DIY Slab Flattening Jig — Build Your Own for Large Projects
Ever struggled to fit a large slab inside your planer? Yeah… me too. That’s why I built this simple slab flattening jig that handles live edge slabs, oversized cutting boards, and more. And the best part? You can make one in your own shop with just a few basic tools.
🎥 Watch the full build process here: Slab Flattening Jig Build Video on YouTube
Why I Built This Jig
Recently, I ended up with a pile of live-edge stock taking up space. Instead of letting it collect dust, I decided to turn it into furniture. A planer wasn’t going to cut it — literally — so I built a jig that:
Flattens slabs too wide for a planer.
Works on projects like large cutting boards.
Can be converted into a track saw or other tools with small modifications.
Materials & Hardware I Used
For my build, I used SBR16 CNC rails (78" for the long rails, 39" for the cross rails). The “16” refers to the rail diameter — thicker rails (like SBR20) are stiffer, which is worth considering for wider jigs.
Other essentials:
3/4" plywood (for rails and sled base)
M5 bolts (size may vary depending on rails)
Wood screws (3/4" & 1 1/4” length)
Countersink bit
Router & flattening bit (I used a 2" bit from Amazon)
Holesaw (size depends on router bit)
Step-by-Step Build Overview
1. Build the Long Rails
Cut plywood 4" wider and 6" longer than your rails.
Mount the rails 1" from one long edge, centered vertically.
Use 3/4" screws and pre-drill to avoid splitting.
2. Add Stops
Install small plywood blocks at one end to prevent the sled from sliding off during use. These don’t need to be fancy, just make sure they’re taller than the rail itself.
3. Install Cross Rails
Adjust the Allen screws on the sliders (sides and top) for smooth movement.
Attach 39" SBR16 rails using M5 bolts in the rear slider holes (furthest from the center) to reduce sagging and increase working space.
4. Make the Router Sled Base
Cut 3/4” plywood strips for your desired length. I went with 2” wide and 13” long.
Mark and drill holes to mount this piece onto your rail sliders from above.
Cut two more 3/4” plywood strips to run along the side of the rail slider and stopping 1” from the height of the longer rails. In my case, this was 2”.
Mount side panels into the top plate using 1 1/4” screws (pre-drill first). I made mine 12” long, centered on the 13” top plate, leaving 1/2” overhang each side.
Cut a 3/4" plywood base to fit under the side panels, big enough for your router to sit and plunge comfortably. Pre-drill and screwed into the side panels from below using 1 1/4” wood screws.
Drill mounting holes for your router base plate (use the router’s base as a template).
Countersink screws so they sit flush and mount from below.
5. Cut the Router Bit Opening
Attach your router to the sled and mark the bit location.
Remove the router and use a holesaw to drill an opening slightly larger than your bit.
I used a 2" bit, but left room for bigger bits later.
6. Add Braces & Stops
Cut 3/4” plywood braces to prevent twisting/racking, match the outside width of your router sled.
Drill holes (if required) to mount support braces on the outside edge of long rails.
Adding these blocks on both ends also stops the sled from sliding off.
Pro Tips I Learned Along the Way
Measure manually with a metal ruler for slider plate hole spacing — templates didn’t work well for me.
Go bigger on the router opening for flexibility and to prevent interference if off-center (but I wouldn’t go beyond 3 1/2”).
Use wing nuts on support braces (step 6) for easy disassembly and compact storage.
The Results
The jig is solid, easy to dismantle, and stores compactly. I’ve already planned:
A slab furniture build using this jig.
An upgrades video (dust collection, storage improvements, and more).
Watch the Full Tutorial
Want the full build with all the details I couldn’t fit here?
▶ Watch my Slab Flattening Jig Build Video
Don’t forget to subscribe if you want to see the upcoming furniture project and jig upgrades.
