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Granite Surface Plate re-conditioning


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  #1  
Old 01-27-2016, 03:03:03 PM
Bill Hazzard Bill Hazzard is online now
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Default Granite Surface Plate re-conditioning

Here is a great video on granite surface plate calibration and lapping.



When I saw the video it inspired me to take a closer look at my 18" by 24" granite surface plate. I suspected it was worn some but I did not know how much.

The only precision measuring device I have is a master precision level with .0005 graduations in 10". The way I checked the surface was to center the level on the plate resting on two 123 blocks and level the plate on the X and Y axis. That gave me four zero reference points that were all in one plane. From those points I checked the level to other points on the plate in a grid. The plate was a grade A in 1989, the last time it was checked, long before I got it. I found that it was high in the center and two opposite corners were low giving a total deviation of .001", not very good.

I have an old beat up cast iron surface plate that I can use as a lap. I did the same checks with my level on the iron plate and that was good to about .0002" I don't have diamond powder but I do have some 280 grit carborundum powder. It won't cut as fast but that would make it less likely to go too far. I spent about an hour lapping and now I am no more that .0005 out. I figure another half hour and I should be up to grade B which is good enough for what I am doing. You can see in the photos the lighter cut down areas.

If I knew what I know now when I bought my plate I would have just bought one of these new import ones for the same price that I paid for a worn out one.

http://www.ebay.com/itm/330370873448...%3AMEBIDX%3AIT
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Old 01-27-2016, 04:23:08 PM
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Default Re: Granite Surface Plate re-conditioning

Watched that done in person on several plates at work.

Can be a very slow process on one that's badly worn from repeatedly sliding an indicator stand back & forth in the same location day after day after day.

Smaller plates they'd just retire and replace, but the big ones got resurfaced no matter how long it took.

I may have to share that video with a few friends that I've explained the process to over the years so they can better see what I was talking about.

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Old 01-27-2016, 11:31:05 PM
dalmatiangirl61 dalmatiangirl61 is offline
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Default Re: Granite Surface Plate re-conditioning

So what causes plates to be out of spec, is it wear or does the granite flex/creep over time if not supported properly?

I saw some big plates on an auction a few years ago, they sold dirt cheap, like under $100, but you did have to move them......I really wanted a 4'x4'x2' with 6"ledge plate for a coffee table, it was a beautiful pink granite, I just could not figure out how to get it into the living room without tearing out a wall
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Old 01-27-2016, 11:43:47 PM
Bill Hazzard Bill Hazzard is online now
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Default Re: Granite Surface Plate re-conditioning

They wear out from repeated use.
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Old 01-28-2016, 12:16:22 AM
Alastair Geddes. Alastair Geddes. is offline
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Default Re: Granite Surface Plate re-conditioning

Wear out....

They Just wear in spots which become low points thus putting a relatively flat table so so flat.
Your working area can wear just like you make tracks with your car in the driveway but on a smaller scale.

With so little removed in reconditioning them in comparison to thickness they would last a very very long time.

Should have grabbed it just for a outdoor table...from what i hear a little rain doesn't hurt them too much, move it inside when you could.
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Old 01-28-2016, 09:54:01 AM
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Default Re: Granite Surface Plate re-conditioning

At work (I just retired, so it isn't work anymore) they were told to put the surface plate out in the open so they could use it from all sides, thus reducing the wear in just one area.

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Old 01-28-2016, 01:09:52 PM
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Default Re: Granite Surface Plate re-conditioning

Guess I should have clarified too that with just a couple/few thousandths of wear in one location even our small plates got resurfaced as they could be many many times with the small amount removed. The small ones that got replaced were ones that were worn AND damaged from people dropping fixtures on them chipping them out bad enough that they would need more work than they were worth compared to the replacement cost.

The big ones that cost thousands of dollars each were repaired as needed and if damaged there was an epoxy they'd mix up with some granite powder and fill the voids before flattening.

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Old 01-28-2016, 01:38:48 PM
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Default Re: Granite Surface Plate re-conditioning

I was always taught you need 3 plates to generate a flat surface. For injector lapping plates, you relap by lapping A to B, B to C, & C to A, cancelling out any error between any 2 plates. Maybe that's the procedure used to generate the cast iron plate used to lap the granite?
I realize it's unrealistic to have 3 of the big granite plates just to keep them true...
There was a 12X18 Starrett pink plate on craigslist here a while back...I passed on it because the guy was a little high on price, plus I was doubtful it was in good condition...you can get a brand new certified one from Enco for a pretty good price if you catch it on sale.
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Old 01-28-2016, 02:55:13 PM
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Default Re: Granite Surface Plate re-conditioning

I don't remember if they use another iron plate to resurface themselves or if the iron plates keep themselves flat by lapping multiple granite surface plates, but yes, your description on cancelling out the errors is accurate.

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Old 01-29-2016, 07:48:32 PM
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Default Re: Granite Surface Plate re-conditioning

The lap does not have to be flat. It is turned 90 degrees every stroke or two to compensate. They first "map" the plate showing highs and lows. Then they work the high spots. This very much a matter of experience and technique as is all honing and lapping. The plate is lapped to millionths of an inch. It is measured with an autocollimator which compares the surface to a beam of light.
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