The general rules* I teach is be a detective and figure out how low the low spots are. If it's a flat plate lay the surface your scraping down on a granite plate and use a feeler gage to and go around the edges and see how much you can slide in. Mark it with a magic marker and how much. Lets say when you blue it up there is a hole in the middle of the plate you can't see how deep that hole is but know there is one because it isn't touching.
With the plate surface facing up lay a straight edge on the plate and slide in a feeler gage under the SE or set a surface gage with indicator resting on the surface and rotate the gage from the higharea to the low now bluing area. Testing from a granite surface plate with a gage and indicator won't work because your part is on as it may not be parallel to the bottom.
Make a drawing or sketch and record the error. Then: 1) If it is out more then .005" machine it if you can. 2) Think that with one pass by with the correct pressure down, a sharp blade ground at about a 60 or 90 radius, scrape like your scraping paint off a surface with 1 to 2 " strokes over lapping each other. Most people when scraping take off .0002 to .0004" per hand scrape unless your Brutus and you may take more. In the classes I set up a surface gage and .0001 or .0005" indicator and measure the depth each student scrapes so they can calculate how many blind cuts they need when roughing down to remove a low area or hole.
I recommend you use a marking medium to roll on like the Canode yellow ink mixed with Windex or we used to use red lead...some use rubbing alcohol to fade the shinny cast iron. With the the facts you know how deep the low area are, calculate how many "Blind" scrapes you will need to scrape paint technique to get it with in .001" before going for a bearing. Also as you get more high spots when you blue, gradually shorten your stroke I say that 100% of the area needs from 2 to 5 PPI on it before starting to scrape the checkerboard shorter stroke method. You gradually shorten the stroke but never lighten the pressure down. Always press the same pressure down when scraping to keep the low and high spots the same all the time. Even when going for 40 PPI press down the same, just use a blade with a smaller radius like 40 R or 20 R and short stokes.
3) Then I say to scrape until you get the surface to .0002" in 12" and have 15 to 20 PPI and then shorten you stroke to 3/16" to 1/16 if your looking to get super precision bearing, ,00005/12" and 40 PPI. You have to scrape diagonal checkerboard during the whole process to set up the pattern.
You can get a good idea by watching the You Tube link from Jan one of my students. He is lifting it the wrong way in this one, but corrected how to break it free in a later video.
https://www.youtube.com/watch?v=O2nyrSq0tY4 he is using a biax, but the technique is the same when hand scraping. * Just so "everyone" knows there are any ways to skin a cat and Low Country is asking about scraping not grinding or milling.... In the old DAPRA catalog they have a chart that explains this too. I will see if I can add the manual or chart. Look at page 6:
http://www.scribd.com/doc/66183655/Biax-Scraper-Literature-1991 Rich
PS: Another thing is to use a soft blow hammer to Wring the plate like a tuning fork now and then to help stress relieve it, also when the plate it laying blue side down tap on the bottom and listen for a solid noise...if it it low and not touching the sound will be different...it sort of slaps. Hard to explain, but once you do it, you will figure it out.
