Reduce router speed
Just as a speeding automobile begins to shake and vibrate on the verge of going out of control, so it goes with high-performance woodworking machines. Each router-and-bit combination has an ideal operating speed range you should stick to for safety and top performance. Push beyond that "sweet spot" and you could damage your bit, router, or workpiece, sometimes beyond repair.
Too fast or too slow problems
Routers with variable-speed motors run between 8,000 and 26,000 rpm. More important than speed at the bit shank, though, is a bit's rim speed, the velocity of the cutter at the farthest point from the center of the shank. For example, a 1⁄2 "-diameter bit spinning at 10,000 rpm runs almost 15 mph at the tip of the cutter. Increase the speed to 25,000 rpm and the bit's rim is traveling at 37 mph. Now take a 3"-diameter raised-panel bit. At 10,000 rpm the rim travels nearly 90 mph, but at 25,000 rpm the rim speed would capture the pole at any NASCAR race: 223 mph!
Speeds greater than 100 mph put excessive stress on bits, especially those with 1⁄4 " shanks, and increase vibration. In addition, cutting performance suffers, as shown below, when speed exceeds the optimum level. Routing at a too-fast speed causes tear-out and burn marks on the workpiece and dulls bits faster. Routing with the speed set too slow leads to choppy, rough, or rippled cuts.
Find your optimum speed
Because bit diameter largely determines the best router speed, you can chart some general guidelines, as shown below. If your router's speed dial features only single numbers or letters with no obvious reference to speed, use the following technique to find the optimum speeds for your router bits. Chuck a bit and slowly ramp up the speed until the router begins to vibrate, then back it off until the vibration disappears. Make note of that setting in the third column of the chart at right for future use with that particular bit. Use this as your starting point; then fine-tune the speed based on the router or materials you're working with.
- When routing cherry and hard maple, slow the bit speed even more for these burn-prone species.
- Make practice cuts in scrap stock, especially when using an unfamiliar bit, router, or material.
- When routing highly figured wood, slow the speed a little more to avoid tearing out wild grain.
- Maintain the same approximate feed rate. Varying the feed rate could affect the cut quality, just as if you had increased or decreased the bit speed.
- When routing in trapped cuts (dadoes, mortises, or dovetails, for example), slower speeds help reduce burning, particularly when the bit can't expel the chips easily.
- Try to finish each task with a light cut (1⁄32 " or less) to get the best cut quality, just in case your speed proves a little too fast or slow.