[OP]
Member
I was pulling out some plywood and melamine scraps tonight for some small sled making. Obviously, the base of the sled ought to be flat as possible. Since these offcuts had been sitting around for a while in my garage, I took out my straightedge and feeler gauges to choose the flattest pieces. Naturally this progressed to a machinist-like obsession for flatness. I can't be the only one.*
What's your personal tolerance for sheet good flatness?
Member
It depends on what I'm using it for. Like you say, for a sled it should be pretty flat.
Not sure where you're buying, but I generally stay away from the borg for sheet goods. Their stuff is usually pretty twisted up.
I get my sheet goods from a plywood supplier nearby. About the same price and much better quality.
Paul
Contributor
'flatness' can't really be measured with a straightedge ( its a theoretical definition but not a practical one ). And sheet goods of any type ( metal or wood or plastic ) are pretty flexible in the vertical direction. Meaning, they do well to locat in two dimensions but the third direction they are not much use for. This is in part why you see embossing on sheet metal, etc.
So what's good enough is a function of design purpose. The way I look at it, for a sled, as long as it conforms against the table top when in use then it's flat enough ( I don't care if it's twisted when sitting against the wall, and since I KNOW it will deflect, as long as it lays against the tabletop such that it doesn't rock then it's good enough ). It really only positioning a piece in the x/y directions.
Big bumps can be a problem, if they cause it to rock across them.
Member
As the above say, it depends.
If I'm cutting out large carcasses, and a piece of 3/4" veneer ply is wavy, (meaning, internal waviness through the veneers, not waviness/bend from me improperly storing it), I'll send it back to my supplier. If slightly bent, and the piece I'm making will be structured such that a) you won't be able to see the bend, or b) I can remove the bend during installation through other joinery, I'll use it - no problem.
What I don't accept are defects with internal veneers that telegraph through to the surface ply. I seem to have gotten into a mess of these lately and I send them all back. I'm buying the better quality paint grade birch, but sometimes I think they grab from the wrong stack.
Melamine better be dead flat.
Sometimes, it really doesn't matter. Today I'll use a part of leftover half sheet of 1/4" birch ply for a back of a large cabinet. Standing on its own, the piece probably has a 2.5' - 3' radius curve to it. We're talking skate board ramp! However, once stapled to the back of the cabinet, it will be just fine.
In Memoriam
"What is your tolerance for sheet good flatness? "
**************************************
Not much.
But it does depend on its end use.
A panel, x itself, hanging in the air is not necessarily found in nature.
It has stuff fastened to it. Will that stuff or can that stuff be made to flatten the panel. & all panels (fiber, plastic, & ply) are cupped to some degree.
If it is badly mishapen you may be able to flatten it locally but not over the whole surface. Does it matter?
Maybe a frame for a panel has the modulus to flatten a panel, maybe not.
Usually the the distortion is shared x the components; that is, the panel flattens some but the frame bends some too.
A constant hassle, and one that is application sensitive.
For jigs, flatness is critical if the application is.
I use nothing less than .50" ground cast aluminum for jig substrate. (It's usually + or - .0015"/sq.foot or I reject it)
The components sections now are job dimensioned, enjoying no part of flattening the substrate. Moreover, if deflection resistance is essential it literally may take a ton or more to bend short sections of 1/2 aluminum out of plane.
Contributor
I tell you what will drive you to drink...start measuring thickness. I have sheets here of ply that reasonably flat measured with a straight edge, but gradually vary in thickness so that the faces aren't parallel everywhere. I have to hand pick pieces to make things like shooting boards. It took me an hour to figure out how I could possibly be getting an out of square cut when everything is dead square! Hah...but the surfaces weren't parallel. Grrrr.....
If you want consistent thickness, MDF is about as consistent a sheet-good as I have found. Ply varies widely which is why those 'undersized' bits don't really work. I just avoid sheet goods whenever possible. We are making furniture after all
"A hen is only an egg's way of making another egg".
– Samuel Butler
Contributor
MDF is what I've been using lately, though it's miserable to work with IMHO. Sometimes I do things like laminating MDF with a thin layer of hardwood, and then passing that through my thickness sander to get a truly flat and parallel surface.
[OP]
Member
John, you shouldn't have mentioned the thickness thing. Quick, someone hide my calipers.
Contributor
Wait, how does a non-parallel surface affect a shooting board? Wouldn't it be equivalent to making a ramped shooting board?Originally Posted by John Coloccia
I tell you what will drive you to drink...start measuring thickness. I have sheets here of ply that reasonably flat measured with a straight edge, but gradually vary in thickness so that the faces aren't parallel everywhere. I have to hand pick pieces to make things like shooting boards. It took me an hour to figure out how I could possibly be getting an out of square cut when everything is dead square! Hah...but the surfaces weren't parallel. Grrrr.....
I just tossed a bench hook that had a low stop for planing thin pieces... the main plywood had picked up a bow and wasn't lying flat on the bench. That was annoying.
Matt
Contributor
If you put one sheet on top of another sheet, and especially if the errors bunch up, you end up with the top surface (the one that the piece rests on) way out of parallel with the bottom sheet (the one the plane runs on) even though the edges are all perfectly square. The only way to detect it is to put a straight edge across the top sheet and measure the distance from the bottom of that to the second sheet. This one really drove me nuts until I figured it out.
Member
I have bought A-1 birch to make cabinets, the sheets varied in thickness & they were from the same lift of plywood. Good luck finding it perfectly flat....That won't happen...
[OP]
Member
I'm still curious to hear numbers for what people find "flat enough" for an application where flatness is important. Obviously plywood is an imperfect substrate.
If you don't mind playing along, next time you're in the shop take out a scrap of plywood you might find acceptably flat for say, a jig surface, and measure how much it's out with straightedge and feelers. I'm finding that plywood/melamine I'd personally consider "flat enough" tends to be out between .005-.01" over 1-2ft. Just wondering what others' personal thresholds are.
Contributor
My sled in free standing state is out about 1/4"
As mentioned, whenin use it lays flat against the taglesaw surface. Functions perfectly
If it were a router table insert and not clamped to be flat, 1/4" would not be acceptable to me. Maybe .005" for that application
Some sheets I used as a cabinet back were out well over 1/2" before assembly. They assembled square and function fine
As John points out, if making a precision fixture I would want much better than 1/2 ". Again likely .040 ish might be acceptable, depending on the design function of the desired fixture
I really don't think you can get a single number .........
Contributor
I came across a stash of NOS birch ply from the 50's.
You have no idea how bad the new stuff is! That stuff was perfect. I kept it around for jigs but it is finally gone. It was dead flat, thick veniers, and voidless cores. Now the "good" stuff from the plywood houses is nowhere near as good.
One would think that with all the advances in technology that it could be better, but its all about production. Don't sand too hard......
Larry
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