Flat(ter) Than a Pancake

A few months back, I was given a fairly large slab of "green" pecan wood. The piece had been chainsaw milled and was left sitting outside, leaning against a fence in the weather. Two things were fighting the original owner here, it was as wavy as the ocean in a storm and was not dry enough to work with. 

Have no fear, we can work with that.

This is what the beast looked like in its new-to-me state. Kind of rough, and you can rightly guess why it was free. I took it because I didn't know any better and always seem to have more time than cents smarts.

You'll notice that it's about an inch out from the two high points on this side...

And about two inches out from the lowest point on what I'll call the "back". Fortunately, shortly after I got this piece I saw a technique that uses some crafty knowledge of geometry and routers to make a sled that allows you to flatten just about anything from giant slab to workbench top.

First off, you'll need some tools and materials to make the jig. Here's what I used:

Tools

• Router
• As big of a flat bottomed router bit that you can get your hands on
• Jigsaw
• Pocket hole kit (Kreg)
• Wire or string
• Clamps
• Drill/Driver

Materials

• Smooth board, wider than your piece by about a foot or two - I used particle board
• Something for the supports, plywood, 1x4, anything to keep it from sagging - I had some weird poly channel beams that were light and strong
• Screws
• Rails - I've seen folks use straight lumber, I had some uni-strut on hand that was perfect

Let's cut the bottom of the jig. This isn't too critical, but needs to be wide enough for your router to sit inside (plus the rail/stiffener width) and long enough for the cutout to extend past the piece to be flattened. Keep in mind that this rig works great for flattening end grain butcher blocks - the orientation of the spinning bit helps prevent tear out - so you don't have to make one this large to be useful to you.

That's it - I made the long cuts with a circular saw, but a jigsaw works just as well, especially if you are not comfortable with plunge cuts using a circular saw. The slot's width needs to be just enough to get the bit through, plus just enough room. Leave enough room for the stiffening rails though. Don't forget that, you'll get upset.

Now, add the side rails to the base. These will serve two purposes: keeping the base from flexing (important for making boards flat(ter) than pancakes) and as rub fences for the router. I just shot some screws in from the bottom, along with a little bit of glue. Who knows if it will stick to that weird plastic, but you don't want it to move around. The rails really aren't in shear, and most of the calculations for fastener spacing are with regard to shear... I hate to disappoint anyone reading an engineering blog by not having an equation to reference for fastener spacing for their DIY project, so, use this one:

one fastener for every 4 to 6 inches of span 

Official enough. Being that official yields results thusly:

Time for the end rails. These also serve two purposes: captivate the router from coming off the end and keeping the skid on the rails. With that being said, I have seen these jigs use sacrificial wooden rails. The operator would just stop moving the router when the bit hit the rail. Bad idea for steel rails. I left about a quarter inch of play in mine, meaning: rail outside width plus 1/4" equals the end cap spacing.

Now it's time for the geometry lecture. Grab a seat and strap in. Planes. A flat, two-dimensional surface. We will use the fact that parallel planes never cross to create a flat work surface and as a result, flat slab lumber.

I cheated a bit. My table top is flat in all directions. Plane 1 has now been established. My rails are also flat through their length, so I can put the bottoms of them on Plane 1 and the tops of them will be Plane 2, now assumingly parallel with Plane 1. Before we check that, I put the slab on the table and check it for rocking. This means that I want to find the side and position of the slab so that it has three points of contact on the table top and consequently doesn't rock. Once you find that spot, lightly clamp it down to the table, but not with so much force that it moves.

Lay your rails on the table and get them parallel to each other so that the skid will slide down them without binding. Very lightly, clamp them to the table in a way that won't interfere with the skid sliding along the top surface. Bust out your wire or string. Tie off (or clamp) some wire across opposite corners of your rails. Go to the other corner (without string) and finish making an X shape on top of the rails, over the slab. On this particular leg of the X, put one piece of wire or string under the leg, like so:

You've now got an X across the rails with one leg elevated above the other. Theoretically they should just be touching each other in the middle where they cross.

In the shot above, the red wire has the "spacer" under it. So, what is this telling us? I'm sure by now, you've noticed that your wire, or string, isn't just touching each other in the middle. That means your rails are not parallel to each other in the vertical direction. You will need to use shims or adjust clamp pressure or whatever method to bring the wires so that they just "kiss". That signals to us that the far left corner is in the same plane as the near right corner and so on.

It should look very similar to this. I know you won't or don't believe me, but trust me, this works. When the rails are firmly secured, remove the wires and place the jig on top, with your router loaded in it. Find the highest spot by just moving the jig around with the router off. Set your bit to take about a 1/8" or less cut lower than that.

Remember, in this orientation, move your router left to right. This will ensure that the bit bites into the wood and pulls the router tight against the piece to prevent kickback.

Start at the back, move left to right at the "front" of your jig, then reset at the "back" of the channel, moving right to left. You shouldn't have any material to cut away here so the bit won't try to kick back on you. Move up incrementally across the piece, repeating this step. It will take a while, but it goes fast.

You'll get something that starts to look like the photo above. Keep on trucking and get the broom. This generates tons of sawdust. Wear a mask, glasses and ear plugs, this is messy and loud work. Remember how far out my slab was, check out the progressive cuts I had to make:

Those steps show you how many times I had to reset the router. You don't want to take too big of a bite at one time, and I was definitely pushing my router on the last one. After a few hours on this horrible slab, I had something that started to resemble a usable piece:

When you get as far as you'd like, flip it over, if needed, and repeat. There are some tricks you can use to preserve thickness, especially if your slab is twisted, rather than wavy like mine, such as re-shimming the slab, but with some poking around the internet you can easily find those kinds of tips. By now, you should have a really flat slab, parallel on both sides that just needs a few swipes with your hand plane to clean up the few router swirl marks...

1917 Vintage Stanley #5C with a Lee Valley PM-V11 blade. Like a boss.

You can see that this slab is trying to check on me and dry funny, a testament to it's rough life. Some grain filler, oil and wax will spruce it up like a spring chicken in no time.

Oh yeah, remember that extreme sawdust warning? In case you forgot:

Leaf blower optional.