What rim speed is and why diameter changes everything
Rim speed, sometimes called peripheral speed, is how fast the outermost cutting edge is travelling through space. It is pi times the diameter times the rotational speed. Because diameter is a straight multiplier, doubling the diameter doubles the rim speed at the same RPM, and the forces trying to tear the cutter off its body rise faster still, roughly with the square of that speed.
Put numbers on it. A half inch straight bit at 24,000 RPM has a rim speed of about 3,140 feet per minute. A three and a half inch panel raising bit at the same 24,000 RPM is at about 21,990 feet per minute, seven times as fast. The carbide, the braze joint holding it on, and the steel body were not designed for that. This is the entire reason variable speed routers exist and the entire reason large bits carry a printed maximum speed.
The published bands, and why they are the second-best source
Most manufacturers publish something close to the following, and this calculator uses it when you have no better figure.
| Cutting diameter | Commonly published maximum | Rim speed at that maximum |
|---|---|---|
| Up to 1" | 24,000 RPM | about 6,280 ft/min at 1" |
| 1" to 2" | 18,000 RPM | about 9,420 ft/min at 2" |
| 2" to 2-1/2" | 16,000 RPM | about 10,470 ft/min at 2-1/2" |
| 2-1/2" to 3" | 12,000 RPM | about 9,420 ft/min at 3" |
| 3" to 3-1/2" | 10,000 RPM | about 9,160 ft/min at 3-1/2" |
| Over 3-1/2" | 8,000 RPM | rises with diameter |
Read the right hand column and the logic of the bands becomes visible: they are not arbitrary steps, they are an attempt to hold rim speed roughly constant as diameter grows. The bands are a fallback. The number printed on the bit, its packaging or the maker's catalogue is the authority, and if it is lower than the band, the band is wrong for that cutter. Enter it in the manufacturer field and this page will use it instead.
Where people get hurt
The router and the table saw are the two machines that put the most woodworkers in emergency departments, and the router is the one whose hazards are least obvious to a beginner because it looks like a drill. Three things are worth naming plainly.
The first is overspeed. A cutter run above its rated speed can come apart, and fragments leave at the rim speed this page calculates. There is no technique that manages that risk; the only control is the speed dial.
The second is climb cutting with a handheld router, where the cutter rotation drives the tool along the work rather than resisting it. The tool pulls, and it pulls faster than most people can react. This page will not tell you feed directions or when a climb cut is appropriate, because that is training, not arithmetic.
The third is that large cutters remove a great deal of material at once and the workpiece is what usually moves. Bits at the top of the diameter range belong in a table, and the setup around them matters more than the bit does.
If you are new to routers, the woodworking beginners guide is a better starting point than any calculator, and the manual for your specific machine and cutter is better than both.
Chip load, and the reason slow is not automatically safe
Chip load is feed rate divided by the product of RPM and the number of cutting edges, and it is what each edge actually takes per revolution. Run too slow a feed against too high an RPM and the edge stops cutting and starts rubbing, which burns the wood and cooks the carbide. That is the failure at the other end of the scale from overspeed, and it is why slowing a small bit down is not free.
The practical version for hand feeding is that you are the feed rate, and the number this page reports is only meaningful if you actually know how fast you moved. It is included because on a table with a power feeder or on a CNC the figure is real and useful. For hand work, treat it as a way of understanding why burning happens rather than as a setting.
For angles on the same parts, the miter angle calculator covers the geometry, and for cutting metal rather than wood the numbers work differently — see the speeds and feeds calculator.
Questions people ask
Why does a big router bit have to run slower?
Because the cutting edge at the rim travels further per revolution. Rim speed is pi times diameter times RPM, so a 3-1/2 inch bit covers seven times the distance per revolution that a 1/2 inch bit does. Run both at 24,000 RPM and the big one is at roughly 22,000 feet per minute against 3,100 for the small one. The carbide, the braze holding it to the body, and the body itself are rated for a speed, and past it the cutter can fail. Slowing large bits down is how the rim speed is held near what the cutter was built for, and it is also why the published speed bands step down as diameter goes up.
What happens if I run a bit above its rated speed?
The honest answer is that it may be fine for a long time and then not be. A cutter run over its rating can come apart, and the fragments leave at the rim speed shown on this page. That is a real injury mechanism, not a theoretical one, and it is the reason the rating is printed on the bit in the first place. There is no feed technique or fence setup that compensates. If you do not know the rating for a cutter you own, look it up from the manufacturer rather than assuming, and in the meantime use the band figure for its diameter, which is the conservative choice.
My router only has one speed. Can I still use a large bit?
Check what that single speed is against the cutter rating before anything else. Many single-speed routers run at or near 24,000 RPM, which is above the published maximum for anything much over an inch in diameter. If your machine cannot go slower than the cutter allows, the cutter does not belong in that machine. This is one of the few places in woodworking where the answer is simply no rather than a matter of care and technique.
Is the rim speed number itself a limit I should check against?
Not on its own. Rim speed is shown here because it explains why the RPM bands exist and gives you a sense of scale, but cutters are not sold with a rim speed rating. They are sold with a maximum RPM, and that is what you set the machine to. Use rim speed to understand the problem and use the manufacturer RPM to decide the setting. Where rim speed earns its keep is in comparison: seeing that the bit in your hand is running at four or five times the rim speed of a small bit at full tilt makes the reason for slowing down concrete in a way that a number on a dial does not.
Does bit sharpness change the safe speed?
It does not raise it. A dull cutter needs more force, generates more heat and loads the braze joint harder, so a dull bit is worse at any given speed, but sharpening one does not entitle it to run faster than its rating. Sharpness also affects the other end of the problem: a dull edge rubs instead of cutting at low chip loads, so burning gets worse. If a bit that used to cut cleanly now burns at the same speed and feed, that is usually the edge rather than the setting.