optical comparator overlays

whats the generally accepted practice for making optical comparator overlays in the computerized world? print out a transparency and verify with the comparator?

You can also create them by hand, very accurately, using a scriber and pointed trammels.
Gene

You could print directly on mylar if your printer and system were properly calibrated. You'd need probably 600 dpi or better, depending on the tolerances of the part.

- Leigh

This is how I do it...

I use the overlay for more difficult to measure features. If I am confirming geometry, I will plot the basic shape, and offset the line on both sides to act as a visual tolerance band.

Depending on the feature callout, the part may be able to be best fit into the overlay.

I plot some calibration feature along side the inspection detail. It is usually a short line, two lines spaced out, a square or a circle. I also label it. The size of the calibration feature is geared to the magnification of the shadowgraph. I can confirm the general accuracy of the overlay from the readout on the shadowgraph.

I usually use transparency type printing media. The material I use has one smooth side for printing and one frosted side to help feed it thru the printer. I believe there is a setting on many printers to support the media. I set the print quality to best and tune the line thickness to as thin as possible while still showing up.

Most of the time an overlay I use only takes up a small amount of the 8.5x11" sheet, so I may print several copies of the same detail on one sheet. Feel free to add text in two sizes, one to be read undermagnification and one for the naked eye.

It seems that once people use the shadowgraph overlay they consider it a useful tool.

I understand that the term optical comparator may be better than shadowgraph, but I don't care.

I use to draw thread profiles in AutoCAD to a certain scale. We would print them on over head projector tranparency flim for lazer printers 1:1 scale and they would be accuarct to within a few thousands in 11" Worked out great'.

Ken