I just got my first mill and lathe. I am gearing up to go to work on hobby parts and basic farm equipment parts. I think I should get myself a surface plate. I found a local add for a 9x12x3 plate from grizzly that should do the job till I can afford a bigger plate.
I bought a plate, long ago, from a woodworking store. It came with a certificate that showed the "map" of the surface. The highest point is within two ten thousandths of an inch of the lowest point. The plate is the exact same size as standard sand paper. So I've never felt the need to check it's accuracy, because A, given what I just said about it, I freaking doubt it, and B, I just don't care. It's small, I use it mostly for sandpaper anyhow...
I have a larger plate, that's not a surface plate at all. It's a square chunk of a "leg" from a granite bench that came unglued and was being thrown away. The polished side, that process inherently puts a "dome" on the surface. In my case, it's a few thousandths, and on the piece I saved, I can find where the "center" of that dome ended up. A few thousandths on a surface plate? That's grounds for disposal and replacement right there, you couldn't pay somebody to lap that in.... But it does fine for me. And if I want, the "saw cut" face opposite the polished face- That's got a "texture", but it's pretty danged flat overall. Perfect? Heck no. But if I can't FIND the error, then the error is within acceptable tolerances.
Both of them do good work for me. Mostly I use "dome shaped" chunk of furniture grade granite though.
But looking at the price compared to a say starrett plate of similar size. The grizzly is less than a 1/4 of the price.
Think that's a bunch? Wait until you find the price of the accessories and metrology that it takes to actually make good use of a "good" surface plate.
When it comes to precision things, the initial "foot in the door" stuff is cheap. Each bit of accuracy you add to it, that takes time, effort, knowledge, well tested processes, and that drives the price up. Add to that, the higher the precision, the lower number of potential customers, therefore lesser sales, so to set up a "run" costs WAY more per unit... So in the end, as accuracy goes up incrementally, the price goes up geometrically.
Is there really that big of a difference in quality to just get a better one? Or will a clean surface plate from any manufacturer do?
There are some quality differences, but mostly it's in the finish, and the precision. If you're coming right out of the gate by spec'ing parts at 50 millionths of an inch,
you need to get your head examined you want a surface plate that's pretty danged spectacular to match that accuracy. And the measuring tools to support that. If you're spec'ing parts to a thousandth, then you want something that "acts like it's flat". If you're scribing cut lines, go to the lumber department at the box store, and grab a premade melamine plywood shelf... Or just do that on your unknown condition local pickup plate that was at a good price. If you're scribing reference lines to keep you honest, it doesn't matter if it's off because you'll be doing it by the numbers, and that really just makes sure you don't cut on the wrong side or end.... If you're scribing actual cut lines because you don't need that precision, well, there ya go. I expect you'll find that a surface plate has a lot less value than it would first seem. I have found that a not flat but close enough that I don't care" plate is a far more useful tool. Most of the things I do on a surface plate, the things that make it useful... you would not do to a good plate, ever.
When and if (and only if) any of these half baked "starter" solutions comes up short, then and only then are you gonna know what you want or need. (The "farm repair" part "needs" only what it needs. The "hobby projects" thing... That's a whole can full of subjectiveness, because "want" becomes a more valid consideration). Do not let me in any way discourage you from something you WANT to have. But I think that the internet in general makes surfface plates appear to be more than they are, NOT because anybody's trying promote them or get you to buy one, but rather it's is a very photogenic way to SHOW you the measurements they're taking. It's (usually) not the most practical way.
My vote is this- If you can get "some" surface plate (of unknown precision initially, used, with unknown wear, whatever...), if you get SOME plate, dirt cheap, that's perfect. (And seriously, I'm only half way joking about the melamine shelf. They're pretty flat if you're just using it for scribing things....) Leverage that cheap plate as far as it will go, that will help you decide what accuracy/precision you want or need, and just as important, what SIZE you need. That's a huge and very important variable too. Not big enough is a waste of money and space. Big enough to be guaranteed to be big enough, that's thousands of pounds and takes up room enough for a couple of machine tools. You really need to know what YOU need, and what YOU want. In the mean time, take the metrology budget and look at-
A mediocre to OK set of micrometers, 3 piece, 0 to 3 inches, and resolving to tenths.
Usually you won't even be working out the vernier, but it's SO little difference in cost, one set is fine.
An EXCELLENT quality 6 or 8 inch digital caliper for general use.
This is a workhorse.
An expendable to excellent (your call) LEFT HANDED 6 inch digital caliper.
Even if you're right handed. Park it right near the lathe.
A good or better quality telescoping bore gauge set.
That list right there is going to do 96 percent of the precision measuring in your shop, especiallly early on, and will never be "obsoleted". And do not involve the surface plate. But just like the plate, the price goes up geometrically for higher precision and niche use cases. Get what you need from there.
I say get the used plate if there's a deal to be had. Even if you decide you need (or want) a lab grade micro inch precision plate, I think you'll find that the beater will get used more often.
