Spindle speed (RPM) for CNC routers
RPM decides how often a cutting edge hits the work. Running high RPM with a feed rate you cannot raise to match is the most common cause of burning. For most wood work on a hobby machine, the middle of the dial with a feed rate to match beats maximum RPM.
What RPM actually controls
RPM sets how many times per minute each cutting edge passes through the material. It does not, on its own, make the machine cut faster or better. Paired with a feed rate it sets the chipload, and that is where its real effect lies.
Why maximum is usually wrong
A trim router at 30,000 rpm with two flutes needs 600 inches per minute to hit a 0.010 chipload. Almost no hobby machine can move that fast in a cut. So the practical result of running flat out is a very low chipload, which means rubbing, heat and burning.
Lowering the RPM raises the chipload at the same feed rate. It is often the single most effective change for a burning problem, and it costs nothing.
Nothing changed except the dial, and the chip almost doubled in thickness. That is often the difference between burning and not burning.
Why maximum is not always wrong either
High RPM genuinely helps in some cases:
- Very small bits. A 1/32 inch cutter has a tiny circumference. It needs high RPM just to get reasonable cutting speed at the edge, and its chipload is tiny anyway.
- Fine detail and V-carving. The tip of a V-bit moves slowly relative to the machine, and higher RPM improves the surface it leaves.
- Very light finishing passes. Where you are not trying to remove material, you are trying to leave a surface.
Routers and spindles behave differently
| Trim router | VFD spindle | |
|---|---|---|
| Typical range | Roughly 10,000 to 30,000 rpm | Often 6,000 to 24,000 rpm |
| Torque at low speed | Drops off noticeably | Holds much better |
| Speed control | A dial, approximate, no feedback | Set numerically, usually with closed loop control |
| Practical effect | Low RPM options are limited by torque loss | Low RPM is genuinely usable, so large bits work |
That torque difference is a large part of why spindles are worth the money for anyone doing heavy work or using large diameter bits. See router versus spindle.
Practical starting points
Starting point These are places to begin a test, not recommendations for your specific tool. Always prefer your bit maker's published range.
| Situation | Typical direction | Reason |
|---|---|---|
| Large bit (1/2 inch and up) | Lower RPM | Edge speed is already high; high RPM burns |
| Small bit (1/8 inch and under) | Higher RPM | Small circumference needs speed to cut properly |
| Hardwood, burning easily | Lower RPM | Raises chipload without needing a faster machine |
| Softwood, fuzzing | Higher RPM with a sharp bit | Cleaner shear on soft fibres |
| Acrylic | Lower RPM, fast feed | Melting is the enemy; thick chips carry heat away |
| Aluminium | Lower RPM than wood | The feed a hobby machine can hold only makes a real chip at a lower RPM; a thin chip welds to the edge |
A note on surface speed
In metalworking the controlling number is surface speed, the actual velocity of the cutting edge, and RPM is derived from it and the tool diameter. That framework matters if you cut aluminium. In wood it is far less critical, because wood tolerates an enormous range of cutting speeds, which is why the woodworking world talks about chipload and mostly ignores surface speed.