Posted by: davidhayden | March 2, 2010

Proven Tips, Tools and Tactics: Understanding G94 and G95


There are only 2 feed modes for cutting a work piece. G94/G95 are the CNC codes you need to select the proper feed mode. 

Code Data

Code: 94 and G95  
Modal: Yes
Group: 05
Action: Sets cutting mode from units per minute (G94) or units per revolution (G95)
ANSI Std: Yes
Requirements: none
Options:
Conflicting G Codes Any other G code in Group 05

Description and How to Use G94 / G95

G94 and G95 are the two common feed modes that CNC machines use. For milling, routing, lasers, waterjet and so on, the most common feed mode is in units per minute (UPM). That could be inches per minute (G20) or mm per minute (G21). G94 is the code that tells the machine control to feed in units per minute

For lathe work it is very useful to program in units per revolution (UPR). Again, that could be in inches per revolution or mm per revolution. While it is unlikely that a mill would ever use UPR programming unless it was a Mill-Turn style machine. It is not unusual to see lathes programmed in a way that uses both UPM and UPR.

Let’s look at some examples using inches per minute and inches per revolution.

An end mill is feeding in three axis to a destination point of X-3. Y2.5 Z-.150 at 5 inches per minute (IPR)
G94 – Sets mode to inches per minute
G01 X-3. Y2.5 Z-.150 F5. – F5. tells the machine to feed the machine at 5 inches per minute

To get a lathe to move to X-3. Z-.150 at .015 per revolution the command would be.
G95 – Sets mode to inches per minute
G01 X-3. Y2.5 Z-.150 F.015 – F.015 tells the machine to feed the machine at .015 inches for each turn of the spindle.

The G94/G95 command is modal and usually set very early in the program.

Unless you are programming a lathe and use G94 to move around the part and G95 to cut the part, it is pretty much a set it and forget it command.

Most CNC machines run boot program when they start up that sets the most common modal commands. If you are running a mill, it will no doubt default to G94 when you turn the machine on.

A lathe could default to either G94 or G95, so make sure that one of the first lines in your program always sets the initial conditions you want default to.

A later article will discuss the Start Up block that you should include in all your CNC programs.

Summary

Using G94 / G95 to select the proper feed mode is pretty basic; you pick one and forget it.


For a complete introduction to the fundamentals of CNC programming, take a look at 7 Easy Steps to CNC Programming . . . A Beginner’s Guide or 7 Easy Steps to CNC Programming . . . A Beginner’s Guide, the Ebook
 

 

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