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Total Indicated Runout (TIR) |
| There are two basic types of
runout in a rotary tool; static and dynamic. As their name implies, one,
static runout (static TIR), is the result of problems with the physical
dimensions of, or arrangement of the components of the
tool/collet/spindle system. The other, dynamic runout (dynamic TIR)
might also result for dimensional infidelity, but can include other
factors such as anisotropic (uneven) material density, worn out spindle
bearings, poor collet to spindle coupling, loose bits and spindle motor
resonances.
Of the two, dynamic TIR is generally much
harder to measure. Luckily, it is also usually smaller than purely
static TIR. Static TIR can be
easily measured by mounting a bit in a spindle and measuring the
concentricity of the mount using a test indicator. It is often a
combination of angular (azimuthal) TIR and radial (offset) TIR.
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Angular TIR is the result of a misalignment (skew) between the
rotational axis of the cutting tool and the central axis of the
collet/spindle system. Causes include improper use of set screws in a
two-point collet (like the CarveWright adapter), poorly aligned
central collet bore, worn spindle taper, and debris between the collet
and spindle bore tapers.
Radial runout is the result of a lateral (parallel) offset between
the rotational axis of the tool and the central axis of the
collet/spindle system. The most common causes are an offset collet bore
and mounting a tool with a shank smaller than the minimum diameter of the collet
gripping range.
To make
life more interesting, measurements of static TIR virtually always
reveal a combination of both angular and radial runout. Wouldn't you
know it? |
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