Setting Part Zero

The program is written from a corner, a centre or a face of the part. Part zero is where you tell the control that point is.

Do it with a probe if you have one. See Part Zero with a Probe, which is faster and repeats better. This page is the way it is done by hand.

The keys that write a datum

Both live on the WORK tab of the OFFSET screen. Pick the WCS row your program uses, the star marks the active one, and the axis column.

Key	Means
INPUT	writes what you type into the cell
+INPUT	adds what you type to what is there
SET	the position I am at now reads what I typed. Blank means zero.
HALF	halves the Work reading, putting zero midway between the two touches

ZERO X ZERO Y ZERO Z on the MANUAL screen are the same as SET with nothing typed: zero, here, this axis. They save walking to the offset screen for the common case.

Z, off the top face

1. Jog the tool down onto the face, slowing to the finest step as you close in.
2. With a slip or a feeler between tool and part, come down until it just drags.
3. Type the slip thickness and press SET.

Z zero is now the face, and the slip is accounted for. Type nothing and press SET if the tool is touching the face itself.

X and Y, off an edge

With an edge finder:

1. Spin the finder at its rated speed and jog into the edge until the tip kicks out.
2. Type the finder’s tip radius and press SET. The finder’s centre is one radius from the edge.

The sign matters: you are saying “the axis centre is this far from zero”. Approaching from the negative side of the part, the radius is negative.

A centre, from two edges

For the middle of a bore, a slot or a plate:

1. Touch one side and press SET with nothing typed. That side is now zero.
2. Jog across and touch the other side.
3. Press HALF.

The Work reading halves, and zero lands midway between the two touches.

Checking it before you cut

Check it before you cut. Jog to X0 Y0 in the Work column and look: the tool should be over the point you meant.

Nudge, do not retype. After a trial cut that is 0.05 oversize, type 0.05 and press +INPUT on the offset screen. It adds to what is there rather than replacing it, so a good datum is not lost to a typing slip.

Set the offset the program uses. G54 through G59 are separate. Setting G54 while the program runs in G55 changes nothing you can see until the tool is in the wrong place.

Where to go next

• Tool & Work Offsets: the tables these write into.
• Part Zero with a Probe: the same job, measured.