Need Help!- Granite or cast iron for surface plate

[technocrat]

I'm want to buy a surface plate for layout work. I'm currently using 2 bits of 3/4 steel plate that I lapped together with valve grinding paste but want something bigger (and flatter).

What are the advantages and disadvantages of granite?

Is wear and damage an issue with granite?

[jashprecision]

Granite Surface Plate - No rust, no corrosion, relatively warp-free, no compensating hump when nicked, longer wear, smoother action, greater precision, non magnetic, less care after commissioning, lower co-efficient of thermal expansion and non glaring surface.

CI Surface Plate - Material of construction is Gray iron casting Grade FG 220 of IS: 210/ 1978. We have vast range of sizes & Configuration for cast Iron flat surface Products. These Plates are strictly conforming to latest IS, DIN, BS & ISO Specifications. We have more than five decades of experience in this field and are foundation Partners of all the Engineering Industries. We are a regular exporter of our products mainly to United States of America and United Kingdom and shall be glad to share our experience with you.



The Calibration equipment used by us has traceability to NPL New Delhi and CMTI- Banglore. The complete manufacturing from pattern making to pouring of metal, machining, and inspection are done in-house as per Standard Sizes and for Custom based items.

[HuFlungDung]

I could be wrong, but I think a granite plate is better for an inspection plate, due to its stability and hardness.

Cast iron may be called for if you want to lap something on the plate, because the cast iron will take a 'charge' of abrasive, ie., some will embed in the surface making it a good lap because instead of the abrasive tumbling around between two surfaces, it will be held by the cast iron, and do the cutting mostly on the other part.

A good example might be lapping a granite plate flat: rather than use three pieces of granite against each other to try recondition them all flat (it takes 3 flats, lapped against each other to guarantee that a flat plane is produced instead of a spherical surface), it would be better to use 2 cast iron flats and the 1 granite. The cast iron laps can be grooved to improve lapping performance. They can also be restored to near perfect flatness relatively quickly on a lathe or mill, without fear of polluting the machine with abrasive grit forever after.

I've also done some lapping of various pieces on my granite plate, but not directly on the plate. Rather, I use some self-adhesive disks (commonly found in body shop supply houses) and stick one of those onto the granite and lap away. These abrasives are easily replaced as the abrasive wears out, and it is easy to stick a few different grits down onto a 24 x 16 granite and make a nice lapping station.

[keebler303]

Around here, you can regularly find granite surface plates on Craigslist in the 24" x 36" range for around $50-100. I picked one up with threaded inserts in it for $35 a while back.

Matt

[HuFlungDung]

Yeah, when I bought a 16 x24 granite plate, the shipping cost more than the plate did

[Algirdas]

granite is unbeatable for inspection and measuring.
disadvantage:
price,
availability,
hard to machine (if fixing thread needed, for instance)

[jashprecision]

Yes I completely agree that for measuring, marking and layout Granite surface plate is quite good, and it is to be kept in cool room environment. Yet for hard work, machining layout etc Cast Iron Marking table to be used.

Though I can offer both but still I prefer to use CI due to price and handling feasibility.