[How do I?] Thoughts on simply scraping a part flat (efficiently)

[Rex Walters]

I'm not entirely sure how to get a higher bearing percentage, but I think the approach would be to have flatter/shallower scrapes. There are two ways to accomplish this: grind the blades with a larger radius, or lower the back end of the scraper to create a larger effective radius at the surface. Either way, I'd need to take a lot of care to keep the scrape depth consistent and not tip the blade (causing scratches).

Here's my thinking on this. If Rich or someone with more experience sees this, please correct me if I'm off base.

Each scrape you make removes a high point and makes a new low point (obviously). If your scraping is consistent (big if) all these new low points should be in roughly the same plane and will eventually become new high points in subsequent passes.

Since the scraper blade is radiused, each scrape mark has roughly the same profile as that an ice-cream-scoop leaves behind in a freshly opened tub of ice cream: it's deeper in the middle and higher on the sides (albeit at a microscopic level). A better analogy than a scoop would probably be a large coin.

The coin in ice cream analogy is pretty good, actually. If you were to furrow two trenches in the surface of some ice cream with a fifty-cent piece, once with the coin almost vertical, and once with the back end lowered almost to the surface, then the first trench would be deeper and with steeper sides. In addition to the shape of the trench from side to side, the depth also increases along the furrow as you plow. The longer the stroke, the deeper you go. The deepest part of the furrow is at the end of the stroke, where the material you've plowed is pushed into a steep steep hill with much of it pushed above the surface (an ice cream burr!).

I need to create some diagrams to accompany these thoughts, but I believe this is why you shorten your stroke and move to narrower, tighter radius blades as you move from roughing to finishing. While you still strive to maintain the same consistent pressure on your stroke, the shorter/tighter furrows allow the high points to get closer together as you create the cross hatch pattern with each pass of scraping. The depth of your furrows is reduced with shorter strokes, but this is counteracted by the tighter radius.

I know that if I lower the back end of my scraper slightly for the last few passes, the bottoms of the trenches formed will be flatter. I think that this is desirable if I want bigger points and a higher bearing percentage. If my reasoning is correct, the flatter bottoms will eventually become the high points (with the slightly raised sides removed by stoning). In other words, with steep sided, short, narrow furrows I'll never get large points. I'm not completely sure about this, though.

I may experiment with something much smaller than the surface plate when I have time.

 

[Richard King 2]

Oh Rex had a good teacher..lol , but he is forgetting to mention the % percentage of Highs and Lows...looks like maybe 30% and we want more like 40 to 60%. Small percentage of small points lets the SE or scraped part wear faster then having 20 to 40 PPI with 50%. Rex how deep are those scrape marks? hardly look a few tenths? Remember min of .0002" and max of 001". Those short strokes make a lot more work too....AI would lengthen the stroke to 1/8". Edit....sorry I missed page 2....you did mention %.....anyway here are the charts I have inside the class work booklet…. Nice work Rex...but practice makes perfect and lengthen your stoke....1/16 is to short.

 

[Richard King 2]

In the last class in VT last week we discovered if you grind the blade to neg 8 to 10 degrees on a 3000 grit diamond wheel you can cut down on all the chatter in the scrape marks too.

 

[Janderso]

Rich,
Maybe I missed it, I haven’t read every word in the post.
Considering this is a surface plate and not a bearing way, is 40 PPI necessary?
Rex, looks good babe! IMHO, my opinion is not certified or bonified.

 

[Bob Korves]

Considering this is a surface plate and not a bearing way, is 40 PPI necessary?

We use surface plates as masters for various accuracy needs, so they ideally need to be accurate to an order of magnitude better than we want our most important finished work to be. Surface plates are fussy things, because they are used as a standard for checking other things.

 

[Rex Walters]

In the last class in VT last week we discovered if you grind the blade to neg 8 to 10 degrees on a 3000 grit diamond wheel you can cut down on all the chatter in the scrape marks too.

That is gold. Thanks, Rich. The chatter was getting annoying.

My scrapes are pretty reliably ~0.0002-0.0003" or so in depth, but I think as I tire I tend to drop the back end of that heavy scraper (creating wider, shallower scrapes). Pretty sure the stroke length never got shorter than 1/8" though (the shiny/chattery strokes in the photos with the PPI gage look to be about an eighth of an inch long to me). I did notice that the finishing blade I was using wasn't radiused terribly well, though.

I know how to increase the number of points, but I'm unclear on how to increase the bearing percentage (three of your classes notwithstanding!). Any further advice on how to focus on bearing percentage specifically will be welcome.

I think now that the part is pretty darn flat, my plan to increase bearing percentage would be to ignore pinpointing for a pass or two and create as even a cross-hatch pattern as possible across the entire surface, with a slightly longer stroke and paying attention not to drop the back of the scraper. By "even" I mean equal sized spaces (both side-to-side between individual strokes, and fore-to-aft between rows). Then go back with no more than one or two passes of pinpointing.

Regardless, I've set the plate aside for now as I need a lighter straightedge more than another plate. I'll get back to the plate eventually, but subsequent photos and posts to this thread will be with the straightedge.

 

[Rex Walters]

We use surface plates as masters for various accuracy needs, so they ideally need to be accurate to an order of magnitude better than we want our most important finished work to be. Surface plates are fussy things, because they are used as a standard for checking other things.

Yup, if you want to scrape something to, say 20 PPI you need a reference that's better than that. But 2'x3' granite plate is my primary shop reference. This small plate is for the very infrequent occasion where I need to move a small plate to a part I'm scraping (rather than bringing the part to the plate).

I'm pretty sure that bearing percentage isn't as important for an infrequently used gage than it is for a machine way that sees constant use (I'll be worm food long before that plate sees much wear!).

 

[Richard King 2]

I wrote a long reply and hit something and it deleted. I will do it again tomorrow....

 

[Rex Walters]

I wrote a long reply and hit something and it deleted. I will do it again tomorrow....

Argh. I know the feeling.

 

[Richard King 2]

I should have included this picture too. As far as PPI on plates, straight edges, Jig Bore table tops and non way wear surfaces, The more PPI the longer they last. Have you ever seen a Moore or Pratt & Whitney Jig Bore that is 60+ years and are as accurate as the day they were built? The reason is they were scraped super precision and 40 PPI. Now if you look at the chart above and say you have 10% and 40 PPI, those small points won't hold the accuracy as long as a 40 PPI at 50%. Remember when Taiwanese machines were called disposable? People said they were super accurate for about 2 years and then went to hell in accurately. The reason was low percentage and no depth of the scrape mark.