Tool Runout vs. Tool Deflection—What Are the Differences?

Tool Runout

Tool runout is the amount by which a tool rotates off its intended axis. For the most useful reading, check near the cutting end of the tool, where runout is often greater than it is near the holder.

To check runout, secure an indicator, bring it into contact with the tool, and zero the indicator. Slowly rotate the tool—typically counterclockwise to avoid catching a cutting edge—and observe the reading. If the indicator changes as the tool turns, the tool has runout.

With excessive runout, one side or flute performs more cutting than the others. An end mill may produce an oversized milled hole even when the program and offsets are correct. A drill may create an oversized or oblong hole.

Runout can originate in the cutting tool itself, including a new tool, because manufacturing precision varies. A defective or inaccurate toolholder can also cause it. With some longer tools, such as drills and reamers, slightly repositioning the tool in the holder may reduce the measured runout.

Tool Deflection

Tool deflection is bending caused by cutting pressure. It commonly occurs during side milling with an end mill and can leave a tapered feature.

For example, consider profiling a part that should measure 1.950 inches wide and long and 2.000 inches tall. If a 3/4-inch end mill with more than 2 inches of flute length produces a top measurement of 1.951 inches but a bottom measurement approaching 1.954 inches, the long tool may be bending under the cutting load.

Reducing Tool Deflection

Use the shortest practical cutting tool and flute length for the feature. A shallow hole does not require an unnecessarily long drill, and a feature only 0.400 inches deep is better served by an end mill with a flute length close to the required depth.

Larger-diameter tools are generally more rigid and resist deflection better. Keep the tool extension from the holder as short as practical, but do not clamp the holder onto the flutes.

Feeds and speeds also matter. An excessive feed rate creates more cutting pressure and can increase deflection. Reducing the feed rate may help, but some features require multiple passes to achieve square, parallel, or perpendicular surfaces.

Key Difference

Runout is an off-center rotational condition that causes unequal cutting between flutes. Deflection is temporary tool bending under cutting forces. Runout is addressed by inspecting the tool, holder, and setup; deflection is reduced by improving rigidity, shortening tool extension, adjusting cutting parameters, and using multiple passes when necessary.