A miter joint is one of the weakest joints in woodworking joinery.
Yet, it is often used, such as in picture frames.
To fix this, these weak connections can be made considerably stronger by using splines.
If you want to apply these splines in the corners of the miter joint, a spline jig will be a great help.
By using a spline jig, you will be sure that each slot can be made at the same distance and depth.
In this article, you can discover how to make a spline jig.
On top of that, there are free spline jig plans available that will help you build this ultimate spline jig for your workshop.
What makes the spline jig in this article so unique, is that it fits perfectly into the T-track rails of my crosscut sled.
As a result, this crosscut sled spline jig can be clamped at any angle, allowing a creative spline to be made.
This jig is adjustable and can be used to make splines in picture frames, as well as in wooden boxes.
I’ll show you how to make this jig!
P.S.: You can make the same crosscut sled as mine.
Keep reading, I will tell you more about it, later in this article.
What are picture frame splines?
A picture frame spline is a thin slice of wood that is placed in a slot on the corner of a miter joint.
Thanks to that spline, the strength of the miter joint increases.
A frame spline can be made from the same wood as the frame, or other woods can be used to create contrasting effects.
What is a spline jig?
A spline jig is an attachment that can be used on a table saw.
This spline jig allows making repeatedly and accurate slots in the corners of picture frames.
Later on, splines can be fitted in these slots.
By using a spline jig, the frame can be held at a 45-degree angle through the angled fences, allowing the blade to make a slot perpendicular to the joint.
By setting the spline jig once, this jig ensures that the slots can be made repeatedly in the same place and with the same depth.
How thick should a spline be?
The thickness of the spline is always the same width as the notch made in the corner of the miter joint.
An easy way to determine the thickness of the splines is to make them the same thickness as the thickness of your blade on the table saw.
Like that, when you make a slot with the saw blade, your spline will always fit perfectly.
You can also make thicker splines, but then you will have to make several saw cuts to make these splines fit.
This increases the chance that the cut will not be made quite correctly, resulting in a spline that is too loose or does not go into the slot at all.
Are spline joints strong?
Yes, spline joints are stronger than regular miter joints.
By increasing the adhesive surface by means of a spline, it adds extra strength to a miter joint, which is otherwise a fairly weak joint.
Because the spline will be placed perpendicular to the connection, the breaking point of the connection will be interrupted, and therefore stronger.
Watch the video & learn how to make a spline jig.
Here you can watch the video and see how to make a spline jig.
After watching the video, you can continue reading the step-by-step guide to make this spline jig yourself.
Don’t forget to download the free spline jig plans on this page, they will surely help you to build this jig.
What do you need to make a spline jig?
To prepare you for building this spline jig, I have made a few handy lists.
In addition to a few personal protective equipment that I use*, I also list the materials as well as the (power) tools you will need to build this spline jig.
* Safety is always your own responsibility!
Safety first! protect yourself!
Materials I used for this spline jig
(power)Tools I used for this spline jig
- Makita MLT100 table saw Watch my Unboxing video and my review video if you want to see more about this tool.
- Makita 3709 router
- Makita AF505N Brad nailer. Watch my Unboxing video if you want to know more about this tool.
- Brad nails 25 mm
- Air compressor
- Drill press
- Hilti cordless drill SF144-A
- Festool sander
- Kreg KMA multi mark and measuring tool
- Pica pencil to mark all the pieces.
How to make a spline jig?
Briefly explained, you make a spline jig by making a kind of gutter, where the picture frame fits in, and placing it at an angle of 45 degrees on a bottom plate.
What makes this spline jig unique is that this bottom plate can be locked into the t tracks of my crosscut sled at any angle I want.
I’ll show you step by step how to build it!
Be sure to watch my blog and video how I made my crosscut sled: The perfect crosscut sled? Accurate | removable zero clearance insert.
There are free plans available to make this awesome crosscut sled yourself.
Step 1 | Preparation
To start making this spline jig, I cut all the parts to size with my table saw and crosscut sled.
The free spline jig plans that you can download on this page contain all the dimensions you need to make the parts.
Don’t forget to download them after reading these step-by-step instructions.
Step 2 | Making the bottom plate
There are 2 slots in the base of the spline jig.
These serve to attach the spline jig to the crosscut sled, but also to place the jig in the angle you want.
To make these slots, I used my palm router with a 10 mm straight router bit.
That way, I was able to make slots that are 2 mm wider than the M8 bolts I use to connect the jig to the crosscut sled.
Making the slots slightly larger will allow the bolts to slide easily through the slots, which will make setting the jig easier.
To make sure the slots were straight, I clamped my speed square to the workpiece and used it as a fence.
When you make these slots, make them in several steps, each time with the router bit set a little deeper.
A slot had to be made on both sides of the bottom plate, and when I was done with that, I could test the bottom plate on my crosscut sled for the first time.
Step 3 | making “the gutter”
Now that the bottom plate was ready, I could start on the part in which the picture frame can be clamped.
I call this “the gutter”, so if you read about the gutter later in this article, you’ll know what I mean by this.
First I made the back of the gutter, where the picture frame leans against, and where the spacers can be clamped.
The gutter has a zero clearance insert that can be easily replaced if it is totally eating away by the saw blade.
For this zero clearance insert, I’m using 6 mm MDF.
That’s why I made a wide slot in this part, into which this piece of MDF fits perfectly later.
I made that wide groove by making several 6 mm deep incisions with the saw blade of the table saw.
By placing the part on my crosscut sled and sliding it over the saw blade, I was able to make this slot quickly and accurately.
In this same part, I also made a slot where the T-track rail could be mounted.
Because this slot had to be made on the side, I was able to make it quickly by sliding this part along my table saw fence.
To do this, I once slide the piece flat over the table saw and once by sliding it vertically against the fence.
By adjusting the depth of the saw blade to the dimensions of the t-track rail, I got a slot where the rail fit perfectly.
As a result, the front edge of the rail was flush with the surface of the wood.
Once the T-track was mounted, I could install the small strip of wood perpendicular to this larger back piece of the gutter.
To make this connection, I used wood glue and brad nails.
The brad nails serve as a temporary clamp while the wood glue dries. That gives me the opportunity to keep working on my projects and not have to wait for the glue to dry.
Don’t just use any wood glue for your projects!
To find out which different types exist and for which applications they serve, check out my blog Understanding Wood Glue + 8 awesome tricks you should know. Here you will discover everything you need to know about wood glue.
Now that the gutter was ready, I was able to make the triangular parts that hold the gutter to the bottom plate at a 45-degree angle.
To make the triangles, I placed my speed square against the fence of the crosscut sled.
By placing the wood against the bevel of the speed square, I was able to cut triangles that are perfectly 45 degrees.
For this spline jig, you will need 4 of these triangles.
Note that 2 of these triangles must be smaller, but all the details for this can be seen on the free spline jig plans.
After making these triangles, I already attached the two largest ones to the back of the spline jig.
Also, here I used wood glue and brad nails for the connection.
I missed a little thing here. Luckily, I noticed it in time.
When I placed the MDF insert into the slot, it was very difficult to get it out again.
Since I had not yet mounted the gutter to the baseplate of the spline jig, I was able to drill a hole in it with my drill press.
I made this hole with a 20 mm Forstner drill.
This hole allows me to later push the MDF insert out of the slot by pushing a finger from the back through the hole against the back of the MDF.
The MDF insert could now be placed at the back and secured with two small screws.
As a result, the gutter was completely ready and could be attached to the bottom plate.
I also used wood glue and brad nails to do this.
After connecting those two parts, I was able to attach the two small triangles that support the narrow strip of wood in the front of the jig.
When placing these small triangles, make sure that they do not end up where the saw blade will later have to pass.
To place them in the correct position, follow the dimensions on the free spline jig plans.
The spline jig was now ready to make the first cuts.
On my crosscut sled, I first placed the jig at a 45-degree angle on the right side of the saw blade.
After a good check on the correctness of the angle, the jig could be clamped to the crosscut sled.
When I had made the first cut, I placed the jig at a 45-degree angle on the opposite side of the blade, and made the second cut.
Finally, I placed the jig against the fence of the crosscut sled and made a square cut in the jig.
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Step 4 | Making the spacer blocks
Now it was time to make the spacer blocks.
These will support the picture frame while cutting the spline slots, but will also serve as stop blocks.
That allows you to make multiple cuts in exactly the same place every time.
These spacer blocks are no more than three pieces of wood joined together to form a solid corner.
In the top corner, I drilled an 8 mm hole for the bolt to pass through to clamp these blocks to the spline jig.
Note that the 2 blocks are the mirror image of each other!
So do not make an exact copy after making the first spacer block.
Step 5 | Homemade knobs
To clamp the spline jig to the crosscut sled, and to adjust the spacer blocks, I used homemade knobs.
In my blog, the 5 best woodworking jigs I made the past year, you can download the free plans for making the star knob jig.
With this jig, you can quickly and very precisely make these buttons.
To be able to make these knobs for your spline jig, you can continue to this blog post to download the star knob jig plans, after reading this article.
Now that I had finished making all the parts, I touched up all the edges and surfaces with sandpaper.
For this, I used P80 followed by P120.
Sanding makes the jig splinter-free and feels nice to work with.
If you want to know more about how to determine the right sandpaper for your project, be sure to check out my blog: The perfect sandpaper grit guide for woodworking.
Everything you need to know about sandpaper can be found in this article. It will no doubt give you a better understanding of the proper use of sandpaper.
There is also a FREE downloadable guide available to help you determine the right sandpaper grit.
How to use a spline jig?
To use the spline jig, position it above the saw blade at the desired angle.
By sliding the spline jig over the saw blade, the slots can be made in which you can place the splines afterwards.
I will explain step by step how to make regular right angle splines as well as more creative splines.
How to make a cross picture frame spline?
If you want to make crossed splines, you have to set your jig as well as possible.
To start, make a mark at the top of the narrow wood strip just at the center of your saw blade.
Do this for every position your jig takes, both when it is 45 degrees to the left of the saw blade, and 45 degrees to the right of the saw blade.
Next, make a mark on the picture frame or box in the exact center.
Now place the picture frame or box with the marking aligned with the marking of the center of the saw blade.
Once this is nicely aligned, you can slide the spacer blocks against the picture frame and fix it.
Once you have made the first incision, glue the first spline into the groove. Use wood glue to do this.
Before continuing, cut away any excess wood from the spline as close to the picture frame as possible.
I used my Japanese handsaw to do this.
To align the wood of the spline with the wood of the picture frame, you can sand until it’s flush.
Now, you set the jig to make the second cut that is perpendicular to the first cut.
Just use the same steps as for the first cut to do this.
After making the second incision, you can place and finish the second spline, just like you did with the first spline.
Keep in mind that perfect crossed splines can only be achieved with a lot of practice, and adjusting your spline jig where necessary.
The more you do this, the easier and more precise you will be able to make the crossed splines.
How to make a regular picture frame spline?
To make regular splines, as with the crossed splines, you must first make a mark on the top of the narrow wooden strip, just in the middle of your saw blade.
This way you can place the picture frame perfectly on the spline jig.
However, this way of creating splines is much simpler than the crossed splines.
Now make a mark in the center, or other place where you want to make an incision in the picture frame.
Place the picture frame with the mark aligned with the mark on the spline jig.
Clamp the picture frame in place using the spacers and make a cut.
After making the notch, you can apply the spline with wood glue and saw away the remnants of the wood with the Japanese handsaw.
Sand it to make the spline flush with the frame, and the spline is ready!
These are two possibilities that can be made with this spline jig, but if you are a bit creative, you can undoubtedly do fun things with this jig.
If you have been able to make some unique splines, you can always send me a picture of your work.
I am curious to see what other people’s results can be achieved with this.
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