Specifying Internal Corner Radii, Cavity Depth and Tool Access on a CNC Milled Part

Rule out sharp internal corners before anything else

A milling cutter is a rotating cylinder, so the geometry it leaves in a corner is curved, not square⁴. An internal corner drawn sharp is therefore not a geometry the shop can cut as drawn: it becomes either an auto-filleted radius, a dog-bone relief, or a feature routed to EDM or wire-cutting^12,29.

One shop states plainly that sharp internal corners are filleted by default depending on cavity depth, and that special square-corner requests may carry extra cost²⁹. That default is the risk.

If you leave the corner unspecified, the shop picks the radius for you, and the value it picks may not suit the mating part you designed against. Decide per corner which of the three it is, and draw it.

Where a true square corner is functionally required, EDM or wire-cutting is the alternative the sources name, and one shop notes that wire-cut and EDM surfaces come off at ≥Ra6.3^12,29. Treat that roughness figure as that shop's stated output, not a general EDM finish.

Snap the corner radius to a standard cutter size

The radius you draw sets the cutter the shop must use, and cutter size drives cycle time. A smaller corner forces a thinner tool, which runs slower and is more fragile, so cost rises as the radius shrinks^9,17,26.

The worked case from one shop: a 3 mm internal radius lets the shop run a 6 mm endmill and cut two to three times faster than it can with sub-1 mm radii¹⁷. The published snap-to values line up with each other.

One shop sets the minimum standard corner radius at 1/8 in (3.175 mm) so standard end mills can be used¹⁰; another recommends matching radii to standard cutter sizes R3, R4 and R5 mm²⁶. Those are complementary rather than competing: both point at roughly 3 mm and the R3/R4/R5 ladder.

No source in this set publishes an authoritative cutter ladder, so the exact stocked sizes are a question for your shop.

Set the cavity depth limit and the ratio that triggers long-reach tooling

Standard end mills have limited cutting length relative to diameter, so a deep pocket forces the shop onto long-reach or reduced-shank tooling that deflects more and cuts slower^17,18.

The screening rule repeated across the sources is a depth-to-width ratio of 4:1² (supplier-reported figure): at or below it, standard tooling applies; above it, long-reach tooling and slower cutting follow^{2,10,12,17,18}. So when you size a pocket, check depth against width, not depth alone.

A narrow pocket that looks shallow in absolute terms can still cross 4:1 because its width is small² (supplier-reported figure). If your design needs to go deeper, the sources do not establish a cheaper alternative to the long-reach step.

Ask the shop what the deeper pocket does to its quote before you commit the geometry.

Check that the cutter can reach the floor without fouling

A radius that fits the cutter is not the same as a radius the cutter can turn without the shank or holder rubbing the wall. These checks catch the cases where the tool reaches the depth but not the corner.

• ✓Check each internal radius against cavity depth: the repeated guideline is radius ≥ 1/3 of cavity depth to allow tool access and limit deflection.
• ✓Check the radius against the cutter: set it at least the tool radius, and consider about 130% of tool radius or 0.02–0.05 in of relief so the tool turns the corner without heavy slowdown.
• ✓Check deep pockets for chatter risk: the deeper the cavity, the more tool stickout is needed, and a larger corner radius lets you use a larger, stiffer tool.
• ✓Check overhangs and undercuts: features not accessible to the cutter need special tooling and raise cost, and one shop recommends splitting the part instead.

Tolerance and finish only where the function needs them

A blanket tight tolerance slows every feature on the part, not just the ones that need it. One shop notes that a general ±0.02 mm callout on every dimension forces the machinist to slow down everywhere, and recommends letting non-critical features default to ISO 2768-m²⁶.

The default machined tolerance is quoted at ±0.005 in, with tighter tolerances requiring slow runs and extra check steps¹⁰. Tightening has a visible cost step.

Moving from ±0.005 in to ±0.001 in is reported at +40–80% per feature^{13 10} (supplier-reported figure). And one shop puts the move from ±0.005 in to ±0.001 in at 2–3x machining cost, with ±0.0005 in at 4–5x¹⁵.

Those are that shop's reported multipliers, not a market rate. The same logic applies to finish: as-machined Ra 125 is the standard, and polishing or grinding adds cost¹⁰.

Decide which bores and mating faces carry the tight callout, and leave the rest at default.

Count the setups before you fix the cavity orientation

Every additional setup adds non-cutting time before any metal is cut. The reported penalty is 15–60 minutes per extra setup¹³, with two other shops quoting 15–45 minutes^17,26.

Features that are not aligned to the machine axes are what force that extra fixturing, and one shop notes that such features push a part toward five-axis machining or complex fixturing⁶. Five-axis machining can reach five of six sides in one operation, which cuts the re-fixturing a multi-setup three-axis job would need³.

Whether that trade pays on your part depends on its geometry and the shop's rates, and the sources do not settle it for a given part. What you can do at the drawing stage is orient pockets and multi-face features so the critical ones are reachable from as few directions as possible¹⁷.

Write the callouts as intent plus a limit

A radius callout that pins one exact value over-constrains the shop; a callout that states the intent and a limit lets it use the cutter it has.

The sources do not publish standard drawing-note wording, so the practical move is to state the minimum radius and the maximum depth-to-width you will accept, and let the shop choose the tool inside that window. Confirm the shop's default rule first, because the defaults differ.

One shop's default minimum radius for a cavity of depth H is R = H/10 + 0.5 mm with cutter D = H/5 mm^{9 26}. And it states it can reach R0.5 mm only at depths of 3 mm or less⁹.

Another shop's stated defaults are a 1/8 in minimum corner radius, ±0.005 in tolerance and Ra 125 as-machined finish¹⁰. A third recommends R3/R4/R5 mm radii and ISO 2768-m for non-critical features²⁶.

Ask which rule applies to your drawing before you release it.

Where the sources disagree on radius and depth

The two rules buyers lean on most — the minimum radius and the maximum cavity ratio — are not settled by a single standard in this set. Keep both figures visible and confirm the one your shop actually works to.

Before you request a quote

• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.