A tightening tool passes the machine capability study (MFU) with a Cmk of 1.85 - yet the customer still complains about faulty joints from series production. Sounds contradictory? It isn't.
The machine capability study (MFU) only evaluates the tool under laboratory conditions. The process capability study (PFU) according to VDI/VDE 2645-3, however, evaluates the real tightening process - including all disturbing influences from people, material, method, and environment. Anyone who skips the PFU or treats it as equivalent to the MFU risks invalid process approvals, costly recalls, and serious gaps in assembly quality assurance.
This guide walks you step by step through how to carry out your first standard-compliant PFU in a structured and reliable way, and how it fits into modern production monitoring, torque monitoring, and manufacturing optimization.
MFU and PFU: Two studies - two completely different meanings
One of the most common misconceptions in tightening technology: a passed Cmk value proves the quality capability of the process. That is incorrect.
Unlike the MFU, the PFU takes into account not only the machine influence but also the influence categories of people, material, method, and environment. The MFU answers the question: Is the tool fundamentally capable of doing this? The PFU answers: Does the entire tightening process consistently stay within tolerance under real production conditions over time?
| Characteristic | MFU - Machine capability | PFU - Process capability |
|---|---|---|
| Observation period | Short-term study (hours) | Long-term study (days / weeks) |
| Influencing factors | Machine & measurement only | Human, machine, material, method, milieu (5M) |
| Metrics | Cm / Cmk | Cp / Cpk |
| Typical limit value | Cmk ≥ 1.67 | Cpk ≥ 1.67 (Automotive) |
| Sample size | at least 50 measurements in one run | at least 25 subgroups of 5 values each |
| Measurement method (torque technique) | Tool shut-off torque under ideal conditions | Continued torque measurement under serial production conditions |
| Goal | Tool acceptance / calibration release | Process release for serial production |
| Standard reference | VDI/VDE 2645-2 | VDI/VDE 2645-3 |
The MFU is always the first step. A PFU can never be successful if the MFU has previously failed. A capable machine does not automatically guarantee a capable process. Both studies must be carried out independently, documented separately, and evaluated individually as part of your overall process capability analysis and repeatability assessment.
What a PFU according to VDI/VDE 2645-3 actually means
The goal of the process capability study for screw joints is to assess and document the quality capability of a tightening process under series-production conditions.
The PFU provides key insights for evaluating and continuously improving the tightening process. This includes: identifying systematic influences for targeted process improvement, evaluating the effect of corrective actions, assessing tolerances, and defining the intervention limits for quality control charts used in assembly monitoring and production monitoring.
The guideline uses the residual torque (turn-of-the-nut) measurement method: after tightening, a calibrated test tool determines the torque required to rotate the screw slightly further. This residual torque provides information about settling effects and their impact on preload force. This indirect measurement method enables non-destructive, in-line assembly testing and series-accompanying quality assurance using modern precision measurement technology.
Understanding Cpk and Cmk - and calculating them correctly
The central key figures of the PFU are Cp (process potential) and Cpk (critical process capability index). The MFU focuses on Cmk as the machine capability index. All three indices are essential elements of professional process capability analysis in manufacturing.
Cp and Cpk are statistical indicators used in production engineering to evaluate a process. They show how reliably the specification targets are achieved. The Cmk value, in turn, quantifies the capability of the tightening tool itself.
While the Cp value only indicates the ratio of the specified tolerance to the process spread, the Cpk value additionally takes into account the position of the mean value relative to the center of the tolerance range. A process can therefore show good variability numerically, yet still have a poor Cpk - because its mean is too close to one of the specification limits.
The calculation formula is:
Cpk = min [ (USL - x̄) / (3σ), (x̄ - LSL) / (3σ) ]
Here, USL is the upper specification limit, LSL is the lower specification limit, x̄ is the process mean, and σ is the standard deviation of the measurement data. This capability index is a cornerstone for assessing process capability and repeatability in torque analysis and measurement data analysis.
A Cpk of 1.67 results in a very low defect rate of 0.57 ppm (parts per million). In the past, a minimum requirement of Cp and Cpk ≥ 1.33 was common and is still standard in many industries today. In the automotive industry, however, the minimum requirement for process capability has risen to Cp and Cpk ≥ 1.67.
Use this to calculate Cpk and your Cmk value directly in the interactive calculator:
The 6 steps of your first PFU according to VDI/VDE 2645-3
First determine the screw joint class according to VDI/VDE 2862 (A, B or C) and set the tolerance limits (OSG / USG) for the retightening torque. Without defined limits, the subsequent Cpk calculation is meaningless. Safety-critical connections of class A require especially tight tolerances.
Before you begin with the PFU: Ensure that the screw process is statistically controlled. Conduct a ramp-up study with a quality control chart over a representative period. If the control chart shows trends, runs, or violations of control limits, the PFU is invalid.
Under real production conditions, collect at least 25 Untergruppen à 5 Messungen (= 125 values). Important: shift changes, material lot changes, and operator changes must be included in the sample. Measure the retightening torques directly at finished bolted connections using a calibrated measuring instrument.
Check the measurement values for normal distribution (e.g., Anderson-Darling test or histogram analysis). The standard formulas for Cp and Cpk apply only to normally distributed data. In the case of non-normal distributions, alternative methods (e.g., percentile method) should be used.
Compute the capability indices: Cpk = min[(OSG - x̄) / (3σ), (x̄ - USG) / (3σ)]. The smaller of the two partial values (Cpo / Cpu) is decisive. In the automotive industry, the lower limit is regarded as Cpk ≥ 1.67. If the value falls below this, the process is not release-ready and actions to optimize the process must be initiated.
Archive all measurement data, calculation bases, and results in a standards-compliant and traceable manner. If the PFU is passed (Cpk ≥ the required limit), grant the process release. At the same time, set the intervention limits for quality control charts and define the PFU repeat interval.
These six steps form a practical framework for systematic process capability analysis, robust torque monitoring, and sustainable manufacturing optimization, from planning through data collection to measurement data analysis.
Typical mistakes - and how to avoid them from the start
A particularly critical point in practice: the confidence interval of the Cpk value becomes very wide with small sample sizes - which significantly reduces the validity of the result. If you work with 30 instead of 125 measurement values, you will still get a numerical result, but statistically you can hardly rely on it when making decisions about process capability.
The same applies to control charts: only if the control chart over a representative period shows no violations of the intervention limits and no unnatural patterns (trends, runs) is the process considered stable and the PFU valid. This is essential for reliable assembly monitoring and assembly quality assurance.
PFU and VDI/VDE 2862: How the standards interact
The PFU according to VDI/VDE 2645-3 directly builds on the screw joint classification defined in VDI/VDE 2862: the class (A, B, or C) determines how tight the tolerance limits must be - and therefore which Cpk value your process must demonstrate.
Safety-critical joints in Class A require the tightest limits and the most comprehensive documentation. They demand particularly robust process capability and precise torque analysis. You can learn more about screw joint classification and its impact on assembly testing in our article VDI/VDE 2862 explained in simple terms: What categories A, B, and C mean for your tightening processes.
The right equipment for a standard-compliant PFU
A PFU is only as reliable as the measuring equipment used. The following factors are critical when setting up high-quality torque monitoring and precision measurement technology:
- Calibrated measurement systems with complete traceability (DAkkS-accredited)
- Residual torque measurements without operator influence for reproducible results
- Automatic data acquisition and statistical evaluation (Cmk, Cpk, histogram, control chart) directly in the system for efficient measurement data analysis
- Standard-compliant archiving of all measurement data for audits and long-term assembly quality assurance
The GWK QUANTEC MCS® is specifically designed for test sequences in accordance with VDI/VDE 2645-3: its patented angle sensor technology with ±1% measurement accuracy across the entire main measuring range, the integrated residual torque measurement, and the direct Wi-Fi connection to the analysis software QuanLabPro make the system a reliable tool for series-accompanying PFU in tightening and assembly monitoring.
All results - tightening curves, Cpk calculations, histograms, and control charts - are documented in an archivable and audit-proof manner. This creates a complete data foundation for process capability analysis, production monitoring, and manufacturing optimization.
If you want to cover your tool requirements for a PFU campaign flexibly, the GWK ToolRent® rental service offers a cost-efficient solution: calibrated, ready-to-use devices without capital investment - available on a daily, weekly, or monthly basis.
Conclusion: Standard-compliant, documented, audit-ready
The process capability study according to VDI/VDE 2645-3 is not a one-off box-ticking exercise - it is the backbone of your quality assurance in tightening technology. Anyone who consistently follows the six steps, avoids typical mistakes, and relies on calibrated precision measurement technology establishes robust Cpk evidence that any auditor will accept.
Three core outcomes every PFU must deliver:
- Proof of process stability (control chart)
- Cpk value ≥ required threshold (e.g. 1.67 in the automotive industry) as a meaningful capability index
- Complete, standard-compliant documentation of all measurement series for credible process capability
Together with you, we develop the optimal solution for your specific tightening applications - from tool selection and DAkkS-accredited calibration through to complete PFU documentation, including torque analysis, assembly testing, and continuous production monitoring.
How often must a PFU be repeated according to VDI/VDE 2645-3?
VDI/VDE 2645-3 does not prescribe a fixed interval for repetition. In practice, PFUs are typically repeated after major process changes (tool change, material change, new operators), after deviations in series monitoring, or at the customer's request (e.g., during automotive audits). As a guideline, an annual cycle is used, or with the introduction of a new part type.
What is the difference between Cpk and Cmk in screw technology?
Cmk is the critical machine capability index and describes the capability of the screw tool under controlled ideal conditions (short-term, without external disturbances) - typical threshold: Cmk ≥ 1.67. Cpk is the critical process capability index and evaluates the entire screw process under real production conditions, including all influencing factors (operator, material, environment). A high Cmk does not guarantee a high Cpk.
Can a passing Cmk value replace the PFU?
No. The MFU (with Cmk) evaluates only the screw tool under laboratory conditions - it is a necessary prerequisite, but not a replacement for the PFU. Only the PFU according to VDI/VDE 2645-3 under real production conditions proves the actual quality capability of the overall process. Both investigations must be documented in accordance with the standard.
Which measurement method does VDI/VDE 2645-3 prescribe for the PFU?
Over-turning torque measurement is described as a procedure for the PFU. After tightening, a calibrated test tool is applied and the torque required to rotate the screw slightly further is measured. This over-turning torque is an indirect measure of the actually achieved preload in the joint.
At what Cpk value is a screw process considered capable?
The required minimum Cpk depends on the industry and the customer. In the automotive industry, Cpk ≥ 1.67 is the standard for safety-critical connections of class A. A Cpk ≥ 1.33 is considered sufficient in many other industries. If Cpk < 1.33, the process is considered not capable and immediate corrective actions must be initiated.
