I have a Starrett precison level I think is not perfectly flat on the bottom. What is the proper procedure for checking the flatness of a part using a surface plate? For example: Do you coat the part with Prussian Blue? How, a finger tip, what thickness? How do you test it on the surface plate - press it down gently, slide it, rotate it, what? How would you prepare the surface plate - clean with alcohol, use a damp kem wipe, what? Do you test flatness on different sections of the surface plate?
I hope these are not to basic. Any other hints, tips, cautions, would be sincerely appreciated.
You coat the plate with touching blue (I think you call it that way). About a fingertip on wipe that you spread over the plate (area where you want to touch). The thicker the blue is, the less precise results you get. Imagine having a layer of 1mm on the plate.*) :-))
Gently lay the part on the plate. Move part with little circular or longitudinal (depending on shape of part) gentle movements. Try not to tilt the part or you will get false results. If the part tilts/rocks, you might get it roughly in shape using a straight edge before proceeding with the plate. Touching surfaces _can_ get very tricky!
There are special cleaning agents. I use petroleum.
Not on mine, I know that it is flat everywhere. :-)
*) I have seen that in a report about a famous airplane manufacturer when they fitted wings to the body. I won't tell names to protect our European industry. ;-)
Put a tiny speck of prussian blue on the surface plate after a very thorough cleaning. Wipe the plate 3 times with your fingers to detect any speck of dust before applying the dye. If you can find a "speedball brayer" (used in the printing and silkscreen industries) this is the best thing ever to spread the dye evenly.
The coating will be about .0001" thick for this type of work! The part to be tested has to be completely clean of dust specs, again, use your fingertips to make a last test of the part before placing it on the plate. Place the part very gently on the plate, do NOT press down at all, and move it sideways by pressing against the bottom. You want to slide it, not rock it. Then lift off from one corner, or just straight up for small items like this.
You can also test the part on the clean part of the plate (no dye needed) with the "spin test". Place the part and gently turn it from one end. If it pivots around the far end, the part is concave, which is good for scraping. If it pivots around the middle, it is convex, and will rock and give a false reading when tested with dye, unless you take precautions to prevent it.
I have some chemical surface plate cleaner. I think it is mostly detergent and water, but I don't know exactly. I suspect alcohol will work fine. Watch out for all types of wipes, they leave lint. You have to remove the lint after the plate dries. A tiny speck of grit or lint will completely foul up a reading of this type.
If your surface plate is in good condition, you don't need to test at different locations. Most granite surface plates, even cheap ones, are incredibly flat. You have to test diagonally across the entire plate to find any error of significance. Cast Iron plates, on the other hand, can warp and wear over time, and can't be trusted until you have verified their accuracy.
That's a good tip on the brayer. I wondered how to get a smooth coating. Too thick and you get a false reading. Think I'll buy one. For
15 bucks it sounds like a good tool if you're going to blue the plate often.
Does your chemical cleaner remove the blue stain from the surface plate? Mine left a blue tinge on the pink surface!!! I hate when that happens.
The KimWipes (I misspelled it earlier) are low lint, non-abrasive, delicate surface wipes made by Kimberly Clark.I thought they would be suitable for cleaning but I would be interested in what you would suggest.
I see your point on using the spin test. It would be easy to rock the part and get a false reading. You'd end up scraping and having it get worse.
Sounds like you've been there and done that. It's tips like these I wanted to get. Many thanks.
| To clean up your hands, scrub with lots of liquid soap and a Scotch | Brite pad. Try not to scratch your nose while you're working!
In the factory washrooms they keep taking the Scotchbrite pads out, and I hear that they're really bad for you. Don't know why, I don't get myself that dirty usually. The other option for stubborn stains is hand lotion. Seems to get the most aggressive sealants right off your hands.
Before you do any scraping or other alteration, it is a good idea to repeat your test in a different orientation and/or with a different method to be sure your indications are not the result of your technique rather than an actual fault. Don Young
Get some shim stock and put a couple of thou. under each end. Then use another piece and run it under the piece. You can feel the high and low spots very easily. Unless you have a little experience in scraping and measuring you can get the wrong readings by looking only at the spotting on the surfaces. Also, any dirt, dust or anything on the surface will screw up your readings and blueing transfer.