Re: [CAD_CAM_EDM_DRO] Re: Just How Accurate Is CNC?

Hi Tony:
I've got a Haas, and I've found the spec is applicable only under the
most exacting of conditions.
For example, if I do a rapid move ten times, using only one axis, from one
position to another and back again with a small mass on the table, my actual
position will vary by about 0.0005" in a pretty typical normal
distribution.
If I do the same movement at say 100 IPM, I will have a similar cluster of
about 0.0002"spread.
If I never exceed a table speed of about 20IPM, I can get within about
0.0001" of repeatability, but NOT nominal position.
My machine has about 3000 hours of runtime; it's almost all toolmaking time,
and it's never been crashed.
If I bore two holes I can expect to get them within 0.0005" of the nominal
spacing using nothing more than rapid moves, but only if the holes are
within 5" of each other, and roughly in the middle of the table travel.
If the commanded traverse is near the maximum capacity of the table, I'm
within 0.001" of nominal spacing on the X axis...the Y is not quite as good.
For most normal machining operations, that's adequate, but it's certainly
not a jig borer by any means!!
For that reason, I still ream the leader pin bores through the guide
bushings when I build a moldbase (the old fashioned way that guarantees an
acceptable match), and I still mike the pocket walls to be sure I'm in the
right spot in the blocks.
I have to reposition the origin about one time in ten due to cumulative
machine errors, and I'm aiming for 0.0005" from nominal for the milling on
most general purpose moldmaking.
Hope that gives you a sense of what the machine will do in the real world,
compared to the accuracies quoted in the brochure.
Cheers

Marcus