A CNC router uses a round rotating cutter. The cutter cannot enter a gap narrower than its diameter, and it leaves a related radius in every concave internal corner. These physical facts affect slots, pockets, small holes, lettering, thin walls and joints.
There is no single minimum feature size that applies to every project. Cutter diameter is only one input; depth, reach, material, workholding, finish and the importance of the detail also matter.
Review width, depth and material together
A feature that appears possible from above can become unsuitable when it is deep. A narrow pocket needs a cutter narrow enough to enter, long enough to reach and stiff enough to cut predictably. Increasing cutter length generally reduces stiffness, while deep narrow channels can make chip clearance more difficult.
For each small or deep feature, identify:
- minimum width or gap;
- depth and reference face;
- internal radius;
- neighbouring wall thickness;
- material and nominal thickness;
- whether the feature is functional or decorative;
- acceptable ways to simplify it.
A smaller cutter can reach finer detail, but may add tool setup, passes and checking. It is not automatically the better or cheaper choice.
Why internal corners remain rounded
The cutting edge rotates around a circular tool. When its centre changes direction inside a pocket or cut-out, the cutter radius remains in the corner. The machine can follow its programmed path correctly and still be physically unable to remove the final material from a mathematically sharp inside corner.
External corners are different. A cutter can travel around a convex corner and create the intersection of two outside edges, subject to the material and finishing result. The round-cutter restriction applies to concave internal geometry.
If nothing square needs to enter the corner, an intentional radius is often the cleanest design choice.
Minimum feature decisions
Narrow slots and channels
The cutter body must fit inside the feature, with a sensible machining route for its depth and length. If the width is not critical, widen it to suit a more robust tool. If the channel is decorative, a shallower engraved line may achieve the visual aim with less machining.
Small holes
A small drawn circle does not guarantee a useful routed hole. State whether it is a clearance hole, locating feature, pilot, recess or decoration. Provide the actual hardware. A marked centre for a later agreed drilling operation may be more suitable than forcing every small hole into the router programme.
Fine text
Lettering contains narrow strokes, counters and acute corners. Review the actual wording at final size. Simplifying the typeface, increasing the size, using a suitable single-line style or changing the depth may improve legibility. Finish can fill fine detail or change contrast.
Thin walls and islands
Material left between adjacent cuts must survive machining, separation, handling and use. Narrow islands can vibrate, chip or break. Their viability depends on height, support, material construction and cutter strategy. Highlight them explicitly instead of allowing them to disappear among decorative paths.
Four ways to handle a square mating corner
When a square tab or shelf must enter a routed pocket or slot, compare these options.
1. Accept an internal radius
If the mating part does not need to reach the corner, allow a radius and position the contact surfaces elsewhere. This creates simple, clean geometry and avoids an unnecessary relief mark.
Mark the minimum acceptable finished radius or visual intent rather than choosing a cutter without production review. A larger radius can permit a more robust cutter and may make finishing easier.
2. Round or chamfer the mating part
Changing the tab can preserve a clean pocket. Round its corners, add a chamfer or adjust the seating surfaces so they clear the internal radius. This works well when both mating parts are controlled and the changed shape does not harm assembly.
3. Add a dogbone relief
A dogbone extends the cut diagonally beyond the nominal internal corner. The extra clearance allows a square-ended part to seat. The relief remains visible beyond the corner and can reduce material locally, so its position and size must be deliberate.
4. Use a T-bone-style relief or open-ended feature
A T-bone moves the relief along one adjacent edge. An open dado removes the stopped corner completely. Either can improve seating while changing appearance, direction and remaining material. Show the insertion direction and visible faces before choosing.
The names are not sufficient manufacturing instructions. Put the approved geometry in the vector and show it on the drawing.
Blind pockets need a section view
Corner relief in a blind pocket also has depth and floor implications. A through-slot dogbone may not give a square part enough clearance at the base of a shallow pocket. The mating part may require a chamfer, or the recess may need another design.
State pocket depth, floor requirement, insertion direction and the surfaces that locate. If adhesive is used, consider glue area and squeeze-out rather than assuming a deeper relief is harmless.
Appearance, strength and finishing
A concealed joint may tolerate relief that would be distracting on a finished face. Relief can also leave a point that is awkward to sand or coat. Closely spaced dogbones can weaken a narrow tab or wall.
Ask:
- Is the relief visible after assembly?
- Does it remove material from a loaded or fragile area?
- Can it be prepared and finished cleanly?
- Will it collect adhesive or debris?
- Does the mating part actually need to reach that corner?
Corner treatment solves geometric clearance. It does not establish joint strength or long-term durability.
Design example: a divider in a stopped dado
For a plywood divider entering a stopped dado, four reasonable options may exist:
- Stop the divider short of the radiused end.
- Round the divider corners.
- Add two intentional reliefs to the dado.
- Open the dado at the panel edge.
The correct choice depends on the visible face, assembly direction, load path, finish and desired appearance. A small labelled corner coupon can compare fit and appearance before a complete assembly is machined.
Cost and tool changes
Fine detail may require a smaller cutter in addition to the tool used for larger profiles and pockets. That adds more than the physical swap: it can add tool identification, reference checking, another group of operations, test work and inspection.
Do not force all material removal onto one tiny cutter merely to avoid a change. Equally, do not add another tool for hidden detail that has no customer or functional value. The cost guide explains this trade-off in the context of the whole production route.
Frequent design mistakes
- Zero-radius corners are drawn for a square mating part.
- Cutter choice is based only on width while depth is ignored.
- Dogbones are added to every corner whether needed or not.
- Relief is placed on the best visible face without review.
- Thin walls remain between adjacent pockets.
- Outlined artwork creates fragile, noisy geometry.
- A long, narrow tool is requested where a shallower feature would work.
- Finish build on the tab or corner is omitted from the fit decision.
- A new cutter is introduced for detail that is not important.
Information needed for review
Send the vector and a marked PDF that identifies every small, deep or sharp feature. Include:
- feature width, depth and acceptable radius;
- material and thickness;
- mating-part geometry and insertion direction;
- visible faces and finish plan;
- final-size lettering or artwork;
- functional versus decorative priority;
- acceptable simplifications;
- the consequence if the feature is changed or fails.
No general chart can guarantee the result for every tool, sheet and depth. A representative coupon gives stronger evidence for a critical fit or appearance decision.
Ask for a feature and corner review, then use the joints and tolerance guide to define the mating behaviour.