3D Milling Strategies

Overview#

When surfaces are sculpted or sloped rather than prismatic, 2.5D strategies are not enough. Fusion's 3D milling operations move the tool simultaneously in X, Y and Z (three axes) to follow complex geometry. Typical uses: molds and dies, organic shapes, sculpted enclosures, 3D relief carving, and any part with fillets, domes or blends that a pocket cannot describe.

A well-planned 3D job has three stages:

1. Roughing: remove the bulk quickly while leaving a uniform skin (stock to leave). Strategies: 3D Adaptive Roughing (previously 3D Adaptive Clearing) and 3D Offset Roughing (previously 3D Pocket Clearing).
2. Semi-finishing / rest machining: even out the skin and clean areas the roughing tool could not reach.
3. Finishing: cut the final surface with a ball, bull-nose, or barrel mill, using strategies chosen to match local geometry: steep, flat, or mixed.

Choosing the right strategy for each region is far more important than fine-tuning the parameters of a single "do everything" toolpath.

Where to find it#

• Milling > 3D on the Manufacture toolbar.
• Toolbar search (S): type the strategy name.

Autodesk marks some strategies as part of the Manufacturing Extension, including Steep and Shallow, Corner, 3+2 Roughing and the Machine Inclined Flats option, plus Surface Manager and undercut machining options. See the manufacturing extensions page.

Strategy overview#

The exact list and names depend on the version and the license. Treat this table as a guide and check the toolbar.

Common parameters across 3D operations#

3D operations share a common set of tabs like 2D ones (Tool, Geometry, Heights, Passes, Linking) with extra concepts.

Geometry#

• Machining Boundary: specifies how the toolpath boundary is defined. Tool Containment and Additional Offset control how the tool relates to the boundary.
• Contact Point Boundary / Contact Only (Parallel, Scallop): limit tool contact points.
• Slope: From Slope Angle and To Slope Angle restrict Parallel and Scallop to a slope range (0 degrees is horizontal, 90 vertical). For example, Parallel from 0 to 40 and Contour from 40 to 90.
• Rest Machining: sources are From Setup Stock, Union of all Dependent Operations, Include all Previous Operations, or a File, with the previous tool's diameter, corner radius, taper angle and shoulder length and Ignore Stock Less Than.
• Surface Manager (Manufacturing Extension): specify surfaces to machine, avoid, ignore or treat as fixtures.

Passes#

Common 3D parameters (names as in the Parallel, Scallop and roughing references):

Scallop height and stepover with a ball mill#

A ball end mill leaves ridges (scallops) between passes. The height h for ball radius R and stepover s on a flat surface is approximately:

h = R - sqrt(R^2 - (s/2)^2)

For R = 3 mm (6 mm ball) and a target h of 0.005 mm:

s = 2 * sqrt(2*R*h - h^2) = 2 * sqrt(0.03 - 0.000025) = 0.346 mm

So a 0.35 mm stepover gives about 5 micrometers of scallop on a flat. On sloped surfaces the effective scallop is larger, which is why strategies like Scallop (constant 3D scallop height) exist. Scallop Stepover is the horizontal stepover (default 95% of the cutter diameter less the corner radius), and you can limit the number of stepovers. The formula above is general geometry, not an Autodesk setting.

3D Adaptive Roughing#

Similar to the 2D version but working with the whole model shape in layered Z slices. Key options on the Passes tab: Optimal Load, Maximum Roughing Stepdown, Fine Stepdown, Stock to Leave (radial and axial), Machine Shallow Areas (with Minimum Shallow Stepdown and Maximum Shallow Stepover), Machine Cavities, Use Slot Clearing, Flat Area Detection, Order by Depth and Order by Area, Minimum Cutting Radius. On the Geometry tab: Machining Boundary, Tool Containment, Additional Offset, Rest Machining (by bodies or file) and Include Setup Model.

Roughing uses stepdown levels, producing visible stair-steps on slopes. A semi-finish pass with a smaller tool or 3D Offset Roughing can even this out before finishing.

3D Offset Roughing#

Level-by-level clearing with offset-style toolpaths. Options include Maximum Stepover, Minimum Stepover, Manual Stepover, Use Morphed Spiral Machining, Allow Stepover Cusps, Smoothing Deviation, Machine Shallow Areas, Maximum Roughing Stepdown, Fine Stepdown, Flat Area Detection and Order by Depth. General guidance (not Autodesk data): it can be more predictable than adaptive on machines that struggle with adaptive motion.

Finishing recipes#

A dome or hill#

Use Spiral or Radial from the center for even cutting, or Scallop for constant surface finish. The tool moves down the sloped surface in climb direction. Finish with Pencil on the base fillet.

A mold cavity#

Steps:

1. 3D Adaptive Roughing with a 12 mm flat or bull-nose, stock to leave 0.5 mm.
2. Rest machining with a 6 mm bull-nose, 0.2 mm stock to leave.
3. Contour on steep walls (slope 45 to 90) with a 6 mm ball, 0.2 mm stepdown.
4. Parallel on shallow areas (slope 0 to 45) with a 6 mm ball, 0.25 mm stepover.
5. Pencil in corners with a 3 mm ball.
6. A final Flat pass for horizontal faces.

A surface with steep and shallow mixed regions#

Use Steep and Shallow (Manufacturing Extension). A Threshold Angle (0 to 90 degrees, measured from horizontal) decides which areas are steep (Contour passes) and shallow (Parallel or Scallop passes); make it slightly larger than a slope you want treated as shallow, for example 21 for a 20-degree area. Surface-finish controls are Overlap Distance, Wall Clearance (Parallel shallow passes only), Smooth Offsets, Remove Cusps at Junctions and Continuous. A steep-first priority machines each steep wall in full before the shallow areas.

Rest machining#

Instead of letting a small finishing tool traverse large areas already cleaned, rest machining computes the material remaining after earlier operations and only cuts there. On the Geometry tab choose a Source (From Setup Stock, Union of all Dependent Operations, Include all Previous Operations, or a File), check the previous tool's dimensions where requested, and set Ignore Stock Less Than to skip tiny remnants. Rest machining relies on stock modelling, so the stock in the Setup matters.

Step by step: rough and finish a simple 3D part#

1. Create a Setup with stock sized 2 mm bigger than the model on each side and the WCS at the top center.
2. Add 3D Adaptive Roughing with a 12 mm flat end mill. Stock to leave: radial 0.5 mm, axial 0.5 mm. Generate.
3. Add 3D Offset Roughing with a 6 mm bull-nose to semi-finish, stock to leave 0.15 mm, with rest machining from previous operations. Generate.
4. Add Scallop with a 6 mm ball mill. Slope from 0 to 90, stepover 0.2 mm, stock to leave 0. Generate.
5. Add Pencil with a 3 mm ball mill to clean concave corners. Generate.
6. Simulate with stock. Check the stock result in Simulate for remaining stock.

Options reference (overall)#

Tips and pitfalls#

Tip: Rotate the pass direction of Parallel by 45 degrees relative to the dominant feature edge. Passes crossing features obliquely produce a more even finish than passes running exactly along or across them.

Tip: Split surfaces by slope: use Parallel (or Scallop) on shallow areas and Contour/Ramp on steep areas. A single strategy rarely does both well.

Tip: Simulate after each stage and check remaining stock; correct early rather than at the finish.

Warning: Small tolerance combined with a complex surface creates enormous G-code (hundreds of thousands of lines). Match tolerance to need; Autodesk warns that very tight tolerances cause large files and data starving on the control.

Warning: Ball mill finishing at the tool tip has near-zero surface speed. Prefer tilting the tool (multi-axis) or avoiding finishing where the surface normal is parallel to the tool axis with a ball tool; use a bull-nose or flat strategy for flat regions.

Warning: A 3D toolpath that depends on selected faces can fail after model edits. After changes, regenerate and inspect.

Note: Older tutorials call these Adaptive Clearing and Pocket Clearing; the current names are 3D Adaptive Roughing and 3D Offset Roughing.

Practice exercise#

Model a 100 x 70 x 30 mm block with a spherical dome (R = 25 mm, 12 mm tall) in the center and a 6 mm fillet at the base, plus a 15 mm diameter pocket at 20 mm distance. Create a Setup with a 3 mm top offset.

Run 3D Adaptive Roughing (12 mm flat, 0.4 mm stock to leave), a Scallop finish with a 6 mm ball (0.15 mm stepover), and Pencil with a 3 mm ball. Note total machining time.

Expected result: the simulation shows a smoothly finished dome, a cleaned base fillet, and no gouges. Change the stepover to 0.4 mm and observe the time drop and increased visible scallops.

Related#

• Pocket and adaptive roughing (the 2D versions)
• Feeds and speeds
• Toolpath simulation (checking remaining stock)
• Multi-axis milling (tilting to avoid the ball tip)