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Today's laser alignment, ballbar analysis and laser calibration services can restore and even enhance your machine tool accuracy—and, ultimately, improve a shop's bottom line.
Helium-neon laser interferometer systems boast an accuracy and resolution down to 1x10-6 (0.000001 inch), a hundred times better than a machine-tool system resolution. They can measure and account for multiple positioning errors due to backlash, ballscrew pitch error and other occurrences, and with a CNC compensation table are able to bring positioning accuracy back to a "like new" condition.
The laser head, interferometer and retroreflector are set up in line with the machine travel (in the working zone of the machine), then measurements are taken at optimal intervals; the results are then added to the CNC compensation file and measurements are re-taken to confirm results.
Although complications can arise with rotary axes and other elements, the laser-calibration process itself remains relatively straightforward. But note: Laser calibration—representing some of the most accurate gauging in industry—should really be thought of as "icing on the cake," the final part of the overall machine-inspection process.
Before bringing in a laser-calibration solution, consider the following steps involved in a machine-tool "tune-up." Machine-tool calibration starts with the geometric alignments—level, flat, straight and square—and a proper machine tune-up ensures all elements are as accurate as possible, before the laser provides the final "icing" in the machine-inspection process.
1. CHECK THE MACHINE FOR "EARTH" LEVEL AT THE MAIN WAYS
This involves simply putting a level on the base ways of the machine. Note, it is not recommended to measure off of the table, because measuring at that point does not take into consideration any potentially out-of-alignment element under it.
For instance, at one company, placing levels on the main rails of a machine found that it was level; but readings from levels on the table-top gave a reading that the table was not level at all. Further investigation found that the turcite material below the table was actually damaged. So imagine if only the table-top was measured. The machine would have been altered to make the table-top level; yet by doing so, everything between the table and the base itself would have potentially been un-leveled and twisted out of alignment to accommodate the worn turcite.
2. CHECK BALLSCREWS FOR BALL-NUT BACKLASH
Often when a machine can't cut parts the ball nut has been worn down, meaning the re-circulating balls inside are under-sized, creating slack that causes backlash. This problem should not be uncovered during the eleventh hour, during the machine calibration, but up-front. Repair or replace the ballscrew before taking the machine through the calibration process. Performing a lead check will ensure you have reasonably tight ballscrew-to-ball-nut tolerance.
3. CHECK FOR BALLSCREW END PLAY
This ensures ballscrew support bearings are taken into account. Many will place an indicator on the machine, grab the hand wheel, and move the machine back and forth, thinking they are checking backlash. But in reality, they are checking the stack-up of elements in the ballscrew assembly causing backlash. Here, as in many cases, there is more than one piece to the puzzle.
The tapered-angular-contact bearings are pre-loaded and the ballscrew tensioned between ballscrew support housings to have zero-degree play, neither axial nor radial. If you have axial play in the ball-support bearings, that separate measurement should be added to the backlash. You may indeed have a good ball nut, but with end play in the support bearing. There are separate pieces that make up the single backlash measurement—ball nut to screw looseness, ball nut flange mounting screws to mounting yoke (usually cast in one piece) and support bearings. All should be measured before making any expensive replacements or repairs.
Imagine again an indicator on a machine giving a reading that says the ballscrew has some significant backlash. If you analyze the ballscrew, however, you may find nothing wrong with it; the backlash problem may be coming from the support bearings. So, readjust the support bearings, and you're back in business.
This solves a common problem. Many spend all kinds of money repairing a ballscrew that doesn't need any repair—all because a needed measurement wasn't made.
4. CHECK SPINDLE TRAM
The head may be hanging slightly crooked off the ways. Inspecting spindle tram checks spindle perpendicularity to the axis and involves placing an indicator in the spindle on a radius and moving it in a large-diameter circle on a known parallel surface. This gives you an indication of where the spindle head is in relation to that axis.
Since an out-of-alignment spindle can indicate an out-of-square reading between axes, check the spindle tram first, and, if needed, tighten the gibs. This alone can sometimes eliminate the squareness error.
5. CHECK SQUARENESS ON ALL AXES
If you do a tram check and don't know if the machine is square, a bad tram reading, even after tightening the gibs, can mean there is a squareness problem between the column and table or other elements.
Note, that spindle tram and squareness are part of a two-measurement dynamic. Adjusting the leveling bolts on the machine to bring the spindle perpendicular to the axis may, in fact, "unsquare" the axes. Making an adjustment in one can affect the other. So for an effective machine tune-up, always perform both measurements.
6. CHECK FOR PARALLELISM BETWEEN AXES
This should include all axes and account for auxiliary ways. Those ways should be parallel to the main way; otherwise, a twisting condition can exist at the machine base—and that twisting will affect measurements of everything above it. But this wouldn't come to light unless you measured this parallelism.
7. CHECK FOR LOOSE GIBS ON ALL AXES
If the head gibs are loose, the head may hang crooked. If you fail to tighten all gibs the table and / or saddle can "duck walk" during operation. If gibs aren't tight, the machine calibration won't be valid because of this "duck walk" effect.
AVOIDING A FALSE SENSE OF ACCURACY
Performing laser calibration on your machine tool without going through a thorough alignment can give a false sense of accuracy. You may think that, after a calibration, the machine is performing plus or minus 50 millionths, according to readings on the laser printout, but if the part is cut and the machine isn't square within 0.002 inch—that 50-millionth accuracy can be thrown out the door.
Editor's Note: Tim Pray owns Measurement & Machine Technology, www.measurementandmachine.com, a machine alignment and calibration services firm based in Roseville, Mich. Photos courtesy of Measurement & Machine Technology.
author: By Tim Pray