A Forstner bit is the best way to bore high-quality holes that are accurately sized, accurately located, and with a cleanly cut rim. Today, the bits come in a variety of designs. To find out which ones are best, I tested 18 brands judging them by several criteria: Size, chip-clearing, temperature, and drilling ability.

Size

I tested 1/2-in.-dia. and 1-in.-dia. bits. I measured the diameter of each bit to see how close it was to the specified size, and then I measured the diameter of the hole itself. Most holes were slightly larger than the bit that drilled them, indicating slight runout. For the Bosch bits the 1/2-in.-dia. bit was actually 0.498 in. and the 1-in.-dia. bit was 0.996 in. The bits drilled holes that were 0.505-in.-dia. and 1.001-in.-dia. respectively.

Chip Clearing

Bits choked with chips don’t cut well and may increase friction and burning. Many of the 1/2-in.-dia. bits became badly plugged when the head went below the wood’s surface. The Bosch, however, didn’t get plugged. The 1/2-and 1-in.-dia. bits both cleared chips.

Temperature

To test the longevity of the bits, I mounted each 1-in.-dia. bit in a drill press. I chose the larger bits for this test because there is a greater area in contact with the wood, and therefore more friction. I set the speed to 480 rpm and drilled 100 holes 1 in. deep in hickory, a wood known for its ability to dull a sharp cutter. I used a digital infrared thermometer to check each bit’s temperature every 10 bores. An increase in temperature could signal a dulling cutting edge. While the temperatures ranged from 134ºF to 268ºF, the Bosch bit measured an average of 181°F. In general, bits that made cleaner cuts generated less heat.

Drilling Ability

No other type of bit can match the quality of a Forstner for the cut around the rim and sides of a hole. The Bosch bit left a bore rim with very good quality.

I also tested to see how the 1/2-in. bit cut overlapping holes and holes drilled at an angle. The rim design in Forstner bits allow you to drill clean overlapping holes, as when excavating a mortise. The Bosch bits scored an “excellent” in their ability to bore overlapping holes. The Forstner bit’s sharp rim also makes it easy to start the bit in a steeply angled workpiece, as when cutting pocket holes in a table apron. The Bosch bits received a very good score when tested for this.

Overall, I picked Famag bits as the best overall in the tests. Grizzly gets the nod for best value.

The Bosch Forstner bits are available through amazon.com.

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