Full indicator movement is a range
The result is the difference between the highest and lowest readings, not the largest reading on its own. Where the operator happened to zero the dial changes both numbers and never their difference, which is exactly why the quantity is defined as a range.
Full indicator movement is the current term. Total indicator reading is the deprecated synonym for the same thing, and both appear on older drawings.
Circular against total
Circular runout is checked at one axial position at a time, with the indicator stationary while the part turns, and each position judged on its own. Total runout traverses the indicator along the surface while the part turns and puts every reading into one zone.
A tapered journal that is perfectly round and centred at every section passes circular runout everywhere and consumes total runout directly. Choosing the circular version on a bearing journal is a classic under specification.
Why a datum is always required
Runout describes movement relative to a datum axis, so with no axis there is nothing to rotate about and any reading can be produced by choosing a different setup. A datumless runout frame is invalid, in contrast to a form frame, which must not carry a datum at all.
Because the setup realises that axis from the physical datum feature, the form of the datum feature contaminates every reading. An odd lobed journal can turn in a vee block with almost no indicator movement while being visibly out of round, which is why datum features usually carry their own form controls.
What runout does not tell you
A circular runout reading collects eccentricity and out of roundness into one number, and the indicator cannot separate them. That is the right answer for an assembly requirement and the wrong tool for finding out why a process drifted.
When the requirement really is the location of an axis, a position tolerance to the datum axis states it directly and can be measured as such. Runout is the functional statement; position and circularity are the diagnostic ones.
One circular element, two readings
An indicator on a rotating shaft reads a high of 0.042 mm and a low of 0.006 mm at one circular element, against a stated 0.05 mm.
- High reading
- 0.042 mm
- Low reading
- 0.006 mm
- Full indicator movement
- 0.036 mm
- Tolerance
- 0.050 mm
- Verdict
- Accept
- Result if the high reading alone were quoted
- 0.042 mm
Quoting the high reading as the runout rejects a part that meets the requirement. The last row is the same arithmetic performed against an assumed zero, which is the shortcut this example exists to discourage.