# Thread: Cutting an arc using a fourth axis

1. ## Cutting an arc using a fourth axis

Imagine an insert mounted vertically in between centers on a fourth axis. I have a grinding wheel oriented like a mill. I'm oscillating the z axis up and down while I rotate the insert back and forth. I have cadded the points, and I should end up with a radius, but I can't get what I want. I usually get two flats or a very large radius. I'm pretty sure that I need to adjust the feed on the fourth axis, but the only setting that will let me do that is the diameter setting in the control (which doesn't seem to help). It is a mini-mill with a collet style fourth axis. I'm just not sure what I'm missing...

2. what are you trying to achieve?
Is the part in the milling spindle or the A axis spindle?

Okay here is another guess, that looks like a square insert, are you grinding the square to a different profile( Radius ),
you will need to use inverse time feed rates. (not the diameter setting)

3. I'm trying to regrind the current radius, and the part is in the A axis. You are correct, the insert is square, but I'm working on the radius, oscillating the z axis up and down to the cad points while I "roll" the a-axis. Inverse time feed rates are new to me, I will start looking into that. Let me know if this makes sense.

GP

4. OK...I've done a little homework on this..Here is the question: if you use inverse time feed, how can I do that on the same line of code, since I need A & Z moving together?

G1 G93 Z.393 A-70.0 F10.

5. ## inverse time

Embarassing to say but the correct format I have forgotten,
I have let my cam do It for me now for so long I have let It slip from my memory. I understand the purpose, as it allows you to correct the feedrate for multi-axis movement. The feed value is the inverse of the time it will take for both axis(s) to arrive at the same point(programmed point). I have a good text file write up on inverse time, expaining It's function and purpose, also how to correctly calculate the actual feed rate (for chip load per tooth). I'll try to find it and post it here. Now I too am on a mission to re-learn the correct format. I believe you can test it's function with a single axis movement as well.
If you program in MDI

(START AT X0. Y0.)
G01 X1. F1.

this will take 1 minute to travel 1 inch

To verify the feed in inverse time you can,

(START AT X0. Y0.)
G01 G93 X1. F1.

I believe this will take 10 minutes, as the value is in time,
whether it is minutes or seconds I can't remember. Try F60. to see if it is seconds, Or F10. to see if it is in tenths of a minute.

I'll just try to find that paper, as it is all explained verry well.

6. Okay, I found a paper copy of the explanaion I recieved. (I need to work on my organization skills)

In this example we will move 3" at 10IPM so it will take 18seconds or .3 minutes. (You can use a stopwatch to verify)

the formula for inverse time is

1/(length/feedrate)

this means...
1/ (3/10)
1/.3
F=3.33

1. G0X0Y0
2. G1X3.0G93F3.3

when you add an A axis move to this the machine will break down each axis separately and make sure each axis finishes in the time allowed.

In typical simultaneous 4 and 5 axis motion the cam will break down the moves to about .003" long. In this case at 10IPM, the move would take .0003 minutes. Since most controls don't like the tiny numbers, they came up with using the "Inverse" of the time. So 1/.0003=F3333, It is common to see Inverse time feeds in the range of F100-F10000. Some controls have a setting for max inverse feed rate you will have to check yours.

P.S. remember to program a G94 to return your feedrates to IPM!!!

hope this helps,
As far as the cad points you have calculated for the radius "interpolation" , how small are your moves? How critical is the radius form?