At 12:05 1.9.1998 +1000, you wrote:
>Hi
>The other day i say a device that was attached to a lathe that gave a
>digital measurement from the tool (ie the position of the tool on the bed)
>to another point. *maybe the chuck, but this box had a zero button.
>it used a sensor that was waterproof, it was parallel to the bed, and had a
>male and female V section that slid over the other.  this unit had imperial
>and metric modes.
>
>my question to you all is, how would you make something like this ?
>how do you determine a distance to a resolution of a maximum of 1/1000 of an
>inch  that is waterproof and can live in an environment that is noisy, has
>oil/ water, metal shavings, vibration etc.
>

This gizmo is called DRO (Digital ReadOut) and is VERY usefull, because one
don't need to count handwheel revolutions like crazy.
Usually, there is optical encoder on the handwheel (or on the motor shaft).
The resolution of the encoder is something around 500 pulses per revolution.
Also, linear encoders are used on some machines.

I made this kind of unit for my Sherline mill (3 axis of optical encoders
plus the tachometer on the motor belt drive) because the commercial one is
expensive as hell. I used one PIC (as always :-), one ispLSI 1016E PLD, LCD
display and HP optical encoders (HEDS 90xx), several buttons and
commercially available plastic box. The whole encoder mount is made from
piece of round aluminium stock which turned and milled. After mounting, I
sealed the optical encoder box with the silicone glue. Not nice, but works
:-))

Best regards

PavelK
    
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