07-25-2012, 02:35 PM
Hll Al,
DA the truth is that the only real way to learn to scrape is to spend time scraping, for the highest precision use a very thin film of dye, I have always used oil based prussian blue and I often set up a lamp on the far side of the job just an inch or two above the face to be scraped with the bulb well sheilded so that I can't see the bulb, this way you don't need to see the difference in colour on the high points, rather you see the difference between the shine off of clean metal and the "oily blue film" from the prussian blue, this lets you get the film thickness really thin and thin = accurate.
Then simply take off the high points and retest on the surface plate,
And one minute of scraping after four minutes spent sharpening the scraper will yeild much better results than five minutes pushing a blunt scraper.
The real skill comes in when creating a master plate, that is using three machined plates and scraping them against each other as references in order to create three properly flat scraped surfaces. I did this once during training and that was enough.
From memory I think it goes a bit like this;
1. scrape plate 1 against machined surface, to 80% or better contact,
2, scrape plate 2 against machined surface, to 80% or better contact,
3, scrape plate 1 against plate 2 to 80% or better contact,
4, scrape the original machined surface against plate 1, to 80% or better contact creating plate 3 which should be the flatest most "True" master surface out of the three plates.
Then keep plate 3 as your master whenever you need to scrape a joint or flat surface of any kind, keep in mind when we did this exercise we started with three sand cast blocks and we were not allowed any machine tools so we had to cross cut with narrow chisels finish with cold chisels then file flat to within 0.002" and scrape from there, our "Machined surface" was the best of the filed blocks as judged by the instructor. Pure torture especially considering we did it in a tin shed in January/February in Australia, most days were well over 30 degrees C up to about 42 degrees C so about 90 - 105 degrees F.
There is little need for this sort of work nowadays, given the accuracy of modern grinding machines.
Best Regards
Rick
DA the truth is that the only real way to learn to scrape is to spend time scraping, for the highest precision use a very thin film of dye, I have always used oil based prussian blue and I often set up a lamp on the far side of the job just an inch or two above the face to be scraped with the bulb well sheilded so that I can't see the bulb, this way you don't need to see the difference in colour on the high points, rather you see the difference between the shine off of clean metal and the "oily blue film" from the prussian blue, this lets you get the film thickness really thin and thin = accurate.
Then simply take off the high points and retest on the surface plate,
And one minute of scraping after four minutes spent sharpening the scraper will yeild much better results than five minutes pushing a blunt scraper.
The real skill comes in when creating a master plate, that is using three machined plates and scraping them against each other as references in order to create three properly flat scraped surfaces. I did this once during training and that was enough.
From memory I think it goes a bit like this;
1. scrape plate 1 against machined surface, to 80% or better contact,
2, scrape plate 2 against machined surface, to 80% or better contact,
3, scrape plate 1 against plate 2 to 80% or better contact,
4, scrape the original machined surface against plate 1, to 80% or better contact creating plate 3 which should be the flatest most "True" master surface out of the three plates.
Then keep plate 3 as your master whenever you need to scrape a joint or flat surface of any kind, keep in mind when we did this exercise we started with three sand cast blocks and we were not allowed any machine tools so we had to cross cut with narrow chisels finish with cold chisels then file flat to within 0.002" and scrape from there, our "Machined surface" was the best of the filed blocks as judged by the instructor. Pure torture especially considering we did it in a tin shed in January/February in Australia, most days were well over 30 degrees C up to about 42 degrees C so about 90 - 105 degrees F.
There is little need for this sort of work nowadays, given the accuracy of modern grinding machines.
Best Regards
Rick
Whatever it is, do it today, Tomorrow may not be an option and regret outlasts fatigue.