See how to quickly make your own variable center point circle cutting jig for your trim router that works for any size circle.
This 3-part series takes you step-by-step through creating this circle cut jig.
See the whole series in order here
See it All
Below you will find:
- Video Tutorial
- Materials List
- Written tutorial with photos
Video
Transcript highlights and images are below the video too!
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Tools, Wood, and Hardware List
This jig takes minimum tools, lumber, or hardware to build.
Below you will find the lists of what you will need.
Tools You’ll Need
Power Tools You’ll Need
Accessories
Safety Gear
Wood and Hardware
Tutorial
See all of the steps in Part 1 for creating this easy circle cutting jig for your trim router.
Choose your Cutting Bit
The first thing you’ll need to choose for this project is the size router bit you will be using.
I chose a ¼” straight cut bit for my project, since I’m cutting circles under 6” into ½” plywood.
A 3/8” straight bit would work fine for this application too.
Some folks like spiral cut bits.
You’ll need to choose which bit will work best for your application.
Measure for Thickness of Jig Wood
The thickness of the wood you use for your jig is based on your application and thickness of the board you’ll be cutting the circle into.
How to Measure for Your Jig Base
Prepare the router for measurement.
- Remove the router base.
- TIP: Get a container to hold your mounting screws!!!
- Put your cutting bit into the router with the shank all the way into the collar.
- TIP: The shank of the bit has to be inserted fully into the router collar. So, the total cutting depth will be limited by the length of the bit, minus at least 1/16” for collar clearance so that it does not scrape across the wood of the jig.
- Lower the router as far as it can go.
Take the Measurement
- Lay the router on its side.
- Shown here is a ½” board, which is the same thickness as the plywood I’ll be cutting.
- You can see that my ¼” straight router bit is long enough to keep the full cutting edge down the length of the wood and still poke the tip of it through end, which will give me a clean cut all the way through.
- Measure from the end of the collar to the tip of the bit
- Subtract 1/16” as you’ll need to back the router up that much to give the collar clearance.
- That measurement is the maximum of the thickness of wood you can cut plus the thickness of the wood for your jig.
- I’m using ¼” plywood for my jig base and I could probably cut ¾” plywood by using this thin board for my jig as well.
Measure Length of Jig
Determine the radius of the biggest circle you want to cut.
(Radius is the distance from the center of the circle to its edge.)
Add to that distance the width of the naked base of your router, meaning without its base plate.
And then give yourself an extra few inches for the outside edges of the jig.
All that added together is the length of your jig.
Here’s an example
The biggest circle I’ll likely ever cut will be 12”, meaning it will have a 6” radius.
The naked base of my router is 3”.
6 + 3 = 9. This is the minimum length of my jig.
I’ll add 3” to give myself a margin of 1.5” on each end of the jig.
6 + 3 + 3 = 12.
I just happen to have 12” x 12” plywood squares for my scroll sawing.
So, that’s what I’ll be using for my jig.
TIP: Start with a bigger piece of wood and don’t cut out your jig yet. You’ll need the rest of the board for clamping as you create your jig.
Router Placement
Router bits turn clockwise. That means you need to cut counter-clockwise with them.
So, the position of your router needs to be on the far right of your wood.
The slot for the center point of your circle will be to the left.
I used the lower right corner of my jig wood as my initial point of reference.
Be sure to always place the router with the opening toward the top of the jig, away from you, as that is the front of the router.
Mark Jig Wood for Width
See video for how this is done.
Put the base plate back on our router.
Place the router base flush with the corner edge of the jig wood so that the bottom and one side touch the edges of the corner. If you have a square base, this is easy. If a round base, check both sides and line up as carefully as you can.
Mark the top of the router base.
Repeat for the other side.
Lay a speed square against the top edge of the jig and draw that line, connecting the two marks.
This is the width of your jig.
Mark Jig for Router Bit
See video for how this is done
Either clamp your speed square or another straight edge snug to the top edge of the jig base.
I hung my jig base off the edge of my workbench and used another square of plywood as my straight edge and clamped them together.
Then I moved the router end to end to ensure that I had clamped my straight edge squarely.
Drop your router bit down until it is touching the jig base. This may make it a wee bit wobbly.
If you want to mark your bit placement on the left, then point the router so that the front is facing you.
If you want to mark your bit placement on the right, then point the router so that the front is away from you.
Attach the battery.
Hold the router securely and turn it on for a split second – long enough to make a mark.
This will be the center hole for your bit.
Mark the Slot
See video for how this is done.
To ensure that your center point for your circle is dead even with your bit, you’ll be cutting slot down the length of the jig base using your trim router.
Now is a good time to mark that slot.
Place your router on the far left in the same spot where you just made the mark for your bit hole. Or on the right if you did it that way.
Hold the router securely and turn it on.
Then slide the router until the base is square with the end of the jig base on the opposite side, or maybe just a little over the edge.
Drill the Bit Hole
See video for how this is done.
Now you’re ready to drill the hole for your router bit.
I did mine on my drill press to ensure I got the center of the drill bit into the center of that mark I made with my router bit.
Drill out that hole using the same size drill bit as your router bit.
TIP: Use a center hole punch or nail to indent the center of that mark so that your drill bit will be centered as you make the hole.
Cut the Slot
See video for how this is done.
Now you’re ready to cut the slot for the variable center point.
You’ll be cutting this with your router bit, starting with the hole you just drilled for your bit as the starting point.
I put my jig base back into the clamp setup I had earlier.
Set your bit low enough to cut through the jig base.
Line up your router base in the left corner of the jig wood, with the front facing you.
Hold the router firmly and cut a slot across the jig wood until your base is flush with the other corner, or however long you need the slot.
Take your time and let the bit do the cutting.
Next Steps
Don’t cut your jig out of the bigger piece of wood yet!!!
You’ll still need that extra board to clamp to in the next step.
In Part 2 of this series we’ll be mounting the router to the jig and I’ll show you tips for marking and making the mounting holes.
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