I doubt JS would use a four flute end mill to machine a perfect slot, it's
not what they are for. I suspect that a two flute cutter aka a slot drill
is probably what you meant.
Is it possible to comment on cutter deflection without knowing the feed
rate, cutter accel and depth of cut?
I don't think I said anything about the number of flutes, but 4 flute
instead of 2 is legitimate if like me your spindle rpm is limited to
in the sense that cutter deflection != zero, and 2 thou backlash is
easily exceeded by cutter deflection, yeah, it is.
in the sense that it is also easy to measure the amount of force
needed to overcome friction and move the tables around backlash (eg
circular pocketing) then you can work back from that force to material
removal rate and work back from that to mill dimensions and mix mill
dimensions with the force to get mill deflection
what gets me here is this whole attitude that just because the cnc
software works in hundreths of a micron bolting it on to a machine
tool is suddenly going to make that machine tool inherently more
accurate, making something fairly simply like a carb manifold accurate
to a thou in every direction is both un-necessary and a bloody sight
harder (and slower) than people assume, but just because the software
says it is so people assume it is so, and don't want to hear
If I had a quid for every time I've offered to take a made part to a
CMM and compare notes to what the gcode said it was making, for free,
hell I'll buy you beer and lunch if you like, and not one person has
ever taken me up on it, why not, what can they possibly be afraid of
from the output of a certified and calibrated CMM?... the ONLY thing
it can tell them is the actual dimensions of the object, ergo they
don't want to know, so why don't they want to know.
but then I'm just ranting aren't I, and those who have splashed out on
ballscrews have the technical high ground merely because they have
splashed out on ballscrews.
Like I said, if my CNC conversion ends up being good for a thou I will
be absolutely over the moon.
well that's your opinion and you're entitled to it, which is why I
suggested the other guy actually measure the force required to move
the table around backlash.
let's wait and see if he is the first example I have ever seen
(excepting linear rails and ballscrews) where the cutter can make
finishing cuts and overcome friction and stiction and move the table
around the backlash slack
Sorry to bring back controversy, but are linear rails much better than
slides? How much better? Bronze or linear ball-bearings?
-- Peter Fairbrother
(who may be overimproving his tiny lathe to the extent that it bends...)
I asked a similar question on the Mach3 forum, here's the reply (for info):
"This reply is contentious but IMO there is no sensible way to
compensate for backlash on a machine your size because the cutting
forces will drag the table around. The control system will be
hopelessly non-linear as the motor has to turn quite a few degrees
befor the table moves at all then it suddenly starts moving at the
commanded speed. Little Taigs etc. have enough friction that some users
get away with Mach's backlash compensation. But it can never be better
that a bodge.
Fit ballscrews or, probably better, sell the Senior and buy a machine
that already has them to retrofit with Mach."
This implies that stiction may be a problem too, have you done much
metal cutting yet to see how well it works in practice?
1/ grab a dial gauge and a spring scale and measure how much force it
takes to drag your table around the backlash.
2/ assume something fairly standard like a 6mm end mill and apply
those forces to it and see / measure what happens.
I could possibly move the table roughing or hogging, but if I'm
roughing or hogging I don't care about a couple of thou, if I'm
finishing and I care about cutter deflection amongst other things, I
can't put enough force into the work to move the table around, but
don't take my word for it, this is easy enough to measure.
what you're getting here is people who wanted ballscrews, who bought
ballscrews, and who don't want to hear anything else about anything
else.... you don't HAVE to go to the expense of a ballscrew to
eliminate backlash either, if it is that much of an issue, but like I
said, go and measure your table, numbers don't lie.
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