There is a relatively inexpensive and easy-to-use machine tool accessory that can deliver significant reductions in production time and cost. That accessory is the machine tool probe.Long used mainly for setup, on-machine probing is gaining widespread application for process improvement—automating and speeding part processing, even eliminating parts of the process. The growth is being driven by shifts to flexible machining, shorter leadtimes, tighter accuracy specifications and automated processing—all areas where on-machine probing yields time, quality and productivity improvements.A traditional objection to on-machine probing—that it diverts machine time away from making chips—can be overcome by measuring productivity in terms of total in-process time, rather than machining cycle time. The view that probing steals machining time focuses on the 10 seconds or so that checking a part feature can require. However, this focus overlooks the fact that checking the part off-line, a step that probing may make unnecessary, can impose the need for additional part handling and another setup, adding to in-process time (not to mention introducing the potential of fixturing error).Considered in the overall context of part processing time, rather than simply machining time, probing can produce vital gains in processing efficiency and manufacturing productivity. In flexible machining operations with small batch sizes, the time that probing saves up front on setup, and at the end on inspection, more than offsets total probing time.Machine tool probing potentially makes sense at nearly every stage of the process:Using on-machine probing to “buy off” a part while still fixtured can greatly shorten in-process time, eliminating the need to remove clamps, transport the part to a CMM, refixture the part, account for thermal effects and re-establish datum points before measuring. Inspecting on the machine is particularly beneficial with large, expensive workpieces such as molds or large aerospace parts that can be difficult and time-consuming to move.The most expensive, non-value-added process in most shops is piece part inspection. When the machine tool is well-maintained, automated probing can assure that the process stays in control and the parts coming off the machine stay on spec. That’s why keeping the machine well-maintained is also essential if piece-part inspection is to be reduced.Today’s standard machine tools deliver accuracy and repeatability approaching levels formerly available only on CMMs. In addition, technology advances are making these machines easier to maintain.Test and calibration technology are now available—and affordable—to enable shops to ensure the accuracy and health of their machine tools. Telescoping ballbars are readily affordable by virtually any shop. A well-stocked toolbox should also contain either electronic levels or a good set of precision machine levels. Plants and large shops increasingly maintain their own laser interferometers and electronic levels, while rental equipment and diagnostics services are commercially available to small shops from various sources.Shops investing in the health of their machines can do many more things with their capital equipment. When the ability of the machine tool to move a probe around accurately and repeatably can be trusted, the shop can inspect the part immediately after cutting it without extra handling and refixturing.In order to move toward 100 percent good parts, it is essential to understand all process variables. This is a requirement of ISO 9000 and QS 9000. A shop must be able to document the process capability and the accuracy of its machines. One way to do this is to inspect them to a nationally recognized and accepted standard, such as ISO 230 or ASME B5.54. Both call for a ballbar and laser to be used with a recommended procedure for checking machine tool accuracy.The purpose of these standards is not to specify an accuracy the machine must meet, but to find out its process capability or what accuracy level it can meet. The part print dictates the accuracy your machine must have to make good parts—that is, where to set the accuracy bar. Testing tells you how high your machine can jump. As long as the machine can top the bar, you have process capability.The industry trend is to calibrate the machine on need, not time. There is no reason for the maintenance department to pull a perfectly good machine out of production for calibration. Let the ballbar and the accuracy of the parts determine when something has gone awry. Meantime, run production!The growth in on-machine probing is also being driven by technology advances that make machine tool probes more accurate, more productive, easier to use, and easier to afford. To illustrate this, here are four relatively new probe developments:As the tool moves through the laser beam, system electronics detect when the beam is broken and issue an output signal to the controller. The system accurately measures tools as small as 0.2 mm diameter anywhere in the beam. The system triggers when the laser beam is broken beyond a 50 percent threshold by the tool being checked. The non-contact toolsetting system uses a visible-red diode laser proven reliable in machining conditions. The visible-red laser operates at 670 nm wavelength at low-power output of less than 1 mW.Advanced electronics permitting simplified design for the first time make non-contact toolsetting an affordable alternative to contact systems. The design avoids the mechanical elements and potentially complex installation of other toolsetting systems—the brackets and actuators with contact-based systems, and the problems with shutters, switches and solenoids on other non-contact designs. A protection system features positive internal air pressure that supplies a continuous stream of air through the laser beam apertures to keep out contaminants.Advances in machine tool accuracy and probing technology make machine inspection viable for process control by allowing a machine to be used to verify the accuracy of parts it machines. Once a machine tool’s performance as a measuring instrument has been established, the probe becomes the operator’s CNC gage.This represents a fundamental change in the way inspection is performed. Rather than a postprocess check performed off-line on parts after machining, probing on the machine makes inspection part of the process, as well as a powerful process improvement tool for machining parts to spec in the shortest possible time. MMSAbout the author: Dave Bozich is business manager, machine tool products, for Renishaw, Inc.