I think that for any given feed vector, you could calculate (or measure
empirically) the stretch, and have a lookup table that adjusted the feed
to compensate. That would probably be a difficult programming job, but
anything is do-able. Back in the early 80's, I wrote Z-80 code that
would allow a bar code wand to scan a code at _widely_ varying speeds
within the body of the symbol (over 20:1 and varying rapidly), and still
decode it reliably. This is that sort of task. A wide range of feed
rates and feed depths would need to be accommodated, and you'd even have
to account for materials in the adjustments -- but it's just software.
I'm not sure what EMC enables you to do in that respect, but since it's
open-source, it's extensible.
The stretch on a belt is not immeasurable, but with modern Kevlar
reinforced belting, I think it would be minimal (supposing the belt is
not used near its design limits). I don't think the BP guys would have
liked not being able to mill round pockets. No... I don't think they'd
have settled for it, at all. Probably, their belts weren't 40 years old,
You'll never CNC Gcode to bore accuracy. You'd have to check your machine
but a circle to +/- .002 with G02/03 would be awful good for your machine.
For accurate bores get a boring bar. I'm partial to devleig microbore - they
do 2 tenths easily and I own a complete set from .250" to over 6.0". I
lock the gibs down tight while boring to get this.
I would think it is minimal since an entire turn of the handwheel is 0.2" or
so. I would guess belt stretch to make less than a half thou difference.
But if it does affect your parts, you have the capability to put DRO scales
on your mill and use them with EMC2 instead of the motor encoders. That's
what my mill has, an Anilam control with Anilam DRO scales , the motors have
tachometers on them, they are stable even with backlash.
Have you found me a Syncrowave yet? :-)
I've found some on eBay for over $2000 but I'm not that interested in one.
If I put my old one on a trailer to I can move it to my current address then
it will probably be sufficient. When I got it I ran some beads on aluminum
but had some trouble welding 2 pieces together, a stainless steel brush and
some more practice would have probably done the trick. I'm hoping to get a
tank of Argon soon but for right now I should concentrate on getting the
truck done and sold.
No, but his point was the one full revolution of the ballscrew is a
pretty small movement, and any belt stretch issues aren't likely to
cause more than a couple degrees of ballscrew position error. I think
you said 2.5mm ballscrew pitch, so perhaps something like 0.0006"
positioning error under belt stretch? Backlash and proper backlash
compensation will be a much bigger factor in accurate circular
The "virtual" handwheel. :-) It is still 0.200" per revolution
of the ballscrew or the nut.
If yours is like mine -- the ballscrew for the X-axis does not
rotate at all -- instead the ball *nut* rotates in matched bearings.
And the ballscrew has a telescoping flat spring steel cover to keep
swarf off of it.
Y-axis, however, does have a rotating ballscrew.
BTW -- looking at the photo you just (re)posted of the mill on
the truck or trailer I now see that the Y-axis motor has the pulley end
towards the vertical ways in the back instead of facing out towards the
operator as mine does. I still should be able to mount the motor on the
side of the knee -- if I can find a flat enough surface. :-)
I saw you had them listed but by that time you had already changed your mind
about selling it. I found a Syncrowave 350 on eBay around 400 miles away
with a 499 starting bid and no reserve. Bigger than I want but a syncrowave
250 isn't exactly portable either. If freight isn't too high for 400 miles
I may bid $750 minus freight charges, thinking it would be worth $750
delivered. It doesn't have the TIG torch, water cooler, and I didn't see a
foot pedal. I already have a torch and water cooler on my old TIG so I'm ok
there. If I knew the foot pedal specifics I could probably get my existing
foot pedal to work on the Syncrowave too.
Do you know if the Syncrowave 350 is the same as the 250 beside the extra
weight and amps? Just wondering if I should bid on this item, assuming
freight is reasonable, or wait for something closer or a syncrowave 250.
He could also move his current encoders to be directly connected to the
ballscrews, eliminating any possible belt issues from the positioning
accuracy. He would cut the counts per inch in half, but I think it's
high enough not to matter.
Baloney! Now, there is some tiny amount of stretch to the belts, but it
is certainly small, in the fractions of a thousandth of an inch.
I can't say about a Series-II BOSS machine, but there are other sources
of apparent backlash. There's compressibility of the screw, rocking of
the table when direction reverses, the constraint of the bearings that
hold the rotating nut, and so on. My gut feeling is that these other
sources of backlash, PLUS finite servo amplifier gain, will be much
greater than the belt stretch. It almost certainly adds up to several
tenths of a thousandth of an inch, and on an older machine, could be
much worse. if the preload on the ballscrew has all worn off, then it
will be in the several thousandth of an inch region.
If you need a really accurate and round bore, for a bearing or an engine
cylinder, you'll never get it on anything but a VERY high-end machine
tool in tip-top condition. When I do any sort of precision bore, I use
a boring head for the final finish. This WILL be round.
I have heard of people in the industry with $100,000+++ machines that
DO, indeed, mill precision bores in production. But, they have
specially tested these machines to be capable of maintaining that level
of precision and lack of backlash, and maintain them to keep them
performing at that level.
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