# Edge Joining: How flat is flat enough?

I'm trying to edge joint boards without a jointer or a table saw....

I have a small router table with an 18" fence (Only 9" outfeed fence) which I tried to use with some jointing shims. I ran two boards through and the resulting edge was fairly flat but if I hold them up to a light I can see some gaps.

How flat is flat enough to edge join boards?

I assume that it might be a slightly different answer for each of the following.

1. Plain Butt
2. Tongue and Groove
3. Doweled
4. Spline

Are there any numerical/measurement based specifications?

• Hi, welcome to Woodworking. It's not clear how much, if any, troubleshooting you've done to try to rectify the problem here and there happens to be this highly related previous Q What are some techniques for jointing boards using a router table? from just a few days ago. "Are there any numerical/measurement based specifications?" There are, but you can ignore them for anything but a plain butt joint. Commented Feb 14, 2022 at 19:05
• Two important clarifying questions. Do you have any hand planes you could use to tweak the edges you already have? And what width is the keyboard tray you're working on, i.e. what length are your boards? Commented Feb 14, 2022 at 19:12
• Tray dimensions are 40" x 25". I have a small lie block plane, but I don't think I could flatten the edge by hand with it. Commented Feb 14, 2022 at 22:28
• "I have a small lie block plane, but I don't think I could flatten the edge by hand with it." I got beaten to the punchline by @VolframK but you don't have to do the whole edge, you only need to improve what you have. Planing off high spots is pretty easy once you know where they are, takes only a little practice to get good at it (and you can do well first time). Plus, learning to do this now will pay dividends any time in the future you get a similar result. But as the above Answer and the link in it show, there are methods which can ensure perfectly jointed edges sans both TS and jointer. Commented Feb 15, 2022 at 13:45
• They used to do it so that the ends of the boards fit a little more snugly than the centers so that the added pressure at the ends makes the joints just a little bit tighter at the edges. Commented Feb 17, 2022 at 0:19

You don't say how big gaps are but if they are small edges are already very close to straight so it is easy to improve them by planing.

• Hold 2 boards together edge to edge to find high areas, mark them with pencil. Or chalk flat surface and rub board edges on it, chalk will mark high areas for you.

• Plane off pencil or chalk.

• Have plane adjusted for fine cut and check progress often, maybe after every shaving. 3 shavings too much!

Jointing by hand is easier with larger planes because of effect of long sole bridging gaps, but you can do it with your block plane. Work carefully to plane only marked areas.

How flat is flat enough to edge join boards?

No visible gap is usual.

With plain glued joint small gap in center only is acceptable, sometimes used deliberately. Uneven gaps and gap at ends of boards never acceptable.

2, 3, and 4 always made straight.

Are there any numerical/measurement based specifications?

|Some say yes, some say no. There is not a single number, it must be small enough that it can be closed by hand.

• on short boards it must be very small.
• Can be bigger on long boards, maximum approx 1/32" (0,8mm). This is total gap, so curvature on each edge 1/64" (0,4mm) or less.
• Gap must be smaller on thicker boards because wood is stronger.
• Valid information on how to fix the problem, but doesn't address the question asked of "How flat is this flat enough"? Commented Feb 15, 2022 at 12:59
• @FreeMan thank you, will update! Commented Feb 16, 2022 at 6:49

"It depends"

the point of a good glue joint, is a good surface area that can be bonded together. The more surface area the stronger the glue joint. Where you can see light that is a weak spot that won't be bonding.

Is it good enough? What materials are you joining AND what kind of finish are you going for? Those 'gaps' can be obvious on a finely finished table top, but maybe less on other works.

I like to get my joins with a clean line across the whole joint. while this isn't needed for a good glue up job, having gaps that let light through I think is a too much.

So if you are doing a straight butt joint you might want to try harder to get a tight fit, you might be able to glue some sand paper down to a flat surface and run the boards like a plane over it, I assume a keyboard tray isn't going to be 3 or 4 feet long. So short enough for this to easily work. I am assuming that you are fairly close so a few passes on a 80-100 grit should flatten it up enough and then a few more with 150 and you should be good to go.

• I'm building a keyboard tray for my wife out of figured maple with a figured walnut boarder. Ideally I would like a no-gap edge, but I haven't figured out how to do this with my equipment.... Perhaps I can lengthen the fence to extend beyond the table to pick up some extra precision. I'm thinking I'll need to use either a tongue and groove or a spline joint since there will be weight/pressure on the boards. Commented Feb 14, 2022 at 18:33
• @bowlturner, as covered many times previously, biscuits, dowels and splines actually aren't reinforcements; as strange as it sounds they (and Dominos!) can actually weaken an edge joint like this! They end up acting purely as alignment aids during assembly. This alone is a good enough reason to use them in some circumstances, but thinking they add strength isn't right. Commented Feb 14, 2022 at 19:16
• @Graphus Thanks for taking the time to help me understand. I'll update this answer soon to reflect this. Commented Feb 15, 2022 at 15:12
• @gnicko, yes, somewhere in that ballpark. It's becoming more and more common for woodworking instructors (on YT especially but in the magazines also) to advise that M&Ts shouldn't be a tight fit, sometimes going out of their way to specifically state that they shouldn't need to be knocked together with a mallet. This is directly counter to old-school advice. And what's more the very same woodworking gurus telling learners this have no problem demonstrating dovetails that need to be tapped together o_O Commented Feb 16, 2022 at 14:47
• Thanks for that last one @Graphus. I always thought M&T joints needed to be snug and was confused by all the more recent advice that they not be. Commented Feb 16, 2022 at 17:31