tapping and single point threading with an encoder, but a poorly
controlled spindle, as might be found on converted manual machines.

I've written up a little simulation of how to catch up to a thread
groove on a running spindle.  What I do is calculate the distance
required to achieve position and velocity lock together, assuming 3/4
of max acceleration for the z axis.  Then I dynamically recalculate
while running, using the actual spindle speed - which could cause the
acceleration to be greater than planned, hence the safety factor in
the inital calculation.

For the moment, I'm assuming the C axis should be used to measure the
spindle rotation (of course that will create problems for a mill with
a rotary table).  Since this is a wrapped linear axis, it simply
increments forever (bad news in a few hours if your encoder line count
is not a power of two!)  What I'd do is multiply this by the thread
pitch to create a linear position.  Then I'd find the thread groove
that was at least the z axis acceleration distance behind the current
Z position and calculate and offset from the absolute groove position.
I'd wait until it was exactly the acceleration distance behind, then
start accelerating the Z axis using the dynamic calculation of
acceleartion (as in the simulation) to achieve velocity and position
lock after the precalculated acceleration distance of travel.

Once lock is achieved, the servo loop can simply be fed a desired z
position which is the thread groove linear position as calculeated
from C axis, pitch, and offset.  It should be able to retain lock
through deceleration and reversing of the spindle when tapping,
assuming the spindle acceleartion parameter is comparable.

One big question I have is, what are the usual parameters to the lathe
threading G33 and G34 codes?

Chris

-- 
Christopher C. Stratton, stratton-at-mdc.net
Instrument Maker, Horn Player & Engineer
22 Adrian Street, Somerville, MA 02143
http://www.mdc.net/~stratton
NEW PHONE NUMBER: (617) 628-1062 home, 253-2606 MIT