Author Topic: Scraping a surface plate. Where to start ?  (Read 4772 times)

Offline DavidA

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Scraping a surface plate. Where to start ?
« on: April 14, 2015, 06:38:44 AM »
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.

Offline awemawson

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Re: Scraping a surface plate. Where to start ?
« Reply #1 on: April 14, 2015, 06:47:19 AM »
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
Andrew Mawson
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Offline vtsteam

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Re: Scraping a surface plate. Where to start ?
« Reply #2 on: April 14, 2015, 09:13:42 AM »
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!  :)
I love it when a Plan B comes together!
Steve
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Offline wgw

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Re: Scraping a surface plate. Where to start ?
« Reply #3 on: April 14, 2015, 11:27:27 AM »
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.

Offline JHovel

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Re: Scraping a surface plate. Where to start ?
« Reply #4 on: April 14, 2015, 12:16:09 PM »
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
Cheers,
Joe

Offline vtsteam

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Re: Scraping a surface plate. Where to start ?
« Reply #5 on: April 14, 2015, 03:24:07 PM »
Wouldn't you use a longer straightedge rather than a larger surface plate, assuming any size to the first one? Unless you have llfting tackle for the rub.
I love it when a Plan B comes together!
Steve
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Offline JHovel

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Re: Scraping a surface plate. Where to start ?
« Reply #6 on: April 15, 2015, 08:45:59 AM »
For a surface plate I would use another larger surface plate. Yes, liftinhg tackle may be required - or a helper.
Staightedge is useful for checking.measuring geometry of surfaces to each other or narrow long slides.
Joe
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Joe

Offline krv3000

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Re: Scraping a surface plate. Where to start ?
« Reply #7 on: April 15, 2015, 05:51:40 PM »
hi well just nick the bathroom mirror or the glass kitchen table put blue on it put it on top of your plate give it a  rub and all the hi spots will be marked  and go from there

Offline DavidA

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Re: Scraping a surface plate. Where to start ?
« Reply #8 on: April 16, 2015, 05:41:19 AM »
Joe,

I was assuming that the person scraping had no access to another plate and would have to start from scratch; or should that be scrape.

I  have a large(ish) piece of plate glass that could be used as per krv3000.

A long time ago I contemplated grinding a ten inch parabolic mirror. Wisely this idea was put in the 'maybe later' file.

My own surface plate seems to be pretty good. At least for the stuff I do. But I do have a source of sg iron that could be milled into 5" squares for practicing on.

One more thing to pass the time.

Dave.

Offline vtsteam

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Re: Scraping a surface plate. Where to start ?
« Reply #9 on: April 16, 2015, 10:09:04 AM »
jhovel are you a scraper hand?
I love it when a Plan B comes together!
Steve
www.sredmond.com