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J Tiers
03-04-2011, 12:43 AM
There has been some talk from time to time about the generation of surfaces and angles by the three piece method(s). I wanted a right angle reference, so I decided to do it.

It was somewhat easier than I expected, but took a while. I did 'cheat" a bit, because while I used Connoley's "one over one" method, I interspersed spotting and scraping with a granite flat as a reference.

That kept things flat, and meant the angle was the target when comparing between the pieces, and flat was the target when spotting on the flat. it also avoids the strange cases that some folks, including Evan, brought up from time to time as a pitfall of the process.

I am by no means sure that the practical process of generation would even allow those oddities, but no matter, the granite flat avoids them.

I also cheated in that I wanted ONE right angle..... so I sliced an angle plate into three pieces, and used the unreinforced parts as "reference tools", being the "other two" of the three. I really don't care if they warp later, they can be used again if I want another angle piece.

I kept the piece with the reinforcement as the "real angle".

I'd show the final spottings, but I find that the marks don't show up well in pictures unless the "blue" is very heavily applied, which is just misleading.

Here they are:

http://img.photobucket.com/albums/0803/jstanley/scrangle1.jpg

The main angle at an earlier stage, with spottings against both another angle and the plate (for bottom) rather heavy blue.... at this stage I believe there were about 25 actual spots per square inch, but distribution was not great, I had probably gotten a bit too aggressive in one or two areas:

http://img.photobucket.com/albums/0803/jstanley/scrangle2.jpg

dian
03-04-2011, 01:27 AM
how big are they and how long did it take you to do this?

J Tiers
03-04-2011, 09:32 AM
The target reference part is a little over 25mm wide, by 100mm tall. The others are similar, I made the target part and the "working references" basically the same since they have to be spotted against one another.

That was the thing, I could only find very large ones, 250 to 300mm tall. Some of the import companies have granite versions, but all big that I found.

Time? I did not keep track, since I didn't do it all at one time. I started off by milling them as closely 90 deg as possible, which was a big help.

rowbare
03-04-2011, 12:43 PM
Cutting up an angle plate was clever. Definitely an idea worth stealing.

bob

dian
03-04-2011, 12:56 PM
you know, i have been wanting to scrape something for a long time. never started though, because im afraid its so much work, that i wouldnt finish it. maybe its not. how much time do you guess you spend on it? thanks.

J Tiers
03-05-2011, 12:30 AM
OK, as a guess, the total probably is under 6 hours of time, but it was split up as a bit of time in the evening every day or so over a couple weeks. Not every evening did I spend time on it.

I have another scraping project underway which this is actually for, which is a mill, and I was working on it also. Probably the 6 hours of time total is close.

And that is longer than it should be, on account of the fact that I had never done this before so I was deciding how to go about the three piece rotation, how often I should compare to the granite flat, etc, etc. Once I got the the gross corrections done after the initial milling so I was close to correct, it went very fast. An expert could cut the time to 1/3 of what I took, maybe less.

For a small area piece like this, it goes pretty quickly, a "round" of scraping and spotting is over in a few minutes.

randyc
03-05-2011, 12:32 AM
Cutting up an angle plate was clever. Definitely an idea worth stealing.

bob

Definitely

lazlo
03-05-2011, 12:40 AM
That kept things flat, and meant the angle was the target when comparing between the pieces, and flat was the target when spotting on the flat. it also avoids the strange cases that some folks brought up from time to time as a pitfall of the process.

The potato chip twist only happens if you're doing the three-plate method. I.e., you don't have a flat reference surface to start with.

Nice job - clever cannibalizing an angle plate :)

J Tiers
03-05-2011, 01:05 AM
And, actually, I think the potato chip is really impossible.

Sure, they fit together statically. But in a practical sense, you move the parts relative to each other to spot them, and so the potato chip will mark unevenly, and I believe it will get corrected even so.

The typical movement is at least a half inch or so, and should start to bring the potato into line. Whatever remains will likely be in the same realm of error magnitude as all the other possible errors of a surface that is imperfect.... as even the finest flat surface is.

Plus, with the one over one method, one will be marked by the movement and scraped flatter as a result. Then when that is used to mark the third, the "chip" contour of the third will be marked in such a manner as to bring it flatter if scraped.

I am not a true believer in these perfect static fits.

The Artful Bodger
03-05-2011, 01:34 AM
The only shapes that fit together while being able to slide in all directions are sections of a sphere and flats. In the case of the sphere sections one section must be concave while the other is convex and as this is impossible with three pieces they must be flats.

Forrest Addy
03-05-2011, 02:40 AM
Perfection is a moving target even in precison scraping. You have three angles scraped to mutual perpenticularity using "averaging of errors" technique.

If you went on to scrupulously sweat every little detail including temperature you could probably refine what you have to 0.000005" per inch of square, within 0.000020" of over all flatness, and 40 spots per square inch. This requires heroic attention to detail and probably performing the last stages of scraping and final inspection over several sucessive days to allow things to return to equilibrium. All you would have in the end is an artifact of patience and precision suited for years on a shelf in a nice wood box. Every once in a while you take it out to show to family and friends and maybe brag. It's not a tool you dare use for anything but checking work of great refinement - but why?

Here's a good answer: first rate work is its own reward. There is a Zen-like satisfaction to the pursuit. If that doesn't work for you consider it a rite of passage for a careful craftsman.

Regardless you done good if my praise is worth anything to you.

lazlo
03-05-2011, 10:29 AM
And, actually, I think the potato chip is really impossible.

Sure, they fit together statically. But in a practical sense, you move the parts relative to each other to spot them, and so the potato chip will mark unevenly, and I believe it will get corrected even so.

Oh, the potato chip happens a lot, especially on long thin parts -- ask Forrest ;)

It's the reason all the old scraping books (Connelly, Moore, that book that Forrest quotes) all tell you to only use square plates for the three plate method, mark one side with a punch mark, and carefully rotate each plate 90 during each spotting cycle. Otherwise, you will get a potato chip twist.

Forrest Addy
03-05-2011, 12:04 PM
Yup. Been there.

There are a number of ways two "flat" surfaces will blue. There's spherical and scalloped (potatoe chip but with any number of scallops. Thus successive prints should not be taken with the same orientation. Randomize them to the extent possible.

The three plate solution will also prove if the scallops are equal and printed when in - um - "mesh(?)". Random, random, random. Deliberately random.

J Tiers
03-05-2011, 12:24 PM
Regardless you done good if my praise is worth anything to you.


It is.

Thank you.