Learn VMC Programming: How and When to Use G90 and G91

 

INTRODUCTION:

G90 - Absolute Positioning

• All coordinates are measured from a fixed origin point (usually machine zero or work zero).
· Every move is calculated from the same reference point.
· It's like giving street addresses in a city.

These are modal commands that control how coordinate positions are interpreted by CNC machines like mills, lathes, 3D printers, etc.

e.g.
G90 (Switch to absolute mode)
G0 X10 Y10 (Move to X=10, Y=10 from origin)
G0 X20 Y20 (Move to X=20, Y=20 from origin)
G0 X10 Y10 (Move BACK to X=10, Y=10 from origin)

G91 - Incremental Positioning

• All coordinates are measured from the current position.
· Each move is relative to where the tool currently is.
· It's like giving directions: "go 5 blocks north, then 3 blocks east."

e.g.
G91 (Switch to incremental mode)
G0 X10 Y10 (Move 10 units right, 10 units forward from wherever you are)
G0 X10 Y0 (Move another 10 units right, stay at same Y)
G0 X0 Y-10 (Move 10 units back in Y direction)

e.g. Drilling Pattern:

G90 G0 X0 Y0 (Absolute: Go to origin)
(Drill hole at 0,0)
G91 (Switch to incremental)
G0 X10 Y0 (Move 10 units right)
(Drill second hole)
G0 X10 Y0 (Move another 10 right)
(Drill third hole)
G90 (Switch back to absolute)
G0 X0 Y20 (Go to absolute position X0, Y20)

Common Mistake:
G90
G0 X0 Y0
G0 X50 Y50

G91 (FORGOT to switch out of incremental!)
G0 X10 Y10 (This moves to X60 Y60, not X10 Y10!)


Use G90 (Absolute) When:

· Programming from engineering drawings (dimensions are usually absolute)
· Setting up toolpaths from a known origin
· Performing operations at specific coordinates
· Most of your programming because it's generally safer and more intuitive

Use G91 (Incremental) When:

· Creating repeating patterns (like bolt circles)
· Making relative adjustments
· Certain canned cycles (like G81 drilling cycle often uses incremental Z)
· Temporary moves from current position

Both are modal, once you set, the mode stays active until you change it. Always verify your mode at program start. Begin programs with G90 to establish known state. Always include G90 in safety block. Some programs use both modes for efficiency.