Square end vs corner end milling cutters
End mills are essential tools in machining, especially for milling operations on CNC machines. Their geometry directly determines the kind of cuts they can make and the surface finish they leave. Two common types are square end mills and corner end mills, which differ subtly in shape but significantly in application and performance.
Geometry and Terminology
Square End Mill
A square end mill has cutting edges that meet at a sharp 90-degree angle, forming a perfectly squared-off tip. All flutes terminate in a flat bottom with sharp corners. These are the standard end mills used for general-purpose slotting, profiling, and plunge cutting.
Corner End Mill (Corner Radius End Mill)
A corner end mill, often called a corner radius end mill, has a rounded radius at the tip rather than a sharp corner. The transition between the cutting face and the outer edge is filleted. This small radius may be subtle (e.g. 0.010") or larger depending on the application.
Functional Differences
Stress Concentration and Tool Life
The sharp corners on square end mills create points of high stress during cutting, particularly when plunging or navigating internal corners. This makes them more prone to chipping or wear at the corners. Corner end mills, by contrast, distribute cutting forces over a larger area due to the fillet, which reduces localized stress and prolongs tool life.
Surface Finish and Edge Quality
Corner end mills produce smoother transitions and less pronounced internal corners in pockets and contours. This is especially advantageous in molds or dies, where stress risers or stress concentrations in sharp internal corners can cause cracking. Square end mills leave sharper internal corners but may create more burrs or chipping if not well supported.
Applications
Square end mills are best suited for parts that require 90° internal corners, such as slots, pockets, and sharp-edged features. However, in practice, true 90° internal corners are rarely necessary in high-stress or load-bearing parts.
Corner end mills are preferred for part geometries where internal corners don’t need to be sharp, especially in aerospace, moldmaking, and structural components where durability and fatigue resistance matter.
Programming and Toolpath Considerations
CAM software often prefers corner radius tools for optimized toolpaths in high-speed machining, as these reduce sudden directional changes and tool engagement shock. Also, corner tools can run at higher feeds and speeds due to their lower risk of chipping.
| Feature | Square End Mill | Corner End Mill |
|---|---|---|
| Tip Geometry | Sharp 90° corners | Filleted radius at corners |
| Strength at Corners | Lower | Higher |
| Surface Finish | Sharper edges, more burrs | Smoother, cleaner transitions |
| Ideal Use Cases | Slotting, sharp internal corners | Moldmaking, fatigue-critical parts |
| Tool Life | Shorter due to corner wear | Longer due to stress distribution |
When to Choose Which
Use square end mills when precision internal corners are required and the part is not subject to high fatigue loads. Use corner end mills when durability, tool life, and surface quality are more critical than perfectly sharp internal corners. In practice, many shops default to corner radius tools for general-purpose work, only switching to square ends when geometry or tolerance demands it.