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.RC.
10-24-2008, 12:18 AM
I am after a surface plate and have come across a local supplier who sells Chinese granite plates...The price for a Grade B in 24" X 36" is about what I want to spend but I am unsure of the accuracy of these plates...

I wish to use it for general marking out plus to scrape a straight edge and make other tooling...Are they generally suitable for this sort of work or does the accuracy of them leave a lot to be desired????

radish1us
10-24-2008, 04:24 AM
A few questions first.

What actually makes it a " B " grade plate ?
How MUCH does this differ from an " A " grade plate ?
Have you run a straight edge over the thing to see if it has any visible highs and lows ?
Would it matter if you used this " B " grade plate for what you intend to do ?
How much accuracy are you really after, do you intend to do work, that is with-in 0.0001'" or would it be close to 0.001",or maybe you could get away with 0.010" ?

The price will dictate, what you eventually DO BUY.

.RC.
10-24-2008, 04:49 AM
A few questions first.

What actually makes it a " B " grade plate ?


I assume the flatness it is supposedly machined/scraped to.



How MUCH does this differ from an " A " grade plate ?

less flat


Have you run a straight edge over the thing to see if it has any visible highs and lows ?

I have not bought anything yet, only got prices



Would it matter if you used this " B " grade plate for what you intend to do ?
How much accuracy are you really after, do you intend to do work, that is with-in 0.0001'" or would it be close to 0.001",or maybe you could get away with 0.010" ?

I would like to scrape a straight edge and other tooling like angle plates to start with...Then after getting some experience use the straight edge to do some scraping to one of my lathes

If the general person's opinion is these cheap chinese surface plates are no good I will re-evaluate my options..Second hand surface plates are non existent here due to us not having a manufacturing sector since the 1980's..

oldtiffie
10-24-2008, 04:56 AM
OK.

Let's get some direct answers - instead of opinions - to a direct question from the OP.

It isn't hard to find - we now move out of the subjective "think so" to the objective "know so" which is better and complies with the request in the OP.
http://en.wikipedia.org/wiki/Subjectivity
http://en.wikipedia.org/wiki/Objectivity_(science)

This should be a start:


Inspection
Surface plates should be checked on a regular basis for wear using a repeat reading gage with a manual indicator. Generally, long before a surface plate has worn beyond specifications for overall flatness, it will show worn or wavy spots, which will produce measurement errors. The reading gage will readily detect these error-causing areas.

Simply set the gage on the plate and zero it at any point on the table. Move the gage over the plate and if there are hand movements in the indicator more than .000025” for a “AA” plate, .000050” for an “A” plate or .000100” for a “B” grade plate then you have indication that the plate may have some high and low spots and be out of tolerance. Specifically, the tolerance must repeat from side to side within the specified tolerance range in order to be an accurate plate.


Also note that actual calibration of overall flatness traceable to N.I.S.T. can be performed periodically using an autocollimator.

When inspection indicates that the overall accuracy of a surface plate is out of tolerance, the plate should be relapped to restore it to its original accuracy. Even with normal use a plate should be calibrated on an established cycle, whether it be 1, 2 or 3 years, depending on frequency of use.

The granite surface plate was invaluable for ensuring flatness accuracy in the 1950's and provides the same benefit today. Following the simple rules for maintaining your plates will provide you with precise measurement readings in your manufacturing process.

From:
http://www.starrett.com/pages/860_granite_surface_plates.cfm

and this:
http://www.tru-stone.com/pdf/Fed%20Spec%20GGG-P-463c.pdf

from:
http://www.tru-stone.com/pages/faq.asp

and:
http://www.microflatindia.com/granite_surface_plate.html

http://www.google.com.au/search?hl=en&q=b+grade+surface+plate&start=0&sa=N

I would think that a "Grade B" SP would be more than sufficient for most jobs anywhere unless there are really compelling reasons or requirements for a higher grade.

Do note the requirement to have the SP re-calibrated etc. on a regular basis if QA or specifications require it. An "out of date" (aka "out of test") SP would be in breach of QA/specifications if these applied.

radish1us
10-24-2008, 05:38 AM
I would think that a "Grade B" SP would be more than sufficient for most jobs anywhere unless there are really compelling reasons or requirements for a higher grade.




Looks like price WILL dictate what to buy.

bob ward
10-24-2008, 08:48 AM
There was a discussion about surface plates on practicalmachinist.com a couple of months ago.

http://www.practicalmachinist.com/vb/showthread.php?t=162871

lazlo
10-24-2008, 02:31 PM
For a HSM'r, the repeatability specs are less of an issue than overall flatness.

A Grade B surface plate is flat to within 2 tenths over an 18" surface plate:

http://i164.photobucket.com/albums/u15/rtgeorge_album/surfaceplate.gif

oldtiffie
10-24-2008, 09:25 PM
This might help as well as regards accuracy.

http://littlemachineshop.com/products/product_view.php?ProductID=3068&category=1438824943

miker
10-24-2008, 09:31 PM
Ringer, just because they are Grade B does not mean you should also use it to chock the wheel of your tractor or use it to give your Bottle Jack a bit more reach. Might be OK if you put a bit of rag on it first though:)

Rgds

dp
10-24-2008, 09:53 PM
I have two granite plates. One is a true surface plate I bought from Forrest when I attended his scraping class in Newport, Oregon. It came with a sheet that describes its imperfections and it's in far better condition than I am.

The second plate is a 3/8" x 12" x 12" black granite tile I bought at Home Depot. I haven't checked it against my true plate but that isn't needed given what I use it for. That being, wet sanding, rough work I don't what to bugger up my "real" plate with, work needs only rough measurements, and for simple work that requires a flat surface such as fit checking.

Both serve a purpose and one is somewhat sacrificial and easily replaced. Now I need a granite brick to check the perpendicular surfaces.

Jpfalt
10-24-2008, 11:00 PM
I have picked up three of the 12 x 18 x 3" chinese plates and find them to be much better than the standard requires.

I realise that the product likely ranges from lot to lot and I may have just been lucky. The problem is you have to have tools accurate enough to verify the accuracy. I went so far as to go to CoorsTek in Hillsboro, Oregon and bought a 37" ceramic straight edge good to 50 millionths and purchased a good dial indicator good to 50 millionths to do the checks.

I found on the chinese plates that the indicator moved less than the width of the needle over the entire surface of the plate using the specification that Forrest provided at the scraping class.

I worked up some hand flats and straight edges from granite tiles and countertop material and found that with the diamond plated hones sold by Crystallite and using diamond lapping compound on a cast iron block, it is possible to make them quite flat in very little time using bright yellow artists pigment as a contact indicator and by using denatured alcohol film to finalize it.

I bought one of the Martin Model 9 x 9 cast iron surface plate castings and a larger surface plate from Forrest to turn into surface laps to restore hand flats and surface plates. I had the chance to watch the process at CoorsTek. They load the cast iron lap by using a tool that looks like a large ball bearing as a brayer on a handle to roll the diamond into the surface of the lap. Then they clean the lap to get any free abrasive off and then use the loaded lap to work the ceramic surface to tolerance. Measurement is done with dial indicators and surface plates and also using a fairly sophisticated camera that magnifies a laser line bounced off the ceramic surface.

macona
10-24-2008, 11:19 PM
Is that what CoorsTek does? I used to live like 3 blocks from them.

I have an old 12x18 DoAll surface plate. Its 4" thick though. Heavy little bugger.

FWIW, We picked up a used 6'x3'x6" Plate for TechShop. At 166lbs per cuft that comes out to 2200lbs! Had a heck of a time getting it up the loading dock plate into the shop.

barts
10-25-2008, 02:17 AM
Macona, that's a little one... when we designed and installed a custom CMM in the Aberdeen, SD FMC plant, the backup plan was manual inspection w/ a 30' x 10' x 2' (approx - it's been nearly 25 years ago) granite plate... I never saw it used. It looked heavy.

dp
10-25-2008, 01:13 PM
This got me to thinking - I would like to have a granite 2 face angle plate but have not found a source I can afford. Anyone know of a supplier of home shop affordable plates?

Jpfalt
10-25-2008, 11:24 PM
CoorTek specializes in sintered ceramic products. This includes straight edges and beams for coordinate measuring machines, which was why I contacted them. Other products include wear plates and edges for paper processing equipment.

I've resorted to making measuring tools, but it takes a fair amount of time and tooling to get a useable result. If trying to do it on the cheap, I'd start by epoxy laminating 32mm granite countertop material that had been roughed out with a tile cutting saw and ground with a diamond wheel on an angle grinder. Finish off with a diamond plated hone. However, getting references to check accuracy will always be a problem.

oldtiffie
10-26-2008, 01:12 AM
This got me to thinking - I would like to have a granite 2 face angle plate but have not found a source I can afford. Anyone know of a supplier of home shop affordable plates?

Dennis.

Why buy one that might crack if you clamp it?

Why not make some cylindrical master squares and then use them to machine the faces of your angle plate on your mill.

Those master squares, being cylindrical are far better than the "brick" I think I heard/saw you mention. "Flat-to-flat" is a PITA to use compared to "round-to-round" and "round-to-flat".

A cylindrical master square is a true cylinder with turned ("squared-off" and "relieved") ends as those ends are flat and mutually square to the cylinder axis and the outer face/periphery of the cylinder.

I did both and it worked very well indeed.

I also re-machined and "trued up" my tilting angle plate.

That went very well indeed.

All of these are very good HSM projects that get a lot of use, are very accurate and if you have the steel cylinder/tube, the tilting angle plate and the angle plate/s will cost nothing.

If you like I may be able to find the threads.

dp
10-26-2008, 02:58 AM
Dennis.

Why buy one that might crack if you clamp it?

I wouldn't clamp it. I currently use my iron angle plate on the surface plate to check squareness of two perpendicular surfaces but I would prefer to not use iron on the surface plate for that function. Rock on rock would be preferred for reasons of wear and tear of the surface plate and temperature balance. But not if it's reeeeally expensive :)

oldtiffie
10-26-2008, 06:00 AM
Thanks Dennis.

A couple of queries etc.

Are you certain that your angle plate is accurate as regards flatness and all faces being square and/or parallel to each other?.

I have three angle iron angles plates and though most were reasonably - but not very - accurate in most respects none were very accurate in all respects.

This neither surprised nor disappointed me as I expected some "movement" as the production process stresses were eased over time and so I had them for over a year before I checked them.

It was after I checked them that I decided to accurately restore/make them as I wanted them.

I must say that the quality of the castings were excellent.

I never had any problems using cast-iron angle plates or steel surface guages, slip guages or vernier height guages or comparators etc. on a cast-iron surface plate either in the main workshops or the Tool Room, Instrument Shop or laboratories that I worked in. We never had granite surface plates. But in all cases all edges were "broken" with a file and then a hand oil-stone. All surfaces on all stuff that went onto the surface plates were either machine ground or hand honed or stoned.

Forrest Addy
10-26-2008, 08:51 AM
Surface plates whether granite or cast iron are tools made to be used. It's long been recognized that surface plates will wear and with inequalities of wear comes deterioration of tte flatness of their reference planes. The action taken by the shop depends on what the plate is used for. Ordinary lay-out and inspection doesn't require extreme flatness of the freference surface it's set-up on. A machine table will do just as well. If your work is high-end metrology then the surface plate has to be correspondingly babied and inspected for flatness at shorter intervals.

The only maintenence required of a granite surface plate is regular cleaning and ordinady care in use. A cast iron surface plate requires cleaning, protection from rust, gliding a worn-out arkansas slip stone over it at regular intervals, ding and scratch mitigation, an in-shop master flat for periodic reconditioning, and daily concern over distribution of wear.

A granite flat wears at about 1/10 to 1/50 the rate of cast iron and it's more durable and resistant to scratching. A knock that would ding a cast iron flat will have no effect on granite. Generally a granite flat is superior to cast iron in every category except weight and ease of reconditioning. A cast iron surface plate is about 2/3 the weight of a grainite equivalent and it can be hand scraped with simple tools provided a flatness reference is available for a master.

Restoring a worn granite surface plate is an easy order of magnitude bigger PITA. It has to go to a cal lab for calibration and lapping. WHile this can be done in the home shop there is mich equipment to accumulate and skills to learn. Unless the home shop owner thrills at the challenge, there's no advantage to leaping into the home granite flat re-calibration cycle. That's a job I would definitely farm out.

A surface plate is made to be used. If ia percectly acceptable surface plate sits under its cover while you're using a machine table covered in chips and dripping with coolant as a lay out and inspecion surface you're missing the whole point of having a surface plate on hand. Cast iron or granite, use it.

While a nice freshly scraped cast iron surface plate makes a "saltier" presence than a grante flat there's no doubt in my mind which I would rather have: a free cast iron surface plate or a cheap but purchased Asian granite one. I'll take the granite every time.

dp
10-26-2008, 11:26 AM
While a nice freshly scraped cast iron surface plate makes a "saltier" presence than a grante flat there's no doubt in my mind which I would rather have: a free cast iron surface plate or a cheap but purchased Asian granite one. I'll take the granite every time.

So where to find a two-face cheap Asian granite angle plate? :)

I used the heck out of the plate I bought from you along with my iron angle plate to finish the project from the scraper class but every time I used the iron plate on the granite block I had the notion I was not doing the granite any favors. That same iron angle plate also gets a lot of use as a clamping device on my mill so I wouldn't swear to it's repeatability.

lazlo
10-26-2008, 11:35 AM
So where to find a two-face cheap Asian granite angle plate? :)

Shars or CDCO sells them, but they're around $100.

I've picked up a couple on Ebay, but you're always taking a big chance buying measuring equipment there, especially when it's a calibration service that's selling them..