View Full Version : Scraping - How many points per inch is this?

01-16-2013, 05:51 AM
Hi folks,

Might seem like a daft question but can anybody with scraping experience tell me how many PPI they read from the picture below?

Also, how's this part looking in general? My sense is that the PPI is good but I'm not happy with the % of bearing, though this seems to be a function of how much blue you apply to the plate... I'd like to have more blue and less silver but the more I scrape I seem to just get smaller and smaller blue spots...

I'm aware the PPI goes up and down in parts, I need to do some more work to get it uniform.




Spin Doctor
01-16-2013, 06:06 AM
I hope the vise is not too tight. to hold parts being scraped we alway captured them against something nailed or screwed to a bench.

01-16-2013, 06:10 AM
The vice isn't very tight. It's not visible in the photos but the part is an angle plate, probably an inch thick, and webbed, so I don't think the vice will distort it at all.

On a critical part (this is just a test piece) I usually retain it with strips of wood nailed to the bench around the periphery of the part.

01-16-2013, 06:23 AM
I do not think your blue is too thick, in the first photo... I dunno 12 or so. 25% coverage.... I probably would be stoning a bit heavier... A good stoning takes off all the higher spots and increases coverage... Of course you do not go silly with it...

In the bottom picture, I see good coverage, that is pretty flat, you only need to break up the dense blue spots, and leave the small single spots, then when you stone, everything comes down together.. And also make sure your stone is flat, and I treat mine not as a deburrer, but as a metal removal tool..... using it lighter and lighter the flatter and flatter you get...

You can be forever chasing your tail when you get like you are now, if you scrape every blue bit there is.. That was the most important thing I learnt at our Melbourne scraping class.... You do not need to get every blue spot, and use the appropriate thickness of blue for the flatness of the work as it shows your progress...

Also see int he below photo, how each blue blob has a sort of shiny centre to it..... The shiny centre indicates where it is actually touching the plate, the blue outside the shiny bits is actually clearance


01-16-2013, 06:37 AM
I was going to do an experiment tonight to determine (and photograph) the effect of progressively more stoning.

Connelly seems to advise against stoning, he prefers a burr file, I have both but lately have been lightly stoning. I confess to having no appreciation for the effect it has on how the part blues.

01-16-2013, 07:51 AM
The vice isn't very tight. It's not visible in the photos but the part is an angle plate, probably an inch thick, and webbed, so I don't think the vice will distort it at all.

are you spotting it while in the vise? one of the basic principles that makes this technique different is that there is no clamping force imposed - but this matters when spotting. If you then imposes some slight amount when scraping it wouldn't matter - its 'was it distorted' when it was spotted that matters.

The blue imo is too thick, the look of RC's pic is more like what you want. I can't tell with yours what is in contact with the plate....with RC's there's the telltaile lighter spot in middle where they blue has been squished out between work and plate. Nick Mueller has a good animation somewhere on utube showing how the blue thickness effects things. You can imagine that with blue thick enough, all kinds of stalactites enter the blue zone that are still quite low compared to the real points of contact. imo, part of not chasing your tail is reducing how aggressive you are, depth of cut, as you approach finish work.

I have both but lately have been lightly stoning. I confess to having no appreciation for the effect it has on how the part blues.

I guess it depends on what you're stoning with and how aggressively. Something like a hard Arkansas stone cuts so slowly you'd have to really work at it materially change things....but it will knock the burrs off. My thought had always been the stoning/burr file is to take the burrs off, the plate and scraping are to create flatness. The burr file works well however you have to be diligent, keep it clean. Nothing more frustrating that having a bit of something caught in the teeth and scratching your work up. I use the file for most of the process but as things start to finish use a hard Arkansas stone. burrs should be smaller with finish work, easily handled by the stone, and it avoids the scratches

01-16-2013, 08:27 AM
I just hold the part in the vice for scraping, I'm putting it face down on the granite plate for spotting.

I will try less blue tonight and see how I get on.

J Tiers
01-16-2013, 08:33 AM
Agree 100% that your blue is WAY too dark, so it seems too thick. Agree that R.C. has a good amount.

Blue varies.... the Dykem seems better once you have got to where you want thin blue, the Canode doesn't seem to thin down as nicely. it seems coarser.....

As for points, it is hard to tell when the blue is thick..... superficially, it looks like high teens to low 20s of points, but that depends on them all being real, and I don;t know if they are. Thicker blue "makes" extra points, but it also "joins" separated points into a "common smear".

At a certain place in the scraping, start to "split" the points, don't scrape them off. if you always scrape them off, you will just have them move around. You probably know that to have gotten to where you are.

01-16-2013, 08:52 AM
I don't know if it's just the temperature around here (-5c, 23f last night when I did this scraping) but I find if I make the blue too thin the part comes off the plate without picking any up at all! I had to push the part into the plate quite hard to pick up any.

The blue is Dykem.

01-16-2013, 09:05 AM
I don't know if it's just the temperature around here (-5c, 23f last night when I did this scraping) but I find if I make the blue too thin the part comes off the plate without picking any up at all! I had to push the part into the plate quite hard to pick up any.

could be temperature is the issue, but you should not push hard on the work into the reference, its defeating the purpose. Be aware you could also run into trouble with temp changes, the work warms up as its being handled etc potentially changing its shape.

Start from brutal un-flatness. If the whole reference surface was blued, and you touch the work to it, there has to be at least three spots where it touches. lay more blue on and these three spots become three zones. scrape the zones - don't pick spots, scrape the whole area (and then some, I draw a magic marker line an inch or so around the edge defining the 'zone'), overlapping diagonal passes. eventually you would get bearing points all over the work, thats when you switch from zones to points. Roughing to finishing.

If you have to press the work into the plate to get anything ( a no-no) , I don't think you've roughed it out....with the blue as thick as you've got it, and without pressing the work down on the plate, what spotting pattern do you get?

Forrest Addy
01-16-2013, 11:12 AM
Have you tried the spin test?

Another point is temperature. If its cold in your shop (like below - say - 40 degrees F) the blue stiffens up and gets reluctant to transfer as was discussed. If you have to push to get blue to transfer, you're kinda in a fix because it's hard to apply a downward force by hand without influencing the pattern. Can you create a little warm zone? Maybe partition off a workspace with sheets of plywood, thick blankets, sheet foam insulation, visqueen etc, then seal it against air infiltration with duct tape, and plant a little space heater in it?

Your surface is coming in nicely but I suggest you use a thinner application of blue on your reference flat. Your points are showing up a nice checkerboard but I can see you're trying for a finer pattern pitch (more points per square inch) than is conveniently possible with your present scraper end radius. A scraper surface "undulates". Think of a lake with a small chop on its surface instantly frozen somehow. The peaks of the little waves are cusps representing spots on a scraped surface. The troughs between the waves represents the path the scraper took removing metal.

It's easier to scrape a finer pattern pitch if the scraper has a smaller radius. This is a two edge sward. If your goal is ultimate smoothness like for a steam tight joint a large as practicable scraper end radius will be your friend. If, however, you are scraping to recondition a surface plate as a flatness reference or for a linear way bearing, greater undulation works in your favor.

Consider the scraper end radius as an arc whose plane is inclined about 25 degrees to the scraped surface. Say the scraper cuts a trough about 1/8 wide. A little calculation using the scraper end radius, the width of the scraped path, and the inclination of the radius' plane will lead you to the height of the chord, that is the depth of the trough. Don't get obsessed with the numbers but look at them if it will help you understand scraping geometry.

Set a DTI up in a little surface gage so it regisers undulation as you glide it around on the newly scraped surface. Take some careful readings noting the difference from crest to trough. If the undulation is less than 0.0002", consider a smaller end radius on your scraper tip. Here is where you will find it convenient to have several scrapers with differing end radii to choose from.

I made up a lame little formula that serves as a rough guide. Don't take it seriously beause I sure dont. Its intended to get you to a starting point. Your personal scraping ergonomics has a profound effect on how many points you can scrape with a given tip radius.

My formula is: r = 40 / n where r = tip radius in inches and n = spots per square inch.

As I said the formula gets you started. If anyone get anal and comes back with "I used your formula and the best I get is 35 points" I will abuse him.

My scraper radius gages are the canned goods. For general purpose scraping I use an end radius matching a coffee can. Fine finishng, a frozen fruit juice can. And in between we got dog food, tuna...

Finally, scrape only the blue part. Never scrape bright metal. As you improve in skill you can take the center out of each spot. Learn that and the spots will proliferate geometrically once basic flatness is acheived.

Stone/burr file? Peanut butter/chocolate? Paper/plastic? Use what works for you. I use an old slip stone I clean up with a diamond plated woodworker's bench stone. The slip stone comes out clean but too smooth to actually cut.

Stoning becomes important when the final step approaches. Reference flats like surface plates require little bearing flats not "points". The percentage of flat area I would suggest as ideal would be 10 to 20% with the space between being scraped trough. Also ideally each flat should be roughly the same size uniformly but randomly spaced all the way to the edges. Final stoning as a step in scraping is often omitted or glossed over but I've detemined this step is where the final "sub-tenths" accuracy is developed. Few who scrape need to journey into persnickety stoning or "pin pointing" but they should be aware of the concepts involved and maybe try it once.

A scraped surface properly done is its own wear indicator. If the reference surface is scraped and stoned correctly, wear as it gradually occurs will become evident to the naked eye. The stoned flats enlarge with wear and in time a few will merge. This is the time to check the scraped reference to a master reference. A touch up early in the game will take only an hour or two. Double the wear and there's a good chance correction time will quadruple. The economics of wear and/or neglect are like that be it surface plates, leaky roofs, or remembering anniversaries.

Richard King
01-16-2013, 02:18 PM
It's nice to see you use PPI for "points per inch". I invented that term when I presented papers at the SME Rebuilding forums at the IMTS show. I also use the terms POP "percentage of points" and DOC "depth of cut".

The part looks high in the middle to me. When your getting the part down and before a fine bearing you can use a little extra blue as your POP is still far enough away you can. I would suggest you spread it on the plate with a foam paint roller, Biax and Dapra call it a Ink roller. Or a ink roller they use in print shops. I teach if you can see the plate thru the blue that's about how thick it should be when rough scraping and as you get more PPI and POP use less gradually. I also like to use the 2 color method when going for 20+ points. ( I grew up using red lead, but now use Canode yellow diluted with Windex, to just offset the shining metal)

On the other straight-edge I would suggest using less red as to much will smear the blue. After you blue up the part, (rub it on one and then pick it up and rub it on the other, a 1 - 2 operation) you can "Highlight" the highest points by cleaning off a spot next to the blued surface plate and rub the plate on an area where there is no blue, this will transfer the blue onto the surface plate and "Shine" up the high spots.

I would not press down on the part as it's own weight should be fine. Just hold it with 2 hands about 1/3 from the ends and rub it on the plate. Don't be afraid to rub it a while. Say rub for 10 seconds in a up and down or left and right or circle or a figure 8 motion and stop then with one hand push it up about 1 inch and pull it back to where you started and then back toward you and watch where the plate swivels or "hinges" ("Rotation of Points" is what the Engineering professors call it) when it is flat it will be hinge or swivel at 30% from both ends. Once you see the hinge on one side, then change hands and hinge the other side. If it hinges say on one corner only scrape there, if it hinges in the middle and there is blue on the ends it is rocking like a rocking chair and you're getting a "false" reading on the ends. Wipe those area off with a rag and only scrape in the middle. You should also check around the edges with a feeler gage. If it is higher then say .001" you can "step" scrape it down. More later if you need me to explain step scraping.

If it looks like it's high in the middle see if you can spin it like a Top (toy). I say there are 4 rules of scraping 1) Individual scraping marks, meaning as you scrape your scrape mark should be the same width of the opening of the no blue. Remember when you scrape your making a low spot, and the spot next to it will become the high spot when your creating the checkerboard pattern. 2) Individual lines, or when your scraping rows of scraping marks ( I suggest 45 deg's or an X design from the bottom of the plate to the top)

Each line you scrape can not touch the last line you scraped as when you look at the part you will see long lines* Next time you blue up scrape the next rows 90 deg's to get the checkerboard or X look. 3) Depth of the scrape mark it should be for .0002" to .0005" deep, everyone should check their depth with a surface gage and .0001" indicator when you're learning so you won't get "chase your tail" . Many screw up and as they get more points they lighten the pressure down and only get .00005" DOC (another reason you chase your tail or flip flops in and out). Always press down the same pressure, you shorten your stroke and blade radius to get more PPI and POP. 4) Check the Hinge each time you blue up (Rotation of points).

I used to use a dull file to de-burr the part, but time's have changed and I use a MS-24 Norton Lap stone to de burr the burrs left over from the scrape mark. I keep it clean by laying some 100 grit sand paper on a flat spot and squirt it with some simple green and rub the stone on it and that cleans the stone.Also after you scrape wash the part of with glass cleaner, simple green or some acid-tone so the stone doesn't get dirty.

I tell students to run their fingernail over the scraped surface to feel the burr, then stone it lightly once and feel again for the burrs. Also check the stone to be sure it if flat with a feeler gage and hinge it. Most assume the stone is flat, but most of the time the stone is not flat. I tell students to use the stone in different directions when stoning.

Your part looks like it does not have the checkerboard look now and I count 29 points and about 20%. When your done you should have 50 % blue spots for "weight carrying or wear high points and 50% low oil pockets. If you buy my DVD I explain this and show how to scrape a plate. All the above info is in it. I also am streaming - selling the DVD over the net now (click here) http://www.indie-ondemand.com/?vidid=39506&oid=474

I like to scrape in a room at 65 to 69 deg. F. Scraping temp as does machining temp needs to be constant
What screws things up is cold then hot or sun shining in a window expanding one side of the machine or plate and not the other. So if your temp stays the same day and night, you should be ok as long as your fingers don't get frost bite...ha ha. I love Dykem blue but use Canode as it is easier to clean up. I got use to it and will sometimes spray Windex on it when it gets a bit thick.
I'm getting writers cramp now...more later if needed.

01-16-2013, 05:53 PM
Hi folks,

Just spent the evening practicing some more, I finished up my test piece.

Thanks a lot for all the advice.

I thinned the blue out on the plate and warmed the shop up a bit with an electrical heater. I discovered a technique which Richard refers to which is to blue the part then slide it about a bit on a clean piece of plate, this seemed to help a lot to identify the different levels of height within the blue. It can help seperate 'really high points' from 'average high points' and false ones.

I also discovered a great contrast medium - breath! If I blue the part then breath on it the metal goes dull/dark gray and the blue is twice as visible. This helped me a lot. Presumably only works in cold conditions though.

After a few passes I ended up with the situation shown below, for now it will do, it's only a practice/test piece! It is now sticking by suction to a totally dry surface plate, so I'm happy it's flat!

Something I began to learn tonight and then Richard refered to in his post is the need to avoid scraping right next to the previous scrape, I think that's how I created some of the voids in the blue in my part. I need to always leave a gap from one scrape mark to the next.

One other thing I found helped was to mark out the "areas I don't want to scrape" with a black marker pen after bluing.

Forest - I use your little PPI/radius formula a few times in the last week. I spent some time experimenting with the 4 different radiuses I have tonight and spent a while lapping them all to perfection on the diamond lap, it's all coming together quite nicely thanks in no small part to advice taken from the members on this board.

Don't have any pics but after finishing off this test part I took my Hilger & Watts precision level and scraped the base, it was ground previously but was only bluing about 40%, the edges having worn from being 'slid' as it was picked up. I got it to the same standard as my test part in probably 20 minutes or less, which I was happy with.


Next step is to move on to some actual tools - got a small dovetail straightedge to make, and then a big precision triangle.


Richard King
01-16-2013, 08:02 PM
Congratulation's.... The edge is still a bit low, but I think you figured it out. I say sometimes it's like snapping your fingers and magic you get it. I can have a student struggling at 5 PM and I say in the morning you will get it and usually that happens in a few hours. I see you have a checkerboard look this time :-) . Many times when one see's it coming in they start scraping 40 PPI techniques (small radius, short stroke) to early and you have a super bearing on 90% of the part and a hole on the other 10%, but the more you scrape the easier it is to "read" the part. In the BIAX /DAPRA book they tell you to rub alcohol on it to dull the finish. On the 2nd to last pass I will lightly wet stone with mineral spirits to flatten out the small pointed high spots. Practice makes perfect . Good Job Rich!