Hints for Huge Holes
Try Routing a Small Hole Larger
In many cases you can bore a smaller hole then enlarge it, using a router, a rabbeting bit, and a pattern bit, as shown in the photo below. The length of the pattern bit presents this technique's principal limiation: The bit's maximum cutting depth limits the depth of a hole that doesn't go through a part. For a through hole, it's slightly less than twice the cutting depth of the bit, because you can work from both sides.
Here's how the process works: 1. Bore the starting hole, shown at left in the photo. 2. Chuck a 1/4", 3/8", or 1/2" rabbeting bit in your router. Rout around the top of the hole, as shown in the center piece in the photo. The diameter of the enlarged hole will equal the diameter of the original hole plus twice the width of the rabbet. So, for instance, if you bore a 1-5/8" hole and rout a 3/8" rabbet around the opening, you'll make a 2-3/8"-diameter hole. 3. Swap the rabbeting bit for a pattern-routing bit. Then, with the top-mounted guide bearing riding in the rabbet, rout the hole to size, as shown in the piece at the right in the photo. You can repeat steps 2 and 3 any number of times to make an even larger hole.
For a non-through hole that's shallower than the pattern bit's length, you'll have to build up the workpiece's top surface temporarily with scrapwood before you drill and enlarge the hole, calculating the thickness to yield the desired hole depth in the workpiece.
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