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DIY surface plate check?

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Rex Walters

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#1
Okay, it may be the bourbon doing the thinking, but I think I've come up with a slow, tedious, and probably impractical way to at least check the flatness of a granite surface plate with just a hand scraper and two cast iron straightedges at least as long as the long diagonal of the plate.

It's more than possible I'm flat wrong about this, but please check my thinking.

To create a truly flat surface plate by hand with no other reference, you actually create three, using what Connelly calls "automatic generation of gages" (chapter 21 in Machine Tool Reconditioning). Basically, repetitively scraping each plate to match the others. Connelly calls this the "Principle of the Symmetrical Distribution of Errors." (Tom Lipton demonstrated the process in a series of videos while creating some small lapping plates.)

Basically, "two things equal to a third thing are equal to each other."

I suspect it would take months to create three plates of any reasonable size this way, but that's beside the point.

A relatively "quick" way to just test your granite plate might be as follows:
  1. Scrape in straightedge A as accurately as you can, bluing up across one of the long diagonals on the plate. Alcohol "bluing" and at least 40 PPI is required in the final stages.
  2. "Blue" up with alcohol along the other diagonal. If the straightedge doesn't mark up evenly, your plate isn't flat.
  3. Repeat step two in the horizontal and vertical directions. Again, if it doesn't mark up evenly, your plate isn't very flat.
  4. Repeat steps 1-3 with straightedge B.
  5. Now, use alcohol to mark up A against B. Any unevenness in the markup will indicate a curve in the plate. If there are no marks in the middle, the plate has a bulge in the middle. If "hinging" the two straightedges together shows they have a high spot in the middle, then your plate is concave.
Step 5 effectively doubles any error, so I think you should be able to detect defects in flatness down to 0.0001" or so.

This is just a thought got experiment, though. I've not actually tried this.

What do you think? Is my reasoning sound?
 

Dabbler

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#5
As a thought experiment, yes, well OK. A good surface plate from a reputable manufacturer, (as they say in a popular commercial) Priceless!
 

Rex Walters

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#6
surface plates are checked with an autocollimator for accuracy
Yup. Cheers, Mike. Got one I can borrrow? ;-)

Buying a decent, certified plate to begin with, mounting it correctly, and periodically having it recalibrated and resurfaced by pros as necessary is definitely the Right Way to Do It.

I'm a hobbyist, though. I tinker and try to get away with stuff on the cheap when I can. I'm sometimes as interested in learning if something can be done as actually doing it.

I've no serious intention of ever trying this, nor do I ever expect anyone else too. I was just curious if the reasoning is sound (and never say never, I've tried crazier things).

I've even less expectation of acquiring an auto-collimator, mapping software, and repeat-o-meter. Worse, I suspect a calibration service would just laugh at me if I asked them to come out and check my cheap little offshore plate with a huge chip in it.

Eventually I'll probably break down and buy a decent certified plate. Until then I'm happy enough with the results I'm getting on my little cheap offshore plate.

I was happy to discover that the sides on my box square that I made using one of Keith Rucker's plates still measured flat and square with my plate at home. The parts I'm making and checking also seem consistent with each other.

That's enough to satisfy me with my current plate. For the moment, at least! ;-)

Regards,
--
Rex
 

Ulma Doctor

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#7
your chipped plate is fine for any class of work you can produce in your shop.
as long as it is relatively flat, you can scrape things mastered from it
i scraped a bunch of stuff from 2 unknown plates BEFORE i had them lapped and calibrated.

luckily my plates were very good before re-calibration, but i had no other uber accurate reference (besides blind faith) that the plates were semi-flat.
i used a surface gauge, then a straightedge backlit with a flashlight or sunlight- they seemed to check good (flat) in those poor checks.

don't let a certification of calibration stop you from enjoying scraping, run what you brung and enjoy the process :D
 

4gsr

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#8
.....................

don't let a certification of calibration stop you from enjoying scraping, run what you brung and enjoy the process :D
My Truestone plate still has its inspection certificate from 2001 on it certifying it to a 'A' grade. Yeah, slightly out of calibration date! But, it was part of a special inspection plate that had a fixture bolted down to the rock. Probably got used a couple times a year, if any at all. I've probably use it more now than it ever got used in its previous life! Mine is flat enough to do what I want it for. The straight edge I made and scraped in was done on this plate. I've done the X-test across the corners of the plate with the straight edge and read the same on all corners across the plate and along the edges. That gives me a fuzzy feeling it's pretty flat!
 
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