has become a catch-all word for the sloppiness, flexibility, non-repeatability or just plain "poor quality" of a machine. Runout could mean many things to many people and the word is often thrown around as a bogeyman.
In the strict sense of the word, runout means the amount of deviation from the theoretical position
The term can be applied to rotary or linear motion, static or dynamic. Because most folk who actually measure runout use dial gauges (as in the pics above), the measurements are normally static, or at hand-turned speeds.
It also stands to reason that if you push/pull, ie. apply force, to the shaft/carriage while measuring, that you will get a bigger reading of runout. (Somebody who wants to prove bad runout will push/pull rather hard - a machine salesman who wants to prove low runout will not push/pull at all)
Why the different readings depending on force applied?
- Because ALL machines are flexible and they WILL bend/deflect. (Cables are much
more flexible than gear-rack teeth)
- Because there is often freeplay/backlash in a connection between parts of a machine and applying force causes the parts to move within this free range.
The other big cause of runout is a manufacturing defect, or damage. A bent shaft will not run "true" even if it was very stiff and had no freeplay. If a machined rail is not produced/setup straight then the rolling carriage will deviate from the theoretical straight line - ie. runout. If a 4-fluted router bit is put in a collet with a bit of dirt causing it to be held slightly skew, a dial indicator will get different readings from the flute edges - runout.
Can one achieve zero runout? No. Any machine that is doing the work it is designed for will bend and deflect. Normally these claims/demonstrations are made without applying loads and with measuring instruments of low resolution.
Is runout always bad? No. In the example with the 4-fluted cutter sitting slightly skew, it means that one of the flutes will take a bigger cut than the others as the cutter "wobbles". And the kerf will be wider than expected. For a single-fluted cutter, the only down side is the wider kerf. But, that wide kerf will be absolutely consistent in its width, and will be as straight as the carriage rails.
Is runout consistent? No. Aside from the fact that it is load dependent (as described above), it is also temperature dependant. A bearing that has a lot of clearance (backlash/freeplay) when cold, will have hardly any clearance when at the right running temperature - if it has been designed and installed correctly.