Re: Digest Number 246
Posted by
Andrew Werby
on 1999-11-12 03:07:02 UTC
Message: 9
Date: Thu, 11 Nov 1999 14:17:50 -0600
From: Jon Elson <jmelson@...>
Subject: Re: Re: What does Tool Radius Compensation do ?
Andrew Werby wrote:
is to mount a hardened and ground rod (maybe 10 mm or so) in the chuck,
and then mount a dial indicator on the spindle, and wipe the dial indicator
along the rod by moving the X axis. Adjust the position of the rotary table's
mounting to the table until the dial indicator doesn't move as you move X
back and forth.
[This is the very thing I need to do- thanks for the suggestion. I ran
another part yesterday, and while zeroing the x and y to the point of a
pencil-sharpened rod held in the chuck did help, I noticed a distinct
increase in my error the farther from the chuck it was. So it seems my
set-up is not quite square to the direction of x travel. Assuming the chuck
holds the rod squarely, the method outlined above should do the trick.]
If the rotary table has a method for tilting it so the rod would
not be parallel to the table's surface, sweep along the top of the rod and
adjust
until that is parallel, too.
[It doesn't have a tilting mechanism, if that's what you mean, but I
suppose it could be slightly off- I'll check.]
Finally, set the Y offset. With a 5mm cutter in the
spindle, say, and a 10 mm rod in the rotary table's chuck, you bring them
together until a thin piece of paper gets stuck between the two. Say the
cutter is in front of the rod. You would then set the Y axis to
Y = - (2.5+5+.1)mm (2.5mm for the radius of the cutter, 5 mm for radius
of 10mm rod, and .1mm for thickness of paper). The command would
be something like G92 Y7.6 in MDI, or whatever your CNC uses.
Raise the tool, and then go to Y=0 and see if the tool looks centered over
the rod. If you work out the trig, you will see that even small errors will
cause pretty large gaps under some circumstances.
Jon
[I was coming to the same conclusion myself- that there is a multiplier at
work exaggerating the small original error I started out with. I was going
with the "eyeball" method for zeroing, and while that usually works for me,
I can tell it isn't up to this task. Zeroing points to points helped, but
I'll give the paper method a try- I think I'm getting closer to making this
work. Does anybody else machine parts this way (superimposing several
contoured toolpaths), or do people using more sophisticated programs
approach the problem of milling fully-round irregular parts some other way?]
Andrew Werby
Andrew Werby - United Artworks
Sculpture, Jewelry, and Other Art Stuff
http://unitedartworks.com
Date: Thu, 11 Nov 1999 14:17:50 -0600
From: Jon Elson <jmelson@...>
Subject: Re: Re: What does Tool Radius Compensation do ?
Andrew Werby wrote:
> [Yes- "exactly" and "accurately" are words I've never been too comfortableOh oh! For this to work, exactly needs to be taken seriously. What you need
> with (I'm an artist, right?) , but I suppose I could get closer.. But I
> have been trying to zero to the rotary (A) axis line- if this is going to
> work, that would have to be the axis of symmetry. Thanks for your help with
> this, Jon- I'll let you know how it goes.]
is to mount a hardened and ground rod (maybe 10 mm or so) in the chuck,
and then mount a dial indicator on the spindle, and wipe the dial indicator
along the rod by moving the X axis. Adjust the position of the rotary table's
mounting to the table until the dial indicator doesn't move as you move X
back and forth.
[This is the very thing I need to do- thanks for the suggestion. I ran
another part yesterday, and while zeroing the x and y to the point of a
pencil-sharpened rod held in the chuck did help, I noticed a distinct
increase in my error the farther from the chuck it was. So it seems my
set-up is not quite square to the direction of x travel. Assuming the chuck
holds the rod squarely, the method outlined above should do the trick.]
If the rotary table has a method for tilting it so the rod would
not be parallel to the table's surface, sweep along the top of the rod and
adjust
until that is parallel, too.
[It doesn't have a tilting mechanism, if that's what you mean, but I
suppose it could be slightly off- I'll check.]
Finally, set the Y offset. With a 5mm cutter in the
spindle, say, and a 10 mm rod in the rotary table's chuck, you bring them
together until a thin piece of paper gets stuck between the two. Say the
cutter is in front of the rod. You would then set the Y axis to
Y = - (2.5+5+.1)mm (2.5mm for the radius of the cutter, 5 mm for radius
of 10mm rod, and .1mm for thickness of paper). The command would
be something like G92 Y7.6 in MDI, or whatever your CNC uses.
Raise the tool, and then go to Y=0 and see if the tool looks centered over
the rod. If you work out the trig, you will see that even small errors will
cause pretty large gaps under some circumstances.
Jon
[I was coming to the same conclusion myself- that there is a multiplier at
work exaggerating the small original error I started out with. I was going
with the "eyeball" method for zeroing, and while that usually works for me,
I can tell it isn't up to this task. Zeroing points to points helped, but
I'll give the paper method a try- I think I'm getting closer to making this
work. Does anybody else machine parts this way (superimposing several
contoured toolpaths), or do people using more sophisticated programs
approach the problem of milling fully-round irregular parts some other way?]
Andrew Werby
Andrew Werby - United Artworks
Sculpture, Jewelry, and Other Art Stuff
http://unitedartworks.com