How do you measure "flatness"

[alexy]

I got ready to measure flatness of my bench, and realized that I don't know exactly what is meant by the various measures I have seen.

I've seen statements about a table's "flatness", expressed in thousandths of an inch or in thousandths of an inch per foot, e.g flat to .002" or flat to .002"/ft. Are people using those interchangeably, or does the first mean literally over the entire surface, deviation from one point to another is no more than .002? And in the second measure, does that mean that using a 1-ft straight-edge, any dip in the middle is less than .001" and any rise in the middle will cause one end of the straightedge to be high by .002" when the other edge is held down?

In a wooden bench, I am much more worried about eliminating twist than absolute flatness, but I wonder what is a reasonable goal? I figure that if I can't fit a .005 feeler gauge anywhere under my 4-ft straight-edge no matter how it is oriented, that I have attained more flatness than I can reasonably expect to keep for any length of time due to normal wood movement. Is that right, or should I strive for better?

This is my first experience flattening with hand planes, and I am pleasantly surprised at how easy it is to get pretty darned good flatness with just a home-made scrub, a flea-market #5c, and my prized "worker" 100-year-old #8.

[Andy Dingley]

Doesn't move when you sit on it, flat and level enough not to spill your coffee mug.

I care much more about stability than I do about flatness. If I was to start obsessing about flatness, I'd go for the assembly table, not the workbench.

[bridger]

yes.

if the high point is in the center, yes.

that would be a nice flat wood bench.

fun, eh?

[alexy]

Yes, it is! Brings a smile to my face in a way that running through a

30" belt sander never could. Of course if this were bus.woodworking rather than rec.woodworking, and I put any value at all on my time, I would be singing a different tune.

[George]

It's a matter of relativity.

Flat is good enough for what's it's doing. If pieces don't rock on your bench, either both are flat enough or both OTL.

[alexy]

Thanks, Andy. I suspected that the right answer for flatness of the bench was "flat enough that flat workpieces don't rock when you are working on them". On the other hand, I am concerned about avoiding twist and that the front of the bench (which will be the rear jaw of my shoulder vice) be perpendicular to the top. Better to plane pieces square and cut square joints than to force them square during assembly!

And yes, I was obsessing about flatness, just for the fun of seeing what is possible with how much work. I suspect that I've got it a lot flatter than I need it, and that I will not maintain it to this degree of flatness.

[Silvan]

Nonsense. If the bench isn't flat to within 0.000001" your projects will explode due to PVC dust in your 18 HP shop vac.

[Silvan]

Oh, for my real answer... I have a hunk of Scarey Sharp(tm) granite. I used that to test the top for flatness. I can slide that thing around pretty much everywhere without it rocking, so that's probably flat enough.

I'm worried about flatness because I also use my bench for gluing, say, chess boards together. That granite thing seemed to do it though. No feeler gauges or anything, just making sure a piece of granite approximately the size of 1/3 sheet of sandpaper doesn't rock anywhere on the bench. I'm sure it's not up to the standard I just posted about, but I'm too lazy to drive somewhere and find a set of feeler gauges anyway.

[Charlie Self]

Silvan responds:

Grizzly has a no-ledge 24"x24" chunk of granite accurate to within .00015". That ought to do it for you. By the time it arrives, you'll have laid out something close to $230, and you'd best have braced your bench to take its 234 pounds. You'll also need a couple helpers to get it ON the bench.

Charlie Self "One of the common denominators I have found is that expectations rise above that which is expected." George W. Bush

[Silvan]

Nah, free is good. I think this was a piece of counter backsplash. I used to deliver to a place next door to a stone counter maker. I picked up a whole gob of this stuff. Unfortunately, I'm down to two pieces. It hasn't stood up very well. The top, smooth layer eventually wants to flake off after a few times in the saddle as a Scary Sharp(tm) stone.

I might have to see about getting some pieces of plate glass or something.

[Charles Spitzer]

go to a place that makes windows and does shower door installs. they won't give you any scrap usually, because of liability, but the one i went to said, that dumpster over there gets filled at the end of the day, and we lock up and go home at 6pm.

that said, i just cut up a 3/4" glass 4'x4' dining table top i got for a few dollars at a yard sale.

[Silvan]

Good suggestion, but actually I know a guy who works in a glass shop, who owes me a favor. :)

(Too bad he doesn't work for a lumber dealer. I would have cashed in the favor much sooner.)

[wolfb]

Depends on what kind of "flatness" you are talking about. In the highway installation sector, the flateness of the road is a part of the installation contract. If there are complaints of the "flatness" of a particular section of road, the DOT has a vehicle that is sent along the offending stretch of road with meters and insturments (obstensibly set to some desired limits) to measure the flatness of that stretch. If the measurements are exceeded, an adjustment is required. I'm not an expert in this, but you can be assured they do require and make adjustments. "Flatness" is a measurable component. Wolf-==-