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J Tiers
07-12-2010, 11:55 PM
I have an 18" straightedge to scrape. I have a flat that is just short of 18" diagonally.

As might be guessed, I am having trouble with getting the surface to "lie down" decently. The flat is OK, I have scraped other things on it and not had a problem. But they all fit it.

I have followed the Connoley suggestions, but I still get inconsistent marks, despite placing in teh same way in teh same place, and holding the same end for spotting. (selected by the "spin test")

Is there any specific thing I need to do differently due to the surface being worked on being a bit bigger than the flat?

My idea was to scrape one section as long as I can get on the flat, without worrying much about the far end, just spotting teh section that fits. Then to move over to the "hanging off" area to get that to come down to meet the rest, by spotting with THAT part fully on the flat.

I was planning to get to a decent basic number of marks all over the surface as per above, then continue to refine by alternate spotting to one then the otehr end.

Sort of as if you machined one part to a level surface, then moved the part over and brought the remaining part down to the same level.

I assumed I would have trouble if I tried to alternate ends too early.

But now I have trouble getting the originally "off the end" part to lay down, despite being careful to scrape only what I think are "real" marks. The first portion got to maybe 5 or 10 marks per inch, and I need to get the rest to match up before I start to refine it.

But scraping is just not moving the marks the way I think it should, and they are jumping around too much. Kinda classic "unstable" markings.

I assumed I would get a line contact or similar mark type where the "step" in the surface is, which I would scrape until I got it to the level of teh first surface..... Scraping ONLY that part which is marked, and where I KNOW there is extra metal, the area that was not originally scraped.

I expected it to tend to "walk off the end" of the surface as I scraped, finally marking more and more of the original surface until they match. But it is not doing that, it moves off towards the end, then comes back.

I am aware I am messing up here, but I am not entirely sure HOW.

Does this sound familiar?

MuellerNick
07-13-2010, 01:58 AM
Does this sound familiar?

It sounded very confusing. But confusion is sometimes familiar to me. ;)

First rule:
One single end never has to leave the plate.

Now if you look at the end that is overhanging:
If that end is to high (needs more scraping) and you shift this end to have a bit more contact, it will lift your work and make a rocking action. The result is, that all your points will wildly jump around.
So, attack that end first to get it halfway flat.
Now here's the pitfall:
If both ends are too high and you move the work lengthwise it again will start rocking. Both ends tooo high means the wear is in the middle.
Solution:
Move the work in a windshield-whiper like motion. Means keep the end that is overhanging steady and only shift the end that has full contact sideways.
Scrape until you have maybe 1/3 of the length contact.
Repeat that for the other end.

Now you should have consistent reading no matter what end is overhanging and even if you shift lengthwise.

You should try to plot a map how the work is out of straight. Put the work on the table with the surface pointing upwards and measure the distance surface::table. Maybe you have to shim the feet of the work (to reduce math, could be fixed with a bit of calculations). With that plot, it will be more obvious what is happening and why you get confusing readings. You might even actually plot the readings onto real paper (with an exagerated scale) and use a ruler to simulate the table.

HTH or was that even more confusing?
Nick

.RC.
07-13-2010, 03:55 AM
http://www.practicalmachinist.com/vb/general/checking-long-straight-edge-short-surface-plate-163600/

Forrest Addy
07-13-2010, 08:06 AM
Thank, Ringer. I wa looking for jut that. Jerry, your decription of your troubles make me think you may have something stuck to the the blue on the surface plate. The least little speck will drive you nuts giving you erratic results. I suggest you stop and conduct a clean up of your work area from bare stone and clean breyer and start over.

I assume your scrapers are sharp, your eyes are good, and no black cats or witch's spells (can't be too careful) are afflictng your run of luck.

You say your straight edge just overlaps the diagonal. This is no big deal and a succesfully scraped edge is just a little work away.

You might also conduct a survey of your stranght edge using the bare surface plate and three 0.005" feelers (taken from the same roll) placed cross ways and distributed along the length under the straight edge. With the reference face resting on the three feelers puill on one after the other. If all three drag then the work i pretty flat already and you only need to scrape for distributed bearing.

Are you lightly stoning after scraping? Got a shop vac handy with a clean flter? Are you handling the work to minimize heat input from you hands. There three details ensure againt the three mot likely problem: residual bumps and burrs, tramp dirt, and heat distortion.

Scraping is where a dirty process alternates with one requiring scrupulous cleanliness. You are constantly cleaning and even the best of care taken to avoid tranfer of swarf from the scraaping bench to the blued reference only means you will probably have to stop perodically and clean everything up, re-apply blue and start over.

If it wasn't a PITA anyone could do it.

J Tiers
07-13-2010, 08:33 AM
Definitely NOT anything on the flat. I actually quit working on it for a while, and went on to some other things, and cleaned the flat between times. Same deal when I re-started. And it has been cleaned since, I am aware of the grit issue. No streaks, no odd 'rolling" feel to it, etc.

The flat is probably good also, since smaller parts reversed, turned 90 deg, and put down randomly still spot sufficiently the same to suggest no humps on the flat.

I alternate stoning and burr file. Burr file works better, the stone loads up and fails to do what it should. Connoley suggests the burr file, which I do find works better.

I put the work straight down on the flat, I hold by the same end and rub about an inch or a bit less of movement 4 or 5 times, I take it off and put in workholder, then cover the flat immediately.

Scrape as appropriate, wipe off twice. Stone or burr file lightly, wipe off again, tip up and brush off with heel of hand, uncover flat, and spot again. if blue is too thin for that stage, it gets renewed, sometimes with a clean-off first.

The holding end is the one farther from the pivot as found by a spin test.

It isn't the first thing I have scraped, and I have done fine with everything that has fit on the flat. I need to finish this straight edge because the next item in line has surfaces which I cannot spot on the flat.

My thought was that the overlap, a couple inches, is throwing me off here.

is the strategy of scraping one part, then getting the remainder to come down to meet it faulty?

MuellerNick
07-13-2010, 08:37 AM
avoid tranfer of swarf from the scraaping bench to the blued reference

If you are accustomed to the sound, you immediately hear that something is wrong.
Burrs/bumps/dirt make a highnoted sound, like draging a stone over glass.

Just by the sound, you can say that a surface is way off.

I think a granit plate does that better than a cast iron one. The CI has more damping.

But also looking at the master after touching tells a storry: If you see narrow circular (if you moved in circles) lines, you know that you have to clean and touch again.

You have a very sensitive device to detect any speck: your thump's ball!
Whipe over the plate with it (yes, it gets blue) and your work and you'll feel where the problem is.


Nick

Forrest Addy
07-14-2010, 02:36 AM
Jerry, sorry if I ruffled your feathers about cleanliness. Swarf transfer happens even in work by the most careful craftsmen; I know you to be a careful worker but I had to ask.

Have you tried the feeler test? It will detect defects that may be linear but inclined, crowned, or sinuous such that the work teeters on the reference to a small degree producing the erratic prints that are driving you bananas. You experiment with how the feeler pulls, slips along the length, baising the pull etc. It takes some discrmination so play with it.

My next suggestion is for you to scrape a slight hollow in the straightedge face; enough to be free of blue indications for a few cuts except for a rim around the straight edge. This will remove all the teeter and rock and maybe allow you to recover in a few cuts and eventually continued scraping will bring the surface down to bearing.

Maybe it's time for a PM. These exchanges go far faster by telephone.

Mcgyver
07-14-2010, 08:52 AM
I agree with the apporach of scrape one section then bring the second to its plane but I wonder about the reference. if there's error on the plate of say .0002 your scraping will follow that....when you shift the work to spot the second section, the work, with error matched to the plate, no longer sits down properly at its new location. I recently had a 12" straight edge I was scraping to do a dovetail. I'd been using a budget surface plate and was having some frustration getting an even set of bearing points to emerge. I switch to a Starrett and and got a COMPLETELY different pattern of blue....

MuellerNick
07-14-2010, 09:45 AM
As a fun-exercise, you could scrape against two masters and rotate work. :cool:


Nick

J Tiers
07-14-2010, 11:07 PM
The flat has been Ok with other work, so while I am certain it has errors, it may not be that bad.......

I think part of the problem is that I thought I was farther along than I seem to have been...... I may have forgotten where I was in this, as the notes are not findable.

I started again, and followed the marks, ignoring those at one end, and "shoveling" at the other...... This has resulted in more consistency.... So I think that I was simply (for now, at least) not moving enough metal to show a decent change of pattern. I'd get no change for a couple of cycles, then one that was larger than expected.

With the break in work on the piece, I may have reversed ends.... So now I have simply picked an end and am going for a good 5 to 10 spots on one end and middle, after which I will bring end #2 down to it....and then refine the whole thing. I am trying to apply just a bit less effort to the end I am not spotting as to the area adjacent to it, so that I keep more-or less "up" with the rest.

So far so good.

While I seem to have a "step" to deal with, it is moving and spreading out OK with teh more aggressive work style.