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How to scrape - with photos

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From the Chaski forum, posted by Stephen Thomas
March 2002

As the article noted, I had (have still not) learned to do "decorative" flaking or frosting. My feeling and informed judgement from numerous sources was that digging holes in a well scraped flat way was counterproductive, reduces bearing, and can sometimes promote wear at an accelerated pace. Recently Forrest has posted a much more thorough (and authoritative) treatise on this.

I am always in a hurry. So taking the time it takes to do it right, going back to dig up something worked so hard to get flat in the first place just never made sense to me. Maybe sometime I will learn so as to impress people.

When I care how it looks, I scrape passes at about 45* to the long edge of whatever it is, then at 45* the other way. IOW, each pass is at about 90 to each other, but at 45 to the edge of the work. This helps keep the scratches and chatter from developing too. You can do 60/60. Or 60/60/60, working right around and across in three differnt repeating directions. Steel, and CI to a lesser extent, will start to scratch when you make more than one stroke over the same spot in the same direction. The shorter the stroke, the less scratching, too.

I scrape with abandon and try to really shovel off the "bad" material when starting in on slick, banana shaped ways or parts. Just keep an eye on aligments from the beginning. It's harder to catch them up later if you get going the wrong way when really hogging of chips.

When the alignments are falling into line, and the surface has begun to develop markings all over at each spotting, I shorten up the stroke, and work more methodically. The spot count is directly related to the stroke lenght and the width of cut. A 1/4 x 1/4 cut yields about 8 to 12 spots per inch; 3/16 x 3 16 gets upwards of 15 to 20 spots, and for gages and straight edges, you will have to scrape with a stroke x width of less than 1/8".

Under about 8 to 10 spots per inch, when you are still shoveling with abandon, the appearance is kind of ropy, swirly, and sort of haphazard, like the picture someone posted. (I am not commenting on their work, just this is how mine is when the spots are starting to come up all over, but kind of sparse and random on really thin making medium. The marks are indicative of long, random strokes. This would be adequate for hard contact surfaces scraped for alignment, like behind vise jaw faces, the nut face, mounting bosses, maybe even a working machine table surface, though the appearance is not as attractive.

Moving up to about 12 spots per inch on a regular pattern, i start to get a pleasing cross hatch finish. In the HSM article mentioned you're looking at about 15spots/inch which is pretty good for vise ways, swivels, etc. might even be adequate for some sliding machine members where you would need some porosity for oil. Most ways are in the 15 to 20 spot range. For me, this upper range (toward 20) is where I start to get the "zinc galvanized" look, only much smaller scales. Past about 20 - 25, the marks become even smaller and more chrystalline, which is a very nice, dense way, but probably overkill. Up towards and past 30, the the marks are small and faint. Your're just easing off pinpoint high spots, mostly going by the shiny twinkles glittering through a slight scum of dirt when the almost bare surface plate is removed. Around 30 spots on a haze of medium, the surface will not tickle a tenths indicator.

I have been exposed to power scraped surfaces since the article, and they have a different feel and character. The marks are a little deeper, even at 40 spots, and may tickle an indicator even though providing a superior surface (at the higher count, not because of hand vs. machine)

It would be difficult to "authoritatively" say how to judge the character of ways by the appearance. It is way too easy to cheat if someone wants to. If I saw new scaping marks over deep scratches and wear patterns, i would be suspiscious. But then again, it could be they scraped flat to adequate bearing but did not get under the worst scratches. This may or may not be "sloppy". It may be conservative and efficient. Any frosting on old ways is pretty obvious and sticks out like a sore thumb. But an all over scraped appearance is hard to judge unless you put a straight edge to it. If you have an appearance scraped surface and can move the mating pieces on it, you can get some idea from the oil tracks if it is really bad. BTW #1, Bugatti had all his engine surfaces hand scraped for appearance to remove the machining marks, probably weren't really flat even though they were blocky pieces. BTW #2, the Bridgeport with the chrome ways was probably factory. All the ones i've seen with chrome looked about like that, a pretty fine cross hatch pattern, chromed over. I did not comment directly because it is hard to base judgement on a computer picture, at least my computer.

There are widely different scraping styles, too. I've seen taft-pierce plates frosted all over,deep cuts, and have no doubt they were at the height of specs for their grade. Also some that were not frosted that you could barely see fine pinpoint little squiggles on a more hazy mirror surface. Best if you can run some sort of alignment tests; or if the seller has known good credibility. An excellent alignment job with mediocre but adequate bearing can have somewhat unattractive appearance if done for efficiency. A beautifully scraped high bearing way set that was never aligned can cause you inconvenience at best and real problems at worst. I'd rather have alignment first, "well" developed bearing second, and appearance last. As with most things though, if you concentrate on the first 2, the appearance will come through. But I wouldn't be comfortable assuming the first two just becasue of appearance.

just to muddy the waters some more! smt
 
I recall scraping flat bed machines during my apprenticeship, and as the no. of spots / sq. inch increased, you could feel a " magnetic" effect when removing the surface plate, this i always assumed was the same as the "wringing"of slip guages/ jo blocks,? Bodger
 








 
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