Keeping machines in line: laser alignment keeps machines running smoothly. (Alignment).
A laser doesn't always need to be the first step when aligning a machine, according to Tim Pray, Measurement & Machine Technology, Roseville, IL-based provider of machine alignment and calibration services. Self-tests can determine if a machine needs alignment or not, he says.
"Some questions to ask are: Is the finish as expected? Are you constantly editing your part programs? Do you bore holes instead of circular interpolating them? Is there only one operator that can make good parts on a machine?" Pray says.
Once those questions are answered, running part programs and machining can answer questions about machine accuracy. "Enter a part program, do not edit it to circular interpolate a pocket or thru hole, remove it and inspect it. Is it round? Is it to size? Relocate the part in a different location and do it again with no metal removal. Does it come out the same?" Repeating this pattern with any machine tool or operation will signal whether or not the machine is repeatable.
"If any of the tests do not come out as expected, your machine tool may need some help," says Pray.
A time for lasers
Once the decision is made to use a laser alignment tool on a machine, Pray suggests a few steps for bringing a tool back to spec. "Precision level the machine base, at the rails, not the table top," says Pray. "Then check and adjust the gibs, squareness between the axes (x to y, x to z, and y to z), and parallelism (spindle tram). These adjustments sound simple and they are for the experienced machine tool alignment technician. Now bring out your laser, verify and fine tune all the necessary adjustments and calibrate the pitch error compensation to bring the machine to optimum running condition."
While linear positioning is the main focus of some calibrations, full alignment is vital, according to Rod Hamar, VP, sales & marketing, Hamar Laser Instruments Inc., Danbury, CT. "In a typical 3-axis machining center, there are 21 possible errors associated with machining a part, 3 of which are linear positioning errors. The remaining 18 errors are geometric."
Hamar adds that another problem often seen on machines is the perception that once a machine has been aligned, it never needs to be aligned again. "We have numerous tales of machines being thrown out of tolerance with simple things, such as the sun shining on it in the morning and not the afternoon." Hamar recommends that equipment be checked for alignment and linear positioning at least annually.
According to Hamar Laser, using too many tools, which require multiple setups, can lessen the accuracy of the alignment and cause idle time. The company says by utilizing multiple alignment targets and laser planes, alignment times can be measured in hours rather than days, at accuracies that conventional tools fine hard to achieve.
"The machinist's level is a good example: it has a resolution of 0.0005" per foot. That translates into 8 arc seconds for flatness, not very accurate for today's ever-tightening tolerances," says Hamar. "Furthermore, using a level to flatten a machine bed can be a long and laborious process, especially on bigger machines where one side of the bed might move when the other is being adjusted."
For 3- to 5-axis machining centers, Hamar's L-743 Ultra-Precision Triple Scan laser has three rotating laser planes that are perpendicular to 1 arc second and flat to 1/2 arc second in a 360 degree sweep and 1/4 arc second in a 90 degree sweep. The laser measures flatness, straightness, parallelism, and squareness of the entire length/area of the main axes of most machining centers, usually in one setup.
The L-743 features 1-arc-second level vials, a magnetic base for stable mounting and pitch, roll, and yaw adjustment for setting the laser parallel to reference points. Errors can be fixed in the same setup since the system provides live measurements. The laser is suited for applications with tolerances of 0.00002"/ft. or greater such as machining centers, injection molding machines, automotive transfer-line wing bases, and aircraft assembly jigs.
The L-733 Precision Triple Scan Laser is a lower accuracy alternative and has a medium adjustment base. Its three rotating laser planes are perpendicular to 2 arc seconds and flat to 2 arc seconds in a 180-degree sweep. This model is for alignments with tolerances of 0.0005"/ft., especially for the alignment of large fabrications, lower tolerance machining centers, laser-cutting machines, waterjet machines, or wings and body sections in large-aircraft assembly.
Stages and tables
While machines are often the culprit in misalignments, auxiliary machine equipment can also cause machine errors. Pinpoint Laser Systems, Newburyport, MA, helped one customer that had doubts about its X-axis stage travels.
Using the Laser Microgage, a laser alignment tool, Pinpoint Laser Systems measured the runout on a CNC machining center with an X-axis stage that travels 70 inches. The Laser Microgage provides measurements to 0.00002" and working ranges of 80 ft. and more. Laser beam accuracy is within 2 arc seconds and repeatability is within 1 arc second.
Over 60 inches of travel, the stage moved only minimally in the vertical direction. At the end of the travel range, the laser measured shifts of 0.003" or more. The problem was most likely caused by uneven wearing of the ways or distortion of the bed.
"Traditional measurements of stage run-out require optical systems, time-consuming techniques, or dial indicators that introduce a variety of error sources," says Pinpoint Laser Systems.
The company's Adjustment Plate, an accessory for any laser alignment system, that moves the laser in four axes, with precision micrometers, to simplify machinery and equipment alignment tasks. The laser beam can be positioned to within 0.001" for checking machine travel, straightness of fixtures and assemblies, precision bore alignment, and verifying parallelism and squareness. Four precision micrometers adjust the position of the laser, two dedicated to straight, linear adjustments in the vertical and horizontal directions, and two are for angular adjustments of pitch and yaw.
Hamar Laser Inc., www.rsleads.com/303tp-217 or circle 217; Pinpoint Laser Systems, www.rsleads.com/303tp-218 or circle 218; Measurement & Machine Technology, www.rsleads.com/303tp-219 or circle 219
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|Publication:||Tooling & Production|
|Date:||Mar 1, 2003|
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