Granite plate

I think you guys already know this but before WWII for the most part all that was used was steel or CI for surface plates. Since there was a shortage of iron the industry turned to granite. After the war this continued since granite has great wear resistance and never rusts. I am fortunate I have both. The one I actively use is the granite one, its around 12 x 18 and is a B grade. I plate I think that that one is 18 X 24. At Least I think it is. I have built covers for both of my plates and keep them covered unless I am using them.


Big John, I for one would like to know how they check the surface plate for flatness. Thanks for the offer.
 
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All,

Within the written word, different interpretations of statements often arise... I don't get the sense of any major dispute here but let's not head down the wrong track and let's stick to technical discussion.

That said, a piece of counter top or a precision plate are likely made of the same substance but the manufacturing process and quality controls for each are very different. That much, I think we can all agree on. In a home shop environment, we improvise according to our budgets and needs. For the sake of those who aren't familiar with things like this, lets just get the message across that sometimes, you can get away using substituted tools with good results but, you can't rely on it in all cases...

Peace...

Ray

EDIT: I want to point-out that counter-top material is usually very thin. A and B grade slabs are thick for a reason and it's only partially what you think... It's to control the rate of expansion and contraction under ever-so-slight temperature variation conditions. The point being, there's a difference between dedicated tools and substitues and it's important to at least understand these reasons so when we take shortcuts, we're aware of the possible pitfalls... In all likelihood, these kinds of factors won't come into play in a home shop environment.

I'd like to keep any discussions on forums like this technical in nature.

An 18 x 24" Grade B plate can be bought for $154 from MSC. That's an import though, much softer than a good pink Starrett, but very serviceable in a home shop where it's use is limited. Used plates can sometimes be bought for much less, although sometimes they may have excessive wear. Again, it depends on the accuracy you need. A worn surface plate is better than no surface plate.

Coincidentally, my wife used to work for one of the largest granite counter top distributors in the US. She got a good laugh out this. She understands the difference.
 
I think you guy already know this but before WWII for the most part all that was used was steel or CI for surface plates. Since there was a shortage of iron the industry turned to granite. After the war this continued since granite has great wear resistance and never rusts. I am fortunate I have both. The one I actively use is the granite one, its around 12 x 18 and is a B grade. I plate I think that that one is 18 X 24. At Least I think it is. I have built covers for both of my plates and keep them covered unless I am using them.


Big John, I for one would like to know how they check the surface plate for flatness. Thanks for the offer.



http://www.qualitydigest.com/aug03/articles/03_article.shtml


Here's a good article on plates and gives some history.

As I explained, plates are checked for flatness and repeatability.

Repeatability is checked with a "Repeat-o-meter" (I didn't make up these names). Basically, it's a sled about a foot long that flexes in the center. An indicator measures the deflection of the end of the sled as it is drawn over the plate. You can do the same test with a gage block, a height gage and a good dial indicator. The block is placed at different spots around the plate and a measurement is made. This will tell you the repeatability of the plate. The Repeat-o-meter is just a lot faster and makes more repeatable measurements.

Here's a picture I.... ummm... borrowed off the web.

Repeat-O-Meter.jpg

The one I use has a better Federal gage head and electronic readout.

Flatness is another measurement. There are a few ways to do this. Laser, electronic levels, autocollimator and a straightedge system called "planakator" is used. I've personally used all those methods. Most common though are autocollimators and planakator. I use both, but for most field work, the planakator system works best for me.

Here's another borrowed image:

2004pricelist_12476_image001.jpg

Basically it's a granite straight edge that is leveled to the top of the plate and an indicator is run between the plate and the straight edge. A plane is established and a grid of check points laid out. Unfortunately, many of the "fly by night" calibration outfits don't do this important test.

Now if the plate doesn't pass these tests and isn't too far gone, it can be repaired by lapping. A cast iron lap (sometimes an old cast iron surface plate) is charged with diamond or carbide grit and the table is lapped back into tolerance.

Here's a video of one of my guys working up a sweat.

th_VID_20121207_102926.jpg


If this really appeals to you, here's where you can buy the stuff. http://www.tru-stone.com/pages/price_lists/price_list_popup.asp?id=8

Most likely, not in reach of the guy in his home shop, but that's what you need to really do it right.

2004pricelist_12476_image001.jpg

Repeat-O-Meter.jpg

th_VID_20121207_102926.jpg
 
An 18 x 24" Grade B plate can be bought for $154 from MSC. That's an import though....

In addition, an import 18 x 24" Grade B plate can be purchased from Enco for $46. I got my import 18 x 24" Grade A plate from Enco for $37 shipped which now costs $114. Shars also offers black granite plates for similar/great prices. For the average home user, I doubt most would even notice a significant difference, if at all, between the ones mentioned. Of course not always true but price does not have to dictate how useful or good something performs. If it works well for you then that's all that matters. There will always be someone who has a different opinion.
:drink:
 
I also saw that granite plat on sale at Enco. It is black granite and is 3 inches thick. Also come with some kind of certificate. Oh, it is foreign made. Sounds like a decent deal.
 
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