How does the Thread Milling toolpath work in VCarve Pro & Aspire for cutting internal and external threads?

Overview

The Thread Milling toolpath machines helical threads directly into material using a thread mill cutter. It supports both internal threads (female, e.g. threaded holes for bolts) and external threads (male, e.g. decorative wooden bolts and lids). It is useful for metals, acrylics, hardwoods, and custom decorative fasteners, and it is available in both VCarve Pro and Aspire.

Two Tool Types

Single Point Thread Mill

Has one cutting tooth. The toolpath spirals the cutter downward (or upward), cutting one thread ridge per revolution.

Pros: Flexible. It can cut any pitch by compressing or stretching the helix, so one tool can cover many different thread sizes.

Cons: Slow. It cuts all threads with a single tooth, so many revolutions are required.

Tool definition fields required:

• S: Tool Size (horizontal width of the cutting face)
• H: Tool Height (vertical height of the widest part of the cutter)
• D: Tool Diameter (tip-to-tip diameter)
• A: Tool Angle (internal angle of the cutter)
• O: Tool Offset (distance from the bottom of the tool to the tip; must be greater than half of Tool Size)

Multi-Point Thread Mill

Has multiple cutting teeth spaced at thread pitch intervals. It cuts all threads in a single helical revolution, which is much faster.

Pros: Fast. One rotation completes the full thread length.

Cons: Fixed pitch, so it cannot cut different thread sizes. The thread length of the toolpath cannot exceed the tool's threaded length.

Additional field: Threaded Length (peak-to-peak distance from the first to the last tooth)

Key Settings

Thread Type:

• Internal: female threads (drilled holes, box lids, nut sockets)
• External: male threads (decorative bolts, threaded posts, lids that screw onto a base)

Handedness: Right-hand (standard, clockwise when viewed from above) or Left-hand

Cut Direction: Spiral downward or spiral upward. The choice depends on spindle direction, tooling, and desired surface finish

Pitch: The distance between thread ridges. Choose from ISO Metric presets (metric jobs) or Universal Thread Standard presets (imperial). Selecting a preset fills in the pitch value and, for external threads, a default fit tolerance.

Fit Tolerance (external threads only): A small clearance added so the external thread fits smoothly into an internal thread. Default values are filled in when you select a pitch preset. Adjust for a tighter or looser fit.

Start Depth: The depth at which threading begins (usually 0). For threading into the bottom of a pocket, enter the pocket depth.

Max Depth: How deep below Start Depth the threading extends.

Thread Length: Calculated from Max Depth, tool offset, and pitch. It is the actual depth of threaded engagement.

Workflow

1. Draw a circle vector representing the thread diameter (inside diameter for internal, outside diameter for external)
2. Select the circle vector
3. Open Thread Milling from the Toolpaths tab
4. Select a Single Point or Multi-Point tool
5. Set Internal or External thread type
6. Select the pitch from the ISO/UTS presets or enter it manually
7. Set Start Depth and Max Depth
8. Set handedness and cut direction
9. Calculate

Thread Standard

Thread Milling is based on the ISO thread standard, which uses 60° tool angles and produces threads with flat crests and roots. A 60° included angle tool gives optimal results. Other angles can be used but will produce non-standard thread profiles.

Practical Applications

• Wooden box lids: Cut matching internal (lid) and external (base) threads for a screwing wooden lid, a popular decorative application
• Threaded inserts: Machine threaded pockets for brass inserts in CNC-cut furniture
• Acrylic and aluminum parts: Thread functional holes for machine screws without tapping
• Custom decorative fasteners: Create unique large-format threaded elements for display or furniture

Pro Tips

These are shop-practice tips to test on scrap.

• For wood threads, slightly increase the fit tolerance on external threads. Wood threads need more play than metal to account for seasonal movement and grain compression
• Start with a larger pitch for wood (coarser threads are stronger in wood grain)
• Pre-drill the hole slightly undersized before threading to clean up the bore
• The software maker's user forum has many wooden box lid projects. Search the forum for "thread milling" for real-world settings