G96 Constant Surface Speed and G50 Speed Clamp
OBJECTIVES
After completing this unit, you should be able to:
- Define G96 Constant Surface Speed (CSS)
- Describe the purpose of G96 CSS
- Identify and Describe G50
- Describe nub removal
Definition of Constant Surface Speed (CSS)
CSS Basic Concept
- The RPM increases as the diameter decreases on the workpiece.
- The feed rate remains constant.
Ideally, the tool cuts at a Constant feed rate that optimizes the cut/finish. When the diameter changes, in order to maintain a constant feed rate, the RPM needs to change.
For example:
If CSS is not used on a facing cut, the finish quality will vary significantly from outside to inside. Using CSS allows the finish quality to remain consistent on the face.
Read this really excellent article
G96 G-Code: Constant Surface Speed CNC Programming by CNC Cookbook.
G96 Constant Surface Speed
G96 controls the speed of the workpiece spindle on a CNC lathe. It appears in the CNC lathe headers but only sometimes.
Two types of lathe headers to choose:
- G96 program with CSS. As the X gets smaller, the RPM increases to maintain a constant speed while cutting away the diameter.
Why would we not want CSS?
2. CSS is not necessary for tools that do not change the diameter of the part. For example, drilling operations, and other operators that start at X0. Operator wants control of RPM.
So, when do we use G96 for CSS? Depends on the lathe operation.
Watch this 7:25 video by Tom Stikkleman: CNC LATHE PROGRAMMING LESSON 7- HOW TO USE CONSTANT SURFACE SPEED
G50 Speed Clamp Terminology and Purpose
What do you know about the word “clamp?”
A mechanical definition relates to a fastener, which is a rigid devise, that holds things together.
For example: Clamps are used in the shop to secure workpieces to work benches. Parts can be clamped to the machine tool table.
Notice the term, to clamp down on something. This phrase means that the clamp is becoming tighter so greater control can occur. This definition is closer to the meaning for G50 Speed Clamp.
G50 Speed Clamp
The G50 is a Speed Clamp for Constant Surface Speed (CSS). The G50 value limits the RPM of the CSS.
There are two reasons why we would want to control CSS.
1.) The G96 CSS command will, theoretically, rev the motor to spin to infinity and beyond.
2.) The practical problem with that is that the chuck or other workholding device may not have the holding power of the lathe motor. The CNC lathe motor may top out at 12,000 RPM. The chuck may only be rated to hold a workpiece up to 4500 RPM. The rating for holding is stamped onto workholding devices.
Collets are better than 3-jaw chucks. The can spin and hold as fast as the machine’s maximum speed.
Depending on the CNC machine tool, G50 may clamp the RPM because of the age or condition of the motor.
G50 defines the maximum RPM the operation will use. It controls runaway CSS speeds, and to ensure the capacity of the workholding device.
No one wants their part getting thrown out of the chuck because the RPM was too high.
Header Examples:
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G28 [G28 controls both x and z away from chuck to machine home] T0101 (first 01 is tool change number) (second 01 is length offset no H01 on lathe) (55 degree Finish tool—ID tool) G50 (Maximum RPM) S4000 (We want to specify max rpm) G97 S500 M03 (G97 = cancel CSS command—starting RPM value, give it somewhere to start) G54 G00 X (stock size—pic. Add .200 to dia) e.g. 1.45 Z1.0 M08 G96 S375 (G96 CSS on)
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Header example with G96 and G50 |
Yes, you read correctly. There are three speeds in the header.
G50 Speed is the maximum RPM
The second S is the starting RPM.
What’s going on with the G96 Speed command?
The third S command is using cutting speed (CS) value! Yes, SFM! No RPM calculation!
Header without G96 CSS and G50 is easier
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G28 (turret home) T0101 (.75 drill) [A cutting tool that does not change the diameter of the part.] G97 S1500 M03 (start rpm value) [based on drill diameter not stock value] G54 G00 X 1.45 Z 1.0 M08 |
Header example without CSS |
Only four lines for CSS depending on tool or operation
Lathe footer:
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G00 Z1.0 M09 G28 (Turret to machine home in X and Z) move to safe place) M05 (spindle off) M01 or M30 |
Footer for a CNC Lathe |
G96 CSS is used for most turning tools, roughing, operations, and finishing operations.
It is not used for threading or parting tools. Do not use CSS with tools in same diameter or X0, e.g. drills, center drills, reamers, or taps.
Finish tool path
Pic
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G28 T0202 [the first 02 is tool change and tool number; the second 02 is length offset] (55 deg finish—note) G50 (max rpm) S4400 G97 (cancels previous CSS) S1000 M03 (Speed higher) G54 G00 X 3.2 Z1.0 M08 [read print to determine X, choose x with .100 off base stock (=.200 dia) recommended best practice] G96 S 1200 [don’t forget work holding capabilities] X -.062 (See explanation below) Z.1 [last check point] |
Header |
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G01 Z0 F.0005 X1.5 z-1 x 3 z-2.6 Tool past part, sitting on part. Don’t return to home.Move off part. .100 off X3.2 |
Body |
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G00 Z1.0 M09 G28 M05 M30 [program end because this is the finishing pass] |
Footer |
X -.062 Best Practice
Command the cutting too to move past the center of the part (X0, point of origin) to catch and remove center nub. This nub is like a burr on the end of the part. It needs to be removed.
Where does .062 come from. It is a value to compensation to go past the tangent point on nose radius. The measurement is common and is a Best Practice.