Re: Cutter Radius Compensation

[Fred Proctor <proctor-at-cme.nist.gov>]

EMC Users,

Frank Hess wrote about cutter compensation. Excerpts from his mail
follow, then Tom Kramer's solution.

Frank:

> I am having a problem trying to use cutter radius compensation with
> circular interpolation.
> 
> We have a Bridgeport milling machine running the emc controller.
> 
> I want to use a 1/2" end mill to "part" a slug from a plate and
> produce a 2" dia. hole.
> 
> I have included a portion of the code below.  Can anyone tell me what
> I am doing wrong?
> 
> I am pretty sure that the problem is in the approach moves.  Depending
> on how I approach it I get different error messages when it reads the
> G3 block.
> 
> Assume that tool 2 is loaded in the spindle and that a .5" dia is
> loaded in the tool table. Center of hole is at X0Y0.
> 
> N0010 G53 G0 Z0
> N0020 X-1 Y1 (load position)
> N0030 G43 H2 (activate tool length comp)
> N0040 G1 G41 X0 Y0 F25 S1000 M4 D2
> N0050 G1 X1 Y0 F25
> N0070 G1 Z0 F5 (position Z axis)
> N0080 G3 X-1 R1 Z-.05 F3
> N0090 X1 R1 Z-.1 F3
> N0100 X-1 R1 Z-.15 F3
> N0110 X1 R1 Z-.2 F3
> N0120 X-1 R1 Z-.25 F3
> N0130 X1 R1 Z-.3 F3
> N0140 X-1 R1 Z-.35 F3
> N0150 X1 R1 F3
> N0160 X-1 R1 F3
> N0170 X1 R1 F3
> N0200 G40
> N0210 G0 Z3

Tom:

If you replace the following two lines with the three lines shown,
it should work (I checked it on my system).

OLD:
N0040 G1 G41 X0 Y0 F25 S1000 M4 D2
N0050 G1 X1 Y0 F25

NEW:
N0035 G0 X-1 Y-1 (start point for first entry move)
N0040 G1 G41 X0 Y-1 F25 S1000 M4 D2  (first entry move)
N0050 G3 X1 Y0 R1 F25 (second entry move)

Fred Proctor asked me to look at this for you.

In general, to start cutter comp on arc A:
1. Pick any arc B tangent to arc A at the start point of A.
   The radius of arc B must be larger than the tool radius.
   Arc B is the second entry move.
2. Pick any straight line C tangent to arc B at the other end from arc
A.
   Line C should be longer than the tool diameter.
   Line C is the first entry move.
3. Write three lines of NC code:
     i) move to the end of Line C farthest from from arc B.
    ii) turn on cutter radius comp and make the first entry move to the
        other end of Line C (this is also an end of arc B).
   iii) make the second entry move to the other end of arc B.

A lot of details are provided in NISTIR 5754, "The NIST RS274/VGER
Interpreter". It may be found in the publications section of my web
site http://www.isd.mel.nist.gov/personnel/kramer/publications.html
Or I can mail you a paper copy if you like.

An example of the method described above is shown in Figure 11 on
page 49.

Tom Kramer
kramer-at-cme.nist.gov