Probing the unknown: use of a measuring probe and CNC measuring cycles can make life much easier than finding the zero point man

In CNC production, it is common to measure tools and workpieces by hand on the machine. The zero point is determined by scratching, and the values are entered into the CNC system. This method is prone to error and is expensive and time-consuming. It also requires a steady hand. Using a measuring probe and the CNC measuring cycles make life much easier.

The probe measures the workpiece using an appropriate measuring cycle from the range offered on the CNC menu. The measured values are transferred to the CNC to calculate the position and zero offset, calibrating the workpiece coordinate system with the machine coordinate system. In a linear process, the machine moves parallel to the X-direction of the workpiece system, even if the workpiece is clamped at an angle to the table.

An advanced CNC offers measuring cycles for a variety of workpiece geometries:

* point/edge measurements,

* right-angled and other corner measurements,

* cavity/drill hole measurements,

* square/round spigot measurements,

* plane/surface alignments, and

* probe calibrations.

Calibration

Before measuring, the probe must be calibrated. This involves determining the switching points and the length correction. First, the measuring probe is set to run concentrically, then it is positioned at the center of a known drill hole or special gauge ring.

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The "Calibrate Measuring Probe" command starts the measuring cycle. "Cycle Start" initiates the calibration process. The center point of the drill hole is then determined and the switching points of the measuring probe are calculated relative to the center of the spindle. The center offset of the calibration sphere also is calculated. By scanning a defined surface in the Z-direction, the control system calculates the length of the measuring probe and then enters it in the tool list.

Quality control

Measuring probes also can be used in the machining process for quality control. For repeated measurement processes or for checking the dimensional stability of workpieces in the machining process, measuring cycles similar to those used to setup the machine can be programmed in the NC program and used. Measurement values are stored in a measurement report and can be used for subsequent machining corrections.

For example, a workpiece requires remachining in serial production. Assume the key reference point for machining is a spigot on each workpiece. Normally, each workpiece would need to be set up by hand. This process can be accelerated and simplified by the cycle for measuring spigots programmed in the NC program via the GUI before machining. The reference position of the spigot is stored and the configured measuring cycle starts. The latter measures the position of the spigot, compares it with the programmed reference position, and executes a new zero offset correction for each workpiece.

Randy Pearson

Sales Support Manager

Siemens Energy & Automation

Randy Pearson is the Siemens sales support manager for U.S. dealers and OEMs. His special interest is CNC machine tool training through seminars and classes the company conducts.

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