Re: G92

[Dam <damg-at-1st.net>]

G92 - position register preset/position preload/load position or 
whatever the Mfg. called it.

G92 is NOT an OFFSET!!! It simply changes the value of the position 
register. It was meant to be used within a part program, not from MDI 
mode to set axis zero or program origin point.

In my nearly 30 years of experience every machine I have ever dealt with 
had a method of setting axis zero manually and it had absolutely nothing 
to do with the G92 code. The earlier machines had no offsets at all, but 
G92 was included in the G & M codes. Next came tool length and diameter 
offsets, program origin was still established by manual setting of axis 
zero. Then came Work Coordinate Offsets. The first machine I encountered 
that used G54,55,... still had the button for setting an axis to zero in 
manual mode. G92 was still separate from the offsets. As for the setting 
of an axis zero point, with the addition of Work Coordinate Offsets I 
found no reason to ever set axis zero the old way and used the Work 
Coordinate Offsets.

A Simple Example:

A drawing calls for a 3.0" diameter bore dimensioned from the edges of 
the part. A 4 hole bolt circle is dimensioned relative to the center of 
that bore, 4 HOLES EQUALLY SPACED ON 4.0" DIAMETER BOLT CIRCLE. The 
operation can be programmed relative to the program origin and all is 
well. That method of programming has one small drawback, the programmed 
coordinates of the 4 holes do not match the dimensions on the drawing. 
Now in order to verify that the program is going to make a good part the 
same calculations used to write the program must be used for those 
numbers to mean anything to the operator. Using G92 to change the 
position register makes it easier to compare the program to the drawing 
reducing the possibility of errors. First position to the center of the 
bore. Then G92 X0 Y0. Next program the bolt circle. With the bolt circle 
finished return to the center of the bore and G92 X?.? Y?.??? (center of 
the bore as dimensioned on the drawing) resetting the position register 
to the original values.

..
..
..
G0X6.5Y5.375
G92X0Y0
G81X2Y0Z-1R0.1F10
X0Y2
X-2Y0
X0Y-2
G80
G0X0Y0
G92X6.5Y5.375
..
..
..

As in the example G92 is a programming convenience which makes a program 
easier to write and easier to verify without any calculations. This is 
just one example of proper use of the G92 command. There are many more.

Setting of the program origin should be done with the Work Coordinate 
System OFFSETS G54, G55, ... That is what they are for!!!! If you have 
more than one part on the table the Work Coordinate System OFFSETS can 
adjust for variations from part to part or fixture to fixture.

There can be many Right ways to do something, but Wrong is Wrong no 
matter how you look at it! IMO to use the G92 code as an offset is wrong 
and can lead to problems as have been described.

Dale