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Measuring flat, straight, and tilt.

Hardly a dimenewcomer to machine-tool calibration, the autocollimator was created for this use in the '40s, replacing cumbersome master straight edges, squares, and flats. Consisting of only tWO components, the instrument and a plane mirror, it remains viable today because of its portability and ease of use.

This simple and highly accurate device has recently been enhanced with digital readouts and computer software to ease the data-reduction chore, and has become easier to use faster, and more accurate. It is now being used to measure linear-axis straightness and squareness, surface-plate flatness, rotary-axis angular index, and tilt-motion error. Makers of ultrahigh precision linear motions and rotary devices will be pleased to know of the development of high-resolution (0.01 arc sec) devices with very low drift characteristics using monochromatic light sources (laser diodes) that permit lens designs based on a single light frequency. And, improvements are continuing.

How it works

The autocollimator is an angle measuring device with a typical resolution of 0.1 arc sec. It is used with a plane mirror, a highly polished 2"-dia surface that is flat to 3 micro-inches. The optical unit contains a light source and detector electronics to measure the angular displacement of the image reflected from the mirror in two axes X and Y) simultaneously. As applied to measuring straightness, X becomes pitch (rotation of the mirror about its horizontal diameter) and Y becomes yaw (rotation of the mirror about its vertical diameter). The measurement of roll (rotation of the mirror about its center) is not possible with an autocollimator alone, but precision electronic levels, can be used to make this measurement in the horizontal plane.

Figure 1 is a block diagram of the optics and electronics of a dual-axis electronic autocollimator. Although the eyepiece is no longer essential, it is useful for initial alignment of mirror and optical unit and also lets an experienced operator judge the quality of the reflecting surface. Straightness

But, you ask, how does an anglemeasuring device measure straightness of a machine tool in inches or millimeters? It's a simple software trigonometric conversion. The altitude of a right triangle whose base is 1" and whose angle is 1 arc sec is 5 microinches (actually 0.000 004 848 19, or for the common mirror base length of 4", each arc sec is worth .000 020". When measuring pitch, Figure 2a, angular data, acquired by moving the mirror sled in constant-length increments, describes the angle at each point between mirror and detector. Assuming a 4" sled, the angle data can be converted to inches of change by multiplication. Because the data probably has slop non-zero start and/or finish, this can be removed by plotting the data incrementally, Figure 2b, and connecting the beginning point to the end point. The perpendicular distance between two lines parallel to this line and including the peak and valley is the total straightness deviation. A relatively simple computer program can do this calculation, speeding the measurement process, improving the accuracy, and producing a graphic display of results. Compared to a manual autocollimator, the new interfaceable models convert what used to be a two-person job into a one-person job because no one is required to read the instrument. Because the instrument measures in two axes, pitch and yaw are obtained simultaneously. In the photo, notice the sled feet engage both the side rail and top rail for additional measurement time savings.

Squareness and flatness

To measure the squareness of axes, a pentaprism is used to bend the autocollimator light beam accurately 90 deg. With a typical accuracy of +/- 1 arc sec, the pentaprism acts as a master square. A word of advice: the best-fit straightness line of the first axis should be square to the best-fit straightness line of the second axis. Thus, the more data available to make this determination, the more accurate the machine tool.

Thanks to the work of J C Moody, Sandia Corp, we now have a procedure to measure surface flatness. His measurement strategy of perimeter, diagonals, and bisectors, Figure 3, resembles the flag of Great Britain, thus the name "Union Jack Pattern. "

It works like this: Each of the straightness lines (AB, BC, CD, etc) are measured independently. During data reduction, the diagonals are joined at their midpoint and the end points normalized, the slopes of the remaining lines are adjusted to the four end points, and the peak-tovalley flatness calculated. The resultant graphic, Figure 4, shows where the errors are. This is very useful to the person who is going to lap or scrape the surface, because it shows exactly where and what to remove.

Again, the computer program that performs this data reduction is a great time saver (often a 3:1 time saver) and makes this another job easily performed by one person.

Angular index accuracy

When used with a precision polygon or index table (such as an A A Gage, Ultradex, or Moore Special Tool precision index or small-angle divider), the autocollimator can be used to determine the indexing accuracy of a rotary-motion device. The polygon or index table is the master angle that is compared to the rotary motion device's measuring system or mechanical stops (shotpins). The instrument reads the angular deviation in arc seconds.

In this case, the polygon is a regular multifaceted mirror, typically with 4, 6, 8, or 12 faces, and the angle between faces is calibrated. Or the rotary index table-typically 180, 360, 1440 position-is a facegear device that is used with a plane mirror mounted parallel to the axis of rotation.

After initial setup, the index table is counter rotated to the rotary table to bring the mirror back into alignment with the autocollimator. Using these devices, the accuracy can be verified in many locations.

A dual-axis instrument is particularly useful for these measurements because the second axis is used to monitor the parallelism of the mirror to the spindle axis. Nonparallelism is the source of a second-order error and should be minimized. Since the autocollimator is not a fringe-counting device, its accuracy is not influenced by breaking the light beam. This contributes significantly to its ease of use.

Tilt-motion error

National Standard, ANSI/ASME B89.3.4, Axes of Rotation, describes the measurement of mechanical inaccuracies associated with rotary motions. Tilt-motion error can be measured by placing a plane mirror perpendicular to the axis of rotation and observing its precision with a dual-axis instrument. This method also works nicely with a load in place.

The error in inches/inch can be calculated from the change in angular deviation. This method has the additional advantage of being independent of faceplate flatness and squareness.
COPYRIGHT 1989 Nelson Publishing
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Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Title Annotation:autocollimator
Author:Moyer, Mike
Publication:Tooling & Production
Date:Jan 1, 1989
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