Measuring runout at the collet, and what the number tells you

Runout is how far the tool tip wanders from a true circle as the spindle turns. It is the number behind poor finish, short tool life and the "it just is not cutting like it used to" feeling, and it is easy to measure.

What you need

• A dial indicator reading in hundredths of a millimetre or tenths of a thousandth of an inch, on a magnetic base.
• A ground test pin or a precision dowel that fits the collet. Not a cutter: flutes make the reading meaningless.
• A clean collet and nut, torqued properly.

Measuring

1. Clean the collet, the nut and the collet seat in the spindle nose. Dirt here is the most common cause of a bad reading.
2. Fit the pin with a good length showing, say 50 mm, and torque the nut.
3. Set the indicator against the pin close to the collet, at right angles, with a little preload on the needle.
4. Rotate the spindle slowly by hand through a full turn. The difference between the highest and lowest reading is the total indicated runout.
5. Move the indicator out towards the end of the pin and repeat. A reading that grows with distance from the collet is angular error; one that stays the same is a radial offset.

Reading the number

Perske specifies radial and axial run-out of 8 microns or better on a new motor; a good rebuild reads in the same range. A collet and nut in good order add a few microns of their own. So:

• A few microns close to the collet: the spindle is fine.
• Noticeably more, and it changes when you rotate the collet in the nose: the collet or the seat. Try a new collet and clean the taper.
• Noticeably more, and a new collet does not change it: the bearings or the shaft. That is a rebuild.
• A reading that changes as the spindle warms up: bearing play or preload that has gone. Also a rebuild.

Keep a log

Measure every spindle when it is new or newly rebuilt, and again every few months. The absolute number matters less than the trend: a spindle that reads 5 microns for a year and then 15 is telling you something months before it makes a noise. Send the log with your quote request; it shortens the diagnosis.