Which Quality For Surface Plate? | The Hobby-Machinist

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.

Surface plates are simple and extremely versatile. They provide a broad, smooth, flat reference surface that can be used to inspect incoming, in-process or finished parts. When used in combination with various gages and accessories, such as height gages, gage blocks, angle plates and squares, they can check a wide range of parameters, including length, flatness, squareness, straightness, angle, feature location and runout.

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Surface plates are available in a wide range of sizes, from about 12 × 12 inches to 6 × 12 feet, and can weigh as much as 10 tons. They are available in three grades, the flatness tolerances for each grade varying with the size of the plate: AA (laboratory grade), A (inspection grade) and B (toolroom grade). Many surface plates can be ordered with ledges and threaded inserts, both of which make it easier to clamp workpieces or accessories to the surface. Granite is the most common material used in their construction; it is harder and denser than steel, has very little internal stress and is less subject to dimensional change due to temperature variations.

Any of the checks mentioned above can be done using a height gage and indicator as the measuring instrument and a gage block stack as the zero reference. However, there is a gaging technique that anyone can adapt to improve the overall performance of the measurement. It’s one that will improve those measurements when gaging performance to 50 microinches or 1 micron is required.

The typical process for making a measurement on a surface plate is to place the part in one spot on the surface, the gage block stack in another and the height gage in a third. This relationship is shown in Figure A. The height gage is first moved to the gage block along path (a) then on to the part along path (b). Once on the part, the deviation between the gage block and the part is read on the indicator.

Fig. A

Now, think about what we are trying to accomplish and what we are actually doing in terms of getting to a performance level of 50 microinches or 1 micron. As noted, surface plates are available in many different grades and, after use in the shop, exist in many different conditions. In other words, there is always some level of variation in the surface plate to begin with, and there will always be some out-of-flatness or waviness condition. Typically, a surface plate might see a total flatness tolerance of 200 microinches or 5 microns over its full surface. Plus, since the surface plate is being used in the shop, there is always the consideration of dirt, chips and scratches influencing the
measurement.

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Thus, if the performance we want to achieve is close to the flatness expected in the surface plate, then measuring just anywhere on the plate would not leave much tolerance available for comparing the master to the part. Since we are trying to get the most out of the measurement, we need to reduce as many sources of errors as possible.

Therefore, to keep the influence of waviness or localized surface plate error to a minimum, the best practice is to keep the distance between the gage block stack and the part to a minimum. This reduces the distance the height gage needs to be repositioned.

Fig. B

As shown in Figure B, placing the gage blocks as close as possible to the part enables the height gage to be moved very little to access the part. Minimizing the path minimizes the area of the surface plate used and thus eliminates the amount of waviness error by which the gage could be influenced.

By using this minimalist technique, we can reduce the chance that waviness will affect our measurement and can help achieve the results we are seeking. 

Contact us to discuss your requirements of Precision Granite Surface Plate. Our experienced sales team can help you identify the options that best suit your needs.