Router Feed Rate Calculator
Work out feed rate, chipload and safe spindle speed for a router or CNC, and see why feeding too slowly is what burns bits rather than feeding too fast.
How to use this calculator
- 1Enter the cutter diameter and flute count from the bit itself, not from the collet size - a 1/4 in shank bit can carry any diameter of cutter.
- 2Set the spindle speed to what the machine is really running. Check the maximum safe speed the page returns before anything else, especially with large profile cutters.
- 3Read the feed rate range rather than the single number, and start in the middle.
- 4If the cut burns, feed faster or slow the spindle. Slowing the feed is the instinctive response and it is the wrong one.
How the calculation works
Feed rate (in/min) = rpm x flutes x chipload
Chipload (in/tooth) = feed rate / (rpm x flutes)
Rim speed (ft/min) = pi x diameter x rpm / 12- rpm
- Spindle speed in revolutions per minute
- flutes
- Number of cutting edges on the tool
- chipload
- Thickness of the chip each edge removes per revolution, in inches - the quantity everything else serves
- diameter
- Cutter diameter in inches
The equation is trivial. What makes feeds and speeds hard is that only chipload has a correct value, and it comes from a published table rather than from the arithmetic.
Note what the equation implies: raising the spindle speed without raising the feed *lowers* the chipload, because each revolution now removes less. That is why a trim router at 30,000 rpm burns hardwood that a slower machine cuts cleanly.
Depth of cut does not appear. It affects how hard the cutter is working and how well chips clear, but not the chip thickness, which is set by how far the tool advances per tooth.
Worked example
A 1/4 in two-flute upcut in maple
- 1.A 1/4 in cutter in hardwood takes 0.009 to 0.011 in per tooth.
- 2.At the middle of that, 0.010 in, the feed is 18,000 x 2 x 0.010 = 360 in/min.
- 3.That is 30 ft a minute, which sounds alarming on a hand-held router and is entirely ordinary on a CNC.
- 4.Slotting in hardwood, depth per pass is about half the diameter - 0.125 in - so 3/4 in of material takes six passes.
Result: About 360 in/min, in six passes
Why a trim router burns
- 1.A fixed-speed trim router runs at about 30,000 rpm and cannot be slowed.
- 2.To keep even the conservative 0.009 in chipload, the feed would have to be 30,000 x 2 x 0.009 = 540 in/min, or 45 ft a minute.
- 3.Nobody pushes a hand-held router at 45 ft a minute, so in practice the chipload collapses to a fraction of that, the bit rubs, and the cut scorches.
- 4.The equipment is the constraint, not the technique. A variable-speed router set to 18,000 rpm makes the same cut cleanly at a feed a person can actually produce.
Result: 540 in/min would be needed - which is why the fixed-speed tool burns
A large panel-raising bit
- 1.A 3-1/2 in panel raiser at 18,000 rpm gives a rim speed of pi x 3.5 x 18,000 / 12 = 16,493 ft/min, which is 187 mph at the cutting edge.
- 2.The published ceiling for a cutter this size is 12,000 rpm, and the reason is entirely structural rather than about finish quality.
- 3.Centrifugal force on the carbide tips rises with the square of the speed, so running 50% fast more than doubles the load trying to throw them off the body.
- 4.The calculator flags the overspeed before it gives any feed rate, because at this diameter that is the answer that matters.
Result: Overspeed - 12,000 rpm is the limit for a 3-1/2 in cutter
Chipload is the whole subject
A cutting edge works by wedging under a layer of material and shearing it away. For that to happen the layer has to be thick enough for the edge to get under it. Below some thickness the edge simply cannot engage - it slides over the surface, compressing and rubbing rather than cutting, and everything the tool would have done as work it does as heat instead.
Chipload is that thickness, expressed as the advance per cutting edge per revolution. Every other number in feeds and speeds exists to produce a chipload in the right range: the spindle speed, the feed rate and the flute count are three dials that between them set one quantity.
The chip also does a job after it has been cut. A properly sized chip leaves carrying most of the heat generated in making it, which is what keeps the carbide cool. This is why thin chips are doubly bad: they generate more heat per unit of material and remove less of it.
Why burning means go faster
A scorched edge looks like the tool was working too hard, so the instinct is to ease off. That instinct is backwards, and it is responsible for an enormous number of ruined workpieces and dead bits.
Burning is a heat problem, and at woodworking speeds the heat comes from friction rather than from shearing. A cutter that is rubbing generates far more heat than one that is cutting, so the burnt edge is evidence of too little chipload, not too much. Slowing the feed lowers the chipload further and the burning gets worse, which tends to be read as confirmation that the cut was too aggressive to begin with.
There are only three ways to raise chipload: feed faster, spin slower, or use fewer flutes. On a hand-held router the practical answer is nearly always to turn the speed dial down, because there is a limit to how fast a person can move a tool accurately. On a CNC the answer is usually to feed faster, and the surprise is how much faster it can go.
- Burning at the start of a cut — Usually a dull bit. A blunt edge cannot engage at any chipload.
- Burning in tight curves — The tool slows through the corner while the spindle does not, so chipload collapses exactly where the cutter is most engaged.
- Burning only in cherry and maple — Dense close-grained woods with low thermal conductivity scorch before oak or pine would. They are the species that punish a marginal setup.
- Chattering rather than burning — The opposite problem - too much chipload for the rigidity available, or too much stick-out on the cutter.
The RPM limit is about physics, not finish
Router bit speed charts exist because rim speed scales with diameter. A 1/4 in cutter at 24,000 rpm has its edge moving at about 1,570 ft per minute; a 3-1/2 in panel raiser at the same speed is at 22,000 ft per minute, or 250 mph.
The force trying to throw a carbide tip off the tool body rises with the square of rotational speed and with the radius, so a large cutter overspun is loaded far beyond anything the brazed joint was designed for. When one lets go it leaves at tangent velocity, and the published maximum speeds are set by that failure mode rather than by cut quality.
This is the one part of feeds and speeds that is not a matter of judgement or preference. A cut that burns is a wasted afternoon. A cutter that comes apart at 12,000 rpm is something else, and it is why any router used with large profile bits needs variable speed.
What this assumes, and where it stops
Assumptions
- Chipload ranges are published tooling-industry figures for sharp carbide cutting edges in a rigid setup.
- Depth per pass follows the trade convention of about one cutter diameter for slotting in softwood and half that in hardwood, reduced further for aluminium.
- The maximum speed table is the standard published router bit speed chart and is treated as a hard limit rather than a guideline.
- Feed rate is the actual cutter path speed, which on a CNC is what the controller reports and on a hand-held router is a matter of practice.
Limitations
- It does not account for tool deflection, which governs small-diameter cutters more than chipload does and depends on stick-out and material stiffness.
- Climb versus conventional milling changes surface finish, tear-out and how the tool pulls, none of which appear here.
- Spiral upcut, downcut and compression geometries behave very differently at the surface even at identical chiploads.
- Machine rigidity is the real ceiling in most hobby setups. A benchtop CNC will chatter long before the tooling table says it should.
- The maximum speeds here are the published limits by cutter diameter. Most full-size routers top out at 24,000 rpm anyway, and a particular bit’s own packaging is authoritative over any general chart.
- For metals other than aluminium, use metalworking speed and feed data rather than this page.
Common questions
My cut is burning. Should I slow down?
Almost certainly not. Burning at woodworking speeds is a rubbing problem, which means the chip is too thin - so slowing the feed makes it worse. Raise the feed rate, or lower the spindle speed, or both. The exception is a dull bit, which burns at any setting; if a fresh cutter behaves the same way at the same numbers, the settings are the problem.
Does more flutes mean a faster feed?
Yes, proportionally - three flutes at the same chipload and rpm feeds 50% faster than two. The cost is chip clearance. Each flute takes gullet space, and in a deep slot the chips have nowhere to go but up the flutes, so a three-flute cutter packs and overheats where a two-flute would clear. Use more flutes for shallow profiling and finishing, fewer for slotting.
Why does my fixed-speed trim router burn everything?
Because it runs at around 30,000 rpm and you cannot feed fast enough to keep up. Maintaining a proper chipload with a 1/4 in two-flute bit at that speed needs roughly 45 ft per minute of feed, which is faster than anyone moves a router by hand. The tool is fine for laminate trimming, which is what it was designed for. For anything in solid hardwood, use a variable-speed router.
How deep can I cut in one pass?
For slotting, roughly one cutter diameter in softwood and half a diameter in hardwood is the trade convention, and small cutters want less again because their shanks deflect. Profiling passes tolerate more, since the chips can escape sideways instead of only up the flutes. If the cutter starts to whine or the finish deteriorates part-way down, the pass is too deep.
Do these numbers apply to a hand-held router?
The physics does, but the feed rates will look impossible. A CNC holds 360 in/min without effort; a person pushing a router does perhaps 60 to 120 in/min. That gap is exactly why hand routing works best at lower spindle speeds - dropping to 12,000 or 14,000 rpm brings the required feed back into human range and stops the bit rubbing.
Sources
- Router bit feed and speed reference for CNC — Freud Tools
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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