# Thread: G96 ... How does it work??

1. ## G96 ... How does it work??

Is there a formula to calculate the real Speed desired on certain X coordinate when using G96???

Let's suppose I'm facing between Diam 6.0" and 4.5" and want to have a real speed of S305 at 6.0" and S405 at 4.5" ... How should these blocks look like (fill in tha blanks)?
G28 U0. W0.
G0 T0101
G54
G50 S????
G96 S???? M03
G0 X6.0
Z.1
G1 X4.5 F.004
ETC
ETC

2. Originally Posted by david rendon
Is there a formula to calculate the real Speed desired on certain X coordinate when using G96???

Let's suppose I'm facing between Diam 6.0" and 4.5" and want to have a real speed of S305 at 6.0" and S405 at 4.5" ... How should these blocks look like (fill in tha blanks)?
G28 U0. W0.
G0 T0101
G54
G50 S????
G96 S???? M03
G0 X6.0
Z.1
G1 X4.5 F.004
ETC
ETC

An S value specified with the G50 address clamps the maximum speed the spindle can achieve irrespective of the surface speed applied with G96 command. This maximum speed is influenced by a number of conditions, but the major consideration would be the balance of the workpiece and any fixturing that may be involved. Accordingly, the G50 S value can be quite variable, but rule of thumb is to apply a clamping speed that is as high as possible.

At a surface speed of say 650 ft/min the rpm at a diameter of 4.5" would be close to 552. Accordingly any clamping speed above that rpm would result in the commanded surface speed being maintained through the whole dimension range form 6.0" to 4.5".

To calculate RPM given the surface speed:
RPM = CS x 12 /Pi/D
where:
CS = Cutting speed in ft/min
Pi = mathematical constant approximately = 22/7
D = work or cutting tool diameter in inches

To calculate surface speed in ft/min given RPM and work or cutting tool diameter.
CS = (RPM x D x Pi)/12

The same references apply as for calculating RPM.

G28 U0. W0.
G0 T0101
G54
G50 S3000
G96 S478 M03
G0 X6.0
Z.1
G1 X4.5 F.004
ETC
ETC

Regards,

Bill

What values did you input to the formula to get S478?

4. Originally Posted by david rendon
What values did you input to the formula to get S478?
305 and 6.0" = approx 479
405 and 4.5 = approx 477
478 is average of the two

Regards,

Bill

5. cool.
How did you get the SFM ?

6. Originally Posted by david rendon
cool.
How did you get the SFM ?
1. SFM = (RPM x D x Pi)/12
2. Plug in your known values
RPM = 305
D = 6.0
Pi = 3.14159265358979
Therefore
305 x 6.0 x Pi = 5749.11455606932 Surface Inches per Min
SFM = 5749.11455606932 /12
SFM = 479.092879672443
SFM = 479 rounded

Regards,

Bill

7. Sorry for being such a kid that won't stop asking ...

So in other words .... in the G96 SXXXX goes the SFM desired value to mantain?? (in this case G96 S479?)

8. Originally Posted by david rendon
Sorry for being such a kid that won't stop asking ...

So in other words .... in the G96 SXXXX goes the SFM desired value to mantain?? (in this case G96 S479?)
Yes. See the example in my original reply filling in the spaces.

If a drill were being used, and therefore operating at X0.0, the RPM would be calculated based on the appropriate SFM for the work piece and cutting tool material. The RPM will then be used in conjunction with the address G97 (constant RPM) rather than G96. In this case the formula RPM = SFM x 12 /Pi/D would be used. This formula can be simplified as follows to obtain a close approximation.

RPM = SFM x 3.8 / D

3.8 becoming a constant

Regards,

Bill

9. It is generally better to first bring the tool near the workpiece before invoking CSS. If CSS is invoked at home position, there would be sudden and large change in rpm when the tool is placed near the job with G00. This causes additional load on spindle (because of inertia), which can be avoided.

I read it somewhere. I do not know how significant the effect is in practical cases.

10. When using CSS, I start the spindle at a target RPM, then rapid X to the starting locatiion, and then command the CSS and SFM.

At the end of each tool, I command G97 before retracting the turret to avoid having the spindle slow way down. If the next tool uses CSS, I command the G97 with the target RPM for the next tool, and if the next tool uses RPM, I command the RPM.

G28 U0. W0.
G0 T0101
G54
G50 S3000
G97 S305 M03 (START SPINDLE AT TARGET RPM)
G0 X6.0
G96 S479 (CSS ON AT 479 SFM)
Z.1
G1 X4.5 F.004
...
...
G97 S1500 (NEXT TOOL TARGET RPM)
G28 U0. W0.
M01