Router bit diameter determines safe operating speed. Running large bits too fast causes dangerous vibration, poor cut quality, and potential breakage. Running small bits too slow produces burn marks and excessive wear. Here’s how to match RPM to bit diameter.
The Physics of Bit Speed
The rim speed of a router bit—measured in feet per minute at the cutting edge—determines cutting efficiency and safety. A small-diameter bit must spin faster than a large-diameter bit to achieve the same rim speed. The formula shows why: rim speed equals pi times diameter times RPM divided by 12.
Most woodworking happens effectively at rim speeds between 9,000 and 12,000 feet per minute. Too fast overheats the bit and burns the wood. Too slow prevents clean cutting and causes chatter.
Small Bit Speeds
Bits up to 1 inch diameter run safely at full router speed—typically 22,000 to 24,000 RPM for fixed-base and plunge routers. These small bits need high RPM to achieve adequate rim speed for clean cutting.
A 1/4-inch roundover bit at 22,000 RPM produces approximately 14,400 feet per minute at the rim—slightly above the ideal range but acceptable for the small cutting load. Reducing speed on these small bits causes burning and poor surface finish.
Medium Bit Speeds
Bits between 1 and 2 inches diameter should run at 18,000 to 22,000 RPM. A 1.5-inch diameter bit at 20,000 RPM generates approximately 7,850 feet per minute—slightly below ideal but safe and effective.
Variable-speed routers let you dial in the right speed for these medium bits. Single-speed routers running 22,000+ RPM still work but run these bits slightly fast. The margin of safety remains adequate for occasional use.
Large Bit Requirements
Bits from 2 to 3 inches diameter need reduced speeds—typically 12,000 to 16,000 RPM. A 2.5-inch raised panel bit at 16,000 RPM produces approximately 10,500 feet per minute, right in the target range.
Running these large bits at 22,000 RPM creates rim speeds over 14,500 feet per minute. The excessive speed causes vibration, poor cut quality, and dangerous bit flex. Variable speed becomes essential rather than optional at these diameters.
Extra-Large Panel Bits
Bits over 3 inches diameter—common for vertical raised panel bits—require 8,000 to 12,000 RPM maximum. A 3.5-inch bit at 10,000 RPM generates approximately 9,200 feet per minute.
These massive bits create significant centrifugal force. Running them too fast risks catastrophic failure. Always start at the lowest speed setting and increase gradually while monitoring vibration and cut quality.
Router Motor Impact
Larger bits require more torque to maintain speed under load. A 1.5 HP router handles bits up to 2 inches adequately. For larger panel-raising bits, a 3 HP router provides the torque needed to maintain speed through the cut.
Undersized routers bog down in heavy cuts, dropping below the target RPM despite the speed setting. The reduced speed causes burning and poor finish. Take lighter passes or upgrade to a more powerful router.
Variable Speed vs Fixed Speed
Fixed-speed routers work fine for bits under 1.5 inches used in light-duty applications. Once you start using larger bits regularly, variable speed becomes necessary for safe operation and good results.
Electronic variable speed maintains consistent RPM under changing loads. Older routers with simple electronic speed controls may show more speed variation during heavy cuts. Professional-grade routers hold speed more consistently.
Practical Speed Guide
Use these speeds as starting points:
- Under 1 inch: 22,000-24,000 RPM
- 1 to 1.5 inches: 20,000-22,000 RPM
- 1.5 to 2 inches: 18,000-20,000 RPM
- 2 to 2.5 inches: 14,000-18,000 RPM
- 2.5 to 3 inches: 12,000-16,000 RPM
- Over 3 inches: 8,000-12,000 RPM
Adjust based on results. If you see burning, increase speed slightly or decrease feed rate. If you feel excessive vibration, reduce speed. These guidelines provide safe ranges rather than exact requirements.
Bit Construction Quality
Higher-quality bits tolerate slight speed variations better than cheap bits. Carbide thickness, brazing quality, and body balance all affect how a bit performs at various speeds.
Budget bits may vibrate even at correct speeds due to poor balance. If speed adjustment doesn’t solve vibration problems, the bit itself may be the issue. Throw away bits that vibrate excessively—they’re dangerous regardless of speed setting.
Feed Rate Interaction
Speed and feed rate work together. Higher speeds generally pair with faster feed rates to prevent burning. Slower speeds need proportionally slower feeds to maintain clean cutting action.
When you reduce speed for large bits, also reduce your feed rate. Trying to maintain your normal feed rate at reduced speed causes the bit to load up and bog down, defeating the purpose of the speed adjustment.
