Cast iron versus granite surface plate

You blue the plate when your hand scraping. Rub the surface being scraped on the flat plate. Thats how you know where the high spots are
 
chuckorlando said:
You blue the plate when your hand scraping. Rub the surface being scraped on the flat plate. Thats how you know where the high spots are
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If this is an answer to my question, I have never and I mean Never seen a granite plate used for scraping. I spent my time scraping just like all apprentices do and it's done with cast iron plates. Granite would not stand up to the job for very long. Granite plates are INSPECTION PLATES.

"Billy G"
 
Walsheng said:
I have been using a CI surface plate for the last 30+ years in the home shop and it is more than adequate for hobby use. I do a fair amount of mold repair using it and I will probably buy a granite one someday, if I can get one real cheap. It's also handy to use with a magnetic indicator base.

John
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During my military engineering apprenticeship days (Early May 1966 to end of March 1969) the cast iron marking plates in the engineering /machining shops were used by many of the 1,300 teenage apprentice hooligans every year , some were 18 inches by 18 inches , most of the plates were big 2 x 3 footers , some were even bigger as well . I suppose the quality of the cast iron used and weathering by thousands of sweaty hands over the years helped keep them in good condition .

At the beginning of each term time we had to check the plates with true straight edges & a torch on the other side ..none of the plates were ever condemed as being unserviceable to the best of my knowlege . Nearly all the marking plates were made around 1933 .


Would a 3 inch thick section of a stone masons workshop ground & polished granite grave memorial / headstone be any good ?
I ask as I have access to such things via a local stone mason who carves/ shapes / polishes / finishes & letters up head stones .
Tthough at present I'm using a 24" x 24" square of 1/2 " of thick plate flow glass from a shop window repair company as it was free.
 
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That would depend on how close you wish to get. Head Stones aren't ground and polished to any great degree of accuracy. They don't need to be.

"Billy G"
 
I think I'll pop in next time I'm near there & see if the grinding /polishing up is done with two faces being face to face & water .
 
Round in circles said:
I think I'll pop in next time I'm near there & see if the grinding /polishing up is done with two faces being face to face & water .
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Please let us know what you find. It would be interesting to know.

"Billy G"
 
I have pretty limited experience but to me they are used for two different things and have different properties.

The CI plate can be used for lapping, which a granite plate should never be. The CI plate will rust, which a granite plate will not. If a CI plate is struck or damaged, there is a raised "lip" around the damaged site which will have to be stoned out before the plate is to be reused.

On the other hand, the granite plate is usually of better flatness than CI. The granite surface plate when damaged does not have a lip, but may chip. The granite surface plate will not rust. I have only used a granite surface plate to determine flatness, and it never occurred to me to use it for lapping!

On the whole, a granite plate seems to me to need less attention and care than a CI plate, but then they can be heavier since they have to be thicker to be safely handled without breaking.

Just my $.02
 
If you get the iron plate, it can easily be made flat, and providing it was properly manufactured so that it is stress free it won't warp or move. Unless temp differences are introduced. Even so, once temp is equalized, the plate will return to shape. You are still left with the need to qualify it's degree of flatness though. A granite stone of known quality can be used for this if used to show bearing and scraping the plate using it for reference. It's more expensive to repair surface would be preserved if used very sparingly, such as to bring the iron plate into shape for use during scraping, and setting up on for inspecting parts and tools. A trio of iron plates of the same size can be marked against each other and scraped as a set until all three are flat to the degree you need or are capable of making happen. I would probably get the iron plate knowing I would need to get something to check it against before I could use it. I couldn't resist buying it at a deal though. As to Bill's observation, I wonder how the iron plates he used for marking bearing were scraped flat. What method is used to check the plates if not against a stone? Is the three plate method commonly used in scraping to preserve your granite surface? I'm always up for adding tricks to my mental toolbox:)) Ever curious.
 
Thank you everyone. I find your responses very helpful and interesting. For the $20 the seller wants I'll take the CI plate. It will be a good learning experience if nothing else.
 
It's flat, and will blue up a part I dont see the problem. I bought my granite plate for the purpose of scraping above all else. Will it last? Long enough for what I need it for.
Bill Gruby said:
If this is an answer to my question, I have never and I mean Never seen a granite plate used for scraping. I spent my time scraping just like all apprentices do and it's done with cast iron plates. Granite would not stand up to the job for very long. Granite plates are INSPECTION PLATES.

"Billy G"
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