Z Axis

Objective

• Describe the difference with Z0 machine home versus Z0 on the part.
• Describe Best Practices for setting Z axis cutting depths.

Z Axis

The Z axis refers to the spindle on a mill.  The spindle holds the tool(s).  On a lathe, Z axis is the carriage.

The Z axis position depends on which coordinate system the program is set. (See G54-G59: An Intro to Coordinate Systems for descriptions.)  In the machine coordinate system, the Z axis will be positioned away from the work surface.  Once the computer program directs the CNC machine to use the work coordinate system, the Z axis will refer to the part coordinates.

The Z axis has only two movements: up and down.  In a work coordinate system, Z1.0 will position the Z one inch above the part.  This is a safe space to send the Z axis or rather, the tool, before cutting begins.

Best Practices for Z-Axis Motions

Making parts with good edges is the goal of manufacturing.  If the cutting tool can go past the edge of the part just a little bit, then the edges of the part or hole will be higher quality.

Each tool has a different shape.  This is called tool geometry.  Some of the tools have angles on the tips. For example, screw drills and taps.  The edges of the part will be better if the measurements for the Z axis go past the tip of the tool. The diameter of the tool should cut the size of the part.  The amount that is added depends on the part, the fixture or vice, the machine, and the programmer.  Be sure to read work orders for each job. They will identify how much extra the Z-axis should go past a part.

The concept of interpolated moves, which we called Linear Interpolation in another section, raises an interesting issue for the Z axis. It is often a good idea to move the Z-axis or “depth-of-cut-axis” on its own.

If the code uses a single line with other axes (X and Y), the controller will coordinate the all of the axes motion into a linear interpolation. The operator may not be able predict a crash or collision (between the tool and the workpiece) as the cutter gets close to the workpiece and fixture.

It is preferred to move in X and Y first. It is much easier to judge whether an accidental collision or crash is about to take place. Also, it is much less likely to hit a random object sticking up, like a clamp, if the tool cutter in Z axis is kept high until it is directly over the position to cut.

Z can be located at one inch (1″) above the part until the X, Y position is located. Then, the operator has more control about moving Z into the part and starting the cut.

For example, the code looks like this.

G00  G54  X0  Y0

Z1.0

The CNC Machine tool will rapid move to the X, Y work coordinate in an interpolated motion.  Then, once in position, the Z axis will go to 1″ above the part in a linear movement.

The Z axis holds the tools.  Each tool is a different length.  Each Tool or each Z axis for each Tool needs to be set. The next section will discuss Tool Length Offsets.