Introduction: Flattening a Chainsaw Milled Slab
Using wood slabs for your woodworking projects can save you a considerable amount of money compared to buying lumber from the big box store. I bought this air-dried oak slab for approximately $45. The amount of boards that I will get from this slab would cost approximately $450 from a big box store!
While you can save a lot of money, the downside is that you have to figure out a way to surface the slabs so that they are flat and usable. To do this, I use a slab flattening sled and my router. A flattening sled allows you to lay the wood slab down on an even surface so that it can be flattened. The side rails of the sled support a plywood jig that holds a router. You slide the jig down the sled, moving your router from side to side. The result should be a slab with a flat surface. At that point, you flip the slab over and do the other side.
My name is Billy and I’m a woodworker and blogger, who shows woodworking tips and techniques. You can find my other tutorials at the following links:
My sled is approximately 10ft long and 3ft wide, but you can make this any size that you want. Here is a step by step breakdown on how to build your own sled, and in turn, hopefully save a ton of money.
Two 2” x 8” x 10ft lumber boards
Three 2” x 4” x 8ft lumber studs
One sheet of MDF
Scrap 3/4" plywood
Step 1: Cut the Sled Sides to Size
I’m using 2x8x10 lumber boards for the side rails of my sled. I want the edges to be clean and flat so I’m trimming each side. The result should be two rails that are approximately 7” wide. My sled will be 10ft long so I’m leaving the boards at the length at which I bought them. But if you want a shorter sled, feel free to cut them down to any length that works best for you.
During this step, I also decided which edge looks and feels like it’s the best surface for my router jig to slide across. I’d prefer not to have any knots that could catch on my jig.
Step 2: Cut the Boards for the Sled Base
The two rails are held together using 2x4 lumber studs. When buying your 2x4s, try to find the straightest boards possible. Usually the home center stores offer a few kinds of studs, one is a little cheaper than another, despite being the same size. Spend the extra dollar and get the more expensive ones. They are usually drier, straighter, and have less defects.
I bought three 2x4s from the home center and trimmed off each side. I want my boards to have nice, crisp edges. They must be the same width so that the slab lays flat in the sled. I cut mine to 3” wide.
My slabs are about 26” wide so I want a fairly wide sled. I cut my studs to approximately 3ft long. That should give me plenty of working space for this slab, and any future ones I may cut.
Step 3: Assemble the Sled Body
With our frame parts cut, it’s time to assemble the sled. I start this by making a rectangle. Using a clamp to hold things in place, I screw one of the studs into the left side of the rails. And then I attach a stud to the right side of the rails. This helps to keep the rails standing upright while I screw in the studs in the middle of the sled. The spacing of the middle studs isn’t super critical. They are only to give a solid surface for our MDF base.
It’s important to ensure that the studs are attached flush with the edge of the rails. I use a square to check that the stud is flush with the top of my rail. I also use it to make sure that my studs are square.
I attach the studs to the rail using long spax wood screws. I use two screws per side. Using only one screw will hold the stud, but it can allow the stud to twist, and the sled wouldn’t be flat. I like using this type of screw because they have a hex end, meaning that they won’t strip out like a regular screw.
Step 4: Attach the MDF Base
I want a flat, slick surface for the slabs to lay on. This helps with loading and unloading the slabs, and for putting any wedges under certain areas of the slab so that it doesn’t rock.
I cut a sheet of 3/4" thick MDF to about 32” wide. I didn’t want the MDF to be the same width as the inside of the sled. When flattening a slab with a router, wood chips and shavings need a place to go. By leaving openings on the sides, the chips can fall under the sled so that they are out of the way.
My sled is longer than a regular sheet of MDF so I added a few pieces of scrap to make up for the extra two feet in length. The MDF was screwed into the studs using some all-purpose 1-1/4” long screws. I counterbored the screw holes so that the screw heads didn’t stick up.
Step 5: Drill Clamp Holes
Even though the sled has some weight to it, I still wanted to clamp it to my bench. This step is optional and dependent on your shop setup. I used a 1-1/2” spade bit to drill a hole into the sides of the rails. This hole gives me enough room for the head of a clamp to fit.
Step 6: Make the Router Jig
With the sled complete, we need a jig to hold the router. My jig is made from 3/4" thick plywood. The width of the jig will be dependent on the type of router you use, and the jig must be longer than the width of your sled.
I started by cutting two strips of plywood for the jig base. These are approximately 40” long for my sled. I then cut two 2” wide blocks from the same plywood, which act as spacers between the strips. Using CA glue and some quick set activator spray, I glued the spacer blocks between the plywood strips, at each end. The result is a channel that is wide enough for a router bit to fit through.
I trimmed each side of the plywood base so that the base was just slightly wider than my router. Then I used CA glue and pin nails to attach two plywood panels to the sides of the base, and two panels to each end. Mine are approximately 3” wide, but this measurement isn’t important. You just need them to be tall enough for the router not to come off the base. The result should be a trough with a channel cut out of the bottom. Your router should be able to slide easily back and forth, but not have a lot of wiggle room.
Step 7: Load the Slab on the Sled
Position your slab in the center of your sled. Try to make it as level as possible so you don’t take off too much wood on one side and not enough on the other. Use wedges or scrap wood to prop up any uneven areas. If your slab is light enough to slide around while routing, clamp a piece of scrap wood on each side of the slab.
Step 8: Routing the Slab
Now comes the fun part, routing the slab! Insert a large diameter bit into your router. Any straight bit with a 1/2" shank should work. I recommend against using a bit with a 1/4" shank as the pressure against the bit could cause it to break. You can buy a specialty slab flattening router bit if you want. But any straight bit will really work for this process. Just note, the smaller the diameter of the bit, the more passes you will have to take across the slab. I often use a large diameter bowl cutting bit.
Find the highest spot of your slab. Positioning the router above that spot, plug the router down (while turned off) until the bit touches the surface of the slab. Lock the bit into place. Move the router to a lower area of the slab and lower the router bit approximately 1/8” lower than its previous position and relock it.
Starting at one end of the slab, turn on your router and move the router back and forth in your jig. Reposition the jig an inch or so (depends on your router bit size) by sliding it down the rails of your sled. Make another pass with your router. Go back and forth with the router, inching your jig along, until you get to the end of your slab.
Once you get to the end, move your jig back to the starting point of the slab and lower the bit another 1/8”. Repeat these steps until all the areas of your slab are routed. At this point, you know that this side of your slab is flat.
Flip your slab over and perform the same process.
During this entire step, please make sure to wear safety equipment. Dust and wood shavings tend to go everywhere. You do not want anything in your eyes or lungs.
Step 9: Enjoy Your Flat Lumber!
After routing both sides you should have a flat slab to work with for your next project. The router bit likely left some routing marks in the wood that will need to be sanded away. But this is a small price to pay for the amount of money saved by using a slab instead of buying wood from the big box store.
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