Ulma Doctor, Scraping Mentor

For instructional purposes mild steel works just fine.
The piece being scraped was not for precision, merely a teaching aid.
The mild steel was used as a comparison of relative flatness achieved by a surface grinder vs. scraping
The surface grinder was flat to about .001"
 
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you are wrong mild steel is not stable for scraping i tried it, one time and the heat from your hands would change it, why arent surface plates and straight edges made of mild steel, because they are not stable
 
OK i'm wrong,


Hear Ye!!!!

Thou shalt never scrape mild steel, even if it is for instructional purposes that will never be used for a precision comparator or reference or facsimile thereof
cast iron will be the only material to ever be scraped from this moment forward!!!:oops:
 
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You wouldn't want a straight edge made from steel flat bar. But I have scraped a many of gibs that were steel. Believe me it will move all over the place on you, not easy to keep a steel gib straight from scraping.

Ken
 
my previous point , whether taken or not, was that most anything can be scraped to varying levels of precision
whether the material will hold the tolerance or not is inconsequential when you are explaining and showing scraping to someone who has never held a scraper before.
 
you are wrong mild steel is not stable for scraping i tried it, one time and the heat from your hands would change it, why arent surface plates and straight edges made of mild steel, because they are not stable

Perception changes as we educate ourselves. Things that may be dropped and bent tend to be made of cast iron or steel that can be tempered. This doesn't mean they can't be damaged, just more difficult to do. I'm just adding two bits here because I think it's unfortunate to get stuck in black and white thinking. It stops the educational process. Many items manufactured from cast iron years ago are now manufactured from steel plate.

The sheetmetal working equipment I own is predominately made of steel that requires scraping to true them up. Should I not scrape them. I have a 10' shear with a bed fabricated from heavy steel plate. I've used many shears where the beds were made from cast iron. It's easy to lose sight of of the lesson being taught if we get tunnel vision on dimensional tolerances or mechanical properties of material. I don't have to be wrong for you to be correct.

I encourage that education and experience be used in a positive manner. e.g. My youngest son came home from wood shop and said; Dad, most of the guys don't even know how to use a measuring tape. I said; Use what you know to help them learn. -Russ
 
even cast iron moves with the addition of heat maybe not as much as other materials. when I was an apprentice I had to scrape the turbine cases of boiler feed pumps they were made of steel and were scraped to allow for sealing steam without any sealant between the faces. never had a leak. the large cast iron straight edges used for restoring lathe ways have wooden handles built in so the heat from holding them do not change them. bill
 
I have a beveled cast iron straight edge dad made out of a section of lathe bed on a machine his company scrapped out back in the 1960's. He never did stress relieve the piece of cast iron before scraping on it. Still have it, it still moves around when you try to scrape on it. One of these days, I'll get it stress relieved. But kid you not, cast iron does move on you if do not stabilized it first. Ken
 
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