See how to put the finally touches on this variable length circle cutting jig for your trim router. And get tips for making accurate cuts, plus how to properly support the circle you’re cutting out.
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
Transcript highlights and images are below the video too!
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Tools, Wood, and Hardware List
Below you will find the lists of what you will need for Part 3.
Tools You’ll Need
Power Tools You’ll Need
Sandpaper – I used 100 grit
Wood and Hardware
In Part 1 of this series we determined the maximum thickness for our jig wood.
Plus we marked the jig’s overall dimensions.
And we drilled the hole for the router bit.
And then we cut the slot for our variable circle center point.
In Part 2 we drilled the holes to mount the router to the jig.
In Part 3 today we’ll test our center point and then cut our jig out of the bigger piece of wood and mount the jig to the board we want to cut and cover tips for making an accurate circle cut.
Cut the Jig to Size
It’s time to cut the jig down to its final measurements.
If you made your jig out of plywood, you’ll want to take a couple of extra steps for cutting.
Plywood is bad about splintering on the edge. So you may want to score a line with a utility knife on the “pretty” face, or top side.
Then turn it pretty face down for cutting.
So, that’s what I used.
And I scored the backside before making the full plunge cut.
Sand the Jig
Use rough sandpaper to smooth the cut edge of the jig.
And then smooth the center slot on both sides of the board.
You can take a long strip of sandpaper and scrub it lightly, shoe-shine fashion, on the inside of the slot.
Just don’t be too aggressive, as you don’t want to widen the slot.
Test Center Point Hardware
Now it’s time to get familiar with using the slot and your hardware stack for mounting your jig to the board you want to cut.
First, I put a Fender washer on the bolt. And then stuck that into the slot.
On the top side, I placed another Fender washer and then my locking washer.
And finally, I screwed in the wing nut to hold it all securely.
Test to ensure that the hardware moves freely in the slot.
And then take it partially apart for the next test.
Drill Center and Router Bit Holes for Circle Cut
TIP: I would use a scrape piece to test your first cuts. I used a scrap of ½” plywood for my tests.
On the board to be cut, mark the center point of the circle.
If you have a compass, you may want to draw the circle.
If not, then you’ll want to mark the radius from the center, and do this in at least 3 other places around the circle, like in the 4 quadrants.
This will help you test the accuracy of your cut all the way around the circle.
Mount the Jig on Board to be Cut
I used a piece of scrap wood for this first mount and cut test.
First you have to make the 2 holes for the center point bolt and for the router bit.
Get a drill bit the same size as your hardware and cutting bit. For me, that is ¼”.
Drill out the center point hole.
For the hole for where the router bit will go through, carefully align the side of the drill bit so that the outside of the bit is on the inside of the circle edge.
I found laying the trim router down on its side made it way easier to tighten up the center point stack.
Ensure that the end of the slot where the router bit will be lines up with the hole you drilled on the inside of the circle.
Now, use a wrench to hold the bolt and tighten the stack as far as you can.
It has to be super tight not to slip as you go around the circle cutting it.
Make a Test Cut
Let’s do a test cut.
Be sure to cut counterclockwise.
Mount Cut Board for Support
The trim router with the battery is pretty heavy. And the board I’m cutting is on ½” plywood and can be bowed.
So, instead of clamping the board to be cut to the edge of my bench, I decided that my Work Mate was a better option.
This allowed me to support both sides of the board to be cut so there would be no bowing.
You could also use a sacrificial board underneath the whole board to be cut. But I found this really heated up the tip of the bit too much.
Spinning the Jig
I found that trying to push the router through the wood to be cut was not the best method. It put way too much pressure on the screws holding the router to the jig.
And yes, it’s a little hard to spin.
TIP: I found using my other hand to spin the jig while simply holding the router gave the best result.
If you push on the router at all, push a little toward the outside.
If you push toward the inside, you will likely move the jig down the slot and start cutting further toward the inside of the circle – meaning that you’ll start shortening the radius and begin cutting a spiral.
Go slowly so that you don’t heat up the bit too much.
Remember, it’s just a trim router and doesn’t have as much power as a corded, standard router, so can’t cut as fast.
If you smell burnt wood, you may be going too fast and the bit is really heating up.
Check Progress Half Way and Three Quarters of the Way Through
There are 2 reasons why you need to stop halfway through and then three quarters of the way through.
1. Check Radius
Turn off the router and check that your center stack is still tight.
Back up the router and check that it is still cutting on the inside of your circle.
If there is an issue, make adjustments.
If all is well, finish the cut.
On my first cut or two, I had to be careful how I pushed the jig around, else it started slipping from the preferred radius.
It may take you cutting a couple of test circles to find your groove with it, literally.
2. Check Mount Support
Keep in mind that you’re cutting out the piece of wood that your jig is mounted to.
I stopped at the three quarter mark and moved the circle of the cut out board onto the far side of my Work Mate so that the circle itself would be supported as I finished the cut.
Hand Cut the Last Inch
Your jig is mounted to a piece of wood that is being cut out and may become too unstable to support the jig as you near the end of the cut.
I chose to remove the jig and put the base back on my trim router.
And then I free-hand cut the last inch of the circle.
That way I could ensure I kept the jig level and the circle could simply fall out.
And there you have it!!
A clean cut circle with your trim router.