Cast Iron VS Granite Surface Plates.

[projectnut]

Recently I've seen a few threads asking questions about granite surface plates. I'd like to expand on the subject and ask why some prefer Granite to Cast Iron and vise versa.

I have used both and don't have much of a preference. I currently have 2 plates in the shop, both are in excellent condition, and are Cast Iron. One is an unknown brand 12"x 18", and the second is a Challenge brand 24" X 36". They both seem accurate enough for my needs, and don't suffer from rust problems.

The smaller one weighs in at less than 50 lbs. It came from the local technical college, while the larger one weighs in at slightly less than 500 lbs., and is from the shop I worked in.

 

[dian]

can you certify a cast iron plate?

 

[jwearing]

I prefer iron for scraping. I like how it holds the blue, I think I can spot with a thinner layer than with granite. Also I prefer the thermal characteristics of iron, it is about 20x more thermally conductive than granite, so temperature equalizes rapidly.

 

[projectnut]

can you certify a cast iron late?

Yes cast iron plates can be certified. This one came from a shop at the company I worked for. If I remember correctly it was last certified in 2015. There is a sticker on it somewhere with the company's name that did it. I'm not at the shop now so I can't give any more details.

 

[Limy Sami]

I like iron for it's magnet holding abilities.

 

[gbent]

I like iron for it's magnet holding abilities.

And I hate iron for the magnetic artifacts left behind. So every so often I have to drag out the plate demag and give it a treatment. A magnetic base is a lot faster than a c clamp to the edge of a granite plate.

 

[Eric M]

In my opinion, granite is better for a surface plate for several reasons:
1. Because it is less thermally conductive, it is less affected by changes in temperature.
2. Here in Florida, humidity is ALWAYS an issue. Since not all shops are air-conditioned, cast iron + humidity=rust.
3. Magnetic attraction is not an issue with granite.
4. If a heavy item is dropped on a cast iron surface plate, it will leave a dent. The material displaced is then pushed up, leaving a raised area that must be flattened or removed, typically by stoning. In the same situation, a granite surface plate will chip, without leaving an associated raised area. Unless a sizable chunk is removed, this chip will not affect the overall flatness of the surface plate.

High quality granite: charcoal black, pink, zimbabwe black, academy black, impala black, imperial black, sierra gray

 

[jwearing]

1. Because it is less thermally conductive, it is less affected by changes in temperature.

"less affected" is not exactly correct. It's affected differently.

Iron will be affected by short-term changes in temperature. However its conductivity also means that it will equalize rapidly. For example it will distort from a beam of sunlight shining on it for 5 minutes. Block out the sun and 30 minutes later it's flat again.

Granite on the other hand will be affected by long-term changes, and will take a long time to equalize. The beam of sunlight is not an issue, but if you left the window in your shop open last night, don't plan on trusting your stone until tomorrow.

The other thing I like about the conductivity of iron is that it will equalize to an iron workpiece. When I'm scraping something, and almost finished, I'll rest the workpiece on the iron plate for an hour before I spot it. I have been able to get really consistent results spotting this way, despite my non-temperature controlled shop. I don't get the same consistency with granite.

I'm sure it's not the same for everyone, but the characteristics of iron work well for me. My shop gets really hot in the afternoon, and I only trust my granite in the morning.

 

[dian]

Yes cast iron plates can be certified. This one came from a shop at the company I worked for. If I remember correctly it was last certified in 2015. There is a sticker on it somewhere with the company's name that did it. I'm not at the shop now so I can't give any more details.

i just thought cast would move a bit (like a straight edge does).

i have a medium sized cast plate that i use for banging and straightening. it is scraped and interestingly has a big hump in the middle. i derusted it and keep it waxed. obviously you couldnt use it for precision with the wax, so i wonder how you keep a cast plate from rusting if used frequently.

 

[metalmagpie]

i just thought cast would move a bit (like a straight edge does).

i have a medium sized cast plate that i use for banging and straightening. it is scraped and interestingly has a big hump in the middle. i derusted it and keep it waxed. obviously you couldnt use it for precision with the wax, so i wonder how you keep a cast plate from rusting if used frequently.

Gage blocks are made of steel and rust. I bought some gage block preservative from a major supplier and it's mostly lanolin. The idea is to wipe on the lanolin, put the block away, get it out, clean off the lanolin, use the block, repeat.

I don't see why this wouldn't work with a cast iron surface plate.

Of course, you could also cut out a rectangle of rust preventative paper and put it between the surface plate and its cover. Uh, you *do* have a cover, right?

metalmagpie

 

[jscpm]

Well, of course, a surface PLATE is by definition a plate normally of cast iron. Granite is used to make reference surfaces.

The difference is that a surface plate is usually movable, so that it can be placed against some other surface to check it for flatness.

A reference surface is a stationary and level surface known to be flat to some tolerance. Granite has an advantage that it is somewhat more dimensionally stable than cast iron, so it can be used to make higher grade surfaces. Also, granite is more rigid than cast iron so it is less likely to deflect under a heavy load or if it is clamped. On the other hand the mica in granite can degrade leading to pitting. Granite blocks are heavy which makes them more solid and less sensitive to vibrations.

Cast iron is easier to scrape and work with in general. Cast iron is magnetic. Cast iron can be drilled and tapped.

As a general rule if you are making measurements, you use a reference surface. If you are checking flatness, you use a surface plate.

 

[dian]

good granite has a modulus about 1/3 of cast iron. gauge blocks (as well as other gauges) undergo very special heat treatment procedures to stabilize them. surface plates probably not..

 

[Milland]

good granite has a modulus about 1/3 of cast iron.

Yup:

Granite:
Foundations - Modulus of Elasticity (around 20-50 GPa)


Cast iron:
Modulus of Elasticity for Metals (around 66-138 GPa)

Granite is close to aluminum in mass and stiffness. Not in electrical conductivity, though...

 

[Mike C.]

Granite for a bench reference plate. Cast iron for a lightweight plate that must be taken to the work. Check the cast iron to the granite with blue and scrape in occasionally.

 

[projectnut]

Granite for a bench reference plate. Cast iron for a lightweight plate that must be taken to the work. Check the cast iron to the granite with blue and scrape in occasionally.

My 24" x 36" Challenge brand cast iron plate weighs in at 475 lbs. A 4" Starrett pink granite of the same size weighs 495 lbs., and a 4" Mitutoyo black granite one weighs 500 lbs. I don't think I'll be taking any of them to the work unless they're on a wheeled cart.

 

[lucky7]

I do what Mike advocates- a 30x48” stationary granite that ain’t going anywhere, and a portable 10x30” Challenge cast iron. Works well for me.

L7

 

[Mud]

i have a medium sized cast plate that i use for banging and straightening. it is scraped and interestingly has a big hump in the middle.

I bought a 12" x 12" cast iron plate, and it also had a big hump in the middle, probably .030" or so. It had an X rib reinforcement on the underside. What causes that?

 

[Milland]

I bought a 12" x 12" cast iron plate, and it also had a big hump in the middle, probably .030" or so. It had an X rib reinforcement on the underside. What causes that?

Improper stress relief, I'd guess...

 

[dian]

mine is a very good quality (orignally) wyhler, reinforced, on a heavy stand, 20" x 24". the hump is just as yours. scraping still visible on whole surface. thats why i said they could move.

my mill is not big enough and you cant scrape that away, so is basically a work bench.

 

[lucky7]

Why can’t you scrape out a hump?

Might want to stress relieve first...

L7