Depth per pass (stepdown) for CNC routers
Depth per pass, also called stepdown or depth of cut, is how far down each pass cuts. A common starting point in wood is one times the cutting diameter for a rigid machine and about half that for a light hobby machine. It is the first setting to reduce when anything sounds or looks wrong.
What it is
If you want a pocket half an inch deep and your depth per pass is 0.125 inches, the machine makes four passes, each one going a little deeper. Depth per pass is that increment. CAM calls it depth of cut, stepdown, or pass depth. All the same thing.
It is axial engagement: how much of the tool's length is in the material. Stepover is radial engagement: how much of its diameter. Together they decide the total load.
Starting points
Rule of thumb In wood, expressed as a multiple of cutting diameter:
| Machine | Typical depth per pass | Notes |
|---|---|---|
| Light hobby gantry, trim router | 0.25 to 0.5 times diameter | Frame flex is the limit, not the bit |
| Solid hobby machine, spindle | 0.5 to 1 times diameter | The common working range for most people |
| Rigid prosumer or industrial | 1 to 2 times diameter | Bit and holder become the limit |
| Any machine, hardwood | Reduce by roughly a third | Higher cutting forces |
| Any machine, small bits under 1/8 inch | Be conservative regardless | These break from deflection, not from load |
So for a 1/4 inch bit on a typical hobby machine, somewhere around 0.125 inches per pass is a sensible place to start in softwood, a little less in hard maple.
Bit makers' own charts often run deeper than this table. Onsrud, for example, publish wood chiploads at one, two and three times diameter of cut, because their charts assume an industrial machine that can hold those loads. The table above is scaled to what hobby frames can hold, which is the limit that actually applies to you.
Why deeper is sometimes better
It is tempting to think shallow passes are always safer. They are not always better, for two reasons:
- Tool wear concentrates. Shallow passes use only the bottom of the cutting edge, over and over. That small band dulls while the rest of the flute stays sharp. Deeper passes spread the wear along the flute.
- More passes means more time and more entries. Each pass has its own lead-in, and entries are where problems happen.
There is a real tradeoff. Very deep passes load the machine and deflect the tool. Very shallow passes waste the tool and the clock. The middle is where good work happens.
Deflection is the real limit
A router bit is a cantilever. Push it sideways and it bends, and the deeper the cut, the greater the sideways force. Two consequences:
- Walls are not square. The tip bends away from the material more than the shank does, so it removes less at depth: a part cut on the outside comes out wider at the bottom than the top, and a pocket comes out narrower at the bottom.
- The last pass cleans up the accumulated error, which is why a light finishing pass at full depth produces a much better wall than the roughing passes did.
Deflection grows extremely fast with stickout. Reducing how far the bit sticks out of the collet is one of the highest value, zero cost changes available to you.
Slotting is the hard case
When a bit cuts a slot, it is engaged on both sides with nowhere for chips to go. It is the most demanding operation a router bit does, and the depth per pass should be reduced accordingly, often to half what you would use for a cut with one side open.
Any full-width profile cut is a slot. This surprises people: cutting a part out of a sheet is slotting from the first pass to the last. If your profile cuts are the ones that break bits, this is why.
When to reduce it
Depth per pass is the first thing to reduce when anything is wrong, because it has the largest effect on load and the smallest effect on finish. Reduce it when you hear chatter, when the machine visibly flexes, when a bit breaks, when steppers lose position, or any time you are cutting something unfamiliar and want the first attempt to be forgiving.