Hints for Huge Holes

Here are some ways to bore (and rout) the big ones
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Chucking a big bit into your drill press offers the easiest method for hogging out a large hole. Large Forstner or multispur bits will drill flat-bottomed holes that don't go through the part as well as through-holes. (Forstner bits have smooth rims; mutlispur bits carry teeth along their rims, as shown in the photo at left.) Holesaws and adjustable circle cutters offer two more choices for big through-holes.

Comparing the Cutters

•Forstner and multispur bits are available in diameters up to 4" from several suppliers, including Woodcraft (800/225-1153) or Woodworker's Supply (800/645-9292). Sizes generally increase in 18 " increments beyond 2". Bits imported from the Far East cost $20-40 each; European- or American-made ones can cost more.

•An adjustable circle cutter like the one shown below, will produce holes up to 8" in diameter. The quality of the cut usually won't equal that of the Forstner and multispur bits, but will be better than a holesaw. The capability of boring odd-size holes is a plus for this tool, widely available from hardware stores, home centers, and tool dealers.

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•Holesaws, like the one shown in the photo below, come in diameters up to 6". Often used by contractors, big holesaws are available at hardware stores, lumberyards, and large home centers. Interchangeable blades—usually sized in 18 " steps—attach to a separate arbor. Holesaw prices are similar to those of the Far East Forstner and multispur bits.

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Big-Cutter Cautions

Here are some points to remember when using these outsized tools.

  • Use a drill press, and clamp the workpiece to the table.
  • Run the tool at a slow speed, about 250 rpm or less, to minimize burning of the workpiece.
  • When boring dense hardwoods, such as hard maple, opt for a Forstner or multispur bit rather than the holesaw or circle cutter.
  • When boring deep with a Forstner bit or multispur bit or holesaw, clear the chips frequently by pulling the bit out.
  • To prevent tearout when boring with an adjustable circle cutter, cut most of the way through; then finish the cut from the other side, inserting the tool's pilot bit into the guide hole.

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.

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Here's how the process works:
1.
Bore the starting hole, shown at left in the photo above.
2. Chuck a 14 ", 38 ", or 12 " 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 158 " hole and rout a 38 " rabbet around the opening, you'll make a 238 "-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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