# Deep cavities

Designing machined cavities around tool reach, corner access, chip removal and the required finished geometry.

Reviewed: 2026-11-01

Tags: parts-and-design, features, machining

Check depth relative to the corner-finishing cutter and its access. A wide opening can still require a slender tool at a tight corner.

The 60 mm deep, 80 mm wide example has 3 mm corner radii. Those corners limit the cutter to 6 mm diameter, giving **at least 10:1 depth to cutter diameter** before holder clearance. At 10 mm corner radius, a hypothetical 16 mm cutter gives **3.75:1**. These ratios screen geometry. Tool projection, flute length and cutting loads still need checking.

## Describe depth and access separately

Record cavity depth from a named surface, opening dimensions, the narrowest internal passage, vertical corner radii, floor geometry and nearby obstructions. Include material and condition: a tool that reaches a feature geometrically has not thereby been qualified to cut that material at the required finish and tolerance.

Protolabs Network recommends cavity depth up to four times cavity width as general design guidance and calls depths beyond six tool diameters deep-cavity milling. These compare different quantities. Its article also states that industry-wide specific design standards do not exist. Treat such ratios as prompts for manufacturing review, not proof of feasibility or a universal rejection threshold. [Protolabs Network, How to design parts for CNC machining](https://www.hubs.com/knowledge-base/how-design-parts-cnc-machining/)

| Quantity | Meaning for review |
| --- | --- |
| Cavity depth | Distance into the workpiece from the specified entry reference |
| Cutter diameter | Cutting envelope, constrained by passages and corner radii |
| Cutting length | Axial length of the tool's cutting edges |
| Tool reach | Manufacturer-defined reach dimension; inspect its drawing |
| Assembly overhang | Unsupported projection in the selected holder/setup |

Do not substitute overall tool length for reach or flute length. The holder, shank, neck, adjacent walls and entry motion all need clearance, including intermediate roughing shapes rather than only the final CAD surface.

## Corner geometry can control the whole process

For a simple vertical internal corner cut with a cylindrical end mill, the cutter radius must fit within the specified corner radius. A radius that is merely equal to the cutter radius also leaves less freedom for a smooth cornering path. Enlarging the cavity opening without changing a restrictive bottom corner may therefore leave the most difficult finishing operation unchanged.

Sandvik's profile-milling checklist asks designers/process planners to establish minimum radii, maximum cavity depth, removal volume and clamping. It distinguishes roughing, semi-finishing, finishing and rest milling of remaining stock. This supports using a larger roughing tool followed by a different tool for restricted regions, rather than assuming one tool removes the entire cavity. [Sandvik Coromant, Profile milling](https://www.sandvik.coromant.com/en-us/knowledge/milling/profile-milling)

Where function allows, increase internal corner radii, reduce unnecessary depth, or provide a wider upper region for tool access. Preserve the mating component's envelope and seating requirements. A corner relief can solve interference for a square mating part, but extra material removal must not cut into a sealing land, fastener seat or required wall.

## Reach is not allowable engagement

Harvey Tool **795322**, a square long-reach/stub-flute end mill for aluminum alloys, lists a 1.000 mm cutter diameter, 1.500 mm length of cut, 8.000 mm overall reach, 4.000 mm shank diameter and 50.000 mm overall length. The published reach multiple is 8×. These dimensions demonstrate that reach, cutting length and overall length are deliberately different. [Harvey Tool, tool 795322](https://www.harveytool.com/products/tool-details-795322)

The tool's 8 mm reach does not mean that its entire 8 mm projection can cut a wall simultaneously. Likewise, an opening that admits the 1 mm cutter may not admit its 4 mm shank. Check the complete tool drawing, holder position, usable neck region and collision envelope before selecting it. Its product category does not establish a recipe for every aluminum grade or operation.

Long assemblies also change force and vibration behavior. Sandvik's 14 February 2024 long-overhang guidance explains that a 90° entering-angle concept generates radial forces that encourage vibration and push-off, while a smaller entering angle directs more force axially. Damped cutters/adaptors are another option where the geometry requires a 90° concept. These are process choices for review, not permission to alter a required wall angle or assume chatter has been eliminated. [Sandvik Coromant, Long overhang—optimizing machining conditions](https://videos.sandvik.coromant.com/tips-film-long-overhang)

## Chips, entry and machining sequence

Removed material must leave the cutting zone. Sandvik's Silent Tools milling guide emphasizes chip evacuation to avoid recutting, particularly in deep cavities, and notes that coarse-pitch cutters provide more chip space. Its guidance is specifically about milling with those tooling concepts; air delivery, coolant choice and containment still need to suit the actual material and machine. [Sandvik Coromant, Silent Tools milling application guide](https://www.sandvik.coromant.com/en-gb/tools/silent-tools/what-is-silent-tools/application-guide-silent-tools/milling-appl-guide)

Access for an end mill is also access for a programmed entry path. Do not assume that every tool can plunge axially, ramp at any angle, or enter a narrow pocket by helical motion. Request a process plan that accounts for entry, step-downs, remaining corner stock and evacuation before assigning a cycle time from cavity volume alone.

For long-overhang or vibration-sensitive work, Sandvik describes plunge milling as an alternative and trochoidal milling as a way to reduce radial forces in suitable slots. Its slot-milling guidance distinguishes open and closed geometry and cutter concepts. Those alternatives require compatible tools and programming; they do not make every deep closed cavity equally accessible. [Sandvik Coromant, Groove or slot milling](https://www.sandvik.coromant.com/en-us/knowledge/milling/groove-or-slot-milling)

## Original geometry screening example

Assume a proposed cavity is 60 mm deep, 80 mm wide at its opening and has 3 mm vertical corner radii. Its depth/opening-width ratio is only `60/80 = 0.75`. Yet a cylindrical cutter intended to reproduce those corners must have diameter no greater than `2 × 3 = 6 mm`. The depth/cutter-diameter ratio is then at least `60/6 = 10`, before adding any extra projection for holder clearance.

If the design can instead accept 10 mm corner radii, a hypothetical 16 mm cutter passes the corner-radius screening check, and the ratio becomes `60/16 = 3.75`. This is a geometric comparison, not tool selection or a capability guarantee. Flute length, neck clearance, holder geometry, entry path, cutting loads and tolerances remain unresolved. A larger roughing tool might already fit the first design; the small corner-finishing tool is the issue being isolated.

This example explains why a “depth less than four times width” check cannot replace feature-level access review.

## Release and inspection requirements

Define what the cavity must accomplish before tightening every surface tolerance. Distinguish bottom depth, floor flatness, wall location/profile, corner radius and surface texture. A single depth reading cannot establish wall straightness or floor flatness. Confirm that the planned inspection probe or other measurement method can reach the required surfaces and establish the intended datums.

For a quote or process review, provide:

1. Material/condition, stock envelope and the complete cavity geometry.
2. Required datums, critical tolerances, texture and functional interfaces.
3. Permission or restrictions for larger radii, reliefs, stepped depths or alternate access.
4. Wall/floor thickness and any acceptable changes in stock or setup orientation.
5. Inspection access and the required treatment of burrs and remaining material.

If no suitable tool assembly can reach the geometry, reconsider the part architecture or discuss another manufacturing route. Splitting a component, changing access, or using another process changes assembly, sealing and tolerance responsibilities; compare the finished function rather than machining time alone.

## Source scope

The Sandvik long-overhang video displays 14 February 2024. The other cited technical/product web pages do not display a technical revision edition; their retrieved specifications and guidance are bounded above rather than represented as standards. Manufacturer dimensions and screening ratios are not a substitute for supplier approval of the actual part and process.

## Related reference

[Part features](index.md)