G96 on a milling machine

[sinha_nsit]

Can somebody please explain the exact meaning of G96 (constant surface speed) on a milling machine. Is it the linear velocity at the periphery of a milling cutter? If yes, then the rpm for a given tool will always remain fixed. It will change only when a tool with a different dia is used. rpm does not depend on workpiece geometry and/or machining method. Are the statements correct?

[Geof]

As far as I know G96 is not used on mills, it really has no application because CSS is only applicable when the cutting radius constantly changes as occurs during facing on the lathe or when taking sequential passes to turn down a diameter. This never happens on a mill, the cutting radius is fixed by the tool size.

[Al_The_Man]

I have a listing that show the normal G codes available for different machines.
The list the Machining (mill) centers as G96 Constant Surface speed Control.
Here is the Fanuc Mill operator manual for G96 description, I am still not sure how it is applied though?
Al.

[Geof]

Originally Posted by Al_The_Man ...Here is the Fanuc Mill operator manual for G96 description, I am still not sure how it is applied though? Al.

I read it and think it seems a bit strange, the description is written in the context of lathe operation not mill.

[Al_The_Man]

That is what I thought at first.
I have seen some applications, one here in fact a while ago, that showed a mill being used with multi fixed tooling and the spindle holding the material, I seem to recall that about the only advantage was no tool changes?
Probably a rare use in any event.
Al.

[Boots]

G96 is used on Mills doing a Taper Pipe Tap threading operation with a single point threading tool. Because the tool whips around in a circle on X and Y and goes down in the Z all at the same time making the hole smaller as it goes G96 is required to maintain proper threading control and orientation of the tool so that subsequent passes can be made on the same cut to inlarge it. As I recall.

[Geof]

Originally Posted by Boots G96 is used on Mills doing a Taper Pipe Tap threading operation with a single point threading tool. Because the tool whips around in a circle on X and Y and goes down in the Z all at the same time making the hole smaller as it goes G96 is required to maintain proper threading control and orientation of the tool so that subsequent passes can be made on the same cut to inlarge it. As I recall.

Huh????

How is the tool made to rotate at a smaller radius?

I have a feeling your perception of threading on a mill is not entirely correct; the tool rotates to perform the cutting but the thread is cut by a combination of X Y and Z motion. The X and Y interpolate a hole of increasing or decreasing radius while the Z moves up or down.

[Boots]

The tool always rotates at the same dia. because that is fixed. I just stated that the hole gets smaller as it goes deeper so it can create a tapered thread. Yes the X and Y numbers constantly change due to the tapered thread. So in order to maintain the same surface footage you would have to use G96 because it is a timed event as well as G41 or G42 Cutter Comp. for right or left hand thread. This is needed to enlarge the hole to accept the proper thread guage. Also I might add that Spindle orientation is required for this to happen, as you have to have a starting and ending point. Otherwise going back into the same hole would not be possible.

[Geof]

Originally Posted by Boots The tool always rotates at the same dia. because that is fixed.... ... So in order to maintain the same surface footage you would have to use G96 because it is a timed event Also I might add that Spindle orientation is required for this to happen, as you have to have a starting and ending point. Otherwise going back into the same hole would not be possible.

Yes the tool dia is fixed.

The surface speed at the tool radius has absolutely no relationship to the changes in X and Y, only to the spindle speed.

It is not a timed event; it is an X Y Z position related event.

Spindle orientation is irrelevant becasue the spindle simply spins like it does for any milling operation; thread milling is in no way similar to thread cutting on the lathe.

Have you ever programmed a mill to do thread milling straight or taper?

[sinha_nsit]

Note 4 of the attached (by Al) G96 page states:

"The constant surface speed control mode (G96) is allowable in G94 (feed per minute)."

This statement needs to be interpreted. I thought G96 has nothing to do with feedrate.

The concept of CSS (constant surface speed) appears to be different on lathe and milling machine. (?)

Certainly, Fanuc manuals leave enough scope for research work. And, I am sure, if you ask some Fanuc engineer to explain such things, he may not have an answer. They are good at maintenance, but do not possess good knowledge of part programming. At least this has been my observation, based on my limited experience.

Thread milling
I believe, thread milling is done using helical interpolation, in a single pass, with a disc-shaped form milling cutter.
rpm is high and has no relation with the thread pitch.
The diameter of the disc is smaller than the hole dia (say, one-fourth of it). So, it is possible to increase or decrease the cutting diameter, as the tool goes down, using a suitable macro. Helical interpolation always uses a fixed radius.
Multi-teethed cutters also can be used. These make several threads in one rotation of 360 degree.

[Geof]

Originally Posted by sinha_nsit ....."The constant surface speed control mode (G96) is allowable in G94 (feed per minute)." This statement needs to be interpreted. I thought G96 has nothing to do with feedrate....

You are correct CSS has nothing to do with feedrate. The statement says allowable which just says it does not matter which feedrate is being used IPR or IPM with G96.

[Boots]

On an Omni Mill 4 Axes Horizontal Machine I did a few parts that required a 2 1/2 in. NPT hole. We did this using a threading Insert that had pitch (It was not a single point threading tool) and a special bore bar for holding that type of insert. The depth of the thread in the side wall of the hole is controled by Cutter Comp. Orientation of the spindle is required. By timed event I ment that mearly the starting point and ending point will remain the same, (X Y and Z arrive at the same time) Feed hold is disabled as using it will result in an undesirable thread. The program readout for this was over 600 pages just for the threading portion. So figuring it out long hand, ta, forget that. Programming with software is the only way. That was back in 1988 but I'm sure the method hasn't changed.