Re: "Machinist Tools For Modeling, Part 1 - Hand Tools" Clinic ar rent NERPM

Dennis Storzek

On Sun, Jun 9, 2019 at 07:58 AM, vapeurchapelon wrote:
I just don't trust those digital things. I have had two failures with them, both showing real crap at the display after some time of usage. (Don't know the make anymore.) I realized this as the display value was WAAAYYYY off the real value - several mm! But what with errors in the decimal digits?
Now that the list traffic has slowed a bit, might be a good time to comment on this. If industry felt the same as above we'd all be in big trouble, since the same digital technology has been in use for digital machine read-outs and the closed loop feedback on CNC machines for about the last forty years.

I suspect what the display was showing was some form of error code; these things are sophisticated enough to react when they are missing counts and give a display that won't be mistaken for a measurement. In interpretation of the code could likely be found in the documentation that came with the caliper, if there was any. There are two main reasons these displays will throw an error code; the scale is dirty or contaminated with oil or coolant, or the reader was pushed along the scale too fast and the circuitry couldn't keep up with the count. I have that problem with the digital read-out on my optical comparator; the read-out is capable of full four place accuracy, and the stage can be uncoupled from the lead screw for quick positioning, but it needs to be done at moderate speed to keep accurate count. The older way to keep a check on this was the use of index marks at intervals along the scale; if the counter reached the next index mark without counting the proper number of steps, the control would fault to an error code on the assumption that it was missing steps.

Did a little more reading on the Mitutoyo Absolute Digimatics... they claim FOUR scales of different pitches, so placed that any point in its range has a unique address. This eliminates the need to re-zero on power-up, and makes them very resistant to over speed errors. Just a bit more sophistication than the Harbor Freight imports.

Dennis Storzek

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