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The accuracy of a tightening tool directly determines how precise and stable the torque output is, and is therefore the key indicator for guaranteeing joint quality.

Yet when we evaluate a tool’s accuracy we often meet two seemingly similar terms – 6σ and ±3σ.

Can you clearly distinguish them?
And what is the critical difference?

1. The concepts of 6σ and ±3σ – two ways of stating the same statistical fact
   In fact, 6σ and ±3σ are only different expressions of an identical statistical fact.
   In a normal distribution the interval ±3σ extends three standard deviations to the left and to the right of the mean, covering exactly six standard deviations in total – the famous “6σ”.
   This zone contains 99.73 % of all values, a probability regarded as very high in statistics.
   The process-capability indices Cp and Cpk commonly used in engineering are also calculated on the basis of this six-sigma band.
   Hence, in the language of the normal curve the popular term “6σ” is essentially the range already described by “±3σ”.

2. How tool accuracy is calculated – different formulas, identical accuracy-definition standard
   Today the accuracy of mainstream power tools is evaluated almost universally with the “6σ accuracy” method defined in ISO 5393.
   Multiple test points are measured on both hard and soft joints; the standard deviation of the torque readings is calculated and used to rate the overall accuracy of the tool.
   Under this standard the highest accuracy class a tool can reach is ±5 %.

6σ accuracy calculation

σ = standard deviation (always expressed as a ± band).

The smaller σ is, the better the process behaves.

In daily engineering practice, however, most manufacturers quote “3σ / mean” for the sake of customer convenience and to satisfy common Cp/Cpk requirements.
According to ISO 5393 this wording corresponds to the same highest class – ±2.5 %.
Therefore, as far as the tool-accuracy grade is concerned, the “3σ / mean” formula is fully equivalent to the ISO 5393 six-sigma approach.
From the formulas it is also obvious: the smaller the accuracy value, the better the reproducibility and consistency of the tool.

More than a “single point” – full-angle accuracy testing guarantees consistent tightening
Besides the statistical method, the way the measurements are taken strongly influences how comprehensive the accuracy statement is.
VDI 2647 requires that, after a certain number of readings, square-drive tools be turned 90° and hex-drive tools 60°, so that one complete 360° rotation is covered within one test cycle.
This stringent rule takes the uniformity of gear-boxes and other transmissions into account and ensures reliable torque delivery over the whole revolution.

At Danikor, every tool leaves the factory only after such a full-angle test, guaranteeing that each angular position around a full turn stays inside the high-accuracy band.
The accompanying report shows that, measured to ISO 5393, the single-point torque accuracy is even within 6σ ±1 %, demonstrating very low scatter.

Key factors that influence screwdriver accuracy

Inside the tool’s transmission the machining quality of the planet gears is the decisive factor for torque-output accuracy.
Most commercially available gears suffer from large module, wide tolerances and low hardness, all of which degrade the tool’s performance.

By building its own machining line and employing high-precision equipment together with a strict process flow, Danikor produces small-module gears that are simultaneously hard, strong and accurate.

This in turn improves the stability of torque output, speed control and service life of the tool.