# First scraping / crude



## rpembry (Oct 22, 2013)

I'd be interested in any feedback. This is from the stock Taig starter vice.


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## Richard King (Oct 22, 2013)

The first picture looks like it will pivot in the middle like a top.  Or as we say "it's high in the middle"   when it s good it will pivot or hing about 30% in from each side.   You need to scrape a checkerboard cris cross pattern.  as you will see from the several pictures posted in the other posts..   Rich


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## Erik Brewster (Oct 22, 2013)

Richard King said:


> The first picture looks like it will pivot in the middle like a top.



Checking the pivot of the part is really important.

What is checking the pivot: If you have a cube-like part, and you push it at one side and it makes uniform contact with the surface plate, it should pivot ~30% from the opposite side. If only the ends are touching, then it will pivot at the end. If it is only touching at the middle, it will pivot in the middle. If your part has the center of gravity to one side, you need to understand it will pivot to that side. I put a weight on the part to get the center of gravity back in the middle, if I can. It makes life simpler when pivoting and interpreting blue patterns.

Why do we check the pivot: If the part is not flat, you can get misleading blue patterns. A couple of examples: a) a sphere will blue perfectly. You can roll it around until you get blue everywhere. That doesn't make it flat, b) if you are applying blue to a four legged stool that rocks, you can get blue on all four legs by rocking it. It may seem that these are silly examples, except when you have a real part with these problems and the problems are less than 0.001", it isn't so obvious.

What do we do with the information if it pivots wrong: If it pivots in the middle, you have a high spot in the middle and you need to scrape in the middle only. If it pivots at the end, you need to scrape at the ends only. When it pivots correctly, you can go back to normal scraping.

Do I need to do this pivoting the whole time I am scraping? I think you should. If all is well, it only takes a few seconds. Once you have the part so it is pivoting correctly, it won't go out of pivot if you are scraping well. If you mess up, it will catch it quickly.


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