Re: 4th axis rotary with a 3 axis control

Message: 12
Date: Wed, 06 Sep 2000 10:28:17 -0400
From: Ron Ginger <ginger@...>
Subject: Re: Re:4th axis rotary with a 3 axis control

Ron Said:

>Are there some rules about how to start and stop the cutter comp? It
>seemed like I had to make an offset move first to get away form the part
>edge, then turned on cutter comp and it ran around a square OK.

>Suppose I had a simple square like this:
>G00 X0 Y0
>G01 X1
>G01 Y1
>G01 X0
>G01 Y0

>That I wanted to do with a 1/2" mill, around the outside, leaving a
>finished 1" square. What gcodes should I add to make this?

The teardrop approach and depart form is very useful for enabling cutter comp.

Most industrial controllers require an initial straight move at least as long as the cutter radius, some as long as the diameter. This line is a G01 only, arcs are NOT allowed and usually the G41 or G42 is on the same line (except A-B which as in most things is messed up)

The tear drop shape starts with the straight line then has a tangent arc to the first entity of the contour to be cut. As the cutter traverses the first line of the cut it moves from on center to beside the line, the controller then processes the geometry of the cut so that the material remaining after the cut is the desired size, rather than the center of the cutter plus the radius. As the cutter finishes the contour, it moves can overlap the starting cut and then arc away in another tangent arc, followed by the final straight line during which Cutter comp is cancelled with a G40.

Vector can draw these teardrop approaches and departs on the inside or outside of any closed shape, with a settable amount of overlap. It can also create approaches and departs between a point and a line or arc, and can make them go to the center by breaking the line or go to each end. By naming the contour with the A/D as G41 or G42 , you can cause it to automatically generate the G41/42 on the first line and the G40 on the last line.

The most often asked question about cutter comp (after how) is why. If you look at a program that is written with cutter comp, all the coordinate values are the finished part sizes. There is no need to offset for tool radius. It makes it very easy to read the program and understand what is happening. it also makes it much easier to manually write or adjust a program. The biggest advantage is that with one constraint, you can use any size tool that you want in the program and it will make the part correctly. You just have to be sure to change the tool radius or diameter offset value to be the same as the tool you use. When you do this you have an opportunity to actually "Tune" the size that will be cut. For example if your 1/4 inch cutter runs out a little and actually cuts .001 oversize, you could tell the controller that the tool was .251 and the controller would move the tool as if it were .251, resulting in a cut EXACTLY to size. If your cutter gets dull and starts to cut undersize, you tell the control that it is actually undersize & the cut stays on size.

The caveat of cutter comp is that you CANNOT cut an internal corner that has a radius that is smaller than your cutter. This is not allowed and on poorly designed controls results in nicked corners, and on most, results in a program interrupt with a cryptic message: #109-CRC error (translated, you can't use Cutter Radius Comp to cut a square internal corner)

Best Regards, Fred Smith- IMService
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