TIR is Total Indicator Runout, the full sweep a dial indicator reads as a rotating part turns one revolution. It captures both how off-center the tool is and how much it tilts, and it directly affects hole size, surface finish, and tool life.

Why does runout get worse with longer overhang?

A holder spec includes a small angular tilt as well as a centering error. Tilt projects outward along the tool: the farther the tip is from the gauge line, the larger the lateral displacement. Doubling the overhang roughly doubles the tilt-driven part of the tip TIR.

Are ER collets less accurate than shrink-fit?

Generally yes. Good ER collets hold about 0.0002 to 0.0004 in TIR at the gauge line, while shrink-fit and hydraulic holders reach 0.0001 in or better. For micro-tools, reaming, and fine boring the lower runout of shrink-fit meaningfully extends tool life.

How is the angular component estimated?

The model treats the rated TIR as the sum of a centering offset plus a tilt measured over the gauge-line reference length. It projects that tilt to the tool tip and adds the centering offset, giving a conservative estimate of tip TIR for a given overhang.

Is this a substitute for measuring?

No. It is a planning estimate to compare holder classes and overhangs. Always verify with a tenths indicator on the actual assembly, because collet cleanliness, tool shank tolerance, and seating force all shift real runout.

What is the Collet Runout Tolerance Calculator?

Estimates expected TIR at the tool tip from a toolholder runout specification at the gauge line and the tool overhang, using a geometric amplification model. Compares ER16, ER32, ER40, and hydraulic or shrink-fit holders so machinists can pick the right holder for precision work. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Collet Runout Tolerance Calculator

Name: Collet Runout Tolerance Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Get one useful tool a week

Like this tool? Enter your email and we'll send you one genuinely useful Gera tool a week — plus a link to come back to this one. No spam, one-click unsubscribe any time.

Runout you measure at the toolholder is not the runout your cutting edge sees. A small angular tilt at the gauge line is amplified along the length of the tool, so a long, slender tool tip can swing far more than the holder spec suggests. This calculator projects a rated holder TIR out to the tool tip and compares common holder classes so you can choose the right one for a precision job.

How it works

The rated TIR is modeled as a centering offset plus an angular tilt measured over a reference length. Tilt projects linearly with overhang:

tilt angle    = ratedTIR / referenceLength      (small-angle slope)
tip TIR       = centeringOffset + tilt × overhang

A conservative split assumes the rated TIR is dominated by tilt over its reference length, so projecting that slope to a longer overhang grows the tip TIR proportionally. Shorter, stubbier tools see less amplification; long reach tools see much more.

Toolholder classes compared

Holder type	Typical TIR at gauge line	Best use case
ER16 collet	0.0004–0.0008 in	Light finishing, small-diameter tools
ER32 collet	0.0003–0.0005 in	General milling and drilling
ER40 collet	0.0003–0.0005 in	Larger-shank tooling, roughing
Hydraulic chuck	0.0001–0.0002 in	Precision finishing, reaming
Shrink-fit holder	0.0001 in or better	Fine boring, micro-machining, high-speed finishing

These are typical ranges from manufacturer specifications; actual performance depends on holder condition, collet wear, and seating. ER collets are workhorses because they are fast to change and clamp a range of shank diameters within each collet range, but that flexibility comes at the cost of runout compared to dedicated shrink-fit or hydraulic holders.

Why runout matters so much for tool life and finish quality

Even a small amount of tip runout means the cutting edge does not travel the same arc on every revolution. In a two-flute end mill, runout causes one flute to take a heavier chip than the other. This unequal loading accelerates wear on the overloaded flute, shortens tool life, and leaves a periodic surface pattern from the varying chip load. For reaming, any runout will enlarge the bore above the intended diameter and degrade finish quality.

The practical threshold depends on operation:

General milling: runout under 0.001 in is usually acceptable
Finishing passes: under 0.0005 in preferred
Reaming: under 0.0002 in for close-tolerance holes

Example and notes

An ER32 holder rated at 0.0003 in TIR over a 1 in gauge reference, holding a tool with 2 in of overhang, projects to roughly 0.0006 in at the tip if the error is tilt-dominated. A shrink-fit holder rated 0.0001 in over the same reference projects to about 0.0002 in at the same overhang — a clear advantage for reaming or fine boring. Keep overhang as short as the part allows, clean the collet bore and tool shank before seating, and verify the final assembly with a tenths indicator rather than trusting any catalog number outright. The tool is a planning estimate; measurement is always the final word.