CNC Router Bit & Feeds Picker
Your bit is not what bends — the machine outflexes it 150 to one. What bit choice actually buys is chipload, through flute count.
What each spindle speed gives you
Who actually bends: the bit or the machine
Why a light cut burns
"Shorten your stickout" is real physics acting on a negligible term
A bit is a cantilever, so its deflection genuinely goes as L³/d⁴ — doubling the stickout is eight times the bend, halving the diameter is sixteen. All true, and almost none of it matters. Carbide is enormously stiff: a 6.35 mm bit at 25 mm stickout under 36 N deflects 0.004 mm. A mid-range hobby gantry under the same 36 N deflects 0.6 mm. The machine is 150× the bit. At an ordinary four-diameter stickout the machine dominates for every bit in the list, and the bit only becomes the larger term past thirteen to twenty-one diameters of stickout — which nobody runs.
The crossover works out as ∛(3·E·I/k), and the cutting force cancels out of it entirely — so the answer doesn't depend on how hard you're cutting. A stiffer machine moves it down, a bigger bit moves it up. That's why stickout advice earns its keep on an industrial router and not on a hobby gantry.
So bit choice doesn't buy rigidity. What it buys is chipload, through flute count — and on a hobby machine that's the whole game. A 1/4" two-flute at 18,000 rpm needs 3,600 mm/min to hit a 0.1 mm chip, which is past most hobby machines. So the chip comes out at 0.056 mm and the bit rubs instead of cutting, which burns the wood and rounds the carbide edge.
And when the feed is capped, the only levers are fewer flutes and lower rpm. Chipload is feed ÷ (rpm × flutes), so with the feed pinned by the machine, rpm is in the denominator. Dropping from 18,000 to 9,000 rpm at the same 2,000 mm/min takes the chip from 0.056 to 0.111 and it cuts properly. The instinct when a cut sounds wrong is to raise the spindle speed — which is exactly backwards, and makes the chip thinner still.
Taking a light cut to be gentle makes rubbing worse, not better. Below a half-diameter stepover the chip comes out thinner than commanded by d / (2√(ae(d−ae))). At a 10% stepover a commanded 0.10 mm is really 0.06, and the feed has to rise 67% just to stand still. The gentle-sounding setting is the one that burns.
- Listen rather than trusting a number. Cutting sounds like a steady tearing and throws visible chips; rubbing whines, smells hot and makes dust. Dust from wood usually means the chip is too thin.
- Climb vs conventional matters more on a light machine. Climb milling pulls the cutter into the work, and a machine with backlash turns that pull into a lurch. The advice to climb for finish quietly assumes a machine that can't be pulled.
- A compression bit needs a first pass deep enough to bury the transition — usually 2–3 mm — or it tears the top face exactly like a plain up-cut.
- V-bits are the exception to all of this. Effective diameter falls to nothing at the tip, so surface speed there is zero and the point always rubs. Depth of cut governs everything; feeds and speeds are nominal.
How to use
- Pick the bit and the machine class you actually have.
- If the feed is capped, lower the spindle speed rather than raising it.
- Correct the feed upward for any stepover under half the diameter.
- Judge by sound and chips before trusting any number.
Frequently asked questions
Does shortening the stickout on a router bit help?
Far less than the advice suggests, on a hobby machine. Deflection genuinely goes as length cubed over diameter to the fourth, so the physics is real — but carbide is so stiff the numbers are tiny. A 6.35 mm bit at 25 mm stickout under 36 N deflects about 0.004 mm, while a mid-range hobby gantry deflects 0.6 mm under the same load. The machine is roughly 150 times the bit.
What actually flexes on a hobby CNC?
The machine, by a wide margin — gantry, Z axis, spindle mount and often the workpiece. At an ordinary stickout of four times the bit diameter, the frame outflexes the tool by ten to a hundred times depending on the class of machine. That is why stiffening a gantry changes results dramatically while fussing over tool length changes almost nothing.
When does bit deflection actually matter?
Past roughly thirteen to twenty-one times the bit diameter in stickout, which almost nobody runs, or on a genuinely rigid industrial machine where the frame no longer dominates. The crossover works out as the cube root of three times the modulus times the second moment over the machine stiffness — and the cutting force cancels out of it, so the answer does not depend on how hard you are cutting.
Why is my CNC burning the wood instead of cutting it?
Almost always because the chip is too thin, so the edge rubs and generates heat rather than shearing material away. A quarter-inch two-flute at 18,000 rpm needs about 3,600 mm a minute to make a 0.1 mm chip, which is past most hobby machines — so the real chip comes out near 0.056 mm and the bit polishes the wood instead of cutting it.
Should I raise or lower the spindle speed if a cut sounds wrong?
Lower it, which is the opposite of most people instinct. Chipload is feed divided by rpm times flutes, so with the feed rate capped by the machine, rpm sits in the denominator. Dropping from 18,000 to 9,000 rpm at the same 2,000 mm a minute takes the chip from 0.056 to 0.111 mm and it cuts properly. Raising the speed thins the chip further and makes the burning worse.
How many flutes should a CNC router bit have?
On a hobby machine, as few as possible — usually one. Chipload divides by the flute count, so a single flute reaches a working chip at half the feed rate a two-flute needs. Two flutes give a better finish and suit a machine that can genuinely feed fast enough. The two-flute bit shipped with most machines is a common cause of burning on light gantries.
What is chipload and how do I calculate it?
The thickness of material each cutting edge removes per revolution, in millimetres per tooth. It is the feed rate divided by the spindle speed times the number of flutes. Around 0.1 mm per tooth suits a quarter-inch bit in plywood. Too thin and the edge rubs, generating heat and rounding the carbide; too thick and the bit deflects or breaks.
Why does taking a lighter cut make burning worse?
Because of radial chip thinning. Below a half-diameter stepover the actual chip comes out thinner than the commanded chipload, by a factor of the diameter over twice the square root of the engagement times the remainder. At a ten per cent stepover, a commanded 0.10 mm chip is really 0.06 — the feed has to rise sixty-seven per cent just to stand still. The gentle-sounding setting is the one that burns.
What is radial chip thinning?
The reduction in real chip thickness when the cutter engages less than half its diameter. The tooth enters and leaves the material along an arc rather than cutting a full slot, so the maximum chip it takes is smaller than the distance the machine advanced. The factor is scale-free — it depends on the fraction of the diameter engaged, not on the bit size — and it reaches 1.67 at a ten per cent stepover.
How can I tell whether the bit is cutting or rubbing?
By sound and by what comes off. Cutting sounds like a steady tearing and throws chips you can see and pick up. Rubbing whines, smells hot and produces dust. Dust from wood is the single most reliable sign that the chip is too thin, and it is one of the few diagnoses you can make without stopping the machine.
Should I climb mill or conventional mill on a hobby CNC?
Conventional, more often than the general advice suggests. Climb milling pulls the cutter into the work, and any backlash in the machine converts that pull into a lurch — so the received wisdom to climb for a better finish quietly assumes a machine that cannot be pulled. On a light gantry, conventional cuts are less exciting even though the finish is marginally worse.
How deep should the first pass with a compression bit be?
Deep enough to bury the transition between the up-cut and down-cut sections, usually two to three millimetres depending on the bit. A first pass shallower than that leaves the up-cut portion working alone at the top surface, which tears the face exactly like a plain up-cut bit — and defeats the entire reason for buying a compression bit.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.