4mm CNC Router Bit: When This Size Is the Right Choice
A 4mm end mill is an unusual choice in CNC woodworking. Most hobbyists default to 6mm (1/4-inch) for general work and 3mm for detail. The 4mm sits between those two and earns its place in specific situations: detail passes that overflow 3mm but don’t need the full 6mm, tight-radius pockets, and precision profiling where the 6mm leaves too much material in inside corners.
A 4mm CNC router bit is an end mill with a 4mm diameter, typically with a 4mm shank. This diameter isn’t among the most common sizes in CNC woodworking but appears in detail work, tight-tolerance fitting, and European cabinetry workflows where 4mm metric sizes align with standard hardware dimensions.
The direct answer: 4mm router bits are most useful for pocket cuts with inside corner radii under 2.5mm, detail features that 6mm leaves incomplete, and hardware fitting (European hardware often specifies 4mm hole dimensions). For general routing work, 6mm and 3mm bits cover most requirements; 4mm earns its place in the tool cabinet for specific precision applications.
When 4mm Makes Sense
Inside corner radius: Any pocket requiring an inside corner radius of 2mm or smaller. A 6mm end mill leaves a 3mm radius in every inside corner. A 4mm end mill leaves a 2mm radius.
European cabinet hardware: Some European hardware specifications call for 4mm pilot holes or 4mm router profile dimensions in hinge and drawer slide mounting.
Detail routing in wood inlay: Wood inlay work with tight detail features that require smaller tooling than 6mm.
Key Specs for 4mm Bits
Flute count: Two-flute for wood and MDF. Four-flute for aluminum.
Upcut vs. downcut: Upcut clears chips from pockets better (cleaner pocket floor). Downcut produces cleaner top surface edges (less tearout on the face side).
Carbide grade: Solid carbide for CNC routing. HSS bits are not suitable for CNC use at production speeds.
Flute length: Ensure the flute length is adequate for your cutting depth. Cutting deeper than the flute length on the shank produces chatter and poor surface quality.
Feed Rate at 4mm
4mm bits are more fragile than 6mm bits. Feed rate and depth per pass must be reduced proportionally:
- Softwood: 1000 to 1800mm/min, 1mm DOC
- Hardwood: 600 to 1200mm/min, 0.5mm to 1mm DOC
- MDF: 1500 to 2000mm/min, 1.5mm DOC
- Spindle RPM: 18,000 to 22,000
Verdict
A 4mm end mill is a specialized tool for specific applications. It earns its place in the tool cabinet when inside corner radius or hardware dimension requirements make 3mm too small and 6mm too large. Not a first-priority purchase; a problem-specific solution.
- 4mm fills the gap between 3mm and 6mm for inside corner radius requirements
- Useful for European hardware fitting and tight-tolerance inlay work
- Reduce feed rate and DOC compared to 6mm due to lower rigidity
- Solid carbide only; HSS is not suitable for CNC production speeds
FAQ
When should I use a 4mm router bit over 3mm or 6mm? When inside corner radius requirements fall between the 1.5mm minimum of a 3mm bit and the 3mm minimum of a 6mm bit. Or when hardware specifications call for 4mm dimensions.
Can a 4mm bit cut hardwood? Yes, at reduced feed rate and depth per pass. Use 0.5 to 1mm DOC in hardwood with a solid carbide bit.
What spindle speed for a 4mm router bit? 18,000 to 22,000 RPM for wood. Calculate chip load: (feed rate) / (RPM x flute count).