Super Slab-Milling Jig

Pair your router with this sliding jig to flatten the widest, waviest planks.

Here at Lohr Woodworking Studio, we work with a lot of large slabs. One of the biggest challenges with this kind of work is flattening the slabs. We have a great router jig that does the trick beautifully. It’s essentially a platform outfitted with rails that support and guide a router back and forth over the slab, while a wide-diameter bit does the milling. We actually have several variations of this jig, having refined the design over time while making new versions to suit different sized  work. I’ve even made a few of these as one-offs for special projects. I’ll share with you here the latest and greatest version, which is an appropriate size for most medium to large slabs.

How it works. With a slab shimmed and wedged in place on the jig’s platen, and the router outfitted with a wide-diameter bit, the tool runs across the slab in a sled assembly that also slides sideways along rails to facilitate a series of overlapping passes. 

Sled essentials. To make the router sled, you’ll need a sheet of 1⁄2"-thick polycarbonate (Lexan), a large, variable-speed router, and a wide-diameter bit. (The “spoilboard surfacing” bit shown works exceptionally well for the job.) UHMW tape used on the platen rails ensures smooth sliding action. 


Go to to watch a video of the jig in action

Easy to build, but accuracy counts

Making the jig isn’t complicated, but it’s important to successful operation that the platen is flat, and that its rails are straight and parallel. The platen rails are rabbeted on their bottom edges to ensure consistent height and positive contact with the platen. (Use lots of glue on the thirsty MDF.) The platform cross rails are narrower than the long rails to reduce the number of contact points that may need to be shimmed to remove any twist during setup. For stability and light weight, I used poplar for the platform, and ultralight MDF for the platen and its rails. UHMW tape on the tops of the platen rails reduces sled assembly drag. For the sled frame, use hard maple for strength and rigidity; you don’t want the router sagging in use. You also don’t want it laboring, so use a strong tool. I recommend a minimum 3-HP machine with speed control. Mount the sled handle at an angle that’s comfortable for your reach.

Sled, Platen, and Platform

The jig consists of a platform that supports a platen with dog holes. Dowel dogs and wedges secure the slab. Platen rails support slotted sled runners that house a polycarbonate sled, which also serves as a chip guard in use. The 3"-tall rails will accommodate slabs from 1½ to 2" thick.  For thicker (or very twisted) slabs, raise the sled by screwing spacers (not shown) to the underside of the runners above the platen rails.

Order of Work

  • Build the platform.
  • Make the platen and rails.
  • Build the router sled assembly.


  • PLATEN & RAILS: Ultralight MDF
  • SLED: Lexan (polycarbonate)

Sizing the runner grooves. Test the runner groove width using a piece of scrap to ensure that the polycarbonate panel runs smoothly in the groove without binding or wobbling. 

Shimming the sled frame. To ensure that the sled assembly slides easily on its rails, make sure there’s about 1⁄16" space between the sled frame cross rails and the platen rails. A few business card shims should do the trick. 

Success depends on a stable slab and steady feed

For cutting efficiency and maximum stock thickness, shim and wedge the slab on the platen as shown. Then, after mounting the sled assembly on the platen rails, adjust the bit projection, and you’re ready to rout. After traversing all the high spots, readjust the bit and repeat, taking a series of subsequently deeper passes until the entire face is flattened. Then flip the slab over, and mill the other side in the same fashion.

Set the bit to nibble. With the router over the highest spot on the slab, adjust the bit projection to cut no more than about 1⁄16". Then take a test cut to make sure the tool doesn’t strain in use.
Forward pass. Begin each pass with the spinning bit hovering off the slab. Then slowly push the sled forward at a consistent rate until the bit has completely traversed the width of the slab. 
Full retraction. Without moving the sled assembly sideways, pull the router back toward you until it’s again hovering off the leading edge of the slab. 
Slide to the side and go again. Slide the sled assembly over to slightly overlap the previous pass, and then make the next forward pass. 
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