Re: Absolute vs Incremental 3D programming
- From: "Bart" <bborb@fusedotnet>
- Date: Mon, 6 Mar 2006 18:37:49 -0500
"Black Dragon" <bd@xxxxxxxxxxxxxx> wrote in message
news:slrne0lpgc.22dd.bd@xxxxxxxxxxxxx
Seeing how we're not far from taking delivery of a machine with a Fanuc
18i control for which we do not have a post processor which means I'm in
the process of tweaking a stock generic Fanuc post, I'm wondering if
there is speed advantage or otherwise using incremental positioning vs
absolute for 3D mold/die type work? I've never posted 3D tool paths in
anything but absolute, but have done manual programming in incremental.
If it's a wash, I'll be giving incremental a try anyway. In that case,
is there anything that might bite me in the ass that I should be looking
for while debugging a post?
--
Black Dragon
Balls Law:
The angle of the dangle is directly proportional to the heat
of the meat provided that the thrusts of the busts are constant
Old toolmakers like myself remember before we had digital readouts, nothing
but an incremented dial at the base of the handles we cranked (all day). If
you remember this, you KNOW backlash doesn't accumulate. If you have .015in
backlash and you bring your edge finder into the side of the part, set the
dial and move half the edge finder and reset the dial to zero, you can crank
anywhere on the table as many times as you want all day long and the largest
deviation from zero will still be in the .015in window. I'm ONLY speaking of
backlash, not pitch error (which is accumulative) or squareness of ways,
etc.
I'll also mention that the CNC control does math in AT LEAST six place
decimal and that math remains used in the background, it is always accurate
to at least the six places regardless whats displayed on your four place
decimal screen, so there would have to be a ton of rounding before it
affected the displayed four place decimal we erroneously assume is the final
result of the math. Motion is determined in revolutions, a single revolution
being an exact number, so rounding errors are contained in just that window
(usually maybe .200in or maybe a metric amount per rev), so a long move is
thought of as "how many revs, then the closest part of a rev (closest part
of the rev being the only place rounding occurs, and its a six place decimal
or more). All math is done from machine zero, no matter how many work zeros
you use.
You can bounce back and forth between incremental and absolute as often as
you please, it doesn't affect accuracy. Absolute is user friendly after we
mark up the blueprint and start programming, I used to like putting my
incremental sequences in subprograms so if I needed to move a part of the
feature it was a simple one or two edits, no refiguring ALL the absolute
numbers that follow or worse yet re-camming and re-posting.
Back to the old handle/dial machines, we COULD increase error if we
loosened our dial to set a new zero and weren't careful about the last
direction we moved when we did this. This is not the case with CNC when you
set work zero(s) because all math is done from machine zero at the startup
of the machine.
Just my opinion,
Bart
.
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