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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 Porter-Cable bits, the 1/2-in.-dia. bit was 0.501-in.-dia.and the 1-in.-dia. bit was 1.002-in.-dia. The bits drilled holes that were 0.503-in.-dia. and 1.002-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 Porter-Cable bits, 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 Porter-Cable bit measured an average of 177°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 1-in.-dia. Porter-Cable bit left a bore rim with very good quality, while the rim quality of the 1/2-in.-dia. was excellent.
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 Porter-Cable bits scored a “very good” 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 Porter-Cable 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 Porter-Cable bits are available from amazon.com.
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