# Thread: 4th axis feed compensation

1. ## 4th axis feed compensation

Does anyone know whether the DRO associated with combined rotational feed rates actually expects the radius or diameter of the workpiece? The Tormach manual refers to it in section 8.1.2.2 as diametral, and this is how it's labelled on the "settings" screen, but then it's referred to as a radius in section 8.1.3.1, under the heading Scaling the "Width" of the Text, and indeed that is how it's labelled on the MDI screen.

The machsupport wiki refers to OEM DRO 825 as "Rot A diameter DRO".

So which is it?

2. Try both and measure the result.

Phil

Originally Posted by flick
So which is it?

3. Originally Posted by philbur
Try both and measure the result.

Phil

So I punched in 1/pi for the size, and made sure the scale was 1. I programmed an A move with a feedrate of 1 IPM. I was expecting to see a corrected feed rate of 360 or 180 (degrees/min). Instead it showed a feed rate of 340, and took a little over a minute for 1 complete revolution. So it's quite a bit closer to diametral than radial, but not exactly.

4. From Scaling the width in 8.1.3.1:

If your diameter is 1/pi then the radius is 1/(2 * pi). The radius correction is given as 57.3/R which is 57.3/(1/(2 * pi) which is 57.3 * 2*pi which is 360.

In the settings page it asks for Rotation Diameter so use diameter 1/pi. (I guess).

You may be thinking to much

Just my take on the words in the manual

Phil

PS: I wrestled with the same paragraphs several years ago.

5. I believe it's radius, and the screen used to be wrong. Newer versions of Mach3 have the DRO's labeled Rotation Radius on the Settings page.

The value you enter should be the distance between Z zero and the center of rotation. If Z zero is the center of rotation, than enter .001, as entering 0 will turn the feature off. This has been fixed in a recent version, I think.

6. Originally Posted by ger21
I believe it's radius, and the screen used to be wrong. Newer versions of Mach3 have the DRO's labeled Rotation Radius on the Settings page.

The value you enter should be the distance between Z zero and the center of rotation. If Z zero is the center of rotation, than enter .001, as entering 0 will turn the feature off. This has been fixed in a recent version, I think.
Are you saying that it compensates automatically depending on your current Z position?

7. Z zero is the top of your work so it's just another way of inputing the radius of rotation as Z zero to the center of rotation will be the radius!

Flick, as I am also interested in learning did my point about 57.3/R in post #4work out or was the solution somewhere/something else.

Phil

Originally Posted by flick
Are you saying that it compensates automatically depending on your current Z position?

8. Originally Posted by philbur
Z zero is the top of your work so it's just another way of inputing the radius of rotation as Z zero to the center of rotation will be the radius!

Flick, as I am also interested in learning did my point about 57.3/R in post #4work out or was the solution somewhere/something else.

Phil
OK, I got it figured out, and this version of Mach3 appears to be faulty.

It does indeed appear to compensate automatically for your Z position by combining it with the compensation factor in DRO 825 but it treats them both as diameters.

Of course it might be reasonable to treat DRO 825 as a diameter, but certainly Z position should be treated as a radius.

As it stands the only way to make the function work correctly would be to write g-code with the machining at Z-zero, and use diameter values in DRO 825.

EDIT: for the record, it's version 3.42.29, came installed on a control purchased last spring.

9. Here's the test code snippets that I used to make my determination:

(#1)
g00 g90
a0 z0
g01 a360 f1.0
.
.
.
(#2)
g00 g90
a0 z0.318
g01 a360 f1.0

I ran each of these code snippets with DRO 825 set to 0.318. The resulting feedrates were 360 and 180 respectively.