For those of us who deal with many boat projects, a set of hole saws is invaluable. Whether you’re installing a new access hatch, through-hull, electrical circuit, opening port, air-conditioning run, or any number of other components, there’s no question that the ability to quickly and accurately remove a perfectly circular chunk of wood, fiberglass, or composite from a deck or bulkhead is absolutely useful and expeditious.
But what if you need to enlarge an existing hole previously cut with a hole saw, either slightly or dramatically, with nothing more than thin air at the center of the smaller hole? How do you keep the saw zeroed in appropriately until its teeth begin to cut the larger hole? Seems just about impossible, right?
I’ve got a simple solution: Begin by using your hole saw to cut a dummy disc from a piece of half-inch or three-quarter-inch scrap plywood—thickness is important since the disc will be used to guide the saw. Of course, the disc will have a drilled hole at its center and it will be considerably larger than the small hole that you are going to enlarge.
The next step entails drilling a couple of very small holes on opposing edges of the dummy disc so that the disc can be temporarily fastened with screws against the bulkhead, deck, or other surface that contains the hole you want to enlarge. Go ahead and fasten the disc so it covers the hole.
The final step? After centering the hole saw’s drill bit in the now-immovable disc, simply apply pressure to your drill in order to cut another hole around the disc, but through the bulkhead, deck, or other surface, thereby enlarging the original, smaller hole. Once you’ve cut through, remove the disc and wastage from inside the hole saw using a screwdriver.
There are, however, three things to bear in mind when employing this nifty technique. First, if you are using a saw with a whopping 5- or 6-inch diameter to cut the larger hole, make sure you also use a variable-speed, half-inch (as opposed to a ⅜-inch) drill. The more powerful tool produces enough torque at safe, slow rotational speeds to deal with a big saw, while the smaller drill will just spin you around like a top and do precious little cutting. Second, if you want to adjust the position of your enlarged hole vis-à-vis the smaller one, all you have to do is fasten the disc in an altered location—just remember all or at least most of the smaller hole must be covered by the disc. And third, if you are cutting a very large hole (for an access hatch) don’t bear down too hard on your drill—slow and steady works best.