Accepting parts off the machine tool is a rapidly growing trend in manufacturing as companies strive to make 100% good parts in the lowest possible cycle time. But for the machining center to serve as an inspection instrument, it must first be process-capable--able to achieve accuracies that meet the design specification for the part to be produced.
Most machining center users are familiar with laser calibration and CNC error compensation for systemic errors in linear and rotary axis motion. Relatively few realize, however, that the spindle-mounted probe can also serve as an error-compensation tool. A probing technique called artifact or reference comparison lets machining centers overcome both angular and transient thermal errors--two types of error that laser compensation cannot address.
Artifact comparison can be used to enable almost any machining center to hold tolerances near its repeatability specification, as well as compensate for thermal effects.
This basic metrology technique uses a pre-measured artifact as a reference master. This means that the artifact's actual dimensions have been measured on a CMM that is calibrated to a traceable standard. The artifact is then located in the machining envelope, typically as part of the setup or fixture. It should be made of the same material as the parts being machined to ensure that it responds identically to thermal changes. By probing the artifact before a critical machining pass, the CNC can check its own positioning against the known dimensions, and then program an offset to compensate for any discrepancy.
Artifact probing makes on-machine inspection seamless and ultra-accurate, and greatly increases the level of confidence in measurements. At our company, we use this technique in our own manufacturing to hold tight tolerances on small, precision components in an automated machining system. An artifact is indexed into the machining envelope as part of every batch of parts. At various points in the machining cycle, a spindle-mounted probe measures the size of a feature on the artifact for comparison against the known dimension. The CNC then compensates for machine scale, geometry, and thermal errors, enabling our machining systems to run unmanned for 140 hours per week with full assurance that we are producing conforming parts.
At the opposite extreme in part size, Pratt & Whitney's commercial jet engine manufacturing operation in Middletown, CT, uses artifact comparison to maintain accuracy to ±0.010" (0.25-mm) on 118" (3-m) diam fan-containment cases. In this application, features on the part fixture itself serve as the artifact master. After measurement on a traceable CMM, the artifact dimensions are stamped on the fixture for reference in determining offsets.
The following techniques will optimize the accuracy gains from artifact probing:
- The reference surface must be located as close as practical to the feature machined, and share two of the feature's three coordinates wherever possible.
- Reference measurements should be taken in the same plane and directions as the machining operation to minimize dynamic errors.
- The critical machining pass should immediately follow the reference comparison.
- Tool length should approximate the length of the probe.
All of these conditions limit the opportunities for angular and thermal errors to affect the process, and minimize accumulated error in the move from the master to the part.
Every machine has its own unique set of numerous small errors in its motions and structure. As a result, there is always 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, and provide feedback for process control, at positioning accuracies that can approach the machine's repeatability spec.
This article was first published in the November 2004 edition of Manufacturing Engineering magazine.