Linear scales provide lift for aerospace accuracy

Linear scales provide lift for aerospace accuracy

Linear scales
An upgrade to linear scales eliminated motion errors and improved the accuracy of a vertical machining center at Votaw Precision Technologies.

Votaw Precision Technologies in Santa Fe Springs, Calif., considers itself a “go-to” source for critical parts for aerospace and defense projects.

Some of the more notable projects that the shop was a part of include the Mars Land Rover, the Aries 1 rocket, and the F-22 Raptor stealth fighter plane.

The shop often is called on to deliver parts for these types of projects mainly because its equipment can handle the demanding work. With that said, machine tool accuracy is paramount, so when a machine’s performance falters, corrective steps must be taken.

A case in point was a vertical machining center that Votaw Precision Technologies had purchased a few years ago.

The machine was previously owned but in practically new condition. However, after about a year in service at Votaw Precision Technologies, it was unable to meet the critical accuracies and tolerances that the shop’s customers demanded.

“We ran this machine on many projects, and time and time again saw that it wasn’t as accurate as we expected and just wasn’t holding tolerances. We had to repeatedly do time-intensive adjustments. Important parts just could not be done on this expensive machine,” Richard Roy, maintenance manager at Votaw Precision Technologies, said.

The machine is a 5-axis milling contour machine employed to serve a wide range of aerospace and industrial customers in the machining of flight components. On a moving table, the machine works a schedule of 16 to 20 hours per day, five and half days a week. Its Y axis measures 8 ft, X axis 15 ft, and Z axis 3 ft, and it has A and B tilting axes. When purchased, its main measurement feedback system consisted of rotary encoders on its ballscrews.

Roy indicated that when the machine began having problems holding tolerances, the shop did some of its own testing and found cyclic errors on one axis. These errors, he believed, originated in the ballscrew and rotary encoders and transferred into axis inaccuracy.

While one outside vendor told Votaw Precision Technologies there was nothing it could do, A Tech Authority, a company out of Chino, Calif., with scale expertise, recommended a linear scale upgrade for the shop’s vertical machining center. Votaw Precision Technologies took A Tech Authority’s recommendation and had the company do the upgrade retrofit. As a result, the cyclic error was corrected, and the machine’s accuracy improved from +/- 0.0005 in. on three axes to +/- 0.0002 in.

“Unfortunately, it is not uncommon for some machine tool equipment distributors to not understand the full benefit of a linear upgrade,” Craig D’Ambrosio, owner of A Tech Authority, said.

Votow specifically requested that A Tech Authority use high-precision scales from Heidenhain Corp. when replacing the measurement systems on the machine’s X, Y and Z axes. Two of the linear scales used were Heidenhain’s LS 100 series, and one was the company’s LB 382. Both these types of linear scales are highly accurate and extremely tolerant of vibration. Once plans were settled and parts on site, the job took only a couple days and looked like an OEM install.

“You know, we had a customer (Votaw Precision Technologies) here that purchased probably a half-million-dollar machine entrusting us to drill as many as 60 holes in their machine and basically give it a new sense of balance. We realized that and worked hard to do the job as needed,” explained Bill Ritter, A Tech Authority sales manager.

“It is also important to note that many people who work in this industry believe rotary encoders are good enough. However, it’s proving time and time again to be untrue, especially as jobs and requirements change.”

According to Votaw Precision Technologies maintenance manager Roy, the vertical machining center has been running its usual schedule ever since being upgraded. And he said that the retrofit made the machine extremely accurate, so much so, that the shop has machined parts for the 787 Dream Liner and the JAF Joint Strike Fighter on it.

“Even our operator who works on this particular machine, and who originally was skeptical that a linear upgrade would fix the problems, was pleased with the retrofit job and noted the amazing difference in machine performance,” Roy said.

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