Machine Tool Inspection

By Rick Glos, HEIDENHAIN CORPORATION

Within the realm of new machine design or rebuilding existing equipment, designers and end users must consider much more than simply how machine tool measurements are encoded. With ISO 9000 methods now being adopted by many companies, special attention must be given to the overall performance of the machine as it is being used in the realm of normal operating conditions.

Though geometric errors on machine tools are easily corrected with state-of-the-art electronics (CNC Controls and Digital Readouts), one must also give consideration to the dynamic behavior of the equipment during machine operation. Temperature and load variables are not easily correctable within CNCs and it would not be reasonable to correct one source of errors and excuse all others.

Many companies are now coming to terms with the realities of this, and are now directing more of their attention in regards to inspection toward the actual machines instead of the finished product.

This discussion will briefly address the types and technologies of the highly accurate, optically-based digital inspection devices that can, and are, being used to inspect machine tools.

SINGLE DIMENSION TESTS

Lasers
The first category, and the one which most individuals are familiar with, is lasers. Currently, the applications for lasers in machine inspection deal with determining distance and, with special optics, pitch, roll and yaw can be evaluated.

The primary advantage of a laser is that the beam can travel over extended lengths where old mechanical mechanisms, such as gage blocks or special step gages, used to have a place. Always of elemental importance, the beam path must be clear.

This has been, and still is, a major tool for determination of some machine errors. Where it falls short is in applications where the machine must be considered in its dynamic role, particularly on CNC machines where two or more axes must move in a coordinated effort such as in a circle or elliptical to one of the axes.

Graduated Standards
Another older method is the use of a graduated standard. This is typically a straight piece of glass with precise lines that are counted, commonly through an eye-piece.

Photo 1: HEIDENHAIN's VM 101

Today's versions offer a graduated standard line grating scanned with a light source and photodetectors. Typical line gratings are 20 and 10 microns. They are often used as position feedback for CNCs.

Linear scales whose scanned light is manipulated into wavelength cancellation are referred to as diffraction-based interference scales (Photo 1). Unlike a laser, the diffraction principle is not affected by the quality of the air, making these measurement tools more effective in many cases. Also, these linear scales are a fraction of the cost of a laser.

MULTI-DIMENSIONAL TESTS AND DEVICES

While the mechanisms and technologies for single dimension measurement are more than adequate for their intended use, they fall short in their ability to take measurements on the machine when a desire to review performance in a dynamic sense is required.

One of the multi-dimensional tests used to review dynamic performance is having the machine run in a programmed 360 degree circle. The test is designed to monitor the machine performance while it is trying to simulate a cut on a 360 degree circle. This check measures how well the machine/CNC interaction is performing while it is interpolating the circle. Naturally, two axes must move together in a coordinated fashion to accomplish this move. The machine geometry, the balance of the two-axes drive systems, and the CNC itself influence how precise the circle will turn out.

Double Ball Bar
Devices used for this circle test attach balls to one end of a spindle and at the other end to a tower placed at a predetermined radius. As the motion swings around the circle center, deviations to the circle are detected by the device as the radius changes. These mechanisms are typically called Double Ball Bars (Photo 2).

Besides circularity, this test can also determine squareness, backlash, directional repeatability, and radial deviation, just to name a few. From pattern generation, one can also determine if the servo motor gains are in balance and need to be adjusted.

Photo 2: HEIDENHAIN's Double Ball Bar

Lattice Grid Plate
The newest and most intriguing of the optically-based technologies is the lattice grid plate. Developed by HEIDENHAIN and called the KGM 101, this is a device with a steel substrate that has been constructed with a grid pattern of squares. A special scanning unit is positioned above, but does not contact the grid area (Photo 3).

Photo 3: HEIDENHAIN's KGM 101

Using the principle of diffracted light, a position can be determined with motion. What is unique is that motion in any direction or bi-direction can be determined. There are no restrictions due to mechanical linkage such as the spindle to the tower, as in a ball bar. Here, the motion is limited to the radius of the ball bar unit. On a laser, the motion is limited to the line of sight of the projected beam and the associated reflectors. To study a diagonal, for example, the laser and/or reflectors must be repositioned.

Applications of the lattice grid technology can now approach the research level. This new lattice grid on a plate still allows for the standard circular tests that the Double Ball Bar devices perform, but it also performs other unique tests. For example, very small circles at high velocities can be achieved. These smaller circles, in a sense, wash out geometric errors leaving only the errors produced by the electronics or the control.

With the lattice grid plate, it is even possible to make high velocity 90 degree turns in order to observe the behavior of the machine. Oblique straight moves can be used to check linear interpolation. It is even possible to test a small contoured move of any shape and determine the X/Y accuracy of all positions of the program as the machine is running.

Digitally-based optical machine inspection devices have come of age. They are easy to use, fast and highly accurate. For machine inspection, they are absolutely state-of-the art, and, in most cases, very reasonably priced. Because of this, the trend toward regular machine tool inspection by both OEMs and end users alike is well on its way.

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