How to test a board's flatness?

[Anna]

I've been using a Starrett straight edge to determine if a board is flat by standing it on its edge and checking for gaps. But when I put two boards that are supposed to be flat (using the straight edge method) against each other, there's a gap in the middle.

So are there other methods to see just how flat my boards are? It's probably an obvious solution, but I'm frankly stumped (no pun intended).

Thanks.

[LarryG]

You mean edge straightness, not flatness -- right?

Two possibilities come to mind offhand.

One is that your straight edge, isn't. That it's a Starrett reduces the likelihood but it could still be this.

The second is that the edges of the boards are indeed straight, but are not perfectly square to the faces; IOW, there's a slight bevel on one or both of them, and this is what you're seeing when you place the two together. This is pretty common even with jointed edges, and is the reason adjacent boards should be run through the jointer with their opposite faces against the fence. This cancels out any perpendicularity errors in the fence.

EDIT: Reading your post again, I'm less sure you meant edge straightness. Sorry for being confused ...

[Otter]

I've been using a Starrett straight edge to determine if a board is flat by standing it on its edge and checking for gaps. But when I put two boards that are supposed to be flat (using the straight edge method) against each other, there's a gap in the middle.

So are there other methods to see just how flat my boards are? It's probably an obvious solution, but I'm frankly stumped (no pun intended).

Thanks.

A quick question, are you refering to the faces so when you set the boards it looks more like this () than ||?

There are 3 axis to deal with, just tring to figure out witch one.

[LCHIEN]

Anna, several things could cause that, a straightedge only checks in one dimension.

For example, the egdes of the board might not be perpendicular to the faces.
Or the board might have a twist
Or your straight edge might not be straight (to check this turn it over and check with the other side, too, after checking a board)

Otter has a good question, is the gap the same all the way down or is it closed at some points and open at others...
|| or () or )(

[Anna]

I was really talking about the board face. The board's edge is a whole other problem. I thought that if get a good flat board face as a reference, I'll just use my adjustable tapering jig to rip the edge straight.

The straight edge just lies on its narrow edge on the board, with no visible gaps between the straight edge and the board from one end to the other. I not only go along the grain of the board, but slanted from side to side, too. I do this throughout the length of the board. I can see no discernible gaps (my board is about 4 feet long). But when I place two boards, face to face, against each other, there's a gap in the center (the ends meet... well, I guess obviously).

Of course my eyes are probably not so accurate anyway. Something I figured out after getting the Wixey digital angle gauge. I was off by 0.1 degree when I did the square and squint method. I was just hoping that there's some more mechanical/objective way of doing the flatness test. Short of buying a new jointer (I hate my #($*&@^ jointer!).

I rely too much on my Starrett, although I'm thinking of buying a new one and keeping it in a velvet-lined box, instead of letting it get knocked around.

Thanks for any help.

P.S. LChien, the boards look like ().

[radhak]

I've been using a Starrett straight edge to determine if a board is flat by standing it on its edge and checking for gaps. But when I put two boards that are supposed to be flat (using the straight edge method) against each other, there's a gap in the middle.

So are there other methods to see just how flat my boards are? It's probably an obvious solution, but I'm frankly stumped (no pun intended).

Thanks.

When I read this (sleepily) at 5am I thought you meant the face rather than the edge; not sure now. Anywhichway, the problem could be if the reference-edge is straight, but beveled, or too short to align along the Starret... ?

[TheRic]

A couple of quick things that is not always obvious. Don't push down on the straight edge, or board when checking, Don't have the board clamped down when checking. Make sure they are on a flat surface.

What I think the problem might be is that you can not see a gap under the straight edge because the bow of the board. Your not getting a true straight line of sight to check it.

Try seeing if you can slip a piece of paper between the straight edge and board. OR put the straight edge over the paper and see if you can wiggle it / slide it along the straight edge.

[LCHIEN]

OK, misread the 1st post, thought it was edges.. not face flatness

Did you flip the 2nd board checked flat face onto the checked flat face of the 1st board so that the flat faces are against each other?
are the board faces perfectly parallel?
is the worksurface perfectly flat?
Are we pushing on the boards when checking (even gravity has an effect on thinner boards).

If gluing two boards together to make a thicker board, a small gap won't make much difference if you clamp them together and especially to a flat worksurface.

[Anna]

Ric, I tried that. I even bought feeler gauges to make sure. I can't get anything between the board's face and the straight edge, that's why I thought it's flat. I even place the straight edge diagonally from one edge of the board to the other to see if there's a cup (or is it a bow? I can't get the terminology straight). None that I can see, though.

If I can get away from the yardwork this morning, I'll take a picture so you can all see what I'm talking about.

I think I'll buy a new Starrett straight edge to make sure. I have two right now - a 6" and 12". I've been using the 12" to test for flatness. I was just wondering if there's any other way to do this before I spend the money on a new Starrett.

[Anna]

OK, misread the 1st post, thought it was edges.. not face flatness

Did you flip the 2nd board checked flat face onto the checked flat face of the 1st board so that the flat faces are against each other?
are the board faces perfectly parallel?
is the worksurface perfectly flat?
Are we pushing on the boards when checking (even gravity has an effect on thinner boards).

If gluing two boards together to make a thicker board, a small gap won't make much difference if you clamp them together and especially to a flat worksurface.

The boards are maple, at least 1.25" thick. I just discovered how hard and heavy maple is, and I don't think I can bend that thing in the middle unless I suspend the edges on something and jump on it. I don't clamp them down.

The surfaced faces are against each other (this is before I start planing to get the other sides surfaced). I stood them on their narrow ends and pushed them together, as well as laying them on the table on top of each other. I've tried this with MDF just to make sure the method is okay (and it is; no gap between the MDF boards). I also tried the boards against MDF, and there's a little gap which, I guess gets amplified when I have the two non-flat boards against each other.

I'm not gluing them together, which would have been okay. Did that with two pieces of bowed (?) plywood, and they pretty much cancelled each other out, giving me a flat surface.

I'm trying to make face frames for my new cabinet. It's probably not as important to have the pieces really flat for this application, but I still want the ability to four-square a board for when I make stuff that really do need that, especially gluing panels together for a top.

[LCHIEN]

...I think I'll buy a new Starrett straight edge to make sure. I have two right now - a 6" and 12". I've been using the 12" to test for flatness. I was just wondering if there's any other way to do this before I spend the money on a new Starrett.

Did I read earlier that these are 4' long boards? (can't seem to find that)
I assume you know you must use straightedges at least as long as the boards?

I use levels from the hardware store as my straightedges. 2', 4', 6' ones.
They don't publish a specification on how straight they are but I would judge them straight enough for woodworking.
they do publish a figure like .001"/ft but that is the angle error form level that the vials have, not the straightness.

[JR]

I've been using the 12" to test for flatness.

You need a 4' straight edge, Anna, and not necessarily a Starett. A 4' level, if straight, would help out immensely.

Oh, and it's not going to make a difference on your face-frame project. I understand the intellectual curiosity of trying to get the material four-square, but if your patience runs out, just go build the darn thing!

JR

[Anna]

Did I read earlier that these are 4' long boards? (can't seem to find that)
I assume you know you must use straightedges at least as long as the boards?

I have a 7' long level from Home Depot (the yellow one that costs more than $60) and I also tested with that. The gap is there, in the center of the board. But I thought my level might not be really level - I've had it for a year and it's been knocked around some. Guess will pick up a new one of those, too.

JR, I think you're right. I've put off finishing this project for two weeks because I've been distracted about getting the faces flat. Might as well get this one over it. Thanks.

[Russianwolf]

one thing I didn't see pointed out is that you need to check the flatness in several directions.

If you check from edge to edge it could appear flat while if you check parallel to the edge you may see a gap.

I have a 2 foot level I use for this and I check about a dozen places. Along both edges, several places along the length from edge to edge. And the kicker, I do "x" patterns along it also.

With your 6 foot level, make a check from opposite corners and see what you get.

[eddy merckx]

Hi Anna

Two things come to mind:
-Are you using your 12" straight edge on a board longer than 12"? This is probably too obvious, but you would need a straight edge at least as long as the board. I apologize in advance if that's not the case.

-Are you using a jointer to flatten the face of your board? If you are, is it possible that you're pressing too hard on the middle of the board?

Best wishes,
Eddy