The Shop > Metal Stuff
Scraping a surface plate. Where to start ?
DavidA:
Never having done any scraping I wondered about the following.
Let us assume you have a large cast iron plate that would make a nice surface plate.
What do you refer it to ?
I can only assume that you will need a small truly flat piece to use on the 'blued' plate so you have some indication where the high/low spots are.
But how does one get this reference plate ? Do you scrape two small plates against each other until both show the same spots-per-square-inch pattern, or what ?
Dave.
awemawson:
The classic way is to scrape three plates to each other to end up with plain surfaces on all of them:
http://en.wikipedia.org/wiki/Flatness_%28manufacturing%29
vtsteam:
Though I own a cast iron surface plate, I've wanted to lap 3 concrete patio flags together to see what happens. Out of curiosity. I suppose a small concrete surface plate might come in handy for some things, too. I'll think on the story of polishing a tile to make a mirror to pass the time! :)
wgw:
I was going to be flipant and say start at the top left corner, but I won't. As said the classic way is to have 3 plates to start. That's how we did it as trainees, of course they were plenty of plates about, and it did take a long time, about a week or more from memory.
JHovel:
If you just want to actually make your own CI surface plate flat, then you need another KNOWN flat one that is BIGGER, not smaller. You may be able to norrow one form another forum member for example.
You blue the bigger one and srape the high spots of yours until you have an even distribution of say 20 or 40 spots per square inch, depending on the purpose. If yours was ground first (not scraped), then you have to 'break up the surface' by a couple of gentle shallow passes 90 deg to each other first. That way you can get meaningful 'prints'.
Once you get the hang of it, it actually doesn't take all that long. Mind you, a medium sized surface plat isn;t the place to start learning! Way too likely to frustrate you. Learn on something about 4"x 6" first.
Cheers,
Joe
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