Have you seen the Home Shop Machinist magazine series on the subject? It's a bit more involved than necessary for just doing a plane sole, but Part 2 (of 5), Jan-Feb. 1998 issue depicts scraper shapes, sharpening, and application.
To cut to the chase: if you just want to see how it works, fit a good, secure handle to the file and wrap the teeth with duct tape to protect your hands from blisters and abrasion. Use at least a 10" file, bigger is better after you get the hang of it. Grind the business end square to both flats of the file, but with a very slight curve from edge to edge. Now use an old oilstone and polish the curve to yield an obtuse but sharp edge with the flats, and polish the flats. Dub off the 4 corners to avoid catches and scratching. Ideally, the flats at the nose of the tool will be nearly mirror polished. I am careful using good stones because the process of polishing the curved nose can dig them up to where they are hard to use for your woodworking tools. You will have a very crude, awkward tool but one that can be used effectively until you determine if you want to buy a "professional" model that accepts resharpenable high speed steel and carbide inserts.
In the "old" days, a shop made scraper would be made by grinding all the teeth off the file. Then it would be pack annealed to cool very slowly. Then it would be completely reforged to have a broad (1-¼" - 1-1/2" wide) nose about 1/16" thick for easy resharpening, which is a constant task, especially as you will probably encounter chilled spots in a plane casting. Lengthwise the scraper would taper gradually from tang to tip to yield whatever characteristic springiness the maker desired. Then it would be hardened and slightly tempered to "glass hard" at the tip. As it wore, it would ocassionally be reforged and rehardened. My suggestion is that if you discover a penchant to continue this skill, at that point it will be more effective to buy a commercial model. (Don't be fooled by the "bearing scrapers" sold for a few bucks in some catalogs; they won't be even an improvement on your duct - taped file for flat work)
Good Luck! SMT
First off, the "superior" part of the scraping process comes from using known references and a disciplined approach. The work itself is fairly crude, dirty, and monotonous. You use the known reference to spot the work, and scrape off the high spots. If you do it often enough and use insight and discipline in your approach, after a while the work will be a mirror image of the reference. Sort of like the dentist touching up a crown. So if your reference is not flat, neither will the work be.
The difficult part, and IMO the most likely pitfall for the novice, is to scrape too carefully, too early in the game. You start to get a beautiful frosted pattern and as the blue gets thin on your reference, you scrape more lightly with the appropriately shorter strokes. Soon there is almost a surface plate quality - but only in a limited area of the casting. Hours go by, and, loathe to scrape out work that took so long, nothing much else ever happens. Or, insidiously, you begin to cheat, and press on the casting, or rub it back and forth vigorously in an attempt to force the marks to appear where you want them. Then things really go haywire, with unstabilised markings, and the work is either put up on a shelf in frustration, never to be touched again, or taken to the belt sander and the whole idea of scraping dismissed as ridiculous and impossible.
What is necessary is to have the judgement to note after the first or second spotting cycle that maybe a bit of work on the belt sander, or with the body grinder might help speed up the rough work. Don't get careless and cause divots that you wouldn't want showing in the finish surface. Mark (spot) often and grind lightly as you get a feel for how things will go. Don't over heat the casting. When you have blue spots everywhere, even if only maybe 1 spot per every 1 or 2 square inch of surface (on a large casting) you are ready to resort to hand methods. Use a thick smear of blue on your reference and scrape vigorously. Each cycle spread out the blue and remove dirt and lint. Always rub down the burrs with a small fine oilstone and thoroughly clean the casting before spotting. Cycle by cycle, the blue will get thinner, the spots will get smaller but more numerous, and you will scrape with shorter lighter strokes. This is the ideal. A complete covering of blue on the work surface never means that it is flat. It only means that you have too much blue (too thick) on your reference to accurately discover or discern the true condition of the work. When the spots are very faint, very uniformly distributed, and very small; the surface will be very flat.
Clearly, lapping will also work, if you prefer. the point is to have an accurate, stable reference to check your progress. I prefer scraping, but the metal removal method is that; a preference.
For tools like plane soles and sides, I seldom go much beyond 6 to 8 spots per square inch on fairly faint blue. The overall appearance of the surface will just be beginning to appear attractive and coherent rather than scarred and chaotic, yet on the high spots, flatness will approach parity with your reference. FYI, a machine way would go to maybe 15 to 20 spots on almost indiscernable blue spots; a surface plate to 30 to 35 nearly invisible "pinpoint" spots set vaguely sparkling by evaporated alcohol.
RJ, I have never used a glass plate. I am told that they are very flat, but not really rigid. I'm guessing here, and bet 3/8" thick would work for up to maybe a #4-1/2, be a bit ambigous for a #5, and marginally inadequate for a #7 or #8. It might hold the bluing better if it were scuffed once all over with 400 or 600 grit. I agree whole heartedly with you about what areas of the sole need attention, and the rest can remain hollow. What is neat about scraping and the characteristic marks is that if you scrape honestly (don't force markings, or superficially scrape low areas for appearance) you can stop at any time, know where you are, and improve things later if you decide it is appropriate, either for appearance or for function.
To scrape fittings to a bedding, pick the one easiest to spot flat surfaces on. I scrape the frog and use small hand scraped flats that are flat right up to the edge so I can butt them into the corner where the casting angles down. A glass plate would probably work well here. (Granite surface plates have a large bullnose radius edge that makes them unuseable into a sharp corner). I do not worry about angularity, very little metal will be removed. If it is a type 9 or later frog, the bottom end will present some difficulty in establishing stable markings, but it is important. Get these surfaces flat and bedded, and a bedrock will have nothing on you (except cachet and ease of adjustment). Again, I use small hand scraped flats, but think that glass strips might work even better. Grip the frog lightly in a padded vise so the surface is horizontal, and balance the flat on it. Nudge it back and forth maybe twice, don't force the markings.
If you work carefully and thoughtfully, the frog is easy because the surfaces are so small. Now the frog is the gauge to mark its bedding area in the plane body. Blue it up and spot it carefully, but you will have to slide both back and forth and sideways to cover all of the bed surface for a good adjustment range. don't force the markings. Here I use what ever works to knock down the protrusions: sharpened end of three point files, gravers, pulling a small metal scraper backwards like one scrapes with a wood chisel, anything to get the job done. I do worry about honest markings here for function, but not at all about scraped appearance. The bottom, at least, of the face of the frog may benefit from a bit of attention. They are often warped, meaning that the iron does not bed the whole way across.
The idea of this approach is that when the parts are screwed together, they do not distort and stress each other; but reinforce each other and act as a solid unit.
A plane casting usually has "chilled" hard spots here and there, especially around the part under the frog bed. For this reason, it is less frustrating to use a carbide scraper. A HS steel scraper cuts almost effortlessly, and more smoothly, in comparison to carbide when it is honed sharp, but you may have to sharpen and hone almost every cycle on any plane larger than a #4-1/2 or #5. The carbide goes on and on with just an occasional touch up with a fine diamond hone. Just to note: it shouldn't be necessary to force the scraper to cut. If it's skidding, or it's difficult to hit your marks with some accuracy, it's dull.
I am advised that I should note that portions of this post, and my previous post in this thread, include excerpts from work copyright by me 1996,1997,1998 Stephen M Thomas. Hope it's useful. I feel like it's long winded, but will be glad to answer further questions if anyone is really interested. Good luck and blue fingers! SMT
Your observation about cast iron is typical, it seems to have a hard slick skin, then when you get under it, scraping goes more easily except in "chilled" areas.
Steel is very difficult to scrape smoothly. A carbide scraper is convenient for roughing, and can be kept in shape with typical diamond hones. You can switch to steel for finishing, or stay with the carbide. Just like in woodworking tools, the steel will develop a keener edge, but carbide wil cut longer while slightly dull.
As Steve K. describes, steel scraping borders on art and seems to vary a little each time it is attempted. With a steel scraper, I will often hollow grind the tip to start. That is, hold it perpendicular (radially) to the round edge of a 6" or 7" wheel and grind the sideways slight curve. Then hone as usual on a very fine stones, which will make the edge razor sharp but slightly obtuse. I prefer hard stones, and use one of those hard fine old synthetic stones used for sharpening razors. They are cheap, and scraper sharpening hollows a stone quickly, which is why I don't use my larger "good stones". But you do need a very fine, very good stone with adequate lubrication.
When you start with a blade sharpened this way, you will have to almost lay it flat to keep it from digging in, but with a light touch, it will shave shiny facets in short strokes. As it dulls, you will have to raise the angle and increase the force. You will have to balance where you want to be on the dullness curve according the stage of refinement of the work, and the amount of effort you want to spend sharpening vs. scraping.
I tend to maintain a broad, flat scraper, but sometimes a little more radius can help, if the scraper is sharp and you are mostly working with it held nearly flat.
In all scraping work, longer strokes = more tear out, scratching, and chatter. Keep the scraper sharp, and use short, narrow strokes for better finish. On some steel you have to get down to nearly 1/8" x 1/8" to get an unscratched effect. Naturally, when you are roughing out, you have to have more force and abandon, or nothing would be accomplished. Work each successive pass at about 45° to 60° to the last. For CI you can use 90°. As soon as most of the surface is roughed out, start getting religious about maintaining the cross scraping ritual at whatever crossing angle works best. This will go a long way to automatically developing a pleasing surface appearance. At this point you might be using ¼" x ¼" strokes, maybe a little longer if you are impatient like me. But as the surface develops, start shortening up. Be disciplined. Here, I am addressing your interest in a better appearing surface. I am not always as disciplined and am sometimes willing to forgo a little appearance for a quicker result. But if you want the appearance, there are no short cuts, except cutting with shorter and shorter strokes in a methodical, repetitive pattern, each cycle intersecting with the last at the optimum angle you discovered earlier in the scraping process.
I sometimes use a steel file to deburr when roughing. I have found that a fine SiC dressing stick (called a "file" but really a 1/2" square long honing stone) really works best for me between cycles after the roughing process. After scraping each cycle, I scour the surface with the stone to knock off all the little burrs. Then clean thoroughly, check with my fingers, and spot again for the next cycle.
I am troubled that your work is coming up crowned. My preference for scraping over any other finish method, including my surface grinder, is that it sort of automatically yields dead flat if your process is conscientious and your reference is true. Glass may not be true enough. I have never used it, and hope never to have to. I do not want to say it isn't adequate, but if you don't know, there goes your reference and all else is guesswork. I suspect, though, if you are working on the smallish smoothers and your glass is, say 1/2" thick or so, possibly you should examine some of your process.
fIRst, never "force" the markings. Set the work on the reference, and nudge it forward once about ¼" to 1/2". Now nudge it back the same amount. That is it. If it isn't marked, probably it isn't supposed to be. When roughing on a big sloppy cushion of blue, you can encourage it a bit more than that, but, to repeat, don't force it. You are probably remembering that a complete coating of blue on the work never indicates flat. It only indicates too much blue on your reference for the current condition. When roughing, keep it copious. As the surface develops, let it thin out more and more. You do not need to take a plane to these extremes, and a window glass reference wouldn't make it sensible anyway, but at the end you will be very delicately scraping only the faintest tiny dots of nearly invisible residue.
For woodworking planes, in practice, largely depending on mood and other pending obligations, I quit somewhere around 8 to 12 spots per sq. inch on a faily faint blue. At that point the work is usually just starting to look "attractive" and somewhat regular as opposed to chaotic and scratched. You can develop it further according to personal preference. If you actually have to go down to 1/8" x 1/8" cuts for the steel, you will be hitting surface plate quality, maybe 30 spots per inch or more.
Hope this helps. SMT
For machinists, a non moving surface that is to be clamped to another is considerd flat with about 5 to 8 "bearing spots" per sq. inch. A precision machine way has about 15 to 20 spots/in². A surface plate, about 35. But these spots are truly "coplanar" as near as can be attained. You have accomplished the condition by comparing discrepancies only the thickness of a thin stain of color, perhaps a few millionths of an inch locally. On larger "precision" assemblies it is common to hold a 50 millionths over 6" to perhaps a ft.
You don't need 50 millionths for a hand plane, but keep in mind that flatness is a collection of discreet points. As soon as you try to show me a "smooth" "flat" surface as one imagines from polishing, I can show you how to subdivide it and demonstrate the points on it that are not coplanar with the rest. The finest flat is the collection of the finest evenly distributed points. Scraping properly can yield an incomparably flat surface, and it is shiny and sparkling.
When you start, the blue will indicate _areas_ that need to be carved off with abandon. Keep the blue thick, and shovel off the metal in the marked areas. Some of us even use non hand tool methods at this point, but don't overheat or cause divots that won't come out later. Add a little blue to the surface plate frequently, keeping it uniformly spread. As you progress, eventually the thick blue will cover the entire surface. Now it is too thick for the condition. Clean the work without scraping it. Smear out the blue on the surface plate without adding any, and try again. Probably now you will have some bare patches (low spots) demonstrated again. Scrape vigorously on the blue areas (high areas). Clean and smear out the blue on the plate, and spot again. Now if the bare patches are bigger than before, go ahead and scrape the high areas, but add a littl blue to the surface plate for the next cycle.
You want to keep scraping the high areas with abandon until the low areas diminish. If you let the blue thin too much, you will waste time and won't scrape agressively enough. If it is too thick and covers too much of the surface, you wont' know where to scrape and may deepen some of the low areas. You will be gradually using less and less blue on the surface plate "automaticallY" if you pay attention to the conditions on the work.
During this stage, you should be starting to establish the crossing pattern of scraping (switching direction at, say 90° after each cycle) and settling into a more disciplined routine of stroke length. It will still be "longish" and somewhat aggressive. Your work will start to show a more complete surface coverage of more or less uniformly spaced irregular blue dots after each spotting. Now you are getting places, this is ideal. Your task now is to keep a routine and rigorously suppress them by dividing them and scraping them off, until like magic, they appear everywhere in burgeoning profusion. By now, you will not be adding any blue to the surface plate. You will be marking (spotting) it by nudging it with the gentlest moves; and scraping with short (¼" or less), controlled strokes in a uniform progression across and up the work.
For a wood plane, you can now quit when you are happy with the looks. If you have a pretty evenly distributed pattern of 10 or more spots per inch and the blue is pretty faint, it will be flatter than anything else you are likely to have in the wood shop except the surface plate used for reference. SMT
Have you tried the plane, as in cleaned it up, sharpened the iron, waxed the sole and went after a twisty old board to flatten it? I don't know if you followed discussion farther down where I asserted that absolutely flat soled planes may not be the best tool for straightening boards. If it aint broke, don't fix it. You don't say if the sole is convex or concave over the length.
As for your tools here's a few remarks and comments on abrasive lapping as I view the process:
First, I confess to a morbid fear of glass in the workshop thanks to an auto accident. I won't have it as a tool in my shop for fear of breakage.
Most abrasive sheet lapping I've read about recommended a thick piece of plate glass - 1/2" thick is mentioned and "Wet-or-Dry" abrasive sheets not sanding belts. I've abrasive lapped plane soles myself but I use a cheap import granite flat from Enco as a platen, not glass.
I'd think your ¼" thick glass would be pretty thin unless backed up by something flat like a table saw top. Glass is among the stiffest of our common materials but if thin it will still flex under load.
What grit are those abrasive belts? It'd start with no coarser that 80 grit. Are you going to bond it to the glass somehow to hold it flat? My concern is their relatively heavy fabric backing may rise up where unloaded to uncontrollably round corners. Conventional wisdom has the abrasive lapping process start with 180 or 220 grit abrasive sheets, and as the sole comes flat progress to finer abrasive.
Moving on to scraping.
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 jointers 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.
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.
Sole prep. Remove all rust from the sole (rust EATS!!! scrapers and files). Install the frog and tighten the screws.
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. Clamp the plane casting in a vise so it's mid-forearm high. Glide the file over the sole 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 sole casting and apply it to the flat. Never apply the flat to the sole clamped in the vise. The force of the vise jaws will spring it a trifle and you'll end up with a plane sole that's concave across the bottom. 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 sole 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 sole 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 sole with a rag and lighter fluid. Let it evaporate and give the scraped sole 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 sole of the plane causing it to go concave - meaning as you work you'll scrape more than necessary off the heel and toe of the sole. I once used a wather 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 plane sole, 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.