OK, I just read through 4 pages of discussion on this "quick question"... mostly it reinforces my belief that you can take any subject, and there's an encyclopedia of information about it, and I usually didn't have a clue about any of it!
But I'm still not sure what to do about my small surface plate, purchased on a whim (it was cheap and easily available). It's a 12x18x3 B grade (or worse, totally undocumented), purchased with an eye towards shop work like lapping and/or marking out parts. It's just laying on my nice solid workbench with no support, other than the bench top itself.
If I have to move it, I'll just slide it around like some millstone.
Should I feel bad or ashamed about it?
I've got no welder, and I'm not about to purchase a purpose-made table that costs 20x what the rock did... not sure what else I'd do to support it, but frankly, I'm not too worried about a chunky thing like this sagging.
Hello Pat! I have a 12X18X3 rock that I've been using for about 20 years now, just as you are. Small surface plates such as this are sometimes known as 'bench plates', because due to their thickness versus their overall size they have very little (hardly measurable) deflection.
I get very accurate results on my offshore, not NIST traceable, cheap 12X18 plate. I won't ever make a stand for it, because it is too handy sitting there on my bench. Note I don't do work to tenths on it either. but for scribing, layout, etc it is better than fine!
For bigger plates (I'm thinking 24" X 36" and above) it is essential you properly mount your plate, but a little one like that is just fine on a bench. My bigger plate is in the grey area, but I'm going to properly mount it with a 3 point suspension, just because I can (read: anal retentive )
my quick questions go forever. But keep in mind I jump overboard in 60 seconds or less. Most people have theirs mounted on a workbench...frequently with just a plywood frame for a cover. If you aren't scraping in parts I wouldn't think sag is going to be of any concern. However, for any precision I would keep grit as far away as possible from the stone.
I don't see any issue or reason to feel bad. But I would keep the grit off of it.