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Preventing Tool Breakage By Controlling End Mill Runout

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When dealing with parts milling on a microscopic level, it pays to consider every facet of the process. This includes the type of tools you use and the methods used to ensure high parts quality, consistent results and optimal efficiency at every juncture. Miniature end mills are unique in the considerations that must be made for quality results. This includes the amount of tolerable runout needed to reduce parts breakage while ensuring a quality product.

The following offers some insight on excessive runout and steps you can take to bring it under control without losing your end mill or your product.

How Runout Affects Miniature End Milling

Sherry DePerno of Advanced Tool defines runout as variations in the end mill's diameter along its outer edge during rotation. As the cutting tool rotates against the milled product, each tooth is designed specifically to strike the exact same spot in the same manner.

Flaws in the tool manufacturing process or prior damage can cause microscopic variations in one or more teeth. When one tooth strikes the surface more than the others, it places a large amount of stress on that particular tooth. Radial and axial variations in runout can create premature wear and tear on the end mill itself or, in more serious cases, cause the tool to break during use.

Measuring Runout

According to Modern Machine Shop's Peter Zelinski, the commonly accepted amount of runout for larger end mills is 0.0005 inch, with many operators pushing the envelope at 0.0002 inch for precision work. However, miniature end mills, available at sites like, require runout that's much tighter than the aforementioned standard—a maximum acceptable runout of 0.0001 inch is needed for improved longevity and performance.

Depending on the type of end mill used, tighter runout can net greater gains in efficiency and tool durability. As Zelinski notes in an example, reducing runout from 0.0006 inch to as much as 80 microinches netted a three-fold gain in longevity for a 3-mm carbide cutting tool.

Because most miniature end mills are relatively microscopic in size, it's nearly impossible to visually inspect and physically measure runout. These tools are also notoriously sensitive—even by placing one down gently on a flat surface, you run the risk of breaking that tool. As a result, non-contact methods are needed to accurately test the amount of runout present in the end mill.

Strategies for Bringing Runout under Control

  • Test for both static and dynamic spindle runout error using non-contact measuring gauges prior to the start of the milling process. As the spindle attains operational speeds, the combination of centrifugal forces, heat and vibration can affect its initial runout. The spindle should be capable of maintaining low runout on its own before the cutting tool is brought into the picture.
  • Take a close look at the toolholder and how it plays a role in helping the mill end maintain strict concentricity as it operates. The toolholder' individual design features work in concert to affect runout. For instance, both the collet angle and collet nut have a sizable impact on the runout error.
  • Use high-quality precision parts that are better suited for microscopic milling work. For example, a precision collet nut features ball bearings that help counteract twisting forces that, under ordinary circumstances, could deform the collet and produce greater runout errors.
  • Consider the quality of the cutting tool itself. As mentioned before, manufacturing process errors can introduce runout errors from the start. Vendors should be made aware of such problems and work to correct these mistakes for future products.
  • Also consider spindle speed. While speeds of 12,000 rpm seem reasonable for larger cutting tools, small-diameter cutting tools require increased speeds to achieve comparable top cutting speeds. Some experts recommend spindles capable of achieving speeds of 80,000 rpm or more for the best results.

Solving the unique challenges surrounding end mill runout allows for longer-lasting tools and a small measure of certainty against parts breakage. It also goes a long way towards creating precision products with excellent results.