Was wondering how much a plate with a B grade versus a plate with an A or AA grading would affect the results. I need to buy a bigger plate but don't know if I should spend the extra money on a higher grade plate.
Flat is flat, A, AA, or B or a tombstone. So I'm gonna guess, and say that the A or AA will be more resistant to going out of flat than a B grade. Thicker rock, maybe. And depending on what you are using it for and how you treat it, B might be plenty good anyway. I think the main thing is about how you mount it, i.e., three points on a dedicated surface or stand v. laying it on the bench and setting stuff on it till you need it.
Using the plate for spotting, you cannot do work to a higher tolerance than the flatness of the plate. In fact, you cannot do work that equals the tolerance of the plate, the plate needs to be more accurate than desired on the finished work. Plates need to be in current calibration, and for used plates, with first hand knowledge of how they have been used and maintained. AA, A, and B plates will all work for scraping. How accurate do you want/need your results to be? Plates with an unknown history are unknown plates, nothing more.
This is more than just a cost issue. If you use an AA plate in a garage or outbuilding with temperature variances, then you might as well have a B plate. An AA plate requires stable temperature, humidity, and must not be moved often to retain the AA cert. It needs to be mounted correctly and leveled to prevent the plate from 'creeping'. -- now you have to know that we are talking of creping in the order of 2-3 millionths of a meter .
It gets a little more fuzzy about A plates versus B plates. What tolerances do you wish to work to? If you are wanting to achieve half-tenths accuracy, you need a minimum of an A plate and take care of it very carefully. If you are using techniques that are closer to half thou or slightly under, then a B plate will usually suffice. (you still have to treat a B plate well, or all you have is a granite mini-bench).
I have 3 granite plates: a 20$ 9X12 B grade that I use for honing wood planes and other less accurate things like woodworking or quick layouts. It sits on my bench. I have a Chinese 12X18 that cost 50$ and is +/- 1 tenth on most of the surface, but is used for most work and to keep my good plate from wearing out.. I don't consider it to be accurate enough or repeatable enough for doing work to better than a half-thou.
I've used the above plates for over 30 years. Last year I acquired an AA grade Mitutoyo in 18X24. It is mounted on a steel frame, supported properly, but not levelled (yet?). I'm sure I can hold to one-tenth on it. If my skills improve I might do better one day. I only use the Mit for taking accurate measurements using my tenths indicator and gauge block stacks.
I will use it for both scraping and part inspection. Again temperature and humidity shifts will be a factor as I expected them to be. It will be in my basement that ranges in the winter 40 to 70 in the summer. Typical CT weather. I was looking at a 18x36 plate. It will probably be an import because I can't swing the cash for a Standrige granite plate. And I plan to have it on a proper stand on 3 points
In our home shops, with often large temperature changes, we do not always attain the stated accuracy of the plates. It is not just the swings in temperature, it it is the diurnal (day/night) changes that cause stratification of the plate temperatures. The top surface can be warmer or colder than the bottom surface with temperature swings, and that causes significant warping of the plate. Open stands, where air can easily churn around, help to get the air in the shop to all the plate surfaces, and then you are only dealing with the actual stratification of the temperatures in the shop which affect it. Hot lights, sun from a window, open doors, air conditioner and heat outlets, and other transitory temperature changes are the big culprits. A surface plate with all the surfaces holding the same temperature will be quite close even if the overall temperature changes from higher to lower. Less stresses...