Cast iron versus granite surface plate

[JPMacG]

What are the advantages / disadvantages of a cast iron versus a granite surface plate? Someone nearby has a cast iron surface plate for sale. I've considered getting a granite plate but never actually got around to it. I never really heard of a cast iron surface plate before.

 

[darkzero]

As fas as I know, surface plates used to all be cast iron back in the day. They have heavy hollow ribbed bases as opposed to solid for granite.

CI is more prone to wear & warpage. When you drop something sharp on a CI plate it will dent & cause high spots around the dimple which then needs to be dealt with. On a granite plate, if you drop some hard enough to chip it, generally it will just chip it without creating high spots.

I took the machining courses at a local community college, they had both CI & granite surface plates. But we only used granite for inspection & the CI for lapping. Not sure if the reason for that was do to wear on the CI plates.

 

[Andre]

CI plates get scratched in short order, and because of the natural properties of granite (surface being slightly porous and irregular) it is more accurate.

Go granite, especially for scraping. But if you can get a CI plate cheap go for it, sure better than a machine table for measuring and layout.

Also, you can drill and tap holes in CI plates, (mine has them) and it makes clamping things to it a breeze, while granite needs special inserts.

 

[Holescreek]

The drawback to the cast iron plates was magnetism and the voids between the spots (when checking small parts). I have one sitting in my shop that needs to be re-scraped. It's pretty hard to beat the very low cost of a 3"~4" thick granite plate from Enco with a free shipping code.

 

[Walsheng]

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

 

[Ulma Doctor]

Unless you're sending something to investigate the rings of Saturn,
Either plate would suit the need, get the easiest to obtain for the best price plate you can.)

 

[samthedog]

Believe it or not but CI is more temperature stable. It is also easier to scrape back in as opposed to granite. Granite lasts longer but once it gets dished out from lapping it is usually a throw away job as getting it flat again takes a lot longer and is very expensive. CI can be scraped in by anyone with a carbide scraper, a reference plate, dye and some patience.

This is also why plenty of old-school shops sell their grantite plates but hold onto their large CI plates. This is the case in Scandinavia at least.

Paul.

 

[chuckorlando]

I would not lap on a surface plate. If you do, then it's no longer a surface plate but rather a lap. Though the cast iron laps I have seen look like SP, they are not

 

[Chiptosser]

When was the last time, that you saw a cast iron Magnet??????????

 

[Andre]

When was the last time, that you saw a cast iron Magnet??????????

:biggrin:
One of CI's properties is that it does not hold on to residual magnetism. CI 1-2-3 blocks are desirable for surface grinding jobs because of it.