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Thread: Optical Comparator Overlays
09-14-2004, 06:58 AM #1
I have just obtained a used Mitutoyo 10 inch screen Optical Comparator (PJ 250). Co does not have any screen overlays, anyone know of a source for such or is it possible to roll your own using a laser printer. Any suggestions appreciated.
09-14-2004, 08:00 AM #2
The overlays that we used at Husky IMS were made by creating a CAD file at whatever scale you use in your comparator (10x) and then plotting it on mylar. Most plotters have a feature in set-up that allows you to apply a scaling correction factor to both x and y axis and if you use that feature on a plotted scale, you will be able to get the accuracy that you need. If it doesn't work out for you, send me the comparator.
Glenn McMullin, Metro North machiine.
09-14-2004, 09:40 AM #3
We used laser-printed mylar overlays too. I was the CAD man there, so I was charged with scaling the printed image to be of accurate size. I used Autocad and it has scaling capability in the software. I printed on paper first to get it close, and it still took a few additional trials on the mylar to tweak the scaling so it was right on. Across the width of the paper the print scaling repeated, but just changing from paper to mylar changed the size of the image across the length quite a bit. It's a fairly simple process, but you might waste a little transparency stock in the process of getting it Nuts On.
Of course, once you figure out the numbers to plug in, you're set as long as you are printing on the same stock. I made a bunch of different overlays with standard acetate transparency stock too.
Occasionally I was inspired to add a "Dilbert" cartoon in one corner. I got good feedback, at least from the people that mattered.
Enjoy the new toy!
09-14-2004, 08:53 PM #4
I've never learned how an optical comparator is used. Can someone give me an education, please?
09-14-2004, 09:02 PM #5
Alan: One more vote for laser on overlay material. We use 10x or 20x optics so the roughly 0.002" (or so) accuracy of the laser becomes about a ten thousandth.
John: An optical comparator is used to inspect parts by examining their blown up silhouette against a template or scale. It usually consists of a positioning stage (left, right, up, down) and transmitted or reflected light source. Newer ones have lots of goodies including DROs (digital readouts). Think of it as a microscope with a large viewing screen 'cause that's basically what it is.
09-15-2004, 04:07 PM #6
Gents, thank you for the reply's, some useful info, I feel another learning curve is about to start.
09-15-2004, 05:19 PM #7
OK. My attempt at optical comparator instruction:
I hope this makes sense and helps a little.
Basic O.C. Operation 101
The obvious feature of an optical comparator is that a magnified image is projected onto a screen, like a microfilm projector at the library. The image can either be a silouette or a reflected image, depending on which lighting mode you choose. The projection screen can be interchanged, or a transparent overlay placed against it, to use different measuring aids for the feature you are checking. The most common and versatile projection screen is a simple cross-hair.
You place the "widget" on the table in front of the lens, focus the image as crisp as possible (as simple as that is, you'd be surprised how many folks I've worked with don't know how to focus an image).
Then you move the table until the cross-hairs on the projection screen are dead-on your first point of reference on the widget. At this point you zero your dials or DRO. Without a DRO this is a little trickier. Since the method varies from machine to machine, I leave that to you to figure it out.
Then you turn the cranks and move the table so the cross-hairs are dead-on the next point to find the distance between this second point and your first reference point. This method of measurement does not depend on the accuracy of the lenses/magnification since you are looking at the exact same spot on the screen (see note below). The table moves and you read the micrometer dials to find how far you moved it.
End of lesson one.
Overlay Measurement Method
Most comparators are equipped with very well-made optics so the magnification is very accurate. But you still should check the comparator's magnification against some kind of precision standard before you depend on overlays/projection screens to tell the truth. An overlay that is printed perfectly to 50X scale will not quite groove with your projected image if the comparator's magnification is 50.03X and may cause problems for you. The solution is to match the scale of the overlay with the actual magnification rather than what the lens says it is supposed to be. There are other factors with optics that can cause distortions on different areas of the projection as well.
Optical comparators have a chain of elements that need to be aligned/calibrated for them to perform their best. I've never looked into it, but apparently it's a tricky thing to do it well. There are engineers that make a business out of calibrating O.C.'s for a list of clients around town, and they aren't a cheap date. My guess is that for home shop use a satifactory job could be done once you learn the basics. I hope this is true anyway, since I have one in my basement that could use some TLC. Hopefully it's not much tougher than collimating a telescope. Maybe Evan has some experience with this.
09-15-2004, 09:01 PM #8
Thanks for the quick education. Am I right that we still have to mike the parts at the end and that the comparison being seen on the screne is a rapid look for anomalies?
09-16-2004, 10:08 AM #9
Not necessarily, John.
A good optical comparator can be depended on to yield accuracy to .0001" so they are very nice (if not essential) to have around for measuring features that are too difficult to reach with standard instruments. However, maybe it's my non-trusting nature or maybe it's just a good idea, but I make as many measurements on a part as possible using a micrometer of some sort. You can always check what you're getting on a comparator vs. a mic and see how they compare.
We had a lot of small parts with many steps, grooves, cross-holes, threads, etc. The magnified image helps a lot for seeing things the naked eye can't, but we also made most of our measurements directly on the optical comparator. They are pretty dependable for accuracy.
An ISO9000 (etc.) company that needs to adhere to beaurocratic standards can't use an optical comparator (or any other gage) for direct measurement unless the gage is certified to be accurate. This "calibration" is expensive for an O.C., but certification is not necessary for the small or home shop. Just checking the comparator against a precision standard should be enough.