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Guide to Slab Flattening Jigs For Smoothing Large Slabs

When a planer or wide belt sander won’t work for your live edge slabs, cutting boards or butcher block tops a slab flattening mill is where woodworkers turn. In this article learn about what jigs are available – and why I decided a combination of a manufactured jig plus a set of linear rails.

Router sleds are one of the best kept secrets in woodworking. From smoothing end-grain cutting boards that cannot be planed to avoiding weight lifting a 400-pound live edge slab through a wide belt sander.

What makes a slab flattening jig unique?

Well, simply put, they can perfectly flatten huge slabs of wood with a single pass of spoil-board routing bit. All with the need for constant flipping OR trying to find a 30-inch wide planer.

And after almost thirty years of woodworking with wide belt sanders, drum sanders and other sanders I added one of these jigs to the shop. 

  1. BEST ALL AROUND – WOODHAVEN was my choice for price, design and wide variety of widths. Noting that the length of the jig is up to your rail design. 
  2. PROFESSIONALS CHOICE – WOODPECKERS SLAB JIG is the premium jig that comes out of the box with both the router sled and top quality sliding rails. But, for a price.
  3. DIY LINEAR RAILS – DIY LINEAR RAILS from a few feet up to six+ feet (2000mm) a set of linear rails, router insert from a router table and some ingenuity can yield a quality slab mill.

While the Woodpecker jig was tempting, at over $1000 at all woodworking stores it was too much to spend. Instead, I combined the Woodhaven jig with a set of 2000mm linear rails, Bosch router and a carbide-insert spoil board bit.

So, after a half day of some assembly and engineering, I’d added a fantastic piece of machinery to the shop.

Read more:

My Slab Flattening Mill

Slab Jig Using Linear Rails

They say you don’t build a house the way you want it until your third one. And, after two bench configurations in my home woodworking shop I am looking to perfect the benches on the third try.

But, while I have a 22″ wide belt Jet sander, the sheer mass of a 24 x 72 x 3.5″ top couldn’t be lifted. 

So after a lot of research, some engineering considerations and budgeting my final design ended up consisting of:

  • Woodhaven Planing Sled with 51-inch rails (for up to 32-inch wide stock)
  • Linear rails that were 2000mm (or just over 6-feet). These were hard to buy, and I ended up going with this 2000mm set and they were straight with decent quality. And with a variety of lengths can make a small and large jig.
  • Specialty metric screws (M6x20) for connecting the slab jig to the linear rail plates.
  • T-track kit to allow the sled jig and linear rails to be quickly separated.
  • Bosch 1617EVS 2.25 HP router to upgrade my old Porter Cable.
  • Carbide Spoilboard Bit with inserts for replacing dull or chipped cutters.
  • Lastly, as seen above, I have a large shop but not a dedicated 4×8 surface to use a jig from. So I went with a BORA Centipede that I also use with my Makita cordless track saw and foam for cutting plywood.

After using a few times it works BETTER than expected with absolutely flat surfaces. However, a few upgrades I’m planning to make are on dust collection and a non-slip setup with quick hold-downs (t-tracks). I just don’t want a dedicated 4×8 sheet of plywood with all of this on it (its clamped to the sheet above, disconnects in minutes), so am thinking through the engineering on that.

Disclaimer: I buy almost all of my woodworking tools from Amazon or Rockler and have shared links to what I buy. 

Router Sled Overview

Once woodworkers discover router sleds the woodworking doors open to projects that just weren’t possible before. From live edge slaps, trimming end grain cutting boards to surfacing 400-pound table tops there are few flattening jobs a router sled can’t handle.

What makes a router sled unique? Well, consider its four basic components of:

  1. Parallel side rails that support a sliding router sled. These side rails can be as short as 24″ and 96″ or longer.
  2. Router sled that slides on the parallel rails and moves perpendicular across the face of the wood.
  3. Heavy duty router mounted on the router sled.
  4. Spoil-board surfacing bit that removes 2-3 inches of material with each pass.

But while router sleds all feature these basic components, the similarities end there as DIY kits and manufactured sleds are built different.

5 Types of Router Planing Sleds

From simple plywood jigs to high grade extruded aluminum rails with machine shop precision woodworkers will find a wide variety of router sleds.

The best router sled for your shop? Well, budget is the first factor. But size of your projects, ability to sand out imperfections left by less precise sleds and shop space all play a factor.

Before deciding on a home-made jig its worth understanding the quality advantages of a manufactured jig:

  • Router deflection (tilting) with DIY jigs can cause damage to the face of the wood
  • Ease of sliding both left/right on the rails and forward/backward over the wood are vital
  • Rails must be kept clear of debris and sawdust to avoid lifting of the sled and creating surface imperfections

Sled #1: Simple DIY Jig

DIY Router Table Sled

With just some scrap wood, a proper router and a spoil board bit any woodworker can have a sled setup in an hour or two. Which makes this an attractive starting point for occasional live edge projects or dabbling in end grain cutting boards (that can’t be run through the planer). 

Considerations when making a plywood router sled :

  • Sliding rails must be perfectly straight and parallel in height
  • Use plenty of wax on the rail tops and sled face to allow smooth operation
  • Use a “C-clamp” design for the sled connection with the rails. Look for a tight tolerance that avoids play in the rail but allows for a smooth left-to-right movement

Sled #2: Linear Rail DIY Design

Linear Rails

Shown: Linear Rails

The top feature of a manufactured router sled is the rails are designed to move left to right with extreme ease. And, for woodworkers that know about linear rails, a perfect use of this little-known hardware.

Since linear rails are self contained problems like dust deflection are much reduced.

Taking these rails one step further, they can be made to work for the sled as well. Simply pair a longer set of rails with a shorter pair. Then attach a pre-fabricated router plate and custom handles.

Sled #3: Rail Kit Only

Sled Rails Only

Advancing into manufactured components, Woodhaven offers a rail kit that lets you design and use your own router sled. Or, for longer projects provides an add-on rail to handle projects over 48″.

Sled #4: Professional Router Sled Jig From Woodhaven

Woodcraft Router Sled

Moving into a manufactured router sled jig woodworkers will find the Woodhaven planing jig that provides a high quality slide with extendable tracks for larger pieces. 

When choosing this jig you’ll want to remember:

  • Double tracks are the left-to-right tracks that are parallel to the length of the wood.
  • Crosscut tracks are the vertical tracks that hold the router sled
  • Be sure to check the Woodhaven manual for more info

Sled #5: Premium Router Planer Jig From Woodpeckers

Woodpeckers Router Sled

Festool. Leigh. Woodpeckers.

All of the top brands in woodworking have both premium quality tools and accessories. And with the Woodpecker router sled you’ll have an out-of-box slab jig that can be quickly setup and taken down. 

Key features of the Woodpecker router sled:

  • Unique angled slides to reduce dust and chip impact on rails
  • High quality extruded aluminum components
  • Large handles for moving the sled across the wood face

The downside to this jig? As you likely know the premium brands come with a premium price. But remember, the cost of the jig is often the same as 3 or 4 of the slabs of wood you’ll put through it. 

So, as experienced woodworkers know, take a LONG view of your purchases and factor your expenses into the projects you produce.

Top Routers for Router Sleds

Plunge Router for Router Sled

While any router over 2-1/4 horsepower will work for a router sled its best to start with the basics: router sleds require a large  diameter bit up to 3-inches in diameter.

And as you start to make pass after pass over the surface of the wood precise height adjustments will be important. 

Which makes the following key features important:

  • Soft start motor to avoid “snapping on” with a large bit
  • Continuous duty motors for larger jobs
  • Easy height adjustment with strong lock
  • Mainstream brand that fits manufactured sled holes
  • Dust collection shroud to capture planer-like volumes of dust
  • At least 2-1/4 HP of power
  • Adjustable speed controls

Budget Router Sled Router

Dewalt Router for Router Sled

Shown: Dewalt DW618

For DIY router sleds where the router will be used for multiple jobs the DW618 is a popular choice for use both on and off the sled.

Key features:

  • 2-1/4 HP
  • Soft start
  • 8,000 – 24,000 RPM adjustment
  • Electronic speed control for constant speed under load
  • Micro-adjustment with simple collar adjustment ring
  • Motor is removable and compatible with router tables

Two-in-One Router for Planing Sled - Bosch

Bosch 1617EVSPK Combination Kit

For both DIY sleds and manufactured sleds a router that works on and off the sled is usually a priority. And in routers, Bosch has defined itself as the market leader with the 1617EVS line-up.

Key features of the 1617EVSPK:

  • 2-1/4 horsepower
  • 1/4 and 1/2-inch collets
  • Electronic variable speed control
  • Soft start
  • Micro-adjustment
  • Plunge and fixed bases
  • Edge guide
  • Dust collection chute
  • Circle guide

Related: Best Routers for Router Tables

Premium Router Sled Router - Festool

Festool 574692 1400 Watt

At the top end of most tools line-ups sits Festool. And, for router sleds these high quality machines offer a few advantages for both on and off sled performance:

  • Top power (measured in watts, not horsepower)
  • Speed control
  • Integrated dust collection
  • Chip deflector
  • 1/256-inch micro depth adjustment
  • Fast bit changing

Router Sled Spoil Board Bits

Router Sled Spoil Board Bit

Finish quality is always decided by where the carbide meets the wood. Which means no matter the quality of your jig you’ll want to invest in a high quality spoil board bit (taken from CNC terminology) that will cut in hardwoods and retain a sharp edge.

Key features to look for:

  • 3-inch cutting diameter to reduce passes
  • High quality carbide
  • Replaceable carbide inserts
  • 1/2″ shank

Populat bits include the budget BINSTAK Carbide Bit or insert style BINSTAK Insert Bit.

Finishing the Job - Sanding After Routing

Random Orbit sander on Wood Slab

While a drum sander is a great next step for finishing a section of wood flattened by a router sled, most woodworkers won’t have this sander or the piece is simply too heavy.

And, unfortunately, a drum sander can cause more damage to large pieces than good.

The solution?  A high quality random orbital sander with a low grit sand paper (24 grit if available) to start and then work your way up. 

Related: Best Orbital Sanders For Woodworking

Alternatives to a Router Sled

Best Drum Sander Guide

You may already own one, or have it on your wish list, but a drum sander is one of the best alternatives to a router sled. Since this sander doesn’t “rip” wood its great for end-grain projects.

Key benefits:

  • Open ended drum sanders can sand up to 50-inch wide wood projects
  • Aggressive 24-grit sandpaper can remove wood fast
  • Unlike a planer, a drum sander will not rip wood grain – making it a great choice for end-grain or live edge projects

The downside? Most drum sanders cost upwards of $1000 or more. 

And planers, unfortunately, are limited in width.

Read more: Best Drum Sanders for Woodworking

Frequently Asked Questions

Frequently Asked Questions

What is a router sled used for?

Router sleds are used for precise flattening of large pieces of wood or on end-grain wood that cannot be passed through a planer. While router sleds can be used in place of a planer for small jobs, they are most commonly used for live edge projects. Key benefits include moving the machine over the wood which is useful for extremely heavy wood pieces like thick tables.

How do you make a homemade router sled?

There are both simple and complex setups for diy router sleds. The simplest designs include a basic set of wood rails and (usually) a plywood sled that a router is attached to. More complex router sleds can be made with linear rails that offer smooth action both left and right as well as (when used in combination) back and forth across the wood.

Who makes router sled jigs?

Both Woodpeckers and Woodhaven make router sled jigs that can be purchased in various lengths and widths. These jigs vary greatly in price, but from $300 to over $1000 offer a ready-made solution with quality sleds to attach your router to.

How are router bit marks removed from a finished surface?

After the router sled is complete the job of hand sanding is started. Most woodworkers will use a low grit sandpaper in a high quality orbital sander, progress to finer grits and then finish sanding with a palm sander.

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