Today’s standard machine tools can deliver accuracy and repeatability approaching levels formerly available only on CMMs. This enables the machine tool itself to be used for probing checks of workpieces during critical stages of the machining process.

Once a machine tool’s performance as a measuring instrument has been established, the probe becomes the operator’s CNC gauge. Probing routines can be programmed as part of the machining process and automatically run at various points to check feature dimensions and locations and apply necessary compensations. This saves operators from using dial indicators and shim stock, or eliminates errors in manually entering fixture, part and tool offsets into the control. Probing on the machine makes it part of the process—a powerful process improvement tool for making parts right the first time in the shortest throughput time.
Setup

 Used to locate the part automatically and establish a work coordinate system, probing cuts setup time, increases spindle availability, lowers fixture costs and eliminates non-productive machining passes. On complex parts, 45 minutes of fixture alignment can be replaced by 45 seconds of probing—performed automatically by the CNC. When starting with a casting or forging, probing can determine workpiece shape to avoid wasted time in air-cutting and help determine best tool approach angle.

  In-Process Control
This uses probing to monitor size and position of machine features during the cutting process, as well as verify precise dimensional relationships between various features at each step to avoid problems. A probe can be programmed to check actual machined results at various stages against the program and automatically apply cutter compensation—particularly after rough machining or semi-finish machining.

 Reference Probing
Reference probing is comparing part features to a dimensional master or reference surface of known location or dimension, which enables the CNC to determine positioning discrepancies and generate an offset to make up the difference. By probing the artifact before a critical machining pass, the CNC can check its own positioning against the master’s known dimensions and program an offset. If the dimensional master is mounted on the machine and exposed to the same environmental conditions, reference probing can be used to monitor and compensate for thermal growth. What results is a closed-loop process requiring no operator intervention.

  Every machine has its own set of numerous small errors in its motions and structure. As a result, there always is a slight discrepancy between a CNC’s programmed position and the true position of the tool tip—even after laser compensation has brought the two into closer agreement. Programmable artifact probing provides a way to further compensate for remaining machine errors. It gives process control feedback to enable positioning accuracy that can approach the machine’s repeatability specification. Such closed-loop process control can allow a machining center to achieve accuracies comparable to boring mills and other high precision machines.

  Fail Safe Operation
Many probing operations are accomplished through the use of memory-resident macro programs. Work coordinate updates, tool geometry changes, part measurement, etc., are automatically determined by the CNC after the successful completion of a probing cycle. This eliminates costly errors resulting from mis-keyed information or incorrect calculations.

  Final Inspection
Used to inspect parts after machining, probing can reduce the length and complexity of offline inspection, and in some cases eliminates it altogether. Inspecting on the machine is particularly beneficial with large, expensive workpieces—such as molds or dies—which can be especially difficult and time-consuming to move.

  Here, too, reference probing can be accomplished using two methods, machine correlation—comparing on-machine measurements to prior CMM data, or artifact comparison—comparing on-machine data to a traceable artifact of known size. When making this comparison, the CNC can determine if the specific machining tolerances were actually achieved. Based on these results, an intelligent decision can be made on corrective actions, while the workpiece is still on the machine tool.        

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