The short answer
A “sharp” internal 90° corner on a CNC pocket is almost never free. End mills are round, so a vertical internal corner inherits at least the tool radius as a fillet. If the drawing shows a knife-edge internal corner with no radius—or a radius smaller than any practical cutter—the estimator must add a smaller tool, an extra finish pass, EDM, a secondary op, or a DFM push-back. That choice widens the quote band and can flip the job into manual review.
When you upload STEP and PDF on the 3DWK quote page (quote-new.php), state the minimum internal corner radius you can accept, mark which corners are critical vs cosmetic, and note if wire EDM or broaching is already planned. This article is typical buyer guidance only—no invented cycle times, scrap rates, or shop certifications.
Why mills leave a radius
An end mill cuts with a circular cross-section. In plan view, the tool’s outside diameter sets how close it can approach two walls that meet at 90°. The leftover material in the corner is a fillet whose radius equals the cutter radius (half the tool diameter), plus any intentional clearance the programmer leaves for finish stock.
Typical shop practice on aluminum housings and plates:
- Rough with a larger cutter for metal removal rate
- Finish walls with a smaller cutter when the drawing demands a tight internal R
- Leave a practical floor fillet or wall–floor blend unless the drawing forces a sharper transition
CNC pockets, thin walls and undercuts covers the related wall-thickness and undercut rules. Deep pockets and tool reach on CNC quotes adds the stick-out problem: a tiny cutter that could hit a sharp corner may chatter or deflect once the pocket is deep. Sharp corners and deep reach stack.
What “sharp internal corner” means on a drawing
Buyers use the phrase in three different ways. Estimators hear all three:
1. **True knife edge**: two walls meet with R = 0 (or a microscopic CAD fillet). A standard end mill cannot produce this.
2. **Tighter than the roughing tool**: e.g. R0.5 mm when the efficient rougher is Ø8–Ø12 mm. Needs a finish tool change or a secondary process.
3. **Floor–wall blend**: the vertical corner looks fine, but the floor fillet or sharp internal edge at the pocket bottom is underspecified.
Clarify which you mean. A note such as “internal vertical corners R ≥ 1.0 mm OK unless marked CRITICAL” saves more quote churn than a silent R0.2 everywhere. Edge break, chamfer and deburr callouts CNC buyers should use is the external-edge cousin; keep internal R and external edge break as separate callouts.
How sharp corners move the quote
Estimators rarely add a line labeled “sharp corner surcharge.” You see the effect as:
- Wider min–mid–max bands when many corners need Ø2–Ø3 mm (or smaller) finish tools
- Manual review when R ≈ 0 on a hardened blank, a sealing pocket, or a deep cavity
- Extra ops when wire EDM, sinker EDM, or broaching is the only practical way to clear leftover material
- Longer prove-out when tiny tools must reach deep features without deflection
Reading a preliminary online quote (min / mid / max) and what needs_manual means on a preliminary quote both apply: automation often assumes a manufacturable internal radius from stock tooling. A STEP that models R0 with no PDF note is a classic reason the band widens or the job parks in manual review.
What drives CNC machining cost and what affects CNC machining lead time put corner radius in the same stack as setups, material, finish, and inspection. How quantity breaks change CNC unit cost matters when a prototype accepts a generous R and a production lot later demands EDM on every corner.
Practical radius guidance (typical, not a shop guarantee)
Use these as buyer starting points—always confirm with the quoting shop for your alloy and depth:
- **Aluminum pockets, moderate depth**: internal R ≥ tool radius of a comfortable finisher (often R1–R3 mm / roughly matching Ø2–Ø6 mm tools) keeps programming simple
- **Deeper pockets**: prefer larger R so a longer tool can still finish without an ultra-small diameter
- **Steel / stainless / titanium**: favor larger R; small carbide tools in hard alloys raise break and deflection risk
- **Plastic (POM, PA66, PEEK)**: sharp internal corners can still be milled, but thin remaining webs and heat at the corner matter—see POM vs PEEK for CNC functional prototypes
A corner radius slightly larger than the finishing tool radius (not equal to zero clearance) often machines cleaner than a theoretical match. Designing bosses and ribs for CNC housings and minimum wall CNC vs SLM for buyers both interact: a rib root with a tiny internal R is a classic thin-tool trap.
When EDM or a secondary process is the honest answer
Milling alone is the wrong tool when:
- The corner must be truly sharp for a mating insert, a sealing geometry, or a die/mold style cavity
- Hardened material must keep a knife edge after heat treat
- The leftover fillet from even a tiny mill would interfere with function
- A keyway-style internal corner is better broached or EDMed than pecked with micro end mills
Wire EDM for precision metal parts: when it wins vs milling and turning is the companion article. Call the process preference on the RFQ (“mill R1.0 OK on non-critical; CRITICAL corners → wire EDM after mill”) so quotes compare the same stack. Prototype vs production CNC quotes often splits here: qty-1 may accept a larger mill radius; a locked production print may add EDM.
Critical vs non-critical corners
Not every internal corner on a housing is functional. Mark the difference:
- **Non-critical / clearance**: state a minimum R the shop may grow to for efficient tooling
- **Critical / mating**: state exact R (or “max R” / “R0 via EDM”) and why
- **Cosmetic only**: prefer a consistent visible R rather than a mix of micro-fillets that show tool marks
GD&T callouts that raise CNC quote bands matter when a positional or profile frame sits next to a sharp pocket—inspection and machining both get harder. Flatness, parallelism and perpendicularity on machined plates and housings is separate from corner R but often appears on the same pocketed plate.
Drawing and RFQ notes that prevent wrong assumptions
Put these on the PDF or RFQ text—not only as silent CAD fillets:
1. Minimum internal corner radius for general pockets (e.g. “R ≥ 1.5 mm unless noted”)
2. Explicit R or “sharp / EDM” callouts on critical corners only
3. Depth of the pocket next to each tight R (reach drives tool choice)
4. Material and heat-treat state when the corner is finished
5. Whether floor fillets may differ from vertical corner R
6. Preferred secondary process if milling cannot meet the print
What files to send for a CNC quote and the DFM checklist before an online CNC quote both apply: STEP + PDF with readable R callouts beat a model full of accidental R0 edges from CAD defaults. How stock size changes a CNC quote and plate vs billet stock for CNC aluminum sit upstream—stock choice does not fix an impossible corner, but a thin plate with deep sharp pockets is a double flag.
RFQ checklist for internal corners
Before you submit on quote-new.php, confirm:
- Global minimum internal R is written once for non-critical pockets
- Every truly sharp or sub-millimeter R is marked CRITICAL with reason
- Deep pockets that also demand small R note depth so reach is visible
- EDM / broach preference is stated when milling cannot deliver
- External edge break is specified separately from internal R
- Inspection expectation on critical corners is clear (visual vs CMM vs optical)
CMM sampling plans for CNC lots and CMM vs optical measurement for machined parts help when corner R itself is a measured feature—rare, but real on sealing and mold-adjacent parts. Datum schemes for multi-setup CNC parts matters when the sharp pocket is cut in a later setup relative to primary datums.
Common buyer mistakes
- Exporting CAD with default R0 on every internal edge and assuming the shop will “just mill it”
- Demanding R0.2 mm at the bottom of a 40 mm deep pocket without accepting EDM or a redesign
- Mixing cosmetic and functional corners under one silent “sharp” note
- Forbidding tool-radius fillets “to match the render,” then rejecting DFM that would cut cost
- Ignoring wall–floor blends while obsessing over vertical R only
- Leaving anodize or hard-coat growth unstated next to a tight internal corner that must still clear a mating part—see as-machined before Type II anodize and how anodize choice changes CNC RFQ cost
When to call out undercuts on a CNC RFQ is related: an undercut plus a sharp internal corner is two process flags, not one.
Bottom line
Sharp internal corners on CNC quotes are really a tooling and process story. End mills leave a radius; smaller tools, deeper reach, harder alloys, and true knife-edge requirements raise cost, risk, and lead time—or push the feature to EDM. Write a practical minimum R for general pockets, mark critical corners explicitly, and upload STEP plus PDF with those notes on quote-new.php so estimators quote the same intent instead of three silent defaults. Ready to check a pocketed part? Start on quote-new.php.