Accurately Routing Large Circles

by Jerry "Jake" Seabaugh

Photos by Jerry "Jake" Seabaugh

Posted: August 24, 2010

Photo 1  The problem. A poorly framed hole.

The problem. A poorly framed hole.

I’m sure most of us have seen or used a router mounted on a pivoting board or arm to cut circles. Well, this past spring I was handed a difficult assignment. Some one needed to mount five, 20” diameter Tiffany glass circles in the walls of a new restaurant. The problem was that someone else had framed the holes very poorly, (Photo 1) and the owner would not let me tear out the framing to build and install window frames to properly mount the circular glass.   What’s unique about my jig is that it allows you to cut rings, not just circles.  Here’s how I built it.

Photo 2  The solution-plywood rings and poly sheet sleeves.

The solution-plywood rings and poly sheet sleeves.

My solution was to make several rings of plywood with inner radii, and rabbets to hold the glass and tubes of 1/16” poly sheets inserted through the wall to hide the ugly framing job, and an outer radius to complete the rings on both sides of the walls. (Photo 2)

Photo 3  How to make very accurate circle cuts to within 1/16”?

How to make very accurate circle cuts to within 1/16”?

The next problem was how to make very accurate circle cuts to within 1/16” diameters, (Photo 3) make them for inner and outer radii, and repeat the exact process five times! Time for another jig.

I made a moveable trunnion arm equipped with a plunge router.

Lay out jig and cut slots with plunge router and the completed jig layout

The jig is a large piece of 3/4" plywood with several 1/4" slots cut to accept 1/4" carriage bolts. I laid out the pattern of the slots and the diameter of the rings, and used a plunge router to cut the slots. (Photo 4). Photo 5 shows the completed layout.

Photo 4 Lay out jig and cut slots with plunge router	Photo 5. The completed jig layout

1-1/2” oak block to be used as an axle bearing and centering hole guide

Next I mounted a block of 1-1/2” thick oak, with a perfectly drilled 1/4" hole, in the center of the underside of the plywood. (Photo 6) This block of wood was the axle bearing for the pivot of the trunnion arm. It also was used to drill the center hole through the plywood blanks after the work had been secured to the base plate of the jig.

Photo 6. 1-1/2” Oak block to be used as an axle bearing and centering hole guide	Photo 7. Trunnion arm and router base plate

 A rectangular nut will make adjustments much easier

Then I turned my attention to the router and the trunnion arm. I wanted a very stable and long trunnion arm to glide over large areas. I chose a piece of high quality 3/4" plywood, 10” wide and 34” long. (Photo 7) I took the base plate off my router to use it as a template to mark the screw holes in the trunnion arm. On the underside of the trunnion arm, I cut a groove to accept the 1/4" x 6” long carriage bolt that I used as the pivot axle, (Photo 8) and a 1/2" rabbet to hold a washer and rectangle nut. (Photo 9).

Photo 8.  Groove and rabbet for pivot axle/trunnion and square nut.	Photo 9. A rectangular nut will make adjustments much easier

View of trunnion arm and underneath of jig base

The top of the pivot axle was epoxied to a wooden knob. Another piece of 1-1/2” oak was sandwiched between the knob and the trunnion arm. (Photo 10) This enabled the pivot axle to be moved and tightened at any desired distance from the router bit, (Photo 11) and helped insure a very stable pivot action as the router was cutting a circle. Photo 12 shows the jig ready to use, and Photo 13 shows the router and trunnion arm next to the underneath side of the jig.

Photo 10  Carriage bolt epoxied to wooden knob, inserted through a block of oak.	Photo 11  Accurate measurements and adjustments can be made and locked in place

Photo 12  Jig ready for use	Photo 13  View of trunnion arm and underneath of jig base

Additional layout reference stick

I added a reinforcing frame under the base that doubled as legs to provide clearance for the axle and other bolts and wing nuts. Photo 14 shows the results of the first pass of an inner radius cut. I added a reference stick on the trunnion arm to keep the upper axle block square to hold an accurate reference mark on the trunnion arm. This reference mark located the exact center of the pivot axle, Photo 15. From then on, repeated accurate settings could be assured. Because of the number of rings needed, I added a second reference stick at the bottom of the base for a quicker set up, Photo 16.

Photo 14  First pass of inner radius	Photo 15  Reference stick and center of axle

Photo 16  Additional layout reference stick

Photo 17 Assortment of hold down blocks

Assortment of hold down blocks

There are two things that make this jig unique. One is the large, stable trunnion arm and pivot axles, with repeated accurate adjustments, and two, was the hold down blocks that kept the rings centered during both inner and outer radii cuts. I made both notched blocks – to hold the blanks from the outside and curved blocks to hold the rings after the inner radii were cut. (Photo 17) 1/4" carriage bolts and wing nuts were used to hold them in place where needed. I used 1/2" plywood so as to not interfere with the travel of the trunnion arm.

Photo 18  Inner hold down blocks in use kept the ring centered and in place

Photo 18  Inner hold down blocks in use kept the ring centered and in place

I must confess that because my blanks were almost as big as the jig base, I screwed them directly to the base at the corners. BUT, the inner curved blocks performed perfectly to hold the rings in place while cutting the outside radii. (Photo 18)

Photo 19  The jig in use

Photo 19  The jig in use (no finger was cut in the making of this article)

The extra long, extra wide trunnion arm. paid off in very stable, accurate cuts, and I had plenty of control over the work. (Photo 19) Oh, the bandage in this picture – I cut my finger at work, NOT in my shop!

It was a challenging but fun jig to make. I hope this helps others.

Here are some finished product pictures: 

Jerry Seabaugh is a registered member of the WWGOA.   

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