Conrad Hoffman
Diamond
- Joined
- May 10, 2009
- Location
- Canandaigua, NY, USA
The profiler in question is basically a collimated light source and a camera system. You hold something in the beam and the camera end sees and measures what is essentially the shadow. If the object is a round pin, I can be highly certain that the light is being cut off tangent to the round surface along a line. IOW, I know exactly what surface is controlling the measurement. The situation is about the same as using an optical comparator or measuring the diameter with a micrometer, and I'd expect them to all be in very close agreement.
Being so fast and easy, somebody decides to measure a hole. If the hole is in something thin, say a photo-etched part, where alignment isn't a critical issue, you get the hole diameter based on the limiting aperture formed by burrs and taper. Probably an accurate result and I wouldn't expect a major argument in getting it accepted.
My problem starts when somebody wants to measure a bore with any depth. The alignment becomes critical and it has to be done in two planes to have any hope of a usable result. Even then, I can't say with certainty what edges are controlling the measurement. IMO, and I think my logic is sound, if you can't say what edges/surfaces are being measured, you can't properly say whether a part meets the print or not. The results may be interesting and useful for process control or whatever, but they can never be used as an argument for compliance to a print. (same issue with any flat part, say a gauge block)
I've never seen any "official" comments on this from NIST or any other authority I can find, but it seems fundamental that you have to know what surfaces/edges control a measurement for it to be a deciding factor, legally or otherwise, in compliance.
Or am I just being too nitpicky? Any comments on this?
Being so fast and easy, somebody decides to measure a hole. If the hole is in something thin, say a photo-etched part, where alignment isn't a critical issue, you get the hole diameter based on the limiting aperture formed by burrs and taper. Probably an accurate result and I wouldn't expect a major argument in getting it accepted.
My problem starts when somebody wants to measure a bore with any depth. The alignment becomes critical and it has to be done in two planes to have any hope of a usable result. Even then, I can't say with certainty what edges are controlling the measurement. IMO, and I think my logic is sound, if you can't say what edges/surfaces are being measured, you can't properly say whether a part meets the print or not. The results may be interesting and useful for process control or whatever, but they can never be used as an argument for compliance to a print. (same issue with any flat part, say a gauge block)
I've never seen any "official" comments on this from NIST or any other authority I can find, but it seems fundamental that you have to know what surfaces/edges control a measurement for it to be a deciding factor, legally or otherwise, in compliance.
Or am I just being too nitpicky? Any comments on this?