MFU vs. PFU: Why Machine Capability Alone Says Nothing About Your Screwdriving Process

Imagine this: your screwdriver passes the machine capability study with an excellent Cmk value of 1.67 - exactly the threshold the automotive industry demands for safety-critical joints. The team is happy, the documentation is complete, the process is released.

And then: a customer complaint. Defective screw joints from series production.

Contradiction? Not at all. This scenario occurs regularly in practice - and is almost always due to the same mistaken assumption: the MFU (machine capability study) is treated as proof of overall process quality. It is not. And that misconception can become very expensive.

This article explains what MFU and PFU actually measure, why the difference is critical in series production, and how to use both capability studies correctly for assembly testing, production monitoring, and assembly quality assurance.

What the MFU Measures - and What It Deliberately Ignores

The machine capability study (MFU) according to VDI/VDE 2645 Part 2 has a clearly defined purpose: it determines the stability and repeatability of the process influence factor "machine."

That sounds precise - and it is. But this precision comes at a price: the MFU is a short-term study under deliberately idealized conditions. Disturbance variables are systematically eliminated:

• No operator change -> eliminate human influence
• No material change -> exclude batch variation
• No work interruption -> avoid shift and break effects
• Stable environmental conditions -> prevent temperature and humidity fluctuations
• No tool change -> eliminate set-up effects

The result is the Cmk value - a meaningful index for machine variation under ideal conditions. In the automotive industry, a Cmk ≥ 1.67 is standard, which corresponds to a level of process variation where statistically only 0.57 ppm (parts per million) of parts lie outside the tolerance.

With that, the MFU answers a very specific question: is the tool fundamentally precise enough? It tells you nothing about what actually happens on the assembly line in a real screwdriving process.

What the PFU Measures - The Reality of Series Production

The process capability study (PFU) according to VDI/VDE 2645 Part 3 asks a fundamentally different question: does the entire screwdriving process under series production conditions consistently stay within the required tolerances?

Unlike the MFU, the process capability study considers, in addition to the machine influence, the influence categories human, material, method, and environment. This is the well-known 5M model of industrial production control - and in torque-controlled screw assembly, all five dimensions act simultaneously on assembly process stability:

The goal of the process capability study for screw joints is to assess and document the quality capability of a screwdriving process under series production conditions. The core tool here is residual torque measurement as a form of torque analysis: after tightening, a calibrated test device is used to determine the torque required to turn the screw a small additional angle. This residual torque provides information about the actual installed preload force - and therefore about the real joint quality in screw assembly quality.

The result of the PFU is the Cpk value - the critical process capability index that reflects both the position of the mean relative to the tolerance limits and the overall spread of the process. In practice, Cpk is structurally smaller than Cmk because it includes all real-world disturbances.

The Crucial Comparison: MFU and PFU at a Glance

Practical Example: Cmk 1.67 in the Lab - Cpk 0.95 in Production

Consider a realistic scenario from automotive assembly:

An electric screwdriver is approved for a Class A joint (suspension component). The MFU under laboratory conditions shows a Cmk of 1.67 - the minimum requirement is met. The release is granted.

Three months later in ongoing production: the PFU shows a Cpk of 0.95. The process is not capable. What happened?

The analysis reveals several overlapping influence factors that affect assembly process stability:

• Variations in lubricant: the supplier has changed the lubricant batch. The friction coefficient under the screw head has shifted - the installed torque systematically deviates from the target value, affecting torque control.
• Seasonal temperature fluctuations: in winter, the assembly hall drops below 15 °C. The screwdriver behaves differently on cold components than in the temperature-controlled lab.
• Operator rotation: various operators apply different reaction forces, which leads to measurable variation when using hand-guided tools.

None of these factors were visible during the MFU - because all of them were deliberately excluded. The tool was capable. The process was not.

What Cmk and Cpk Really Tell You

The relationship is clear: the minimum value for machine capability (Cmk) must be higher than the threshold for process capability (Cpk), because beyond the MFU time frame, additional disturbance variables act on the process and increase process variation.

In other words: a Cmk of exactly 1.67 leaves almost no buffer in real production for inevitable disturbances. A good Cpk in series production requires a significantly higher Cmk. Planning this CmK Cpk gap from the start is a key part of a robust capability study.

Try It Yourself: The MFU-PFU Influence Simulator

How strongly do the 5M factors change your Cpk in practice? Adjust the disturbance variables yourself and watch how the process capability value reacts:

When Do You Need Which Study? The Right Use of MFU and PFU

The basic rule is clear:

• MFU = once during tool procurement, after every repair, and after major retooling
• PFU = regularly in ongoing production, after process changes, and whenever anomalies occur

A successful MFU is a prerequisite for PFU - but not a replacement. Running only an MFU without a follow-up PFU is like a car that passes its inspection on the test bench but is never actually driven in traffic.

What the Standards Specifically Require: VDI/VDE 2862 and VDI/VDE 2645-3 Working Together

The two guidelines are closely interlinked and are central references when you design your screwdriving process and overall process capability:

VDI/VDE 2862 classifies screw joints into three risk classes:

• Class A - safety-critical: risk to life and limb in the event of failure (e.g. suspension, brake system)
• Class B - function-critical: product failure in the event of failure ("breakdowns")
• Class C - non-critical: no safety- or function-relevant consequences

The classification defines the minimum requirements for tools, monitoring, and documentation in torque control and screw assembly quality. VDI/VDE 2862 has been valid since 1999 for the automotive industry and since 2015 for all plant, machinery, and equipment manufacturers. When people refer to VDI 2862 in assembly quality assurance, this is typically the standard they mean.

VDI/VDE 2645 Part 3 (PFU) provides the procedure to prove the process quality required by VDI/VDE 2862. The principle: the higher the risk class, the tighter the tolerance limits - and the more demanding the required Cpk value.

The PFU provides guidance for evaluating and continuously improving the screwdriving process, including identifying systematic influences, assessing process improvement measures, and defining control limits for quality control charts based on real measurement data analysis.

If you have not yet fully understood VDI/VDE 2862 and its screw joint classes A, B, and C, start there - because the classification is the basis for all capability requirements in your screwdriving process.

The Right Measuring Tool for MFU and PFU in One

A PFU is only as reliable as the measuring system used. Clear requirements apply to both types of study if you want robust repeatability and trustworthy torque analysis:

• DAkkS-accredited calibration with complete metrological traceability
• Residual torque measurement without operator influence for reproducible PFU results
• Automatic Cmk/Cpk calculation with statistical evaluation and archiving
• Standard-compliant documentation of all torque curves and measurement data for audits

The GWK QUANTEC MCS® was developed precisely for this application: as an electronic torque and angle analysis tool with patented angle sensor technology and ±1% measurement accuracy, it covers both types of capability study in one device. The integrated residual torque function provides the PFU data basis directly on the component - without sending parts back to the lab.

The analysis software QuanLabPro automatically evaluates Cmk and Cpk values, provides control charts and histograms, and archives all measurement data in a tamper-proof way. This allows you to conduct MFU and PFU from a single source - with one calibrated tool, simplifying measurement data analysis and supporting ongoing production monitoring and assembly quality assurance.

If you want to avoid investing in your own measurement equipment, or you only need a torque analysis tool for a time-limited PFU campaign, you can also use QUANTEC MCS® flexibly via the GWK ToolRent® rental system - including certified calibration before delivery.

Conclusion: MFU Is the Starting Point - PFU Is the Proof

The machine capability study is indispensable - but only the first step. It demonstrates that your tool works precisely under ideal conditions. Nothing more.

Only the process capability study shows what really happens in production: with changing operators, fluctuating material batches, seasonal temperatures, and the hundred small disturbances that shape day-to-day assembly. The Cpk value is not a bureaucratic formality - it is your earliest warning signal, long before a quality issue leaves the assembly line.

Practical takeaways:

• MFU with Cmk ≥ 1.67 is mandatory for procurement and after repairs - but it is not a free pass for series production
• PFU according to VDI/VDE 2645-3 is the standard-compliant proof of process quality in series production
• Cpk is structurally smaller than Cmk - plan for this CmK Cpk gap from the outset
• Both studies must be performed independently and documented separately
• Any relevant process change (material, lubricant, operator, environment, method) requires a new PFU

Anyone who wants to carry out a PFU step by step will find a detailed guide to the first standard-compliant process capability study in our handbook - from sampling plan through to Cpk calculation.