The Key to Lean: Machine Tool Calibration

Post-process part inspection is one of the leading components of waste in a lean manufacturing operation, according to Clive Warren, calibration products business manager, Renishaw. Instead of giving a picture of future process capability, it can only provide a historical record.

“It is the manufacturing equivalent of looking in the rear view mirror to drive a car,” Warren says. “Lean manufacturing dictates you see ahead and know beforehand that a machine tool can make parts to the spec the customer just handed to you. Post-process inspection only tells you what happened, not what is going to happen. It is the antithesis of lean.”

Warren says advance knowledge of the machine’s capability is worth money, especially to manufacturers bent on lean shop floor practices, faster throughput, and reduced WIP. “The most expensive, non-value-added process in manufacturing today is post-process part inspection,” he explains.

“Documenting the capability and accuracy of machine tools and proving you have control of the machining process is the foundation of ISO 9000 and QS 9000,” Warren explains. “To determine that accuracy, machines should be inspected to a nationally recognized and accepted standard, such as ISO 230-2 or ASME B5.54.”

Both standards delineate the use of telescoping ballbars and laser calibration systems to measure the accuracy of 3-axis CNC machines, baseline their capabilities, and perform critical geometric and contouring checks.

The prescribed tests include linear displacement accuracy, bi-directional repeatability, volumetric performance through diagonal laser displacement measurements, and contouring performance. The latest laser calibration systems provide linear measurement accuracy better than ±0.5ppm, with a resolution of just 1nm over linear distances up to 80m, enabling calibration of both large and small motion systems with state-of-the-art precision.

“To meet the demands of new high speed machines and devices, slew rates for new lasers have been increased to 4m/s,” Warren says. “These new systems can deliver this performance even on a shop floor, where an unstable environment plays havoc with lesser systems.”

In fact, the best laser interferometer systems maintain accuracy and performance over a wide operating range, from 0°C to 40°C, extremes not seen in any normal machine shop.

Calibration system software enables both static and dynamic data capture, and provides data analysis to national and international machine tool testing standards. Besides determining current machine capabilities, linear error compensation software can create new compensation values for a CNC machine’s controller to improve positioning accuracy, significantly. While many CNC machines are mapped and compensated by OEMs and third-party service companies for axis positioning errors, wear, damage, and misuse can cause these errors to change over time.

One of the latest calibration software developments allows dynamic data from the laser to be streamed to a PC at up to 50kHz and displayed, in real time, on an oscilloscope-style software display. The operator can immediately see the effects of vibration and deviations from; programmed axis motion.

According to Warren, there is no good reason to pull a perfectly good machine out of production for calibration. “Routine testing with a telescoping ballbar only takes about 15 minutes to complete and will reveal if a machine is still performing to specification or not,” he explains.

A compact, portable tool introduced to industry by Renishaw, the telescopic ballbar can identify 22 different dynamic and geometric errors affecting machine axes. Attached magnetically to the spindle housing at one end, and the machine table on the other, the ballbar monitors movement of the machine to an accuracy of ±1.0µ, as it follows a programmed circular path, comparing the test path to that of a perfect circle to identify errors. Sophisticated algorithms in the ballbar software break down these errors to produce a comprehensive diagnostic report featuring a color-coded polar plot and tabulating up to 22 error values, ranking them in order of importance to part accuracy.

A test history review function allows tracking of machine performance over time to identify wear/process deterioration and schedule predictive/preventive maintenance. A machine error simulator function enables maintenance technicians to try out the effect of different adjustments and fixes without ever touching the machine.

“By diligently applying these diagnostic and corrective tools to pre-qualify machine tools and processes, manufacturers can lean out their operations, reducing the demand, time, and costs for post-process inspection,” Warren explains. “In short-run, nimble, high-flex production – where lean manufacturing is trending – inspection only tells you where you have been, not where you are going.

It shows there is an error in the part, but cannot tell you if it is a repeatable error traceable to the machine. When part runs are small, there is often no trend to monitor with SPC. By defining and monitoring machine capabilities – inspecting before the fact – manufacturers can lean out their operations with high confidence that parts will be produced to spec, right the first time.”

Renishaw Inc.
Hoffman Estates, IL
renishaw.com