Granite Surface Plate - AA, A, or B | Practical Machinist

So I'm referring to Starrett's crystal pink series here (seems to be desirable since they claim the pink granite surface to be very long wearing).

There are a whole set of specs here

Crystal Pink

But just to abbreviate, in 24x36, AA = ." unilateral flatness, A = ." unilateral flatness, and B = ." unilateral flatness.

Looking at MSC prices, the B-grade is currently around $, A is $200 more and AA is $400 more in round numbers.

Not suggesting that I have a pile of cash around to hand to Mr. MSC for his finest, if my cast iron plate can't get qualified easily, it looks like there are some "used B" plates out there locally that I could potentially acquire.

I realize that the easy answer is nothing but the finest AA Lab grade will do, but I'm actually using this in a minimally temp controlled shop and not in a CMM metrology room.

I'm wondering what some of the thoughts are out there about what grade of plates everyone else is running for scraping purposes and what kind of detriment there is to using one bought with personal shop budget thinking in mind

I know Prussian Blue will permanently stain a Crystal Pink plate. I don't know if the water based Canode will be completely removable. It would suck to spend the longer dollar for the pink plate and then stain it the first time you use it.

Canode Blue stained my white granite plate.

honestly if you want a big'un if you can find one on local auction you'll be in luck. I have seen big plates struggle to get a bid,, even a 6' x 12' granite AA plate at one sale that went for about $75.00.... but you have to move it. Freight is the biggest player in granite.

Around here, 3X4 plates are giveaways, apparently. I was given one, it was even dropped off at my shop. The second one I had to pick up. The shop tried to sell it for 6 months, finally called me and asked me to take it away- has a few nicks but was recently calibrated. Both were on rolling stands. I tried to trade one to a dealer, got an emphatic NO.

BTW, Gallery of Machines in Marathon NY has a truckload of plates to sell cheap, like $200 for a 2X3. Said they came from IBM. Lots of sizes.

I'm wondering what some of the thoughts are out there about what grade of plates everyone else is running for scraping purposes and what kind of detriment there is to using one bought with personal shop budget thinking in mind

My first thought is outside of checking your parallels what good is a 24x36 plate for working on machines?
Seems small for even checking a 6x18 grinder table or a toolmakers sized B-port.

For heavy use, yes the pink plates are worth the money but I may be biased as I used to sell them.
My use has shown that they do not wear out as fast as most black plates. A undocumented guess would be 3 times the life.

For light use, it's hard to beat the cheapo black imports.
You can buy much larger and when they go bad they are useful for landscaping around the house.
I like using these and consider them "throwaways" , in the smaller sizes they make great workbench tops for your rollaround toolbox.
(Yes, I am not nice to my plates, they work fine with a few chips here and there.)

All plates need to be supported correctly, pay attention to this.
It's amazing how much even a 24" thick 60,000 lb. one bends. (the kind you drive cars and trucks onto to check them)
They are only flat when supported in the right spots and on a body checker you have to fire supports or do crazy compensation as the arm moves.

Used plates are such a crap shoot.
Most will have been used in only a few convenient areas and will have hollow spots where used.

Nothing like a "AA" pink if you have GM style funding behind you.
Two and four ledge plates can also be very handy if you need to clamp a guide plate down.
Spending your own money one can buy cheap black and maybe get into something that works fine and won't break the bank.

OTOH, since you have budgeted it buy a pink and at least a "A" and maybe one step up in size if you can move and fit it.
Given a choice for small machine scrapping I'd buy a pink "A" in a 36x48 two ledge. A darn nice thing to have.
Spread the use around it and you will get a long life between relaps and you can will it to the grandkids.
It will get stained (which you can remove with some effort) but I have no idea how that is a concern at all.
Bob I think I paid $75.00 for my last AA Grade 48" x 72" plate I bought at an auction. It had a inspection tag on it and it was spot on. It wasn't a surface plate so to speak, but I used it as one. It was an obsolete CMM that the computer died so no one bid on it. I brought it back to the shop and took off the bridge and gave it to a guy. It came on a 3 point stand and I saved kept the steel beam that was straight so I could measure parallelism of ways of machine tables, lathe beds I would check on the plate. You will be surprised as to how cheap CMM's are and how accurate they are. Rich My Ottavino black granite plate was covered without washing, left sitting for a prolonged time, and the blue area shows clearly. I assumed black wouldn't show any discoloration but was wrong. In the scheme of things it doesn't matter much, though I suppose if I'd paid for pink granite it might. I really am concerned about support placement though, I just put three supports in the areas that looked like they shared the weight best. Now I'm told there is data available on exactly where they should be.

BTW, one black granite plate maker says this; "We produce our granite products from several types of granite, depending on the customer's needs. Laboratory tests have shown that fine-grained black granites are stronger, denser, and less absorbent than other granites. For this reason precision machine bases, air-bearing surfaces and accessories are best made from black granite."
I guess their claim is more about strength under load rather than hardness or wear resistance, but it goes to show that every type of granite has some kind of claim of excellence.
In a related thread a poster mentioned cast iron laps being used on rock plates, I always wondered how they were brought true.

Dear all,
First thing first i'm very new to forum so i would like to salute you from Turkey. My name is Yavuz and have mechanical engineering degree. I'm also hobby machinist, don't have a dedicated garage. Just working on my spare times in a dedicated room inside my flat.

Nowadays i am looking for a granite plate in size of max 400x400 mm (~12 in) and reasonable weight to handle it. Just looked for suppliers and learnt about their different standarts, quality etc..
One of them returned me that they have A and B grades in din 876 standart, also, for both south african and chinese stones. I think that A and B reffers to 0 and 1 grades in standart. (attached image)

I would like to have your opinion, in my case which of them should i choose? I'm not so pro right now and my machines / tools are not in space engineering grade Actually i would like to have that plate for some bed way scraping. With a bit of limited budget i'd like have the most suitable one.

Can you help me on that issue, would grade B (1) and chinese stone be so bad?

thanks

Yavuz, welcome the forum. I think you will find this a friendly and helpful forum.

I am not so sure that your attached flatness chart corresponds at all to the grades A and B for surface plates, but I am in the US, and we have our own standards. Here in the US we also have grade AA, along with A and B. The tolerances are different for different sizes of plates, and also include repeatablility limits.

http://standridgegranite.com/products/surface-plates
http://standridgegranite.com/images/stories/docs/forms/fed spec ggg-p-463c.pdf

What you need in a surface plate depends on what you are going to measure, to what tolerances, and to what repeatability. Grade B is commonly used for in-shop surface plates. If you plan to scrape things into higher tolerances, or do high precision grinding or layout work, you may want to go to a higher grade. Grade AA only really makes sense in climate controlled metrology labs. All that said, inaccuracies are to some extent cumulative, so a higher tolerance surface plate helps to build confidence in your home workshop measuring capabilities.

In my opinion, everyone making accurate parts needs measuring tools including flat (surface plate), length (gage blocks), and angles [sine bar/table, known accurate square (certified granite angle block and/or cylinder square.)]

You should be able to prove your tools and machines to your needed accuracy... Firstly, please accept our condolences for the attack on your airport.

Second, from your chart I see that the DIN876II is listed as accurate to within 1.06 mils (0.) in your chart. That's almost twice as accurate as the lower grade one.

In my experience, when I'm not exactly sure what to buy, I have a simple rule: Never buy the cheapest or the most expensive. I use this for all types of purchases.

I think I'd go with the one I mentioned above unless the price is really the most important factor. Hello Yavuz,
welcome to the forum!
i try this rule when buying, buy the best you can afford.
you can upgrade later, if necessary, as your needs demand.
sometimes you can sell the old items you no longer use to fund the purchase of something you will use.
all the best
mike Yep, consider your projected needs and your current budget.
Get the best you can afford.
But, also consider the sequence in which you need/want to purchase machines/tooling/inspection/etc equipment.
Obviously, if the deal of the year comes along. then you may need to scramble.
But, keeping priorities in line is critical.

Also, (and don't tell my wife I wrote this) don't forget the kids need winter boots and breakfast.

Daryl
MN

Firstly, please accept our condolences for the attack on your airport.

Thank you for your kind feelings, also same here about Orlando, Paris, Brussels, Ankara also.. A wicked universal thing.

I think i will go with sum of your suggestions, top quality until to affordable budget (i dont need winter boots ). But i think for hobby purposes 12x12 in^2 is more than enough.
In the first time i will use it to take referances while reconditioning my lathe bed ways and gibs mostly. You see there are visible cutting left overs on gib surface, of course that's a cheap mini lathe i own (7x12).
While making projects (double acting steam engine / bollaero diesel) i clearly understood required tolerances and finishing detail.

Thanks again for your help, i'd like to join other issues and threads later.