When making a wooden box, picture frame, or any project with mitered corners, adding splines is a great way to strengthen the joint and make the corner look much nicer. The spline reinforces the miter, adds more glue surface, and also creates a small decorative detail that can really stand out.
There are several ways to cut miter splines. You can do it with hand tools, on a table saw with a spline jig, or on a router table. For this build, I made a router table spline jig that works as an add-on to my benchtop router table.
The jig slides over the router table using wooden guide rails and holds the workpiece at 45 degrees while the router bit cuts the spline groove. This makes the cuts repeatable, cleaner, and much easier to control compared to trying to hold the workpiece by hand.
This build is made from MDF, Baltic birch plywood, and a few simple hardware pieces. It is not a complicated jig, but accuracy matters — especially when setting the sliding base and aligning the 45-degree supports.
In this article, I’ll show you how to make a router table spline jig step by step, how to set it up, and how to use it to cut clean miter splines. I also have downloadable plans available if you want the dimensions and build details in a cleaner format while working in the shop.
This jig is part of my small router table system, together with the benchtop router table, router table tenon jig, and router table box joint jig.
DIY Router Table Spline Jig Video
If you want to see the full build process, watch the video below. I show how I make the sliding base, build the 45-degree supports, add the stoppers, and test the jig by cutting spline grooves on the router table.
If you enjoy the video, I’d really appreciate it if you subscribed to my YouTube channel. It helps me keep creating more woodworking jigs, router table add-ons, and step-by-step build guides.
Table of Contents
- Router Table Spline Jig Plans
- Materials and Tools
- Quick Build Overview
- Before You Start
- What is a spline in woodworking?
- What are the different ways of cutting a miter spline?
- What router bit for cutting spline grooves?
- How to Make a Router Table Spline Jig
- Â Step 1: Build the Sliding Base
- Step 2: Build the 45-Degree Spline Cradle
- Step 3: Attach the Cradle to the Sliding Base
- Step 4: Cut the Kerf in the Base
- Step 5: Build the Adjustable Stoppers
- How to Set up and Use a Spline Jig for a Router Table
- My Take
- FAQ
- Woodworking Jigs used for the project
*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!
Router Table Spline Jig Plans
I also offer a premium version of the Router Table Spline Jig plans, which includes:
✔ Metric and imperial measurements
✔ Detailed materials and hardware lists
✔ Exploded views and assembly drawings
✔ Additional build information and workshop tips
✔ Improvements and other resources
Browse all my premium woodworking plans.
Materials and Tools
Before starting the build, prepare the materials and tools for the router table spline jig. This jig is mainly made from thin MDF, Baltic birch plywood, spruce guide strips, and a few bolts and wing nuts for the adjustable stoppers.
I built this version as an add-on for my benchtop router table, so measure your own router table first and adjust the sliding base if needed.
Router Spline Jig Base
- MDF baseboard – 470 mm x 160 mm x 5 mm
- Spruce wood strips – 2x (220 mm x 28 mm x 18 mm)
- Beechwood Strip – 260 mm x 7 mm x 6 mm
Spline Jig
- MDF – 2x (300 mm x 120 mm x 8 mm)
- Plywood – 250 mm x 35 mm x 6 mm
- Plywood triangles – 2x (60 mm x 60 mm x 12 mm)
- Plywood triangles – 2x (35 mm x 35 mm x 12 mm)
- DIY T-track bolts
Spline Jig Stoppers
- Plywood – 4x (40 mm x 100 mm x 12 mm) – https://amzn.to/3TPULp2
- Plywood – 2x (28 mm x 40 mm x 12 mm)
Tools:
- Trim Router – https://amzn.to/3HbQcOg
- Table Saw / Proxxon Mini table – https://amzn.to/47mWiGf
- Ryoba Japanese Saw – https://amzn.to/48if4Qz
- Cordless Hand Drill – https://amzn.to/3HqLEnH
- Corded Hand Drill / Drill Press – https://amzn.to/3NSA3Bo
Machinist Square / Speed Square - One Hand Clamps – https://amzn.to/48jGxkZ
- Spring Clamps – https://amzn.to/41Nuh9D
- F-Clamps – https://amzn.to/48ArzY1
Other:
- Wood Glue – https://amzn.to/48ltKhJ
- Forstner bits – https://amzn.to/3Hd1aDb
- Drill Bits – https://amzn.to/48Kaaf7
- Straight router bit – https://amzn.to/47prNQi
- Spiral bit – https://amzn.to/48pJJv9
- M6 Bolts, Washers, Wing nuts, Wood Screws – https://amzn.to/41PodgQ
Check all the Tools I Use
Disclosure: Some of the links above are affiliate links. If you purchase through them, I may earn a small commission at no extra cost to you. Thanks for supporting AllFlavor Workshop.
📌 Save THIS PIN to your Board on Pinterest!
Quick Build Overview
Here is a quick overview of the router table spline jig before we get into the step-by-step build. The jig is simple in principle, but accuracy matters because it needs to slide smoothly on the router table and hold the workpiece at the correct angle.
| Item | Details |
|---|---|
| Project | Router table spline jig |
| Main use | Cutting miter spline grooves in boxes, frames, and other mitered projects |
| Router table type | Built as an add-on for a benchtop router table |
| Finished size | About 470 × 220 × 110 mm |
| Main material | MDF, Baltic birch plywood, spruce guide strips |
| Router bit | Designed for small straight or spiral bits; this build uses a 6 mm bit |
| Difficulty | Beginner to intermediate |
| Best for | Mitered boxes, picture frames, decorative corner splines, and repeatable spline cuts |
Before You Start: Splines, Router Bits, and Miter Joints
Before building the jig, it helps to understand what a spline does and why this jig is useful. A router table spline jig is made for cutting clean, repeatable grooves across mitered corners, usually on boxes, frames, and similar projects.
What Is a Spline in Woodworking?
A spline is a thin strip of wood that is glued into a matching slot or groove. In mitered corners, the spline adds extra glue surface and helps reinforce the joint.
Splines are not only functional — they can also be decorative. If you use a contrasting wood, the spline becomes a nice visual detail and makes the corner stand out.
What Are the Different Ways to Cut Miter Splines?
There are several ways to cut miter splines. You can cut them by hand using a saw and chisel, use a table saw with a spline jig, or cut them on a router table with a router table spline jig.
The router table method works well because the jig holds the workpiece at 45 degrees and slides over the router bit. This makes the cuts more repeatable and easier to control, especially when you want all splines to line up the same way.
What Router Bit Should I Use for Spline Grooves?
For cutting spline grooves on a router table, a straight router bit or a spiral bit works well. I used a 6 mm router bit for this build.
A straight bit is simple, affordable, and works fine for most spline grooves. A spiral bit usually gives a cleaner cut and better chip removal, but it is more expensive.
If you are just starting out, a straight router bit is a good option. You can also use painter’s tape on the workpiece to help reduce tear-out and keep the cut cleaner.
Let’s Start Building
The router table spline jig consists of two main parts: the sliding base and the 45-degree spline jig body.
The sliding base fits over the benchtop router table and lets the jig move forward and backward across the router bit. The spline jig body sits on top of the base and holds the workpiece at 45 degrees while cutting the spline groove.
I built this jig from MDF, Baltic birch plywood, and spruce guide strips. The finished size is about 470 × 220 × 110 mm, but the exact dimensions depend on your router table. Before you start cutting parts, measure your own router table and adjust the sliding base so it fits snugly.
The jig should slide smoothly, but it should not wobble. That fit is important because it affects how clean and repeatable the spline cuts will be.
This version is made for my clamp-on benchtop router table and a 6 mm router bit, but the same idea can be adapted to other router tables as well.
How to Make a Router Table Spline Jig
Step 1: Build the Sliding Base
Start by measuring your router table first. My benchtop router table is 400 × 300 mm, so I made the sliding base slightly longer to leave a small overhang on both sides.
Cut the MDF baseboard to size. For my jig, the base is 470 × 160 × 5 mm. Then cut two small cutouts on the sides of the base. These allow the jig to slide farther forward over the router bit and give you a little more usable cutting travel.
Next, cut two spruce guide strips and glue them to the underside of the MDF base. These strips wrap around the sides of the router table and keep the jig aligned while it slides forward and backward.
The easiest way to position them is to use the router table itself as a reference. Turn the router table upside down, align the MDF base on top, apply glue to the guide strips, and clamp everything in place. Once the glue dries, secure the strips with screws.
Finally, drill a 20 mm hole in the center of the base, aligned with the router bit. This opening gives the bit room to pass through when cutting the spline grooves.
💡TIP: The sliding base should move smoothly, but it should not wobble. If it feels too tight, sand the inside of the guide strips lightly. If it has a little play, add painter’s tape to the strips until the fit improves.
NOTE: Measure your own router table before cutting the base. The dimensions in this build are made for my benchtop router table, but the same idea can be adjusted to fit other router tables.
Step 2: Build the 45-Degree Spline Cradle
Now it’s time to build the part of the jig that holds the workpiece at 45 degrees. I like to think of this as the spline cradle — two angled supports that form a V-shape and keep the box or frame corner steady while cutting the spline groove.
Start by cutting two MDF boards to size. These will become the front and back parts of the spline jig. On each piece, cut one long edge at a 45-degree angle. When both parts are mounted on the sliding base, these angled edges will face each other and support the mitered corner.
The back part of the jig also gets a small T-track slot. This is useful for adding adjustable stoppers later, so each spline cut can be made in the same position. I cut the slot with my router dado jig, but you can use any accurate method that gives you a straight, clean slot.
To keep the T-track bolt from falling out, add a thin plywood strip over the slot. Cut a shallow rabbet on one side of the strip, remove the center section, and glue it to the MDF board. The opening makes it easier to insert and remove the bolt when needed.
Next, cut the plywood support triangles and glue them above the 45-degree edges. These triangles strengthen the cradle and help keep the workpiece supported during the cut.
Repeat the same basic process for the front part of the jig, but without the T-track. Once both parts are done, you should have two matching cradle pieces ready to mount on the sliding base.
💡TIP: Make sure the 45-degree edges are clean and accurate. These surfaces hold the workpiece during the cut, so any error here can affect the spline position.
NOTE: Do not place the support triangles in the router bit path. Leave enough clearance in the center so the bit can pass through safely when cutting the kerf later.
Find This Blog Post Useful?
Join my newsletter to receive the latest news, tutorials, and project plans sent directly to your inbox!
Step 3: Attach the Cradle to the Sliding Base
Now that the front and back cradle pieces are ready, glue them to the sliding base. This is one of the most important parts of the build because the cradle needs to be aligned correctly with the router bit.
Place both 45-degree supports over the center hole in the sliding base. The angled faces should point toward each other and form a V-shape that will hold the mitered corner during the cut.
Before gluing, make sure that:
- both cradle pieces are centered over the router bit opening
- the angled faces line up evenly
- the pieces are square to the sliding base
- there is enough clearance for the router bit path
I like to draw a reference line on the base first and use a square to keep everything aligned. Once the position looks right, apply glue to the bottom of the cradle pieces and clamp them in place.
Let the glue dry fully before moving to the next step. The alignment here affects how clean and centered the spline grooves will be, so take your time.
NOTE: The cradle does not need to be complicated, but it needs to be aligned well. If the front and back supports are slightly twisted or uneven, the workpiece may not sit correctly during the cut.
Step 4: Cut the Kerf in the Base
Once the glue is fully dry, mount the spline jig on the router table and cut the first kerf in the base. This kerf shows the exact router bit path and becomes your visual reference when positioning the workpiece later.
I used a 6 mm router bit for this jig. Raise the bit slowly and make the cut by sliding the jig forward over the router table. Don’t rush this step — the jig should move smoothly and stay firmly guided by the wooden strips underneath.
After cutting the kerf, you can also mark both sides of the cut with a pencil or a thin marker. This makes the cutting line easier to see when setting up spline cuts, especially when the workpiece covers part of the base.
At this point, the jig already shows where the spline groove will be cut. The next step is to add adjustable stoppers, which will help you make repeated spline cuts in the same position.
💡TIP: Make the kerf slowly and keep steady pressure on the jig while sliding it over the router bit. A smooth first cut gives you a clean reference line for later setup.
NOTE: Use the same router bit size you plan to use for cutting the spline grooves. If you change the bit size later, the kerf reference will no longer match exactly.
Step 5: Build the Adjustable Stoppers
The last part of the jig is the adjustable stoppers. These are simple plywood blocks that slide in the cradle (attached through the T-track) and help you cut each spline groove in the same position.
Cut the plywood pieces for the stoppers and glue them together. I used two layers so the stopper has enough surface to register against the workpiece and stay stable during setup.
Once the glue dries, drill a 6 mm hole through each stopper for the T-track bolt. The bolt passes through the stopper and locks it in place with a washer and wing nut.
The stoppers are especially useful when cutting splines on multiple corners. Set the position once, tighten the wing nuts, and then repeat the same cut on each corner without measuring again.
💡TIP: Make the stopper faces square and smooth. The workpiece will register against them, so a clean edge helps keep the spline position repeatable.
NOTE: The stoppers are adjustable, so you can change the spline position depending on the size of the box, frame, or mitered project you are working on.
Below is a picture of the finished router spline jig.
How to Set Up and Use a Router Table Spline Jig
Once the jig is finished, setting it up is quite simple. The main idea is to mount the jig on the router table, set the stoppers, hold the workpiece securely, and slide the jig over the router bit to cut the spline groove.
Use all necessary safety equipment, especially safety glasses. Chips can fly out quickly when cutting splines on the router table.
- Mount the spline jig on the router table
Place the jig over the router table and make sure the sliding base moves smoothly without wobbling. - Choose the router bit
Install the router bit you want to use for the spline groove. I used a 6 mm bit for this jig. - Apply painter’s tape to the workpiece
Add painter’s tape where the router bit will exit the cut. This helps reduce tear-out and keeps the spline groove cleaner. - Place the workpiece in the jig
Set the mitered corner into the 45-degree cradle. The corner should sit firmly in the V-shape and stay flat against the supports. - Set the stoppers
Slide the adjustable stoppers into position and tighten the wing nuts. This lets you repeat the same spline position on every corner. - Secure the workpiece
Use a hold-down clamp to keep the piece from shifting during the cut. Do not rely on your hands alone, especially with smaller boxes or frames. - Make the cut slowly
Slide the jig forward over the router bit in a controlled motion. Take your time and keep steady pressure on the jig. - Repeat the cuts
Rotate the workpiece and repeat the process on the remaining corners. Since the stoppers are already set, each spline groove should line up in the same position
TIP: Painter’s tape makes a big difference when cutting across mitered corners. Place it directly over the exit side of the cut to reduce blowout. Always keep the workpiece clamped or held down securely. Small frames and boxes can shift easily, and your hands should stay away from the router bit path.
My Take
I really like this router table spline jig because it is a simple add-on that makes the benchtop router table much more useful.
If you make boxes, picture frames, or other mitered projects, splines are a great way to strengthen the corners and add a nice decorative detail at the same time. With this jig, the process becomes much more repeatable — set the stoppers, place the workpiece in the cradle, and make the same cut on each corner.
What I also like is that the jig does not take much space. It slides over the router table when I need it, and when I’m done, I can store it flat with the rest of my router table accessories.
The most important part of the build is accuracy. The sliding base should fit the router table without wobbling, and the 45-degree cradle should be aligned properly. Once those parts are right, the jig is easy to set up and use.
This spline jig is part of my small router table system, together with the benchtop router table, router table tenon jig, and router table box joint jig. If you are building a similar setup, this is a nice add-on to have in the workshop.
If you want to build this jig, you can follow the steps in this article and check my woodworking plans page for current and upcoming plans. I’m also preparing more detailed paid plans for my router table add-ons, so the build will be easier to follow directly from the workshop.
📌 Save THIS PIN to your Board on Pinterest!
FAQ
What is a router table spline jig used for?
A router table spline jig is used to cut spline grooves across mitered corners. It is useful for boxes, picture frames, and other mitered projects where you want to strengthen the joint and add a decorative detail.
Can you cut miter splines on a router table?
Yes, you can cut miter splines on a router table with a spline jig. The jig holds the workpiece at 45 degrees and slides over the router bit, which makes the cuts repeatable and easier to control.
What router bit should I use for cutting spline grooves?
A straight router bit or spiral router bit works well for cutting spline grooves. I used a 6 mm bit for this jig. A straight bit is affordable and works fine, while a spiral bit usually gives a cleaner cut with less tear-out.
Are splines only decorative?
No. Splines are both decorative and functional. They reinforce the mitered corner by adding more glue surface, and if you use contrasting wood, they also create a nice visual detail.
Is this router table spline jig beginner-friendly?
Yes, but accuracy matters. The build is simple in principle, but the sliding base should fit the router table without wobbling, and the 45-degree cradle should be aligned well. Once built correctly, the jig is easy to set up and use.
