Re: CNC lathe threading?

[jmkasunich-at-ra.rockwell.com]

Matt wrote:

> I wanted to exhibit rigid tapping at NAMES, but with 6 weeks
> to go, I don't know if there's time.

> The sequence of operations would be:

> a. Position X & Y over the hole to tap.
> b. Start the spindle & wait for the speed to stabilize.
> c. If the spindle isn't going too fast, sync the Z axis to
>    the spindle encoder starting on an index pulse. This will
>    cause the Z axis to move toward the workpiece.
> d. At some Z depth, cut the spindle power and wait for it
>    to stop rotating.
> e. With Z still synced, start the spindle in reverse causing
>    the tap to be withdrawn from the hole.

> We also need to be able to do this with regular AC motor
> driven spindles, not servo motors.

> It may require (later on) the ability to brake the spindle in
> cases of tapping a blind hole, either mechanically, or by
> injecting a little DC current into the windings. I don't really
> know that much about how to achieve this, only that large,
> multi-horsepower machines don't have DC servo spindle motors,
> and since they can do rigid tapping, it must be possible.

Large machines use three phase motors.  If running on VFDs, they
can be ramped down and reversed fairly quickly, depending on the
VFD's ability to absorb the kinetic energy of the spinning spindle.
The better VFDs have braking resistors in them to burn off the
excess energy.  If a VFD is not used, then the three phase motor
can be plug reversed with contactors, which reverses it very
quickly, but not as smoothly.

> Since this is a more difficult problem, my hope is that lathe
> threading will almost fall out of this as a byproduct of
> developing the rigid tapping technology (the exception being
> the interpreter differences).

Lathe threading has its own challenges.  Simply reversing and
backing out will wreck single point tooling, instead you must
retract the tool between passes.  And of course once you retract
the tool, there is no longer a need to reverse the spindle.
The two projects certainly have common elements, especially
syncing the axis with the spindle, but they are quite different
too.

In theory rigid tapping can keep a 1-to-1 relationship between
the spindle and Z axis, since the spindle must stop and reverse.
With the lathe, the relationship must be re-established before
each pass, since the spindle keeps turning.

Ray mentioned the term "electronic gearing".  This is very
important!  I'm familiar with the term as used on paper making
machines, where fragile wet paper runs through many sets of
rollers.  In the old days, the rolls were gear driven from a
common line shaft.  When individually driven rolls started to
appear, "electronic gearing" was the term used to describe how
all of the rolls were slaved together so that each one ran at
the correct speed.  Before microprocessors, this was done by
pulse multipliers that took the pulse train from one encoder,
and multiplied it by the "gear ratio" to create a reference
pulse train for the next set of rolls.  This is exactly what
is needed for both rigid tapping and lathe threading.  For
rigid tapping, electronic gears alone are enough.  For single
point threading, we also have to be able to re-sync the gears
for each pass.

High quality electronic gearing implies a moderate to high
resolution spindle encoder, because spindle position completely
replaces time as the independent variable.  I think this is the
right solution, but not everyone will have a high resolution
encoder.  With a low resolution encoder or a 1 PPR index pulse,
time based interpolation is needed, and things get more complicated.

The most elegant design is one that would interpolate between
encoder pulses, regardless of encoder PPR.  If the encoder has
a low resolution, there would be lots of interpolation.  With
higher PPR, the algorithm would automatically do less interpolation,
and with very high PPR, there would be no interpolation, for true
electronic gearing.

Ray also mentioned something called "EMC Monday" at NAMES.  This
is the first I heard of it.  What is it?  Is it open to all
interested parties?  If I'm gonna stay another day, I'd like to
know soon.

John Kasunich