How do I know when my wood is flat? - Hand plane.

First thing, is your combination square blade perfectly straight ? Check it against a table saw top or a granite counter top and see if you can still see light. You need to have a reference surface that is guaranteed straight/flat to start from.

A piece of aluminum angle extrusion stock is usually very straight as well.

How flat you get your wood depends on your own personal preference for exactness and the nature of the project. If it’s a garden planter then it doesn’t need to be dead flat. If you’re cutting joinery on the piece and you want it to be crisp with no gaps then those light showing low spots will be visible in the final piece. A tenon shoulder or dovetail baseline with a low spot on the face of the board will show as a gap.

Can I offer “if it feels flat, it is flat.”

That means, of course, if you’re happy with the feel (and fit), it doesn’t really matter what the straight edge says…

-- Don't anthropomorphize your handplanes. They hate it when you do that. -- OldTools Archive --

Run it through a jointer and planer….it’ll be flat enough…

-- My grandpa used to say: "Y'all come back when you can....come after dinner, and leave before supper.."

One: A #4 is too short to make a table top “flat”

Two: Need a much longer plane. It will take down the “high ” spots, while skipping ove the low ones. When you can get a shaving full a full pass of that longer plane, stop, and move a little to the next high points. When all points can produce a full shaving the length of the pass, top is flat.

Three: Wood will move over time, so what is flat now, may not stay that way over the years. Nature of the wood. Get it as close as YOU feel is flat enough, and call it good.

-- A Planer? I'M the planer, this is what I use

Can I offer “if it feels flat, it is flat.”

That means, of course, if you re happy with the feel (and fit), it doesn t really matter what the straight edge says…

- Smitty_Cabinetshop

This!

You are making a project from wood. Even if it were possible to get it dead flat, as soon as temperature or humidity changes, it’ll be slightly out. Get the piece to where you are happy with how it looks to the eye and feels to hand and call it good.

-- In matters of style, swim with the current; in matters of principle, stand like a rock. Thomas Jefferson

Flat is dependent on the project. You will never get wood “truly flat” and have it stay that way; it moves and always will. If you are going to build something like a box, getting the faces flat may be somewhat less critical than other projects. Also, checking for light is a good technique, but only if you know how much light should come through. The human eye can see light through an opening of less than a thousandth of an inch, which is way more than wood needs. A set of feeler gauges comes in handy, because then you can quantify how out of flat you are. If you are 2 thousandths out of flat, I would say that is more than good enough. If you are 10 or 15, then your application may require further flattening.

One final note on the light thing, if you are using a hardware stole metal ruler, it may not be perfectly straight.

-- Brian T. - Exact science is not an exact science

I’d reccommend Rob Cosmons rough to ready.

His technique where he would take the cut piece and rub it down on a known flat surface. I used my TS top. Lift up the piece and the high spots will be burnished and easily visible. Worked great. For Twist, I use winding sticks.

A Good square will tell you a lot for a specific point but not the entire surface.

-- Sooner or later Liberals run out of other people's money.

Rule of thumb. Place the wood on a level surface. Try to rock it on the sides. If no rocking it is generally good enough. If the surface is expected to as precise as a machine…........ The poor plane will suffer.

1-3 thou is good enough in all conditions I can think of. Just remember that wood moves too.

Thanks for all the replies!

I took a piece of oak that I flattened and put a small piece of printer paper underneath the ruler from my combination square, apart from a 0.5” strip along one side, I couldn’t pull the paper out from underneath the edge of the ruler. Is this flat enough? (I’m mainly making candle holders but I’m starting a foray into dovetails boxes!)

Double post.

You need a jointer and a planer. Any amount out of flat is going to translate the error to the rest of the project. You’ll just frustrate yourself and waste wood until you do it right.

-- --Dale Page

You need a jointer and a planer. Any amount out of flat is going to translate the error to the rest of the project. You ll just frustrate yourself and waste wood until you do it right.

- bannerpond1

That’s not possible for me – I do woodworking in my living room. I also want to use hand tools as much as I can – people managed for hundreds of years without a jointer and a planer so I’m sure I can too.

The paper test sounds like you’ve got it flat enough.

AESamuel, you bring up a very good point. People have done it by hand for thousands of years…I think we sometimes get wrapped around the axel about using machines to reach perfection when it’s completely within our grasp to accomplish the same by hand. In my opinion, slight imperfections are a sign of hand workmanship and should be embraced over sterile machining. Machines make the process easier not necessarily better.

-- Do or do not...there is no try - Master Yoda

I moved up to a 4 1/2 and will begin using a 5 1/2 as soon as I get in on line. My projects have been slightly smaller 10”-20” If I was going much bigger I’d pull out my 7. I picked up a woodpecker tri square last month and using it like you for light gaps. Sounds like you right on the money. I’m tackling half blind dovetails with a Leigh jig. Having a bit of trouble getting the joint snug. The wood flatness is not the issue for me, it’s the adjustments.

I have to agree with Warrior. You can get to wrapped up looking for perfection.