Re: MaxNC modifications

Tim,

Will have to sit down with my dad and gather up the details, but here's
what I can tell you right now.

Not sure what you mean by controller, Ah-ha communicates directly with
the stepper drivers via a special card.
Power supply is 40 volt more or less, unregulated, based upon the
schematic supplied by CyberPak for their drivers. Dad built the PS, I'll
get details from him.
Don't recall motor specs, will have to look them up again.
Think they are rated for 5 amps, we are running about 3 now, or so we
think. That is based upon the current limiting resistor.
Dad thinks we're not getting full power to the motors and tried to
measure amp draw INTO the drivers, came up with something like .5 amps
draw at rapid (40"/min). However, it's a chopper driver, not sure if
he's checking properly.
I've written little test routine programs. Single axis linear moves,
looped and run for 5-10 minutes show no lost steps. Interpolated linear
moves (XY) show no lost steps. Circular interpolation seems to be the
problem, but not always.

My dad machines little resistive elements for a customer of his. These
are generally under 1" dia and we run multiples of up to 8 at a time on
a custom pallet setup. We've tried using G92 to set offsets for each
nest, no subroutines; G92 with subroutines; G91 with and without
subroutines; and if a particular job is going to show lost steps, it
doesn't seem to matter how we approach it. Some parts do not lose steps.
Each part will show a different pattern of lost steps. The pallet base
has a .500 dia hole that we indicate from to establish datum, all
pallets and all programming is referenced from this hole. It's real easy
to drop an indicator in and check for lost steps.
For example, one particular part run in multiples of 8 parts per pallet
might show .0013 lost in X and .0003 lost in Y every for every pallet
load. With the tolerances on this job, Dad can let this accumulate over
two pallets before having to rezero. This is so consistent that he
stopped checking after every two pallets and simply would jog X.0026
Y.0006 and rezero. After running several hundred parts, he'd check with
the indicator after the last two pallet loads and be off exactly
.0026/.0006. (I don't recall exactly the sign (+/-) of the error or
corrections, but the numbers are real close to what he runs into)
This is with a program that does not use G92, it's done with geometry in
a subroutine, we move to a set entry point, enter the subroutine, enter
G91 and machine features, then back to G90 and exit the subroutine.
We lose NO steps in the Z axis.
Motors are wired with shielded cable, grounded at one end to the motor
PS ground. Logic lines are also shielded cable, grounded at one end to
the computer ground. Care has been taken to keep power and logic lines
well separated. If I didn't mention before, we've also run the MAX
software on two different computers, a 486-33 and 486-66 with identical
results. I'm close to getting my Cyrix P200 box done and will try
the Ah-ha from it, but don't think that's going to solve the problem.
Ah-ha warns about possible rounding errors and recommends programming to
4 decimal places. I edited a problem program to accomodate this and
there was no change in the lost steps, so that's not likely to be the
problem.

I printed your message, when this gets back to me I'll print it as well.
Dad will be by tomorrow and I'll give him both copies to go over and
fill in any details I missed or don't know.

We would both LOVE to solve this problem. Dad's good with basic
electricity, but we are both stumbling in the dark on this, and it's
getting expensive running around buying stuff trying to fix the problem.

As for desired speed, would like to see 60"/minute. This was part of the
reason I went with 16 pitch screws. I can accept the slight loss of
accuracy, the tradeoff is that for a given linear velocity we are
running the motors 20% slower. I may be all wet, but my seat of the
pants guess was that we were coming out ahead over a 20 pitch screw by
keeping the motor down in a broader part of the torque curve.

I have some top quality linear rail bearings and ballscrews, and want to
build a CNC conversion for my Hardinge. However, I have a hard time
justifying the time and expense in finishing it with the lost steps we
are getting. Even a single lost step per part would quickly cause major
problems in the X axis of a lathe.

Thanks,

Jon
(who btw, is highly impressed with the quality and quantity of posts to
this list!)