Runout is not a property of the holder. It is the sum of the spindle, the interface, the holder body, the clamping element and the tool, and each contributes independently. Replacing the holder when the spindle taper is the dominant term is the most common and most expensive mistake in this area.
7 min read · Last reviewed 28 September 2026 · All resources
Measuring runout only at the cutting edge tells you the total and nothing about where it came from. The diagnostic version is to measure at each interface in turn: a test bar in the spindle, then the empty holder, then the holder with the tool.
Each measurement isolates one contributor. If the test bar already shows most of the error, the holder was never the problem.
Rotate the holder in the spindle by a known amount and measure again. If the high point rotates with the holder, the error is in the holder. If it stays put, the error is in the spindle.
The same test works one level down, rotating the tool within the holder. It is a few minutes of work and it usually settles an argument that would otherwise be resolved by buying something.
After the stack has been isolated, and the holder is demonstrably the dominant contributor. That is a real and common outcome — holders wear, particularly the clamping element, and a holder that has been in service for years may simply be at the end of its useful accuracy.
It is also the right answer where the holder was never suited to the duty: too much overhang, a clamping method that cannot hold the tool concentrically, or a body that flexes.
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It depends entirely on the operation and the tolerance you need to hold. A figure that is fine for roughing may be unusable for a reaming or boring operation. Rather than a universal number, work back from what the feature has to achieve.
No — only if the holder was the dominant contributor. That is why the isolation test matters before purchase rather than after.
Sometimes, where the wear is in a replaceable clamping element or where a seat can be reground. Whether it is economic depends on the holder type and how much material can be removed while keeping the interface correct.
Yes, with more contributors. Driven tool holders add their own drive train and bearings to the stack, so isolating each stage matters more, not less.
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