Correct Use of the Tool Measurement Device

• 1
  What the tool measurement device actually measures
• 2
  The skip signal and why G31 controls the stop
• 3
  Mounting and aligning the probe on the table
• 4
  When a tool measurement device is the wrong choice
• 5
  Calibrating the probe against the setting block
• 6
  Contact probe vs laser setter vs offline presetter

A tool measurement device is a contact probe mounted on the machine table or on a dedicated bracket, with a hardened measuring face at a known height. The spindle brings the tool down toward that face. When the stylus is touched, the probe output flips and the control freezes the axis position. The difference between the frozen position and a stored reference is the tool length.

Nothing about this is optical. The system measures a mechanical contact event, not a light beam, so the number it produces is only as good as the position where the machine stopped. That is why the feed rate at contact matters: a fast approach gives a fast stop, and the control only records the position on the next interpolation cycle.

Diameter comes from the same idea in two directions. The tool is moved to one side of the measuring face, touched, retracted, then moved to the opposite side. The distance between the two contact points, minus a known constant for the probe tip, is the cutting diameter. Runout shows up here as a difference between the two readings.

The output is not a finished offset. It is a number fed into the tool offset table, where wear compensation and cutter radius compensation are added later. Keep those two separate in your head and most confusion about drifting offsets disappears.

On a Fanuc or Mitsubishi control, G31 is a one-shot move that watches the skip input. As long as the probe signal reads 0, the axis keeps moving. When the signal changes to 1, the control stops the axis and stores the position. There is no alarm and no overshoot correction. The stored value is simply where the machine was on that cycle.

The skip input is a discrete DC signal, normally 24 V, wired from the probe interface in the electrical cabinet to the control terminal. Wiring is where most installations fail. A screened cable with the screen grounded at the cabinet end only, a regulated 24 V supply, and a separate input card from the spindle drive are the three details that decide whether the signal is clean.

A no-contact reading usually means the tool never reached the face, the probe cable is broken, or the skip input was assigned to a different terminal than the macro expects. Check the input diagnostic screen before you touch any parameters. If the bit never changes state when you press the stylus by hand, the problem is electrical, not mechanical.

The macro that follows the G31 move matters as much as the G31 itself. A typical macro commands the approach, records the position, retracts the tool clear of the face, and writes the value into the offset table. If the retract move is missing or too small, the next tool change drags the tool across the stylus. Two or three of those and the probe is out of tolerance.

The probe body sits on the table, clamped so it stays put through a full shift but can be removed when a large part needs the space. Position it outside the normal cutting envelope and away from the chip conveyor side. Chips piled against the measuring face are a common cause of a length reading that is 0.02 mm to 0.05 mm short.

Align the measuring face before you calibrate anything. A dial indicator swept across the face shows parallelism to the table in the Z direction, and squareness in X and Y. For most work, keep the face parallel within 0.01 mm over its length. If it is tilted, every tool reads long or short depending on where the flank touches.

Air blast matters more than most people expect. A short burst across the face before each measurement clears coolant mist and fine chips. On aluminium, coolant residue alone can add a measurable film thickness. Set the blast to fire on the approach move so it does not blow the tool off the face.

Use a probe body rated for the environment you run. Flood coolant, cast iron dust, and interrupted cuts all shorten probe life. If the machine runs lights-out, the probe is the single point that decides whether an unattended tool change produces a scrapped part or a good one.

Very small tools are a hard limit. Below about Ø1 mm, the contact force of a standard probe bends the tool before the signal trips. The reading comes out short and the machine cuts undersize. Use a non-contact laser setter, or set the offset off the machine on a presetter and accept the transfer error.

Heavy roughing tools with unequal flutes give a diameter reading that depends on which flute happens to touch. For a three-flute cutter with runout, the probe cannot tell you the effective cutting diameter without a rotation. Measure length here, not diameter.

On a machine that runs one family of parts for months, an offline presetter plus a verified tool holder taper can be faster than touching off every tool in the spindle. The measurement device earns its place when tool variety is high or when a broken tool must be replaced mid-run without a presetter nearby.

Thermal state changes the answer too. A probe calibrated on a cold morning will read differently after four hours of cutting. On a machine without compensation, calibrate after the spindle has run for a while, and recheck if a long run starts drifting in Z.