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Choosing between inverse time G93 and feed per minute G94


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That is extremely deep and complex conversation. Do a search for these topics on the forum in the last 18 years and see what comes up. Then go search 4 or 5 other forums about this topic and then we can go from there.

There are well over 100 brands of machines that can use these. In that depending on the builder can be 10 to 300 different models of machines that have it. Then add material, tooling, work holding and 20 other factors to that conversation and we start getting into the middle of where the conversation can go. Now think about all the different controls out there and how they did and currently support things. It gets really deep really quick. Narrow it down to a specific machine, control, holders and material and what options it has then it might get to a start.

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Inverse time is the old school way of defining feedrates when applied to rotary motion. You can't really use IPM (inches per minute) on a rotary motion. And when you add linear motion to a rotary motion you really can't use degree's per minute to define the feedrate.

So you are left with time. Basically you are telling the machine controller how much time to take between moves, from point A to point B. The moves are typically very small so 1/time is used, and that is why it is called inverse.

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On 10/12/2021 at 8:58 AM, tilikoom said:

@cncappsjames thanks.
 

If anyone else can share an example of when/why G93 inverse time feed was used, it would be highly appreciated. 

Thank you

G93 was the "original" method of synchronizing "linear motion" with "rotary motion".

You XYZ axes move in linear motion, with respect to time. "Inches per minute" or "millimeters per minute".

Your ABC rotary axes move using different units entirely. "Degrees per minute". (Same units, in Inch or Metric mode).

"Inverse Time", means that we specify "the inverse of the amount of time it takes to complete each move.

When using G93, every line of G-Code must have a unique F value, that tells the machine "how long it takes to complete the move".

Because we specify the "feed rates" on our machines in terms of "1 minute", that becomes the Numerator of the equation. (1 / x) The Denominator is the "Time" component, which is simply the 3D Distance divided by the velocity.

1 / Time, where [Time = 3D Distance / Velocity]

For example, in Inch units, say we are making a 4" long move. We want that movement to happen at 25 IPM. 

4 / 25 = 0.16, so that is our "Time" component of the equation.

1 / 0.16 = F6.25

Now, we can vary either the "3D Distance" or we can vary "the velocity" component. (How fast do we want to complete the move?) If we programmed the same 4" cut, at 210 IPM, then we have

4 / 210 = 0.0190476, and our equation is:

1 / 0.0190476 = F52.5

What if we changed the "amount of distance"? Let's change that same equation from a 4" move, to a 0.020" Move. With the same "25 IPM" velocity:

0.02 / 25 = 0.0008

1 / 0.0008 = F1250.

Now, if we want 120 IPM velocity:

0.02 / 120 = 0.00016666

1 / 0.000166666 = F6000.

One thing you should notice based on the equation above, higher "F" values equal "shorter or quicker" moves. (Remember, it is a combination of distance divided by velocity)

Lower "F" values indicate "slower or longer" moves.

It is not uncommon to see "very low" or "very high" Inverse Time Feed Values.

You should be careful to check and be certain that you aren't hitting the "Inverse Time Feed Limit". This is typically "F9999.999" in many Post Processors, when some machines are actually equipped with larger Inverse Time F Registers. (Newer Haas Machines, for example, use F40000. as the max.)

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On 10/12/2021 at 7:18 AM, MIL-TFP-41 said:

Inverse time is the old school way of defining feedrates when applied to rotary motion. You can't really use IPM (inches per minute) on a rotary motion. And when you add linear motion to a rotary motion you really can't use degree's per minute to define the feedrate.

So you are left with time. Basically you are telling the machine controller how much time to take between moves, from point A to point B. The moves are typically very small so 1/time is used, and that is why it is called inverse.

And back in the earlier days, the controls did not have as much processing power in order to make the UPM-DPM time calculations necessary to just input a single feed command.

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On 10/18/2021 at 5:49 PM, Leon82 said:

our Siemens and yasnak Matsuuras use inverse time to tap

I know on our Yasnac I-80 & J-300 controls it calls out G93 for IPR feed (or what the books calls "Solid Tap Mode")...there is no G95 code

Our 840Di's are all close to what Fanuc is for tapping:

G0 G17 G90 G54 X0. Y0.
(OP 1)
G43 H1 Z1.
G95
G98 G84 Z-1. R.05 F.0769 S260
G80

Note no need for the M29 S260 or M80 S260

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39 minutes ago, MIL-TFP-41 said:

I know on our Yasnac I-80 & J-300 controls it calls out G93 for IPR feed (or what the books calls "Solid Tap Mode")...there is no G95 code

Our 840Di's are all close to what Fanuc is for tapping:

G0 G17 G90 G54 X0. Y0.
(OP 1)
G43 H1 Z1.
G95
G98 G84 Z-1. R.05 F.0769 S260
G80

Note no need for the M29 S260 or M80 S260

I know there are fanuc and yasnac mode for the 840. They are in yasnac mode I believe

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