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Increasing surface coverage and PPI

SuperSpinach

Plastic
Joined
Sep 15, 2023
Location
Southern France
Hello everyone,

I have been scraping a little bit for a couple of years but I recently got serious about improving and obtaining a higher quality surface finish.

I am still very much a beginner and the information on this forum has been invaluable (and also the videos made by the students of Richard King)

Right now, I use a hand scraper and I know how to rough scrape, to go from a rough machined surface to a fairly flat one, but I really struggle to increase the bluing coverage during the finish scraping.

I have been training on a small surface plate (about 10"x5") and managed to get a flat surface that hinges well (it evens sticks to my master plate when bluing) but the points are quite spread out with an average point size of around 1/16". I measured the depth of "valleys" and it's about 0.02mm (1 thousands of an inch). The result is far from some of the outstanding work that can be seen on this forum and I'll try to post some pictures in the following days.

Since I couldn't get a good result even after a lot of passes I decided to start again on a new piece of cast iron that will later become a straight edge (hopefully).

I tried following the 4 main steps given by Richard King and again I get a good flat surface that hinges between 25 and 33% from the ends but the more I finish scrape, the smaller the points get.(or so it seems)

Here are some pictures of the straight edge after some fairly heavy bluing (by my standards) :

Straight edge.jpg

Close up.jpg

Close up with gauge.jpg

Close up with gauge 2.jpg

I do a scraping pass at 45° trying to only scrape the very high spots (the bright ones surrounded by a dark ring) while leaving the lower ones. I then deburr and clean, blue again and do a pass at -45° (again only removing the highest ones).


I am not too sure what I'm doing wrong and any help or feedback would be appreciated to improve the overall coverage of the surface.

Thanks in advance
 
You're actually doing fine if you can get a fairly even spread of points (no vacant lots anywhere) and it hinges as it should.

If you stone it more heavily it will lead to larger areas of blue. Then start splitting those up, but not necessarily trying to remove the whole blued area.
 
Splitting is key.

You have already discovered removing the "bull's eyes" where a light area is surrounded by darker blue... the light area was high, and had the blue squeezed out. You can remove those, but split the other blue areas.

To improve (change) the spotting, you have to change the spots. Splitting larger spots increases the PPI and also often minutely changes the high spots, lowering them as they are split. Otherwise, the spots move around, but they never improve.

When I was first clued in on those two things, bull's eyes and splitting spots, I was amazed that all of a sudden the surfaces started rapidly "coming in" to a good PPI and coverage percentage once I arrived at the point to use them. It seemed almost like cheating.
 
Thank you for the feedback,

I'll try the stoning method although I don't have proper flat stones. Right now I use very fine swiss watchmakers files to deburr but I will try to lightly draw file across the surface with a slightly coarser file.

Regarding the splitting it's not always feasable as sometimes the very high spots are narrower that my scraping marks but I'll try it on the larger ones.

I guess there is no magical trick to instantly improve the surface so I'll have to go back to the workshop and spend some time to improve my technique also. 😀

I'll keep you guys updated on the progress
 
.............................................

I'll try the stoning method although I don't have proper flat stones. Right now I use very fine swiss watchmakers files to deburr but I will try to lightly draw file across the surface with a slightly coarser file.

.............................
Suggestion:

Get an old file, and break or cut it into handy chunks. Round off the broken edges, and then rub the file pieces on the flattest fairly fine sharpening stone you have. Get it to where there is a flat spot on top of every tooth, or most all of them.

Now the file will only cut things that stick up, such as burrs, etc. It will not dig into a flat surface, it will just glide over the surface. When you use it, you know to stop when it no longer "drags" on anything.

There are benefits to using a stone, but the "burr file" is very useful also.

A standard file CAN cut into a flat surface, so while you can use one, it is safer and more effective to use the flattened file.
 
That's quite a neat trick to get a cheap equivalent of a precision groundstone !

Right now the file that I use is ridiculously fine with (if I remember correctly) between 60 and 70 teeth per inch. It also tends to glide over the surface and I usually just rub it a couple of times and it just polishes the high spots.

Also I scraped a bit today, trying to just remove the very high spots as described before and it worked fairly well. I still need to improve my technique and my consistency in the mouvement tho.

By reducing the pressure on the scraper I also managed to reduce the depth of the valleys at 0.01mm (half a thousands) and i'll try to reach a maximum depth of 5 microns (a quarter of a thousands) but I feel like I'm not consistent enough to achieve that, especially in the checkerboard pattern.

Also I found the magic trick to instantly improve surface bearing : you just need to apply a very thick layer of bluing on the master :D.

Here is the result for today (with heavy bluing) :

Heavy bluing.jpg
Heavy close up 1.jpg

Heavy close up 2.jpg


There are some shallow spots but i'm not too worried about it for now as this is just a test piece and my goal is to improve my technique.
 
Is your scraper blade radius small enough? As I recall, you should use large radius for roughing and small for finishing so you can mre easily split the high spors.
 
With a tighter blade radius and a shorter stroke, you should have no difficulty cutting those high spots in half, or even thirds. If you aren't already doing so, you need to start aiming your strokes at specific places. Get into a groove of tapping or dabbing the scraper right onto the target.
But as already noted, it's too soon to go for high PPI on that. Your test piece needs to be flatter with more even coverage before you spend a lot of time increasing PPI.
It's difficult to read scrape marks from one photo angle, but you may need to take slightly shallower cuts. 0.001" depth (mentioned a few posts back) is a nice roughing cut, but not so good when you are refining things. What rake angle does the scraper have on the surface, and what angle does the scraper handle have to the surface?
 
If I had the result your pictures show, and was wanting seriously to flatten it, I would go after that center area of high spots, and I would not just scrape the high spots. I would probably just do a pass or two of "area scraping", only on the high region, and then see where I was getting to.

You can waste a lot of time getting down to the low spots if you just take off the high spots too early in the process. Generally, unless you have better information, the high spots are seriously high, so just area scraping to reduce that region is best and most efficient.

Checking after a pass or two of area scraping is good, to avoid drastic overshooting. Once you have reasonable coverage in all areas, you can get more persnickety. But when you have virtually no coverage in a large area, get on the high areas and make the chips fly.
 
@marka12161 Right now I use a 40mm radius with 5° negative cutting edge for finishing but I recently managed to get a box of 5 new 25x25mm sandvik inserts so I might try to sharpen a blade with a 30mm radius to see if it helps.

@sfriedberg and @JST

Yes right now there are still some low spots in the part that I need to fix. They actually come from me lightly draw filing (in the hopes of improving coverage) . It does work somewhat but it also leads to some low areas so I probably wont do it again.

Regarding the finish scraping technique, I experimented today and it sort of started to "click". By applying less pressure I can manage to get a better bearing on my surface. Before I was pushing too hard, nearly as much as during roughing and it would lead to very deep valleys.

I also experimented with scraper angle and by holding the scraper at 25-30° (I think but I didn't measure it) I can make very narrow scrape marks so now I can split the high points. I also tried shortening the stroke length and the PPI started to increase as well.

I don't have any good photos of my work since as you suggested I went back to doing "semi-finishing work" to lower the middle and get better overall contact. I'll post a picture when I get a better surface.
 
1697310833390.jpeg

What you have here is what I call over-scraping. The areas with no blue you've obviously scraped a bit lower than the areas with blue, but the areas with blue you have over-scraped a bit and what I mean by that is that your scrapes are too close together. Get a sharpie and draw and island around the blue banded area. Now, inside that area, scrape it twice - once in both directions - but leave generous spaces between each scrape, at least the width of each scrape gap if not a bit more. Ignore the actual blue marks themselves, you are not ready for point-splitting quite just yet just do the whole area you have outlined. Do it once, re-print, outline again then do it again the other way and you should find a much improved pattern. If it still hasn't come in, keep doing that until the spread of blue leaves no distinct spaces. Discipline yourself to spread those scrapes apart and you'll get over this hump.
 
This is something I thought about during my scraping sessions but I'm not sure how to avoid this over scraping thing. I feel like I am bound at some point to overscrape by having scrape marks being right next to each other after multiple scraping passes when trying to scrape in the checker board pattern like you suggest (for example at some point a scrape mark that I do at pass number 8 ends up landing between 2 scrape marks made at pass number 5).

Right now I try to follow Richard King's advice of always leaving a gap between each mark but after many passes old scrape marks line up with the new ones

I don't know if my explanation makes sense ?
 
Keep your scraping tip very keen and try to scrape for a little more depth. Keeping the tip keen will let you cut consistant depth scrapes without putting excessive pressure into it which will cause you to lose control.

TBH your scrapes are very nicely formed, a reasonably consistent size and shape and not at all scratchy, just try for more depth because if your 8th pass scrapes match depth with your 5th pass then you're scraping shallow. As I said above, when you have an island like that draw around it then scrape inside it to a pattern ignoring the blue marks. Keep your gaps and you'll see the pattern emerge like magic. You really want the exact opposite of what you have there - heavy blue spots around the edges and lighter in the centre island. It's lucky really that you have some spots around the edges to prevent it rocking on the plate or else you risk getting a very false print.
 
Ditch the file and get a stone. Any medium of fine India stone will work.

Scraping makes the low spots, stoning creates the bearing surface.

Getting a good print and sliding surface is a combo of precision stoning and scraping. It takes time to figure out stoning, once you do you can confidently scrape and predict what your print will be
 
This is something I thought about during my scraping sessions but I'm not sure how to avoid this over scraping thing. I feel like I am bound at some point to overscrape by having scrape marks being right next to each other after multiple scraping passes when trying to scrape in the checker board pattern like you suggest (for example at some point a scrape mark that I do at pass number 8 ends up landing between 2 scrape marks made at pass number 5).

Right now I try to follow Richard King's advice of always leaving a gap between each mark but after many passes old scrape marks line up with the new ones

I don't know if my explanation makes sense ?
I don't have much experienmce but it'smy understanding that if you are going to do area scraping as has been recommended, your radius might actually be too small. I think my roughing scraper is something like 90mm radius
 
Ditch the file and get a stone. Any medium of fine India stone will work.

Scraping makes the low spots, stoning creates the bearing surface.

Getting a good print and sliding surface is a combo of precision stoning and scraping. It takes time to figure out stoning, once you do you can confidently scrape and predict what your print will be
Don't ditch the burr file, use both. The file is fine when getting things roughed into finishable shape (no low spots etc), but the stone does help out in finishing.
 
Don't ditch the burr file, use both. The file is fine when getting things roughed into finishable shape (no low spots etc), but the stone does help out in finishing.
I disagree, why use two tools and two techniques when you really need to be proficient at stoning? Its very important to be an expert at stoning. The less tools you use for any given project the better.
I messed around with a file for a hot minute and scratched the soft cast surface I was scraping. This was with a virgin Grobet tapered file, that was the last time i touched a file on a flat surface. I do use files to chamfer corners and what not. If you are producing big burrs during scraping that need to be filed then one should adjust their technique to minimize burrs.
 
Your not following the kingway Scraping rules 1 and 2. 1. Scrape diagonal rows of scrape marks that don't touch each other. They should be separated by the same with of the of the scrape mark. If the scrape mark is 1/8 wide the separation gap should be 1/8".
2. The rows should be separated by too. The first and second row should be separated by 1/16 to 1/8". So all rows should be separated. Last week I taught a class and a never scraped before hand scraped this part. I am having another class November 9 - 13. And we have room for 2 more students. He brought his Harig surface grinder for a project.
 

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When I was an apprentice 55 years ago I used a dull file I know use a med grit Indian stone. Every 5th stone I wet the surface with penitrating spray. As Matt said it spread the small points. You said you have a copy of my DVD so you should see how I do it as I show how to handscrape 40 PPI. If you are having issues email me. My email is on the sheet I send with it. Or [email protected]. or call me 651 338 8141.
 
Hello everyone,

Sorry for the lack of answer, I was quite busy as I recently finished my engineering degree and I had a few job interviews lined up (and still have a couple more to do). I probably won't be able to scrape until this week-end but I'll send picture of my work asap.

Regarding the edge, I try to keep it as sharp as possible because I realised as you said that it becomes quite hard to scrape correctly with a dull insert.

Regarding the stoning, it was my belief that india stones aren't flat and I thought they wouldn't be suitable for deburring. Do you use your stones out of of the box or do you grind them first on a surface grinder ?
I do have a Norton india stone that I use to sharpen HSS tooling so I can use that but if a precision ground stone is required i'll have to wait to buy it as my finances won't allow for it 😀.

@Richard King I have been applying technique 1 and 2 but I think there is some confusion here (on my end). I have applied it during the point splitting technique (that led to the last picture I posted). As mentioned by everyone here, I probably need to go back to area scraping.

I am still trying to figure out when to switch from area scraping to finish scraping. Although I watched videos on youtube explaining it and I understand the theory, I still need to figure it out by actually practicing it if that makes any sense. I probably need to improve the consistancy in my technique also.

I do not have a copy of your DVD, but I would have liked to get one. Unfortunately I saw on your ebay store that you don't ship to Europe anymore. I would have also liked to attend a scraping class but the trip to the USA makes it too expensive for me right now. Bringing parts of my lathe or my shaper would also probably prove to be a challenge :D
 








 
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