Excessive tool runout: causes and solutions
Runout concentrates in a single measurement the combined effect of several error sources — spindle, interface, collet, tool. That is what makes it a valuable indicator, and a tricky diagnosis without method.
What runout is, and why it concentrates several errors
TLDR: Runout measures the gap between a tool's real axis of rotation and its theoretical one. It can come from the spindle, the toolholder, the collet or the tool — which makes diagnosis delicate without a rigorous method.
Runout denotes the measurable gap between the real axis of rotation of a mounted tool and its perfectly centred theoretical axis. It is a revealing indicator because it concentrates, in one measurement, the combined effect of several sources: the condition of the bearings, the accuracy of the spindle-toolholder interface, the state of the collet, and even the straightness of the tool itself.
Its effects are direct: uneven wear on the cutting edges — one tooth working more than the others — degraded surface finish, dimensional tolerances missed, and tool life sometimes spectacularly shortened.
The possible sources of excessive runout
TLDR: Bearing wear, interface defect, damaged collet, or a tool of insufficient quality. Each source calls for a different diagnostic method, and they differ enormously in the cost of correction.
Bearing wear is the deepest and costliest source to correct: abnormal internal clearance lets the shaft drift from its theoretical axis, and the defect passes down the whole chain. The role of bearings in accuracy.
An interface defect — damaged taper, worn face on a dual-contact interface, toolholder deformed after an impact — introduces a centring error even when the spindle is perfectly sound. Toolholder interfaces in detail.
A worn collet, or one unsuited to the application, is a very frequent cause — and often the easiest and cheapest to fix. The signs of a worn collet.
The quality of the tool itself, finally: shank straightness, concentricity between shank and cutting portion. This is particularly true of low-grade tools, or ones already damaged by an earlier impact.
How to isolate the source methodically
- Measure runout directly on the spindle taper, with no toolholder, using a dial gauge or a test bar, to isolate any spindle problem
- Test with a reference toolholder and collet, known to be in good condition, to rule those components in or out
- Run a quality reference tool rather than any tool to hand, so as not to add another error source to the diagnosis
- Compare measurements at different speeds — runout that worsens with rpm points to dynamic unbalance rather than a static defect
What tolerances to target by application
Acceptable tolerances vary widely. General-purpose roughing can tolerate several tens of microns with no noticeable consequence on the result. Precision work — moulds, aerospace, medical — or a finishing operation may demand under 5 microns, and under one micron on the most critical applications.
That is why the same level of runout can be perfectly acceptable on one application and unacceptable on another. The diagnosis must always be set against the real specification of the machining concerned, not against an absolute figure.
Frequently asked questions
What level of runout counts as normal?
It depends entirely on the application. Roughing tolerates far more runout than precision finishing. There is no universal threshold.
How do I tell whether runout comes from the spindle or the toolholder?
A direct measurement on the spindle taper, with no toolholder fitted, isolates the spindle’s own contribution from the rest of the chain.
Can a poor-quality tool really cause significant runout?
Yes. Concentricity between the shank and the cutting portion varies with build quality: a low-grade or damaged tool introduces a significant defect regardless of the spindle.
Can runout worsen with no impact or particular incident?
Yes. Normal progressive wear of the bearings and interfaces makes runout drift over time — which is why periodic checks pay off.
Do you carry out this check on site, or does the spindle have to come out?
A first dial-gauge measurement is done on site, machine stopped. A full diagnosis, with a test bar and comparison across several speeds, is done in the workshop.
Runout drifting on your machine?
We measure and isolate the real source of runout, from spindle to tool, before committing to any repair. That is what avoids replacing sound bearings when the real cause was a collet worth a few tens of euros.
Our teams are available from our sites in Beaurepaire (France) and Lussery-Villars (Switzerland).
Request a quote