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Re: When even a Davis Mark 3 is too much
From: Brad Morris
Date: 2017 Jan 15, 19:03 -0500
From: Brad Morris
Date: 2017 Jan 15, 19:03 -0500
Örjan
You wrote
As to centering error, centering is more or less automatic with hirth rings, but mounting whatever is to be indexed true to center... Another story entirely.
Precisely! The standard by which I measure accuracy involves the use of a hirth device (commercially known as an Ultradex). The one in my tool kit has 360 divisions, 1 degree apart, with any one division to any other division accurate to 0.5 arcseconds (repeatable to 0.05 arcseconds). I measure that Ultradex with an Nikon 6D Dark Field Autocolimator, accurate to 0.5 arcseconds. The combination permits me to measure rotational axis performance (extremely analogous to index arm and arc) to about 1.05 arc seconds over 360°. I can see the angular error from any one whole degree to any other whole degree, with NIST traceable devices.
When using commercial encoders (arcs) on rotational axes (index arm), it is quite easy to see the effect of centering error when using my equipment. In simple fact, it is a mechanical requirement and the mechanical arrangement of mounting (eccentricity) is specified by encoder manufacturers.
Indexed true to center indeed!!
Brad
