The Role of Stoning in developing Spot pattern

I'm new to scraping, but have attempted to do my homework before asking this question.

When scraping, we are looking for a good pattern of points of contact at 20-14 PPI. for these to be contact points, they need to be slightly proud of the average surface, so that each point is a "peak". (understanding that the actual topography created by scraping is still very flat).

Where I get confused though, is in determining:
1. how large these spots should be? is larger or smaller better?
2. how much does/should stoning process affect the spot pattern?
3. is stoning really lapping the high spots? If so, doesn't the flatness of the stone become critical, or does stoning technique need to make up for this by rotating the stone in some way?

A frequent result of my inexpert scraping is that I end up with large patches of blued surface. Say, 1cm[SUP]2[/SUP], separated by large areas of no contact. I'm assuming that either my scraping marks are too close, or my scraper geometry is not good for the attack angle that I am using on the part. Or I am stoning too much between passes and removing small high spots.

Stoning is more about removing minute burrs from edges of the scraped cavities. Flatness of the stone is not at all critical, and you should not be using enough pressure to remove measurable amount of non-burr surface anyway. Just rub the stone on the work, you are not trying to sand it, grind it, or grain it.

Large patches of blue, with large areas of no contact? You may have specks of dirt on your work or the surface plate. Both must be scrupulously clean before trying to blue the work. Like surgically clean, ignoring the blue pigment. Dirt is especially likely if your large patches move around between scraping passes. If your large patches are always in the same places, it's also possible that you just aren't digging deep enough in your rough scraping passes.

Either way, rubbing a stone over the work is not going to remove contact patches or significantly change the spot pattern. It may substantially improve the accuracy of the next bluing (by removing burrs), and it may very, very slightly increase the size of contact patches.

Spot size is related to points of contact. More points of contact necessarily requires smaller spots. Once you have established plane and level, you can increase the PPI by reducing the spot size. Shorten the stroke on your scraper, and deliberately try to divide existing spots, rather than removing them entirely.

sfriedberg has summarized it very well in his post.
Regarding the big patches and radical changed due to stoning makes me wondering about the depth of your scraping marks: have you measured them? They should be at least 0.0002" deep and, when scraping, you should hear a "crunch-crunch" sound of the cast iron scraped away and you should see some coarse dust accumulating.
Make sure your blade is sharp and re-sharpen it frequently.

Paolo

Make sure you sharpen your scraper with a 90mm radius for roughing and 60 to 30 mm for finishing. You want an undulating surface right from the start. If you get big flats it means two things, you are not bearing down enough and your blade has a very large radius. Stoning does two things, it removes burrs, and also slightly widens the top of the peaks. It is not meant to adjust their height significantly, so flatness of the stone is not all that critical.

dee
;-D

Thanks for the tips. I have switched to a finer stone, and in addition changed a few other things about my setup.

1. the sharpening angle of the scraper is now >90°, I didn't realize that from the posts I had read, but saw one of Richard's demos that showed it in video.
2. switched to allowing more space between each stroke, rather than having them all touch. (I am pretty sure this is why I didn't get good results before, since I was scraping away large high spots completely, rather than dividing them up)
3. switched from Permatex to Canode blue. Canode is much more visible and easier to clean up, which improved the accuracy of my scraping.

Here are some pictures of a vice base that I scraped in for practice.

There seems to be a pattern in your scraping to me you are not cutting but scratching. I see no depth of cut and your blade is not sharp enough to dig in or you are not putting enough pressure on the scraper. I also see lines on some of those scrapes like the edge was broken down or from a carbide edge that was nicked.

You need to scrape deeper/harder. It looks like you are kind of willy nilly with the direction of scraping. It's hard to read the blue when your scrape marks are not in the same direction. The light reflect differently off the scrape marks, depending on the angle.

Also, I don't see much blue. The pattern you are showing is the final step in scraping. Did you rough scrape first to get blue over the whole surface?

Those other guys are experts, I am not.

However, as a non-expert, I think I see what you are doing. It looks like you are scraping all over, but you do not have SPOTS all over, at least as the pics show up (I know that blue can be hard to show in pics). Looks like you are not regulating your scraping to the spot pattern, also.

For a vise base, you are being WAY too fussy. You should be good with 5 or 10 spots per inch, big ones with good distribution and coverage even for the contact from base to rotating part,and that looks like the underside of the base.. If it's practice for ways, OK, I'll assume that.

What you want to do is to start with larger/longer strokes, around a half inch or so. As the pattern of spots gets finer, you shorten up the strokes, so that you end up with a stroke that can take off just one spot, and maybe just the middle of that (for a bulls-eye mark). That will maybe be around 3/16" or 1/8", depending on what you are scraping. For ways, you are not going for as many points as your scraping would be for, maybe 15 or so per inch, for reference surfaces, the more the better up to 40 or so,. Your scraping is too fine for a mating surface like a vise base if that were the surface that the rotating part contacted. Maybe too fine for ways, I am not sure of the scale.

Then also, scrape the blue, and let the rest take care of itself. The exception to it is that I like to go over the whole surface fairly lightly first, just to get an even surface as far as texture, so all the blue "has the same meaning". I don't mean "chicken scratches", but real scraping, just not digging too far.

So, when you finish a pass, do you have a pile of shavings? If you do not see shavings, you are just chicken scratching, it does no good.

You seem in the pics to have areas of no blue. If those are real, then quit with the little strokes. Go over the whole thing with strokes about 3/8 or a half inch, and ONLY go "from south east to north west".

Now spot it, and see where you are. Make a note of where the blue was. Scrape just the blue, but go "south west to north east" ONLY. Make sure you see shavings, so you are doing some actual work.

Spot that again. See if there was any change. If you didnt get much change in blued area, start "digging". Just go after that blue area hard, make the shavins fly off the front of the scraper. Don't do full coverage, scrape about 1/3 as far apart as the stroke is long. ALTERNATE the two directions, and maybe throw in an east-west every so often. SCRAPE JUST THE BLUE.

Keep that up until you get full coverage with a coarse pattern.

Next start in a bit shorter and lighter, and split every bigger spot. Cut it in half. If you see spots where the blue is squeezed out so they are a bare spot with blue around it, cut that bare spot off. It's a "bull's eye" that is too high and has metal to metal contact.

Your surface should be developing a good distribution of smaller spots. You should be surprised as how fast a decent surface will come up. When it stops changing, and you have 50% coverage with 15 spots, you are probably done as far as slideways.

I always have more trouble keeping percentage coverage high. You have to leave the lower spots, and just take away high ones, or you will not increase coverage. You want to bring the surface down to include the "lower high spots".

And sharpen that scraper with diamond, not green wheel, that should help with the scratching marks that look like they come from defects in the scraper. You can use diamond hone sticks if you have to.

Probably others can do a better job of explaining, and correct what I said. I only went this far because I am closer to your status than the guys wo do it every day, so I can kinda understand what you may be having trouble with.

sfriedberg said:

Stoning is more about removing minute burrs from edges of the scraped cavities. Flatness of the stone is not at all critical, and you should not be using enough pressure to remove measurable amount of non-burr surface anyway.

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I disagree. I think stoning is an integral part of the metal removal process.


You do not go silly and rub and rub though. Just do it like it is shown in the various videos. I do not think it is something you can easily describe.

JST said:

Those other guys are experts, I am not.,<snip>Probably others can do a better job of explaining, and correct what I said. I only went this far because I am closer to your status than the guys wo do it every day, so I can kinda understand what you may be having trouble with.

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For fucks sake, say it isn't so. I can't rely on you to parrot what you read from Forrest?
2,500 times per year across a multiple of forums? Shucks.

machtool said:

For fucks sake, say it isn't so. I can't rely on you to parrot what you read from Forrest?
2,500 times per year across a multiple of forums? Shucks.

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No, sorry Machie, it's out of my own hard-won experience. I don't remember what Forrest wrote sometime.