Laser Calibration Insures Machine Tool And CMM Accuracy

As a code-one flight item vendor for more than 40 years and an approved source for fracture critical, level 1/sub-safe and high-strength steel components, Tapemation Machining, Inc. (Scotts Valley, California) relies on laser calibration for ensuring the accuracy of its machining processes. But calibrating five-axis machine tools and CMMs is a difficult and expensive process, especially when the cost of machine downtime and outside services are considered.

Because it makes large precision-machined components for the aerospace, defense, medical, scientific, electronic, marine and petroleum industries, which require tight tolerances, laser calibration is a competitive advantage for Tapemation. In its 50,000 square foot facility with 22 major machines, including five-axis Anayak, Mazak, SNK, Rambaudi and LeBlond-Makino machining centers, Tapemation has an internal quality control system that meets or exceeds MIL-I-45208A.

“Calibration isn’t mandated by our suppliers, it’s mandated by the type of work we do,” says Bruce Erickson, the company’s president. “We don’t include laser calibration as a part of our regular maintenance plan, except for our CMM. We calibrate the CMM at least once a year for ISO 2000 and 45208 inspection procedures. We calibrate the machine tools to ensure accuracy, so if we get a part that we have to hold flatness or position over a distance, we know the machine is capable.

“We were unable to find an outside service with the experience to calibrate many of our five-axis machine tools and rotary tables,” says Mr. Erickson. So the company purchased a MCV 4000 laser calibration system from Optodyne, Inc. (Compton, California) with a dual beam laser head, dual channel processor and squareness optics for running linear, angular (pitch, yaw, straightness, flatness) and squareness calibration. In addition to machine tools, the MCV 4000 is used for calibrating Tapemation’s Brown & Sharpe MDL 3000 Validator CMM with travels of 132 inches (X), 47 inches (Y) and 78 inches (Z).

Most of the components machined at Tapemation are close tolerance, for example, ±0.001 inch over a 40-inch diameter or ±0.005-inch flatness over a 10-foot-long part. “Our QA manager, formerly a machinist, runs the calibration,” Mr. Erickson says. “Because we do five-axis and rotary table calibration, his training required about a week to be proficient. Still, that’s not bad when you consider the high cost when a machine is down waiting for an outside service to show up.”

Data collection is manual, automatic or on-the-fly with a notebook computer. At Tapemation, the MCV 4000’s velocity trigger is used for automatically recording position information, reducing operator errors. For example, the operator uses the control to move the table into position. Because the system automatically senses table movement, data collection is triggered after a user-defined interval as the table starts to move and stop. This ensures measurements are taken at uniform intervals.

The rotational axes A and B are calibrated with a precision metal sphere mounted on the spindle, and the laser head with focus lens is mounted on the table. A 4-inch focal-length lens is used to center the laser beam on the surface of the sphere. The laser head is pointed in the X-axis direction, and the sphere is aligned with the laser beam by moving the spindle. By rotating the A or B rotational axes, the distance variations between the laser head and the sphere surface are measured.

The setup process for calibrating a rotary table is automated by using the motorized, programmable rotary table to eliminate the manual return movement. This reduces the time it takes to calibrate a rotary table, minimizing operator error.

With the rotary table calibration accessories and software, the rotary table platen stops and settles for 3 to 5 seconds after each rotational movement of the rotary table, then angular data is collected.

“An outside service for just our CMM was charging us $6,500, two times a year,” Mr. Erickson says. “The service technician couldn’t calibrate some of our other machines. And they had a very difficult time doing our five-axis and rotary tables. We used to spend half the cost of the Optodyne system every year calibrating all our machines. A 2-year ROI is a pretty good investment.”