It also does not give zone information although with modification that is also possible.
This method relies on a cheap laser level as a light source. Although I use a surface plate qualified to 0.0001" as a reference, in this test it isn't necessary once you know what to look for.
In pic 1 is the test setup. The laser level is on the right, the test surface just to the left of it (surface plate) and on the far left of the bench is the white screen.
The laser level is set to project a horizontal line. It is adjusted so that the line just grazes the surface of the test object with the angle of incidence nearly zero. The angle and height are adjusted so that it is possible to split the thickness of the line with the far edge of the test object. A portion of the beam illuminates the screen directly while the rest of the thickness of the beam is reflected from the surface at grazing incidence.
Note that at grazing incidence it is not necessary for the surface to be a reflective polished surface or even metallic. It merely needs to be fairly smooth.
There are three main considerations in this setup. You must be able to adjust the angle of incidence, the height of the laser and the parallelism of the beam to the test surface.
If the beam is not parallel to the surface it will result in this condition shown in image 2.
When properly adjusted in all axes it will look like this:
In most of these images a six inch scale is included.
Image three is the signature produced by my surface plate which is certified to .0001" flatness.
The bottom line is projected directly on the screen and represents a reference as it is a portion of the top edge of the projected light from the laser that overshoots the test object. The top line on the screen is the portion that is reflected from the test surface. Because of the extremely low angle of incidence the entire test surface is illuminated and the entire surface contributes to the reflected image that composes the top line. This is obtained by careful adjustment of the height and angle of incidence of the laser.
The angle of incidence is adjusted so that the beam sweeps the entire surface of the test object. When it is correct the separation between the top and the bottom lines on the screen will be about equal to the thickness of the original laser line multiplied by the ratio of the distance of the laser to the center of the object and the distance from there to the screen. In this setup it is about 5 to 1.
By sprinkling a bit of salt on the surface plate it can be seen in image 4 that the entire surface is illuminated by the beam.
Because of this a multiplication of any errors of slope is obtained. In this setup it amounts to about 240 times. This is arrived at from the ratio of the width of the laser line, about .1 inch, compared to the length of the area it illuminates, about 12 inches. This gives a factor of about 120 but is doubled because the angle of reflection is opposite and equal to the angle of incidence. That gives 240. The amount of multiplication will vary depending on the angle of incidence. If the angle of incidence is kept the same for other test objects the multiplication will remain the same. The lower the angle of incidence the greater the multiplication. The angle of incidence must of course be greater than zero.
The amount of error in slope on a surface depends on what distance that error covers. A concavity of .001" over ten inches produces much less error in slope (about 1/10) than the same .001" over a distance of one inch.
Now that the setup is adjusted for use we can proceed to test some surfaces. The first is a section of 1/4" thick aluminum mill plate. It hasn't been polished or otherwise altered, only the edges have been machined.
It is apparent with the rule that there is a slight gap in the center. It measures with brass shim stock to be about .002" bowed downward.
In the laser signature it is obvious that this piece isn't flat as this error shows as a much thickened top line compared to the bottom reference line. Not all parts of the laser light are reflected to the same part of the screen because of the slight curve in the part.
The thickness of the top line is about .5" which corresponds to the .002" error magnified around 240 times.
The next part to be tested is a piece of 3/4" tooling plate. This is pretty flat stuff and an eyeball check with the rule confirms this.
But, it isn't all that flat. A check against the surface plate revealed that it is bowed slightly, about .0004" in the middle. The laser signature confirms this as it produces a double line.
Last is my mill table. There are some gaps and spurious reflections produced by the edges of the four rails that make up the table. Also, the very ends of the center rails (last 1/4") are very slightly turned down by a few tenths from the finishing process. In all though it compares very favorably with the surface plate and approximates it in flatness. Keep in mind that any out-of-flat condition in side to side or end to end of each rail relative to the others would result in a horizontal or vertical misalignment of the line segments.
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