Whenever tightening tools are discussed, the conversation inevitably turns to their accuracy—because accuracy is the decisive factor for joint quality. Different classes of tools carry different accuracy ratings, typically in the ranges of ±20 %, ±10 %, or ±5 %.

Examples
• Low-assembly-requirement applications (e.g., electronic toys) often use pneumatic screwdrivers or basic electric screwdrivers with 10 %–20 % accuracy. These tools meet basic tightening needs but cannot detect or report skipped, cross-threaded, or floating screws.
• In automotive plants, even non-critical interior trim stations generally require current-controlled tools with 5 %–10 % accuracy. These tools guarantee correct torque/angle and can flag tightening anomalies.
• Safety- or function-critical joints demand sensor-controlled tightening spindles with 3 %–5 % accuracy. These units monitor torque and angle in real time, store the data, and ensure full traceability.

How to Compare Tool Performance
Performance cannot be judged by intuition; it must be quantified. The German automotive industry introduced two rigorous indices: Cm and Cmk.

Definitions
• Cm (machine capability, no offset)
• Cmk (machine capability with offset)

Rule of Thumb
A tool is considered “good” only if
Cm ≥ 1.67 and Cmk ≥ 1.33.

Analogy
Two shooters both hit a tight cluster—similar stability (Cm). Shooter 2’s cluster is closer to the bull’s-eye, so Shooter 2 has better combined stability + accuracy (higher Cmk). Use Cm and Cmk to rank tools objectively.

Formulas
THI = upper torque limit
TLO = lower torque limit
μ = mean of test values
σ = standard deviation of test values

Cm = (THI − TLO) / (6 σ)
Cmk = min[(THI − μ)/(3 σ), (μ − TLO)/(3 σ)]

Worked Example
Target torque = 0.8 Nm
Allowed tolerance = ±10 % → THI = 0.88 Nm, TLO = 0.72 Nm
25 test readings: μ = 0.81 Nm, σ = 0.011 Nm

Cm = (0.88 − 0.72) / (6 × 0.011) = 2.42
Cmk = min[(0.88 − 0.81)/(3 × 0.011), (0.81 − 0.72)/(3 × 0.011)]
  = min[2.12, 2.73] = 2.12

Because both indices exceed the required limits, the tool is performing well and no re-calibration is necessary.