Flattening Wood Slabs & Large Boards

When it comes to flattening a rough piece of wood, there are 3 methods I can think of quickly off the top of my head. I’ll talk about these in a minute but first, why do we need to flatten a piece of wood anyway? Isn’t it ready to use straight from the store? That depends on which store! Keep reading!

I don’t really buy much wood from a store like Home Depot, Lowe’s, etc. I only go there for basic construction lumber like 2x4’s, 4x4’s, treated lumber and junk like that. If I’m going to build a piece of furniture I go to a hardwood dealer. This is where I’ll find my walnut, white oak, wenge, and other domestic and exotic hardwoods. Some Home Depot locations actually have some hardwoods like walnut but they’re limited in thickness and width and I’m usually in the market for thicker stuff.

I buy 8/4 walnut frequently (learn about thickness measurements here), and I’ve even had a nice piece of 12/4 walnut to work with recently (stay tuned to the blog and my portfolio page to see that project when it’s completed). These thicker pieces are great and even though they may look nice and smooth at the store, that doesn’t mean they’re flat. They may have some bowing or some twist going on, and that stuff has to be dealt with. So what does a woodworker do? We flatten it.

One method of flattening is to use a jointer. This would probably be the quickest way to do it but depending on the size of the jointer, you’re limited to how wide of a board you can flatten. While I have a larger floor standing Ridgid jointer, I can only do a 6-inch wide board. Now, if you need to flatten a board larger than 6 inches wide there’s a workaround that includes removing the safety guard and running the wider board over the blade like usual. This leaves you with a lip on the underside of the board, then you need to make a special “sled” for the board so you can run the opposite side through the planer a couple times, then remove the sled and flip the board over and send it through the planer without the extra sled, lip side up this time. This is a little bit of a hassle for me so I never really do it like that.

The next method is to use a cnc machine if you have one. I have a Shapeoko3 XL, so I could potentially do a board close to 15-inches X 30-inches (which I have done). The non-flattened board gets secured to the cnc bed, shimmed if needed, and then you run a surfacing toolpath down to the lowest point of the uneven board. Now you can run that board through the planer opposite face up (if your planer can handle the width of the board). If it cannot handle the width of the board you can just flip the board over and run another surfacing toolpath until you get to the desired thickness.

The last method I’m going to talk about is a slab flattening mill, which has been my go-to method lately. Slab flattening mills can be very basic and low-tech, all the way to overly-complicated and over-engineered. Rockler, one of the most popular woodworking tool stores, has a couple different models. One is a benchtop model for smaller slabs and can handle up to an 18”x28”x1-3/4” slab, while the larger model can handle a 28-1/2”x78-1/2”x2-3/4” slab. Both seem like great options but at $200 and $600, that’s not really in my tool budget.

A newer company named SpeTool has some flattening mill options as well. They’ve got a very basic model that allows for unlimited length because it rolls on guide bearings along the edge of your work table. Ehhhh, I don’t know if I’d trust the accuracy unless you’ve got a pretty solid table and at $390 I’m gonna pass. SpeTool also has a beefed-up version called the XPERT for $900 with an interesting router lift mechanism and some dust collection features. I watched their product video on YouTube and it looks like it’s definitely been thoughtfully designed with a lot of great consideration put into it, but it just seems like…..a lot. SpeTool also has variations of the XPERT with rail extensions and router bits and those are all at different price points.

If you’re a woodworker you’ve most likely heard of Woodpeckers. Their red and silver colored woodworking tools are easily recognizable (unless of course you’re looking at a red and silver knock-off on Amazon, haha!). A few years ago I think the Woodpeckers flattening mill was the first one of these I’d seen….and by far the most expensive, but then again, all the Woodpeckers tools are the most expensive woodworking tools. I will say for sure, they do make the slickest looking tools and I’m sure they are fantastic AND they’re made in the USA which I love, but due to their prices I have yet to add a Woodpeckers tool of any kind to my shop. Their large version flattening mill comes in at almost $1500 while their basic version is still $1000. They’ve also got extension rails and dust collection accessories but for those of us trying to make more money than we spend, I’m super happy with the flattening mills I’ve built from scrap plywood.

My flattening mill setups are super easy to build. I basically built small versions of the one Andrew from Infinity Tools built. See the video below:

I have one that I built specifically for the tops on my Zelo Side Tables, and I use that one pretty often. I also added some brush strips to “try” to keep the sawdust contained under the sliding router bridge.

I also built a flattening mill for a walnut cookie I was working on, and another larger mill for a longer piece of walnut. I was able to use the same router bridge for both, which saved time and plywood!

One little accessory I did spend money on was these little steel claw hold-downs. I bought them from a company called O’Skool on Amazon and apparently they’re called “flattening dogs”. Anyway, to use them you simply let whatever claw end works best for your piece of wood and you screw it down to the mill. Driving the screw down sends the claw into the wood, securing it in place.

My router of choice for flattening in my DIY flattening mills is the Bosch 1617EVS, and I use the plunge base. With 2.25 horsepower it’s got plenty of muscle to power through the wood. Even still, I usually don’t take more than a 1/8” off per pass. Currently, for budget reasons, I’m using this cheap Uxcell 2-inch wide surfacing bit. I think before long however, I’ll upgrade to a 3-flute replaceable blade version like this one from Binstak. I also use this Taytools collet extension for 1/2” shanks if I need a little extra length.

So, after flattening the one side I can send it through the planer or flatten the other side and it’s ready for the rest of the build!

I hope you pulled a little nugget (or more) from this article that you’ll be able to use on your projects. I have a large olive cookie that I’m looking forward to flattening. It’ll probably become a coffee table but who knows. Stay tuned here and when that project comes around you’ll be able to see it…..and maybe buy the final build. Thanks for stopping by the blog of Seth Mondragon - American Craftsman.