How deep a cut can I take with a router in one pass?
Why routing is a stepped operation, how to judge a pass by sound rather than by a rule of thumb, and why the last pass should be the lightest.
- Difficulty
- beginner
- Time
- 12 min
- Read
- 4 min
- Safety
- caution
Short answer
Take several shallow passes rather than one deep one, lowering the bit between each. Judge the depth by the router: if the motor note drops, the cut burns or the tool pulls, the pass is too deep. Go shallower in hardwood, in dense man-made board, with 1/4in shanks and with large-diameter profile bits — and always make the final pass a light skim, because that is the pass that leaves the surface.
A router removes material with a small cutting edge travelling very fast. The limit is not the motor's power but how much waste the flutes can clear and how much sideways load the bit and the tool can take without deflecting. Both are exceeded long before the motor stalls, which is why people take cuts that produce a poor result rather than an obvious failure.
Safety
Step by step
- Plan the cut in steps before you start.Decide the final depth, then divide it into passes. On a plunge router, set the turret stop so each step is preset and repeatable — that is what the rotating turret under the depth rod is for.
- Judge the first pass by sound and feel.There is no universal number, because it depends on the bit diameter, the timber, the shank and the router. Set a modest depth, make a test pass in scrap, and listen. A clean cut sounds even and the motor holds its speed. A drop in pitch, a burning smell, or a router that wants to run means back it off.
- Go shallower for a bigger bit.A large-diameter profile bit has far more cutting edge in contact at any moment, and much higher rim speed. Reduce both the depth of each pass and the router's speed setting for anything with a big diameter.
- Go shallower with a 1/4in shank.A quarter inch shank flexes noticeably under load, which shows as a wavy or stepped cut and, at worst, a bent or broken bit. Half-inch shanks tolerate more, but the rule is the same: several light passes.
- Go shallower in hardwood and dense board.Oak, beech, maple and dense MDF all load the cutter far more than softwood. The same pass depth that works in pine will burn in beech.
- Clear the waste between passes.Chips packed into a groove get recut, which generates heat and burns. Blow or brush the slot out between passes, and use extraction if the router has a port.
- Make the last pass a deliberate skim.Set the final pass to take only a whisker — a fraction of a millimetre. It removes the marks left by the heavier passes and produces a surface that needs almost no sanding. This single habit changes the quality of everything a router produces.
- Keep the same reference for every pass.Do not move the fence or the guide between passes. Only the depth changes. If the guide moves, each pass cuts a slightly different line and the walls of the cut end up stepped.
- For very deep work, remove the bulk another way.A deep mortise or a long housing is quicker and easier on the bit if most of the waste is drilled or sawn out first, leaving the router to cut only the walls and the floor to their final dimensions.
Tips
- A plunge router's depth turret is one of its most useful features and one of the least used. Setting three stops before you start means all three passes happen without any measuring.
- If you get an unwanted step in the wall of a groove, it is almost always because the router tilted on a narrow base or the guide moved between passes. A larger sub-base helps.
- Burning marks the exact place the router paused. Keep the tool moving continuously through a pass, including at the start and the end.
Questions people ask
Is there a rule of thumb for pass depth?
The common one is that a pass should not exceed the bit's diameter, and for a 1/4in shank considerably less than that. It is a reasonable starting point, but the honest answer is to set it by test cut and by the sound of the router, because a shallow pass in oak is a deep one in pine.
Why does my groove come out wider than the bit?
The bit is deflecting under too heavy a load, or the router is tilting on the workpiece. Lighter passes and a wider or steadier base fix both.
Can I take one deep pass on a router table?
It is safer than by hand because the work is supported, but the same limits on chip clearance, bit deflection and burning apply. Multiple passes still give a better surface.