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Putting
a Value on Cutting Speed
The cutting speed of any material is based on the speed of the
material passing over the cutting tool.
This speed is measured as SFPM – surface feet per minute.
The measurement is based on the circumference size of the part
or cutter. The
circumference of a circle is the distance around the periphery.
With milling, this is the peripheral speed of the cutter
acting on a stationary workpiece.
With turning, this is the peripheral speed of the workpiece
material acting on a stationary cutting tool.
The speed that the surface of the part or cutter moves each
minute is measured in feet, not inches.
Constant surface speed (CSS) is applied to cutting tools to
provide the correct machining conditions.
The use of a constant surface speed (sfpm) results in a
spindle speed being relative to the part diameter (turning) or to
the cutter diameter in milling.
The correct sfpm values for machining are available for each
particular material type. A
first-class machinist must develop a knowledge of the various sfpm
values for each material type. There
are many variables in the choice of the correct sfpm.
Here are factors to consider:
material type, rigidity of the machine, material hardness,
type of cutting tool material, coolant type.
Programming considerations:
One of the major benefits of using a CNC lathe is the G96
command that engages the constant surface speed mode.
The formulas for the correct rpm for a specific sfpm are easy
to calculate:
Rpm =
sfpm
x
3.82
part diameter or cutter diameter
Sfpm = rpm x part diameter or cutter diameter
3.82
Rpm =
sfpm
part diameter or
cutter diameter x 0.262
Sfpm = rpm x part diameter or cutter diameter x 0.262
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A
word of caution regarding G50/G92 code:
Do not exceed speed.
When you are facing to the centerline of the part, the
spindle of a CNC lathe will attain the maximum possible RPM
available on that machine. There
is a standard G code that must be programmed to prevent accidents.
The standard G50/G92 code must be set at a specific speed
prior to the control reading the G96 command for css.
If you are facing on a machine with a 5000-rpm spindle, the
machine will attain that maximum rpm unless you specify a safer,
lower spindle speed.
This must be considered prior to running the program.
We must limit the machine’s maximum speed with a sensible
G50/G92 command prior to running the part.
There is a danger any time a large-diameter part is run or
there are any compromises in the workholding.
Excessive rpm can overcome the chucking force, resulting in
the part flying out of the chuck.
Always consider the centrifugal force.
Set the G50/G92 to a safe maximum speed.
When facing to centerline, you will produce a dull finish at
the centerpart of the face. This
is caused by the css reducing as the tool approaches centerline.
You can calculate the diameter that the machine will attain at
the maximum rpm. Once
that diameter is calculated, you can reduce the facing feedrate in
an attempt to improve the surface finish.
Use this formula:
Diameter at maximum rpm =
sfpm x 3.82
Machine maximum rpm
Here’s an example: face
an 8” diameter part to centerline at 600 sfpm.
A g50/G92 command of 2000 has been programmed.
Starting rpm at 600 sfpm = 600 x 3.82
= 279 rpm
8.2
Diameter at maximum rpm = 600 x 3.82
= 1.146” diameter
2000
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