Posted 6 Oct 2026 15:15 | 57 views

Automatic Screw Tightening: Torque Learn how torque and angle control work in automatic screw tightening, compare mechanical clutch, current control and transducer methods, and see what to look for in an electric nutrunner for Industry 4.0.Control, Angle Control and Cm/CmK Explained

Automatic Screw Tightening: Torque Control, Angle Control and Cm/CmK Explained

Why does a small tightening error matter?

On a line that tightens thousands of screws a day, a small deviation can become a big problem. A screw that is too loose can work free in service. A screw that is too tight can strip the thread, snap the fastener or crack the part.

Worse, these faults often stay hidden at the station and only show up after the product reaches the customer. Controlling the tightening process is therefore a question of product quality and accountability, not just line speed.

Three parameters to watch: torque, angle and time

Torque (Nm): the main indicator of clamping force.
Angle: how far the screw turns once it starts to load. It reveals problems that torque alone can miss, such as damaged threads, the wrong screw size or a missing washer.
Tightening time: a useful extra check. A screw that was already tight, for example, will finish in an unusually short time.

Looking at all three together gives a much fuller picture than torque alone.

How automatic tools control torque

Method	How it works	Strengths	Best for
Mechanical clutch	A clutch disengages when the set torque is reached	Simple design, good repeatability even when joint stiffness varies	Applications needing consistent torque without detailed data capture
Current control	Torque is estimated from motor current, angle is read by an encoder	Flexible programming, one tool can serve several products	Lines running many product variants
Built-in transducer + encoder	Torque is measured directly by a transducer in the tool, angle by an encoder	High-resolution torque and angle measurement	Tight tolerances and critical joints

The more accuracy and feedback an application needs, the more directly the tool should measure.

Choosing a tightening strategy for your joint
Joint type matters. A hard joint shows a steep torque rise once the screw seats. A soft joint, such as parts with a thick rubber seal, builds torque gradually, so angle becomes important too. Three strategies are common:

Torque Control – Angle Monitoring (TC-AM): tighten to a torque target and verify the angle falls within limits.
Angle Control – Torque Monitoring (AC-TM): tighten to an angle target and verify the torque stays within limits.
Torque and Angle Control: both conditions must be met for an OK result.

Measuring repeatability: Cm and CmK
Tool capability is commonly assessed with Cm and CmK.

Cm (machine capability) shows how tightly the results cluster compared with the tolerance range.
CmK also accounts for how far the average drifts from the centre of the tolerance.

Higher values are better. The required minimum depends on the customer and the industry, and automotive work often uses 1.67. Tool performance testing is commonly referenced to ISO 5393, the standard for rotary tools for threaded fasteners.

Poka-yoke and traceability
Modern tightening systems help prevent human error. They can enforce the correct tightening sequence, count screws, limit re-tightening attempts after a NOK result and store every result for later review.

Connected to a PLC or MES through fieldbus such as PROFINET, EtherNet/IP or Modbus TCP/IP, tightening data flows straight into the plant's systems. That is the foundation of an Industry 4.0 workflow.

A real-world example: FIAM E-MCB + E-TCS
FIAM, an Italian manufacturer with over 70 years of experience in industrial tightening, designs and builds both the motors and the control unit in-house. Its new generation puts the ideas above into one system.

E-MCB electric nutrunner motors

Item	Specification
Sizes	3 sizes: Ø 32, 36 and 53 mm, 44 models in total
Torque range	0.1 – 18 Nm
Speed	22 – 2,100 rpm (adjustable)
Weight	0.51 – 2.1 kg
Control modes	(1) Built-in transducer + integrated encoder (2) Current control + integrated encoder
Motor	Brushless, low voltage (32 V), ESD certified
Status LEDs	Green / red / yellow for tool status and tightening result

Other design features include thrust bearings for high-cycle slide applications, mounting along the full length of the aluminium housing, and a centring system for stability in both vertical and horizontal installations.

E-TCS power and control unit

Colour touchscreen with real-time torque, angle and time display, plus graphs
100 Jobs × 100 Tasks × 12 Steps, all configurable
Wizard programming and poka-yoke programming from the interface
Three strategies: TC-AM, AC-TM and Torque and Angle Control
Real-time Cm and CmK calculation
Logs and graphs of the last 200 tightenings, with storage for over 1 million cycles
Fieldbus output via PROFINET, EtherNet/IP and Modbus TCP/IP (interface modules ordered separately), plus USB and SD card ports

Working with automatic screw feeding
The E-MCB and E-TCS also work in CA automatic handheld screwdrivers with screw feeding, suited to manual stations, and in MCA automatic tightening modules, which are plug & play and ready to install on automatic or robotic lines.

Planning an automatic tightening system for your production line? Mostori can help you choose the right motors, control units and accessories for your application. Contact our team to request information or discuss your project.

Need an air motor sized to suit your machinery? The Mostori engineering team is ready to provide expert advice and a quotation. → Request a quick quote: http://shorturl.asia/sU1wR