Simple Router Slab Flattening Jig (Step-by-Step with Pictures)

Learn how to effortlessly flatten slabs with a slab flattening jig. Discover the step-by-step process of creating and using this DIY router sled, ensuring perfect results for woodworking projects.

If you don’t have access to a fancy planer or a CNC machine, you can still achieve a smooth surface on slabs, cutting boards, serving trays, coasters, and more with a simple jig called a slab flattening jig (also known as a slab flattening mill or a router sled). Plus, this simple router jig is a lifesaver for larger pieces of wood that won’t fit into a small benchtop thickness planer or drum sander.

Building your own DIY router sled is easy and affordable. You won’t need a bunch of materials, and the great thing is that it can be taken apart and stored anywhere in your workshop. Plus, you have the freedom to customize the size of the jig to fit anything from small cutting boards to big wooden slabs for table tops.

In this article, I’ll walk you through the step-by-step process of creating your own router sled for flattening slabs and cutting boards. As for the material, I’ll be using plywood and MDF. Let’s get started and turn those uneven surfaces into smooth perfection! 😉

DIY Router Sled for Flattening Slabs

If you want to see how it is done, watch the video below for a step-by-step guide on how to make a simple router sled.

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Table of Contents

  1. DIY Flattening Jig Video
  2. The Material you will need
  3. General Questions
    1. Benefits of a Router Sled
    2. Slab Flattening Router Bit
  4. How to Build a Router Slab Flattening Jig (Step-by-Step)
    1. Step 1: Make the Base
    2. Step 2: Make the Side Rails
    3. Step 3: Attach the Rails to the Base
    4. Step 4: Build the Router Carriage
  5. How to Use a Router Sled for Flattening Surfaces
  6. Conclusion
  7. Woodworking Jigs I used to build it

*Safety is your responsibility. Make sure you know what you’re doing and take all necessary safety precautions while working with power tools. Safety comes first!

The material you will need to build it

Router Sled
Sheet of plywood for the rails (stands) – 12 mm, 9 mm –
MDF/HDF Base Board – 760 mm x 515 mm x 16 mm (29.92″ x 20.28″ x 0.63″)
HDF Strips for T-slots
Aluminum Square Tube (20 x 20mm) –

Slab Flattening Router Bit –
Forstner Bits –
Wood Glue –
Sandpaper Sheets –
Sanding Discs –
M6 Bolts, Washers, T-Nuts, Wood Screws –

DIY Tools:
DIY Multi-Purpose Router Base
DIY T-track Bolts
DIY T-track Clamps

Trim Router –
Circular Saw –
Cordless Hand Drill –
Table Saw –
Orbital Sander –
Guide Rail Clamps –
One Hand Clamps –
F-Clamps –
Hold Down Clamps –
Narex Chisels –
Tape Measure –

Check all the Tools I Use

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Benefits of a Router Sled

These are the advantages of making a DIY router sled and using it for your woodworking projects:

  1. Cost-Effective Solution: By building your own DIY router sled, you save money compared to purchasing an expensive commercial alternative, making it a cost-effective solution for achieving flat surfaces.
  2. Minimal Materials Required: To build the sled, you don’t need an extensive list of materials. It can be constructed using just a few components – a sheet of MDF (HDF) for the base plate and plywood for the side rails.
  3. Portable and Space-Saving: The detachable nature of a router sled enables easy disassembly, allowing you to store it conveniently in any corner of your workshop, saving valuable space.
  4. Customizable Size: One of the significant advantages of a router sled is the ability to accommodate workpieces of different sizes. You can flatten anything from small cutting boards to large wooden slabs.

Slab Flattening Router Bit

When it comes to flattening surfaces with a router sled, there are specific router bits designed for this purpose. These are the two commonly used types:

  1. Double Flute Straight Router Bit:
    • Price Range: Typically affordable, ranging from $20 to $40.
    • Benefits: The double flute design helps in efficient chip evacuation and provides smoother cuts. It is versatile and suitable for various woodworking applications. In addition to surfacing, it can be used for cutting mortises, dados, and rabbets.
    • Use: This router bit is ideal for initial rough leveling and removing material quickly. It can handle both softwoods and hardwoods effectively.
  2. Spoilboard Surfacing Router Bit:
    • Price Range: Relatively more expensive, ranging from $60 to $100.
    • Benefits: The spoilboard surfacing bit is specifically designed for wood surface flattening tasks. It has a large cutting diameter and multiple carbide inserts, ensuring longer tool life and improved surface finish.
    • Use: This specialized router bit is primarily used for final surfacing, providing a smooth and flat finish on large wood slabs and tabletops. It is ideal for achieving precise and professional results.

When selecting a bit for surfacing, it is important to consider the bit’s diameter, cutting length, and shank size.

Let's start building!

Making a router sled involves making the base (including T-slots), the side rails, and the carriage for the router. 

The sled features a modular design – you can easily assemble a disassemble it as needed. This build is reusing my multi-purpose router base. It is easy to set up and can be easily adjusted as needed. Also, it can be used with different types of router brands.

Router Sled properties:

  • Material: Baltic birch plywood, MDF/HDF
  • Total dimensions: 760mm x 515mm x 80mm (29.92″ x 20.28″ x 3.15″)
  • Clamping and attachment: Homemade and Shop made T-track clamps

NOTE:  When it comes to smoothing out wood, using a plunge router is preferable as it grants superior hold, steadiness, and handling. 

How to Build a Router Slab Flattening Jig (Step-by-Step)

Step 1: Make the Base

First, cut the base to the desired dimensions. For the base, I am using a 16mm MDF (HDF) panel. MDF is a robust material, that does not warp or bend easily and it boasts a smooth and even surface.

You can cut the board on a table saw, but if you don’t have one you can use a circular saw with a straight guide. 

Router Sled Cutting Base

Then, draw the T-tracks on the base and cut them out using a trim router. You can use a long piece of lumber or aluminum as a straight edge or a router slot cutting jig.

NOTE: I am not trimming the slots right to the end of the board. Instead, I am halting my cuts just before the edges and creating wider gaps to allow for easier access from the top. 

Router Sled Draw T-tracks
Router Sled Cut T-tracks

Sand the tracks and square the ends with a chisel. Cut thin HDF pieces and glue them to the outer slots. Use just enough glue to secure the strips in place and make sure it does not get in the central track (It would be quite difficult to get rid of the wood afterward).

Using just glue wouldn’t be enough so add a few screws to reinforce the attachment of the strips. Make sure the screws are flat with the work surface.

Router Sled Glue T-tracks
Router Sled Attach T-tracks
Slab Flattening Mill Finished T-tracks

Step 2: Make the Side Rails

Cut four 45mm wide plywood (12mm) strips and glue them together two at a time. Additionally, use screws to reinforce the attachment.

Slab Flattening Mill Side Rails

Then cut two additional 55mm wide strips of plywood (9mm). They serve as the base for the rails and are used to attach the rails to the base board.

Drill 2 6mm holes on each side of the strips and then glue them together with the side rails (rotate the rails vertically and attach them to the base plywood strips).

TIP:  It is better to drill the holes in the base plywood strips before gluing the rails. Align the strips with the edges of the base board and drill through to make sure the attachment holes match. 

Slab Flattening Mill Cutting Side Rails
Slab Flattening Mill Drill holes

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Step 3: Attach the Rails to the Base

Turn the MDF base upside down and use a Forstner bit to enlarge the attachment holes. Make shallows cuts just enough to flush a T-nut.

Then insert 8 T-nuts and use M6 screws to attach the rails to the base. 

Check the height of each rail. If they are not level use shims of paper to make them even.

Slab Flattening Mill T-nuts
Router Sled Attaching rails to base

Step 4: Build the Router Carriage

When using the sled, the router moves across the workpiece using a carriage. There are a few ways how to build it. You can either make a container and place the router inside (the router plate fits in perfectly), or you can make a construction of two rails that allow the router to slide back and forth. I am making the second type which allows me to use my custom router base.

Cut two aluminum square bars to size. The length of the bars must surpass the width of the sled. Then drill a 6mm hole on each side of the bar.

Make the holes slightly wider for better maneuverability.

DIY Router Sled Cutting aluminum bar for carriage
DIY Router Sled Aluminum bar with holes

Cut two pieces of scrap plywood. These pieces serve as runners that move along the side stands.

Decide on the distance between the aluminum bars (mine is 50mm). Accordingly, drill 2 holes in the runners and insert T-nuts.

NOTE:  The distance between the bars should be wide enough to well support the router and provide enough room for any surfacing router bit to fit.

DIY Router Sled Drilling holes in runners
DIY Router Sled T-nuts in runners

Use the pre-drilled holes to attach the bars to the runners with bolts. Make sure the bars are perpendicular to the sled.

Tighten the screws once you have found the right position and add small stop blocks if you needed to disassemble the sled again.

NOTE: The carriage should wrap around the sled tightly without any wiggle room. However, it should still allow for smooth and effortless movement.

Assembling runners
Adding Stop Blocks

And this is what the finished slab flattening sled looks like.

Finished Slab Flattening Jig
Router Sled ready for use

How to Use a Router Sled for Flattening Surfaces

These steps provide a more detailed guide on how to set up a router sled and how to effectively use it for flattening and surfacing work pieces.

  1. Attach the trim router securely to the router sled, ensuring a stable connection without any wobbling.
  2. Place the workpiece onto the sled and use clamps and stop blocks to firmly secure it in place, preventing any unwanted movement.
  3. Lower the router to the desired depth, taking care to only remove a small amount of wood with each pass. This helps avoid excessive pressure on the router bit and minimizes the risk of burn marks.
  4. Move the router back and forth across the surface, gradually removing thin layers of wood at a time. Take your time to maintain control and ensure even and consistent cuts.
  5. Once you’ve finished flattening one side, rotate the workpiece upside down, secure it again, and repeat the process for the other side.
  6. To achieve a perfectly smooth and straight surface, use a sander with progressively finer grit sandpaper. Start with a lower grit and gradually work your way up to higher grits to eliminate cutting marks and achieve the desired finish.

TIP: For better control and precision, it’s advisable to use a router that offers variable speed capabilities. This feature allows you to adjust the router’s speed according to the cutting depth and the type of material, giving you enhanced control over the woodworking process.

How to use a slab flattening jig
Surfacing a slab
DIY Router Sled
Surfacing a cutting board
How to make a Router Sled Flattening Jig
Surfacing a wood cookie


In conclusion, if you want to achieve flat and smooth surfaces on your work pieces and you can’t afford a CNC machine or a large planer a DIY router sled is a great solution for any woodworker.

The great thing is that you can build a sled of any size you want. That way you will be able to flatten anything from small serving stays to larger slabs.

There are two main reasons why to build a router sled. Firstly, it will help you out with some of the main issues and defects of wood slabs (twisting, warping) and secondly, you will find it very useful when making cutting boards, or table tops of large atypical shapes.

This build does not feature a dust collection port which is something definitely worth adding. Even a small project can generate a significant amount of debris and shavings, so be sure to wear a dust mask and appropriate eye protection.

How to Build a Slab Flattening Router Jig

Woodworking Jigs I used to build it:

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About the author, Lukas
About the author, Lukas

Meet the creator of AllFlavor Workshop! As a passionate DIYer and woodworking enthusiast, Lukas is always looking for ways to make things himself rather than buying them off the shelf. With a keen eye for design and a knack for working with wood, Lukas enjoys sharing his craft with others and helping them discover the joy of building. Whether you're an experienced woodworker or a novice looking to try your hand at a new hobby, you're sure to find plenty of inspiration and tips on AllFlavor Workshop.