# 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

Chiptosser said:


> 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.


----------



## 4GSR

Back in the days years ago, your cast iron plates are generally used for taking impressions when your scraping and fitting.  That way, you don't put wear and tear into your granite plate.  Your granite plate is used to check the cast iron plate and straight edges, and make adjustments to them.  At least, in some shops, that's how it was done.

Now days, granite plates are more readily available than cast iron surface plates are.  In fact, I haven't seen a new CI plate for sale for many years now.


----------



## Fredz1

Sure do like my 12X12 shiny black bathroom tile. Mounted my dial indicator off about 4" and ran it around the surface, could not see the needle move. 
Sure is accurate enough for any of my .....-punching layouts.
If/when I break it, very low cost replacement.
When I'm done with it, it stands out of the way, with the refrence books on the book shelf.
Just my 1 1/2 cents worth.


----------



## benmychree

An advantage for cast iron plates is their lighter weight, when used as a reference surface for scraping where the plate has to go to the job rather than the job on the plate.  All the other reasons posted are also of note, especially the magnetic base use on the plate; as far as scratching, one needs to take care to clean the plate and the work piece of the debris that does the scratching (in any case).  For me, I'll take cast iron as a first choice, and this applies to all forms of scraping masters, such as straightedges. Another thing, aimed at scraping use, is that granite plates, so far as I have ever seen, lack the handles used to lift and manipulate the plate.


----------



## Bill C.

JPMacG said:


> 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.



All the inspection and layout plates I have used were granite.  They are heavy and are not moved once set up.  Cast Iron by it nature can scratch and likely needs a oil coating to protect it from rust.  Doesn't hurt to use a cloth or plastic cover on both of them.


----------



## silence dogood

If the CI is in good shape,  good price, no shipping costs,  then what is the problem?  I just buy it.  Mark


----------



## Andre

Also, cast iron does not transfer blue as good as granite. Okay, but not great.


----------



## Bill Gruby

Andre said:


> Also, cast iron does not transfer blue as good as granite. Okay, but not great.






 Why would you want to blue up a granite inspection plate? Granite plates are not lapping plates. What you are lapping could damage them beyond repair.

 "Billy G"


----------



## Andre

Bill Gruby said:


> Why would you want to blue up a granite inspection plate? Granite plates are not lapping plates. What you are lapping could damage them beyond repair.
> 
> "Billy G"



Not talking about lapping.... just saying that granite holds and releases blue better than CI. Prussian blue doesn't hurt them from what I've read, and you have to blue them up anyway to scrape your CI plate in every once in awhile. It might stain the plate though.


----------



## Bill Gruby

Andre said:


> Not talking about lapping.... just saying that granite holds and releases blue better than CI. Prussian blue doesn't hurt them from what I've read, and you have to blue them up anyway to scrape your CI plate in every once in awhile. It might stain the plate though.



 I will ask the same question another way.

 "Why do you wish to blue a granite plate in the first place?"

  "Billy G"


----------



## 4GSR

Andre said:


> Also, cast iron does not transfer blue as good as granite. Okay, but not great.



That's a first for me.  I've never had a problem using Prussian Blue on any CI plate I've dealt with.

Now, on the subject of Prussian blue, there is a difference between Dyken and Permatex brand of Prussian blue.  The Permatex brand will not lay very good, where the Dyken will do fine.  Since I changed over to the Canole Blue, I'll never go back to Dyken brand. 

(excuse my spelling here)


----------



## chuckorlando

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


----------



## Bill Gruby

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



 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"


----------



## Round in circles

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




 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.


----------



## Bill Gruby

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"


----------



## Round in circles

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 .


----------



## Bill Gruby

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 .



 Please let us know what you find. It would be interesting to know.

 "Billy G"


----------



## fgduncan

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


----------



## LEEQ

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.


----------



## JPMacG

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.


----------



## chuckorlando

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"


----------



## Bill Gruby

JPMacG said:


> 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.



 If I had know it was only 20 bucks I would have saved you a lot of reading and just said go buy it. :lmao:


 Now that the OP has spoken on buying the Plate I bid you farewell. As for paying for something and not using it for the purpose it was made for, that's up to the owner.

 "Billy G"


----------



## Round in circles

LEEQ said:


> 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.



I seem to recall from my apprenticeship lessons , that the once the webbed cast marking tables were precision ground with grinders they were then put face to face with another and then lapped flat ... as in ..... whetted with distilled water ,oscillated & moved in circular movements with the top plate being moved about over the bottom plate for a couple of days or so .  Apparently this gives you one of the greatest accuracies you can or could get at the time  and is much more accurate than scraping .  The tables were set on a rigid steel benches and very accurately leveled by using their adjustable feet with engineers/ machinists levels .  It was a fool who marked out with the plate without ever bothering to check to see if the plate was true level as some of the more childish apprentices seemed to delight at undoing one or more of the levelling screws for quite often the instructors would come along and say , "Now prove to me that your plate is supported level & true " 

Our marking out plates  never ever got rusty , for at the end of most working days the top was wiped with a clean oiled cloth then polished off with another clean cloth before the wooden made protective lid was put in place . At the end of term & for the holidays the plates were left smeared in clean engine oil which was cleaned off with meths when we came back off leave.


----------



## Todd

I almost bought that plate from the guy when it was posted on cl it would be a good scraping project.  Good to see someone else in the area on the site I'm in the chalfont area


Todd


----------



## janvanruth

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"



Isnt blueing supposed to be a way of inspection?
It will make detectable faults in flatness that are not measurable with mechanical instruments.


----------



## Kernbigo

I have a 2 sided ci plate, 1 side is a lapping plate and the other is scraped in for inspection etc.The 1 side i made into the lapping plate buy cutting the grooves in it with a ball end mill, took some time to do, but works great.


----------



## janvanruth

Round in circles said:


> I seem to recall from my apprenticeship lessons , that the once the webbed cast marking tables were precision ground with grinders they were then put face to face with another and then lapped flat ... as in ..... whetted with distilled water ,oscillated & moved in circular movements with the top plate being moved about over the bottom plate for a cpuple of days or so .  Apparently this gives you one of the greatest accuracies you can or could get at the time  and is much more accurate than scraping .  The tables were set on a rigid steel benches and very accurately leveled by using their adjustable feet with engineers/ machinists levels .  It was a fool who marked out with the plate without ever bothering to check to see if the plate was true level as some of the more childish apprentices seemed to delight at undoing one or more of the levelling screws for quite often the instructors would come along and say , "Now prove to me that your plate is supported level & true "
> 
> Our marking out plates  never ever got rusty , for at the end of most working days the top was wiped with a clean oiled cloth then polished off with another clean cloth before the wooden made protective lid was put in place . At the end of term & for the holidays the plates were left smeared in clean engine oil which was cleaned off with meths when we came back off leave.



it may only be part of the story....
they probably had three plates and lapped them against each other


----------



## fastback

I have one of each, but have never used the CI.  The only reason is because it is much larger than the granite surface plate. I believe my granite one is 12 x 18 and the CI is maybe 18 x 24.  Lately, I have been looking for a place to set it up.  If you have it you may as well use it.

Oh BTW, up until WW2 most all surface plates were made of CI, but with the shortage of metal during the war the switch was made to granite and has never returned.


Paul


----------



## DaveSohlstrom

test


----------



## Kernbigo

that is the way i scraped in the straight edges, and the cast iron plates in our shop. The granite plates were lapped in buy a outside co.


----------



## The Liberal Arts Garage

janvanruth said:


> Isnt blueing supposed to be a way of inspection?
> It will make detectable faults in flatness that are not measurable with mechanical instruments.


Think about it-- you must use three plates of cast iron to check one. I f you need everyday reliability get granite........BLJHB


----------



## Tozguy

What about quartz?
We installed a kitchen counter top made of quartz which is synthetic granite I believe.
Seems pretty flat to me. Please tell my wife that there is around 20 sq. ft. of surface plate in the kitchen that I can use anytime.


----------



## fgduncan

Without some testing , you can't really tell exactly how flat the counter top is. But I can see a really major problem with using her brand new countertop for a surface plate! Unless you have a MUCH more amiable wife than I do, using her new counter top for anything other than a kitchen counter may lead to something less than marital bliss. Let me rephrase that. You might find out what a furious woman sounds like the hard way because it will be your own woman. And you might find that you are going to be spending more time in the shop than you had planned since that is where you might be sleeping, if not living. You might find it a whole lot better to buy a piece for your shop and keep her happy. They say that when Mama is unhappy, everybody is unhappy. You really do not want to find out if that is true or not. It is also said that there are some things you don't want to learn, and I'm quite certain that is one of them.


----------



## DaveSohlstrom

Go to the people that sold you the counter top and see if you can get the drop where the sink goes. You will not be able to use it as a scraping master but it would work fine for a lay out table using a height gauge and other lay out equipment.
If you need a true surface plate then go to Grizzly or Enco and buy one that is big enough for your needs. Look on the internet on how to set up a 3 point mount that insures that the plate is not distorted by the mount and away you go.

I have 2 one 18X24 and a 9X12 Keep your plate clean and protect the top when not using it. Don't lay lots of stuff on it like I do and should not do.

Dave


----------



## John Hasler

Tozguy said:


> What about quartz?
> We installed a kitchen counter top made of quartz which is synthetic granite I believe.
> Seems pretty flat to me. Please tell my wife that there is around 20 sq. ft. of surface plate in the kitchen that I can use anytime.


No, quartz is a mineral.  https://en.wikipedia.org/wiki/Quartz  What you have is probably a synthetic material consisting of quartz grains embedded in some sort of plastic.


----------



## Tozguy

Apologies to Jon if I am getting off topic but it might be possible that 'quartz' countertop material might be easy and cheap to come by for a hobby surface plate.
As John wrote it is a composite material. It can replace granite in many applications. Around here it is commonly referred to as quartz countertop.
My experience with it in the kitchen is very positive for stability and durability. It does not stain like granite does. If I have written this it is because I have not tried our kitchen counter as a surface plate and am still alive to wonder about it. 
Nevertheless if a CI plate came up at a bargain price I would buy it.


----------



## stupoty

I like the cast iron surface plates their very good value second hand, mine was a bit knocked about on the edge when i got it but they buffed out ok.

Interesting side note, when i first used it for seeing how flat somhing was the blue brought out a distinctive scrape patern that you can't even feal when you run your fingures over it.

Stuart


----------



## John Hasler

Tozguy said:


> Apologies to Jon if I am getting off topic but it might be possible that 'quartz' countertop material might be easy and cheap to come by for a hobby surface plate.


I doubt that any of the "engineered stone" materials are stiff enough or stable enough.    I'm sure it would make a fine workbench though.


----------



## Bill C.

Anyone consider tombstone granite? I bet a some of those monument companies have a mistake or two laying around. Just curious.  

I remember McDonnell-Douglas used granite and they had a inspection team that used a laser to check for flatness.


----------



## Jonathans

I only use my CI plate for bladesmithing.  It's flat within a .001, and does the job when scribing center lines or checking the straightness of a blade.  I do occasionally use it for a finish lap for flat surfaces and find that it is much easier to clip a sheet of abrasive to it than to a thick granite plate. Why, may I ask, is lapping with a sheet abrasive on a CI plate bad for it?


----------



## joshua43214

Jonathans said:


> I only use my CI plate for bladesmithing.  It's flat within a .001, and does the job when scribing center lines or checking the straightness of a blade.  I do occasionally use it for a finish lap for flat surfaces and find that it is much easier to clip a sheet of abrasive to it than to a thick granite plate. Why, may I ask, is lapping with a sheet abrasive on a CI plate bad for it?



It is about impossible to keep the debris from getting under the paper. It is bad for both CI and granite plates. Even if you think the paper is not moving, and therefore not abrading the plate, it is moving and abrading the plate. Even in a hobby environment, you will cut a dish into a granite plate pretty fast, and even faster in CI.
It is perfectly fine to use a spray adhesive to tack the entire sheet to a plate that is the same size or smaller. I use my little granite plate for this purpose as a rough "lapping" plate.
There is a video where the guy from Bar Z went to that surface plate maker in California that makes all the top end plates, and he discusses doing this exact thing, and even recommends this as a valid use of a surface plate. I figure if the guy that makes and services plates for the space program and the Navy says that a granite surface plate is an excellent lapping plate, then I guess they are not just for inspection after all


----------



## anadeau

Good info. I'm in the market for a surface plate right now. Found this thread helpful.


----------



## Highsider

chuckorlando said:


> 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


There is a difference between a CI lapping plate and a CI surface plate.  The lapping plate has a cross hatched groove pattern to hold the lapping compound.  The surface plate does not.   The lapping plate is expected to wear and be resurfaced periodically.  If your surface plate is getting worn, it's probably from using it as a lapping plate.


----------



## kingmt01

fgduncan said:


> But I can see a really major problem with using her brand new countertop for a surface plate! Unless you have a MUCH more amiable wife than I do, using her new counter top for anything other than a kitchen counter may lead to something less than marital bliss.


My wife would likely even suggest it.


----------



## Kernbigo

The counter top is not accurate enough even if the wife would let you. I have a cast iron plate, double sided, i turned the one side into a lapping plate.I setup my mill and cut groves in it with a ball end mill, took a day of cutting, but works great.


----------



## benmychree

Kernbigo said:


> The counter top is not accurate enough even if the wife would let you. I have a cast iron plate, double sided, i turned the one side into a lapping plate.I setup my mill and cut groves in it with a ball end mill, took a day of cutting, but works great.


All the lapping plates that I have seen have had grooves about 1/16" wide and deep, cut with a slitting saw or gang of slitting saws.  A much faster method, if you have a horizontal mill.


----------



## planeflyer21

benmychree said:


> All the lapping plates that I have seen have had grooves about 1/16" wide and deep, cut with a slitting saw or gang of slitting saws.  A much faster method, if you have a horizontal mill.



1/16"?  The only ones I've seen have 1mm grooves, wtih about 3/8" squares forming the grid.

Must be a regional thing.


----------

