Scraping is a process that makes things very flat. You need a precision flat surface for sliding surfaces on precision machines. Well with that goal in mind, I have to learn how to do it. I'm getting the hang of it pretty well! First undertaking was the surface plate at left, mainly for practice.
This thing is 7" square on the top, 3/4" thick for the most part plus a 3/4" square bar running around the bottom perimeter for extra stiffness (as if the 5 pound monstrosity needs it). So overall thickness is 1 1/2". (Draft is something like 10 degrees, real steep but makes for easy molding. :)
So anyway, what is scraping? It's a process where you compare the work to an already-very-flat surface and knock down the high spots, ten-thousandths of an inch at a time. After carefully scraping away many layers like this, you can eventually get within millionths of the master surface plate!
Here's what I'm using. A piece of plate glass (usually pretty flat - I think I found a small bump in it, then again that might have been a bit of grit stuck under it) with dye/stain/paint/ink/etc. smeared on it. You place the work on and maybe slide it around a bit, pull it off and get to work scraping. The little pink circle is a chunk of house insulation foam I use to smear the stain (rich mahagony wood stain :) over the plate.
Oh and by the way, here's the new casting with ways already attached. You can maybe see some stain spots on the top surface (kinda brown/pinkish): these need to be scraped off. A high point will touch the master plate and pick up some dye. Scrape down the peaks, repeat ad nauseum and it'll get real nice and flat.
Which reminds me, the scraper! I just used a small bastard file whose tang was broken off.. spent a few minutes with the Dremel grinding it down to a nice rounded nose then took it to a diamond hone and got it scary-sharp (well, the total angle is maybe 120°.. not particularly scary to flesh but works great for metal).
First I scraped down the bed part, the aluminum bars of the casting. Then I drilled, tapped and countersunk holes for the way itself and got to scraping that.
Let me tell you, after scraping aluminum, steel seems hard as nails (erm, duh :p ) at first. But ten minutes with it and it scrapes just like aluminum, to me at least. And I don't see any reason yet why I should be afraid of using a carbon steel scraper here - I've only sharpened it once while scraping the steel and it's still going strong. OBTW, make some sort of handle. I shant regret the cheezy notched dowel I made for mine!
Oh, cutting fluid -- you might want to lubricate your scraper with some paint thinner/diesel/kerosene/etc. when scraping aluminum. Really cuts down on BUE (Built Up Edge, metal that sticks to the cutting edge, digs in and makes a nasty looking cut). I don't think there is much need for cutting fluid on steel...just seems to make it chatter more. Would probably work fine if I just clamped the thing tightly (with just the one clamp shown, the far edge of the plate is hanging in midair!), but meh.
About technique, you hold the scraper at a good angle to the work (maybe 30°), push, pull, push, pull...(reciprocating motion). Pressure on the cutting edge makes it cut so you can control depth of cut. (Remember to keep the cut even across the entire surface, don't let up just because the spots are smaller or anything.)