Here's something I wrote for Village Press to get you started:
Forrest Addy's Take on Scraping
First of all, don't even think about any form of plate glass as a scraping reference. Granite flats are typically 2" to 4" thick for a good reason. Glass is among the stiffest of our common materials but if thin it will still flex under load.
Scraping is back breaking work. Unless you're young and fit and have a strong back you'll wish you were long before the scraping job is done.
The tools are simple and not that expensive. A $40 import granite flat (glass will not work - no texture), a couple of good second cut 12" square files with handles, a 2" - 3" long stencil or rubber ink roller from the art supply store, some old mill files ground to make scrapers, a bench grinder and an oil stone - plus a $4 tube of Prussian blue from the auto parts store. Oh yes, plenty of rags and hand cleaner. Also have on hand a shop vac with an old hose to pick up the scrapings and filings. And a can of hand cleaner.
First an important message. Prussian blue is a pigmented transfer medium. Be aware of your fingers when they are smudged with Prussian blue. You will have visible evidence of their progress between initial smudge and realization all over your shop. One smudge on your finger will transfer to the light switch, your shirt, your nose, your ear, the back of SWMBO's silk blouse when you smooch her "thank you" you for the coffee she fetched. Wherever there's a smudge is another transfer-to-finger waiting to happen. Prussion blue is communicable like the plague.
The granite flat (Many call them "granite surface plates" but to me a surface plate is cast iron and made to be portable. Somehow I can't get past my training: to me the granite gizmo that does the same thing as a cast iron surface plate is called a flat.) is available from Enco Mfg. 1-800-873-3626 or
www.use-enco-.com. Their Model 640-120 for $39.96 12" x 18" flat may seem largish for a small shop but it's long enough diagonally to rework most machine slides with a minimum of overhang. A little overhang properly addressed poses no problem to final flatness.
You can get import granite flats for about the same price from most any catalog site serving machine shops. MSC, Travers, tec.
10" and up mill smooth flat files are the best starting point for home made scrapers. Any junky old thrift store file will work. Dust off the worst of the teeth with an angle sander and provide it with a handle.
Advance to the bench grinder. Holding the file at a 5 degree (roughly) angle to plane of rotation and using the side of the wheel, remove the teeth for 5/8" back from the tip. Then grind an arc on the end of the scraper using the wheel periphery. The arc should be about the same as the rim of a 3 lb coffee can. Stone both surfaces smooth but leave a sharp intersection. You've made what amounts to a chisel with a 95 degree edge.
There are superb storebought carbide scrapers on the market. I suggest the hand scraper sold by the Dapra corporation (
http://www.dapra.com/html/biax.htm) but they are expensive ($90 to $150 depending). The home shop typically doesn't have diamond sharpening equipment. If you know a rock hound having lapidary equipment consider him a scraper sharpening resource. A keen carbide scraper outlasts carbon steel about 100 to 1 (I'd almost swear). If you can fit a carbide scraper in your budget and you have several projects to justify it, I'd reccomend the purchase - or make one.
Here's a flat statement: you cannot satisfactorily sharpen a carbide scraper on a green silicon carbide wheel. The edge micro-chips and the effects of the crumbled edge will show in the ugly appearance of the scraped surface and the sweat dripping from your brows and elbows. Only diamond abrasives work well on carbide.
Carbide stones remarkably easy on those flat diamond plated stones such as those made by DMT and Norton...
Carbon steel doesn't last long scraping. It will dull rapidly. Resign yourself to touching up the edge every few minutes.
Work prep. Remove all rust from the work surfaces (rust EATS!!! scrapers and files).
The first step is filing. A second cut file removes metal much faster than scraping. Note the square file has a "belly" a convex portion. It will allow you to concentrate filing the high metal. Position the work so it's mid-forearm high. Glide the file over the aurface to see of there's anything sticking up. Scraping and filing is a task involving all the senses and lends itself to a Zen-like contemplation.
Apply six or eight 1/8" dabs of blue on the plate and spread them with the ink roller. Vacuum clean the work and apply it to the flat. Never apply the flat to the work if its clamped in a vise. The force of the vice jaws will spring it a trifle and you'll end up with a work that's concave. be careful you don't rock the work on the flat. You can scrape a perfect convex if your're careless when taking a "print".
There will be a few smudges of blue on the bottom. File out only the blue using the belly of the file. Refresh the blue with the roller. Take another print and file out the blue. File crossways to the first strokes. Proceed for up to several evenings until the work shows a scatter of blue patches all over. Sooner or later you'll discover just how hard to file (don't rub the file on the work, make it cut) and just where.
As work progresses the transferred blue will fade. You'll need to roll out more dots of blue to the flat every now and then. How much is a judgment call. Gain the skill.
Cleanliness is important. a single filing chip or piece of lint will screw up a blue reading (called a "print". Get a foot switch for the shop vac. If you see flecks of metal stuck in the blue, wipe the flat clean and re-blue. Use that shop vac and keep the scraping area clean. I've seen scraping benches that looked like a coal cellar. The work produced there was poor and expensive. Clean is cheap.
Proceed with the scraper. Practice a little on scrap cast iron until you can predictably make little flaky chips and nice parallel scrape marks. You'll soon learn to tip the scraper this way and that to catch blue spots not quite in line and to relax the downward force so the scraper's edge glides across low metal without cutting. You'll also learn to consciously relax your shoulders and neck. The little short controlled movements required for scraping force you to oppose one muscle groups with other muscle groups. Persist and your whole shoulder girdle and lower back will become one big knot you'll have to stew loose in the hot tub.
It does no good to rub the blue off with the scraper. You have to apply enough force and effort to remove metal. The skill is acquired through skill and practice. Make chips and study them through a 10X loupe. When old timers hand scraped for stock removal the powdery cast iron fountained 3" above the scraper but we're looking for control just now.
After scraping, lightly stone the work with lighter fluid (evaporates quickly, has a low latent heat, and helps keep the stone free from pinning) to remove any raised burrs. Clean the work with a rag and lighter fluid. Let it evaporate and give the scraped work surface another rub on the refreshed blue. Scrape some more from another direction. I like to scrape from two or three directions; each in rotation.
A Norton made, 2" x 5" fine India stone is an invaluable aid. When new they're too sharp. Condition one side by rubbing it on 220 wet or dry. Be sure to clean it. Tramp abrasive is worse than scraping crumbs.
Some of you may be alarmed at using flammible stuff like lighter fluid as a cleaning agent. I can only say it's the best solvent I've found. It's flammibility is balanced by its other properties mentioned above plus the stuff I prefer, "Ronsonal" in the yellow bottles, has little propensity to rust cast iron.
There's a time element as well. Lighter fluid evaporates in seconds and doesn't carry away much heat from the work. It's a major time-saver. Lacquer thinner is harsh on the hands and has a higher latent heat. Plan on waiting up to 30 minutes for mineral spirits paint thinner to evaporate and an hour for real Stoddard solvent. Flammibility isn't a real issue in my shop. I don't smoke, I'm careful with rags, and my scraping area is well ventillated.
Water based cleaners have a high latent heat of evaporation. They cool the work causing it to go concave - meaning as you work you'll scrape more than necessary off the ends of the work. When the work reaches thermal equalibrium. I once used a water based machine cleaner while scraping a 60" precision straight edge. Worked great!!! But: the next morning the straightedge had reached thermal equalibrium. My first check against the master flat showed the straight edge turned 0.0060" convex over-night. It was the cooling effect of the water based cleaner that did it. If you don't think water based cleaner won't have an effect on the scraping of a little old machine tool slide, try it and report back.
Proceed with scraping until all the file marks disappear and you get 4 spots per square inch. If you're careful with heat input at the final stages, and you scrape with a keen edged scraper, final flatness will be equal to the granite flat you're using. A small import flat is typically in the 0.0002" to 0.0003 range of a geometric plane.
I wouldn't suggest you lap out the scraper marks when done. For one thing there a belief that the slightly interrupted surface results in lower working friction. I believe this to be true but I have no numbers to support it except a maybe little less sweat. For another thing, a hand scraped surfacee is a bragging point and a handsome feature in its own right.
Precision scraping is a skill best learned under the guidance of a mentor but a crafty individual can discover most of the tricks for himself after getting a push in the right direction.
