In CNC machining, many factors can lead to machining errors, and one significant contributor is radial runout of the cutting tool. Radial runout directly affects the minimum achievable form error under ideal conditions, as well as the geometrical accuracy of the machined surface.
In real cutting scenarios, tool runout impacts dimensional accuracy, surface roughness, uneven tool wear, and the performance of multi-flute tools. The greater the runout, the more unstable the machining process becomes—leading to inferior results.
Causes of Radial Runout
Radial runout in CNC milling can be caused by manufacturing or assembly errors in the tool or spindle, misalignment between the tool axis and spindle rotation axis, improper clamping, or specific machining conditions.
1. Runout from the Spindle Itself
Common sources of spindle radial runout include:
- Coaxial misalignment between spindle journals
- Bearing inaccuracies
- Misalignment between bearings
- Spindle deflection
These factors originate from the machine’s manufacturing and assembly stages. As such, they are hard to eliminate during operation but still affect spindle rotation accuracy to varying degrees depending on the machining method.
2. Misalignment Between Tool Center and Spindle Center
When installing a tool, if its center does not align with the spindle’s rotational center, radial runout is inevitable. This misalignment can be caused by:
- Poor fit between tool and collet
- Improper tool installation
- Tool quality issues
3. Effects of the Machining Process Itself
Radial cutting forces generated during machining can intensify tool runout. These forces (a component of the total cutting force) cause tool deflection, workpiece bending, and vibration—significantly affecting surface finish and dimensional accuracy. They depend on:
- Cutting parameters
- Tool and workpiece materials
- Tool geometry
- Lubrication method
- Machining strategy
Methods for Reducing Radial Runout
Since radial runout is primarily worsened by radial cutting forces, reducing these forces is key to minimizing runout. Below are several practical methods to achieve this:
1. Use Sharp Cutting Tools
- Larger rake angles make tools sharper, reducing cutting force and vibration.
- Larger relief angles reduce friction between the tool’s flank and the workpiece’s surface, lowering vibration.
However, rake and relief angles should not be too large, or tool strength and heat dissipation may suffer. For roughing, smaller angles are acceptable; for finishing, use larger angles to ensure sharpness and minimize runout.
2. Use High-Strength Tools
There are two main ways to increase tool stiffness:
- Increase shank diameter: A 20% increase in shank diameter can reduce runout by up to 50% under the same cutting force.
- Shorten tool overhang: Longer overhang increases tool deflection, causing fluctuating runout and rough surfaces. Reducing overhang by 20% can also cut runout by about 50%.
3. Ensure Smooth Tool Rake Surface
A polished rake surface reduces chip friction, cutting forces, and ultimately tool runout.
4. Keep Spindle Taper and Collet Clean
Always clean the spindle taper and collet before installation. Avoid dust or machining debris.
Also, choose tools with shorter overhangs, and apply even, appropriate clamping force—neither too tight nor too loose.
5. Use Proper Depth of Cut
- Too light a cut may cause the tool to slip, leading to variable runout and poor surface finish.
- Too heavy a cut increases cutting force and tool deflection, also worsening runout and finish.
Choose a balanced depth of cut based on the tool and material.
6. Use Climb Milling for Finishing
In conventional milling, backlash between leadscrew and nut changes constantly, causing uneven feed, impact, and vibration. This affects surface finish and tool/machine life.
In climb milling, chip thickness and tool load increase gradually, leading to a smoother cut—especially suitable for finishing.
For roughing, climb milling is still preferred due to its higher productivity and longer tool life.
7. Use Appropriate Cutting Fluids
- Water-based coolants mainly aid in cooling and have little effect on cutting forces.
- Oil-based lubricants, on the other hand, can significantly reduce cutting forces through lubrication—lowering friction on both rake and relief faces and thereby reducing tool runout.
Final Note
In practice, as long as the machine is accurately manufactured and assembled, and proper tooling and machining processes are used, the negative effects of radial tool runout on workpiece accuracy can be significantly minimized.
