The story shows how to mark and cut a circle hole for a ceiling canister.
Cutting circular openings for ceiling canisters or other electrical boxes can sometimes be quite easy. If the canister is within a couple of feet of the edge of the drywall, you can hoist the drywall into position and reach between the joists to trace the can's perimeter onto the back of the drywall panel. A drywall lift is a valuable helper in this process.
But if you can't reach the canister, you can utilize the procedure outlined here to produce professional results.
Making the jig takes a few minutes but it will last through many remodeling projects. You probably have enough material in your scrap bin to make it, so its cost is virtually nothing.
Working with a helper, about 5 minutes per ceiling canister
Tape measure, compass, jigsaw, plumb bob with nylon line, permanent marker, drywall lift (optional but recommended)
Accurate measuring, using a plumb bob
Install ceiling canisters and have wiring inspected, if necessary.
Scrap plywood and 1x2s, screw eye, ceiling canister with supplied hole template
Measure the inside diameter of your ceiling canister and use a compass to draw a circle of that size onto a piece of 1/4-inch plywood. Carefully cut the circle with your jigsaw, and drill a 1/8-inch hole at the center. Also drill a hole in the perimeter of the circle. Nail the disc to a pair of 1x2 strips so the assembly won't fall into the canister. Attach one end of a nylon mason's line to a plumb bob (or use a chalk line and case) and thread the other end into the center hole and out through the perimeter hole. By pulling on the free end of the line, you can adjust the line's length while the plumb bob stays centered.
Recruit a helper for the next steps. One person working at the canister adjusts the length of the string so that the plumb bob is as close to the floor as possible without touching it. The other person steadies the plumb bob until it settles. Make a dot on the floor with a permanent marker at the tip of the plumb bob, then draw a circle around the mark so that you don't misplace it.
Put the drywall panel for the ceiling onto a lift and carefully position it against the joists. Don't exert too much pressure, or you could crack the panel. Thread the plumb bob's line through a screw eye, and use its tip to transfer the mark on the floor onto the drywall panel. Again, you'll benefit by having a helper working at floor level.
Lower the ceiling panel several inches to make it easier to cut. Get the hole template that's usually furnished with the ceiling canister. Center the template, and draw its perimeter with a pencil. If you don't have a template, measure the canister, then mark the circle with a compass. Using your jab saw, cut to the outside of the line so the fit isn't too snug.