Problem with scraping

I am having a problem and I cannot find a solution. I am trying to make a reference surface and no matter how I alter my method of scraping, I keep pitting the surface of what I am trying to scrape.

Is this normal?

Are you tying to scrape steel?

Should have specified material.

Cast Iron. Using a Sandvik Coramant clamp on scraper with carbide insert.

Lower your approach angle and a fresh -5deg hone on the carbide.

Lucky7

About how much pressure?

I asked a question on YouTube and the only answer I got was to use enough pressure to get a decent chip, whatever that indicates.

Feather touch, mild, moderate, heavy, skull crushing?

It depends entirely on what you are trying to do. If you are roughing, lean as much weight on the scraper as you can and still push the scraper forward. If you are finish scraping for nice spread of points, you can't lean on the scraper very hard or you lose the ability to control the length of your stroke.

ewlsey said:

It depends entirely on what you are trying to do. If you are roughing, lean as much weight on the scraper as you can and still push the scraper forward. If you are finish scraping for nice spread of points, you can't lean on the scraper very hard or you lose the ability to control the length of your stroke.

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Thanks for the reply about pressure but can you answer the pitting problem? Lucky7 indicated I should lower the angle and put a new hone on the blade. I am using a new blade and have the scraper just high enough not to tip back.

What radius is your scraper? You want a 60 - 100 mm radius for most work.

I think I remember reading that the "out of the box" insert geometry isn't ideal for scraping, and you should hone the edge first for best results. The -5 deg is what I understand is for cast iron. As Wes says, check your radius.

Is that part you're working on previously machined? It looks sort of shiny, as if it was glazed by a dull cutter. This could create a hard skin that makes your first pass difficult.

Part of your issue may be the milled finish is telegraphing through your scrapes. You need to get through that surface.

Make your first pass perpendicular to the lines in your milling cut. Or re mill the part with a sharp 1/2" endmill with a .0005" finish pass. I do this last milling pass with the vise, clamps, or fixture quite loose. I dont want to distort the part. The end milled finish is easy to scrape.

Also remember to approach the part from different sides. For example: start at 0° next pass @ 90° next pass @ 0°next pass @ 90°

Your scraping swarf should look like black pepper. The heavier the cut the coarser the pepper grind. If the swarf looks like dust, you're not bearing down hard enough.

I've run some experiments. Find a bathroom or kitchen scale. Place it on the bench, grab your scraper in a normal grip and use it to push down on the scale to about 15 lb (Block of wood to protect the surface? SWMBO may disapprove of trifling disfigurements.) Remember the sensations and use them to calibrate your scraping. FWIW,

I've determined a few very rough figures for hand scraping"

- Light finish scraping using a 1 1;/2" radius (35mm) scraping tip. - about 5 lb.

- Developing bearing using a 75 mm - radius about 15 lb

- Heavy roughing using a 4" radius (100 mm) - about 50 lb (push with your hip or belly. Swarf should puff up ahead of the scraper)

YMMV a lot. Much depends on the material, habitual angle of attack, keen-ness of the scraping edge, personal factors, power Vs hand, push vs pull scraping, etc.

Efficient scraping where stock is removed in an expeditious manner can be hard work.

You do not seem to have done much to that piece of cast iron yet.

Since the milling marks are still there you are not going hard enough.

Bill COmbs said:

I am having a problem and I cannot find a solution. I am trying to make a reference surface and no matter how I alter my method of scraping, I keep pitting the surface of what I am trying to scrape.View attachment 206629

Is this normal?

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Scraping is not really for looks . Although a nicely scraped surface can be beautiful. At that early stage do not even worry about the pitting or anything. Even some chatter is just noise. First order of business is to get rid of all your milling marks with a heavy paint scrape. Wipe some yellow or orange canode on the part and make sure you scrape all of it off with a R90-100 mm blade. The blade should have -5 degree grind. For soft cast it may need to be as high as -10. Repeat the yellow canode and full scrape in a perpendicular direction. At that point. stone, yellow, and blue it up on the stone. At that stage I almost immediately start developing the waffle pattern by scraping (still with the 90 mm blade) with 3/4" stokes, 3/16" to 1/4 between scrape marks, till i get to about 8 to 10 PPI. At this point I scrape the surface once in both direction, without stoning and spotting in between, and do not care much about hitting only high points, I just avoid low areas. Stone, put on a haze of yellow or orange, blue up on stone, scrape repeat. Once you get to 10 PPI, switch to R60 mm blade shorten the strokes to 3/8" to 1/2" get them about 1/8" or less apart and repeat the waffle patter till you get to about 20 or s PPI. Still not hitting only high spots just mowing down high areas, avoiding low areas. Once you have an even 20 PPI you are either done, for most machine ways, or it is time to switch to an R30 blade, and hit the high spots for a marking surface. I look at it one of two ways, i either chaise high spots or split flats. If you stone your surface harder, you develop wide, flat high spots that need to be split, or cut the connections between them, or if you stone light, then you have peaky highs that need to be scraped down. either way it is a tedious process and you need to learn how to read your blue. Things that shine with a dark atoll around them are really high. Things that are just dark blue are medium high, and things that are light blue are barely touching. The differences between the shiny and light blue spots are smaller then a pimple on a fleas ballsack, assuming that you spread you ink right. Think of it this way, the ink forms a film on your surface plate. the part will only rest on the high points. the points that rest on the stone will get shiny as you rub it, the once that are barely touching the surface of the ink film will be light blue, and the ones in between will get smeared with blue and get darker, but not get shined by the surface plate. One more thing, the closer you get to the finished surface the harder you should rub the part. don't just swirl it round a few times. bear down and rub it for a few minutes.

And here is the real rub on this...If you are making a marking surface, you want it to be as accurate as your surface plate allows it to be. For all practical purposes it is about 0.0001"/12" if you have an AAA surface plate you might be able to get it better, but the limiting factors become too numerous. You can only spread the ink so thin, if you rub the part too long the heat form you will deform it enough that it changes shape, etc. Going beyond 40 PPI requires all the stars to line up and stay that way till you finish scraping the part. Not very likely to happen. Finish scraping is an exercise in knowing when it is "Goodenough", "Flatenough" or some other Russian sounding word.

Relax and have fun at it.
dee
;-D

I can't tell enough from the OP's description and photo, but I wonder if he's misunderstanding the quality of a scraped surface. A good surface is not a smooth surface as though ground or polished, but an undulating, scalloped surface with all the highest points being the same level surrounding by dished low spots several tenths lower.

What did you do with the corners of the scraper? I like to round them and blunt them a bit so that they will not dig in.

I cannot be sure just what you are pointing out in the pic, but it does look as if some marks could be from corners.

As for the surface, the first thing to do is to go all over it, scraping aggressively (make the chips fly), but evenly, from several directions, until all the machining marks are gone, and the surface is covered with scraping marks. THEN you can get down to business.

There is no harm in doing prints while you rough it down, that can help keep you from putting in irregularities and low spots, etc, but the main thing is to get to a decent starting surface that is reasonably flat. Obviously the flatter the better, since that is your goal.

I see we have the consigliere of scraping here. Tiers & Addy.




Apart from R.C, he hasn't got under the surface finish yet. What ever happened to the break up pass?

machtool said:

......
Apart from R.C, he hasn't got under the surface finish yet. What ever happened to the break up pass?

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You mean like I said in the post before yours?

No need to mention me and Forrest in one sentence.... he's an expert and I'm not. Maybe that helps me spot rookie problems.... or not.

Maybe these will help.

Are these scratches and pits normal?

The reason I am asking is that when I scraped the top portion of the cross slide of a mini lathe with a cheap steel "scraper", there were no scratches, no pits and it had a beautiful finish with so many bearing spots, I couldn't count them.

I am currently using a sandvik scraper, a carbide scraping insert and I am gouging the piss out of the surface.

And yes, every one of those lines you see is a scratch.

The lathe cross slide was soft cast iron. Grab your cheap steel scraper and try to scrape this piece. You will see that it doesn't scrape like the cast iron of the cross slide. There are different grades of cast iron and some almost as hard to scrape as a hardened way. Send me your address and I'll send you a Sandvik 2530 blade that has 8 cutting edges with 40mm on the narrow and 60mm radius on wider side and all lapped to a negative 5 degree. Just send me your blade back to replace the one I send you.

Daryl

Bill COmbs said:

I appreciate the reply but I give up. Scraping is beyond my means for physically, mentally and financially. I don't have the space for all the tools I need to shape and hone the scraper and then work with the piece being scraped in addition to all the money I have to spend and I am tired of throwing good money after bad.
All I do is gouge the crap out of the piece I am trying to scrap and I don't have the patience for it.

There has been good information posted by very competent individuals and I am sure some others will benefit from it.

Thanks.

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Well heck, if you are giving up, send me the scraper and the inserts. No point in letting them rust.

CarlBoyd