Richard King
Diamond
- Joined
- Jul 12, 2005
- Location
- Cottage Grove, MN 55016
Many new to scraping start to evaluate the project and over do what they need. I was telling my group about a Psychiatrist who emailed me with photo's showing how he was indicating a Rung-Fo Mill table using an Inter-rapid .0001" indicator and how bad it was. If my memory serves me right it was off .008" When he bought it he thought it would be less then .001" He held a class inside his 3 car garage and the first thing we did was to scrape the 4 feet so they were all touching at the same time. It also bolted to a metal stand and the stand was bad.
We drilled and tapped the cabinet legs to add leveling screws then we took the machine base and set it on the cabinet and shimmed so it set solid on the stand. I told him he could not expect better then .0005" per foot after we were done as the machine wasn't designed to hold .0001". In good German, American, European conventional machines were built correctly and the spec's were .0002" per 12" on machines like a lathe, mill. If I was scraping a surface plate or jig bore you would expect a new machine to be .00005" in 12".
So the Doctor wanted tenths and settled for .001" Now think about the work envelope. Most of those cheap machines only had a travel of 12" and if he put a vise on the table, his work envelope was the width of the vise. Most vises are 6" wide, so his spec then was .0005" in 6". If he had a lathe he was scraping the bed and it was 10' long then it could be out .002" as you have to think of the multiplication of error is .0002" x 10' = .002" Also when scraping a conventional machine you only need to have is 20 PPI (Points per inch) and 50% contact. I
I taught a class 40 years ago at Gallmeyer and Livingston Surface Grinder Company in Grand Rapids, Michigan and before the class they only used hand-scrapers and on there surface grinder builds to 12 PPI. Using a BIAX power scraper one can get 20 PPI by accident. Another thought is the more PPI and POP (Percentage of Points) of 40 to 60% contact, the longer it will last. This is why we scraped straight-edges to 40 PPI. You can use it longer.
We drilled and tapped the cabinet legs to add leveling screws then we took the machine base and set it on the cabinet and shimmed so it set solid on the stand. I told him he could not expect better then .0005" per foot after we were done as the machine wasn't designed to hold .0001". In good German, American, European conventional machines were built correctly and the spec's were .0002" per 12" on machines like a lathe, mill. If I was scraping a surface plate or jig bore you would expect a new machine to be .00005" in 12".
So the Doctor wanted tenths and settled for .001" Now think about the work envelope. Most of those cheap machines only had a travel of 12" and if he put a vise on the table, his work envelope was the width of the vise. Most vises are 6" wide, so his spec then was .0005" in 6". If he had a lathe he was scraping the bed and it was 10' long then it could be out .002" as you have to think of the multiplication of error is .0002" x 10' = .002" Also when scraping a conventional machine you only need to have is 20 PPI (Points per inch) and 50% contact. I
I taught a class 40 years ago at Gallmeyer and Livingston Surface Grinder Company in Grand Rapids, Michigan and before the class they only used hand-scrapers and on there surface grinder builds to 12 PPI. Using a BIAX power scraper one can get 20 PPI by accident. Another thought is the more PPI and POP (Percentage of Points) of 40 to 60% contact, the longer it will last. This is why we scraped straight-edges to 40 PPI. You can use it longer.
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