Re: What does Tool Radius Compensation do ?

Message: 11
Date: Mon, 08 Nov 1999 01:19:22 -0600
From: Jon Elson <jmelson@...>
Subject: Re: Re: What does Tool Radius Compensation do ?



Andrew Werby wrote:

> >
> > "TOOL RADIUS COMPENSATION "G" codes (G40, G41, G42)"
> >
> > and the Program by Doug Yeager "CNCPRO.EXE" does not have this. I
> [Actually, the reason I was searching in there was related- or might be
> related- to this tool-offset issue. I've been trying to make 360 degree 3d
> parts using a mill equipped with a rotary table, taking instances of a part
> and milling one "view" up to about half-way down, then indexing over 120
> degrees and milling another "view", etc. Since I'm zeroing the y and z axes
> to the axis of rotation, and am rotating the part representation along an
> axis established in the CAD geometry, it seems to me that everything should
> line up nicely, but instead I'm finding that the instances are offset from
> each other by a discontinuity that seems suspiciously like the radius of
> the 2mm cutting tool I'm using. Am I zeroing it wrong, by making the Y
> zero-point the middle of the tool and not one edge or another? Am I off in
> the Z-axis by zeroing to the tip of the ball-end tool and not the point
> where the ball diameter falls? Should I be invoking tool-offset
> compensation? (This would be bad, if so, because the MaxNC software I'm
> using gives a choice of this or unlimited file-sizes, which I need.) I
> haven't reground the tools, so I thought DeskProto should be giving the
> correct commands. I'm using a "spiral out from the center" milling
> strategy, where the tool cuts on each side of the part alternately. Is it
> assuming that I've got a mill that automatically compensates for tool
> offset? Any ideas out there?]

If it does not have the G41 or G42 code anywhere in the program, then it
does NOT assume the CNC is compensating for the tool.

[It doesn't, so is there a way DeskProto could be compensating in software,
independent of the milling program? Or is tool offset compensation
irrelevant to my problem?]

Well, I don't know how you are making the initial alignment of the tool
to the work. I also don't know what plane anything is in. It doesn't make
any sense to use a rotary table laying flat on the machine table, with the
axis of rotation parallel to the spindle. So, I'm guessing you have the
rotay table on its side, with the axis of rotation parallel to either X or Y.

[Right- it's parallel to the x-axis.]

I really don't see how you are doing this, anyway, as the 120 degree
rotation would not allow you to use an offset from one rotary setting
to the next, since that would not be parallel to any machine axis.

[Well, I'm still unclear as to how tool offsets function, but as it stands
I wasn't using any, as far as I know. The question was- should I be? The
next question: Is it possible?]

Even a 90 degree rotation would complicate things, as what was the
X axis would become the Z (assuming rotation parallel to Y, which would
make the rotary axis a 'B' axis).

[Actually, I wasn't changing the names of the axes, just rotating the part
geometry along its own axis (parallel to the x axis) and instancing it as
another part. DeskProto put in this facility with its recent upgrade to
version 2.0, and it was my hope that the various instances would line up,
since it seemed that the axis of rotation and the axis of construction
would be congruent if I zeroed y and z to the tailstock center. It almost
works...]

But, maybe that brings us to the problem, being that if ONE axis
is calibrated to the CENTER of the ball, ALL axes must be calibrated
to the same point, the center. If you are applying a 1 mm offset for the
cutter radius, when you turn the coordinate system, then the other axes
must also be offset from the same point.

Does this make sense, what I'm saying here?

[Except for the x axis, all axes are calibrated to the center line. It
didn't seem to matter where the x axis started, except that x=0 should be
toward the left, where the chuck is, in order for there to be room to cut
the part. There doesn't seem to be any discontinuity between the instances
in the x-dimension; the problem seems to be confined to the y-axis, or
perhaps the z as well.]

Your description of the setup and the way you've programmed the cutting
from the different directions is so vague, I can't analyze the problem any
better than this.

Jon

[Sorry- does any of the stuff I just wrote make things clearer? Basically
I'm carving 3 separate reliefs based on 3 different positions of the same
part offset 120 degrees from one another around a central axis parallel to
the x axis, and hoping I can get them all to merge into one fully round
part. It seems like this basically works- I just have this "silly little
millimeter" discrepancy to figure out.]

Andrew Werby


Andrew Werby - United Artworks
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