Scissor Blade Shear Stress-Strain Measurement Based on Strain Gauges

1. Overview of the Solution
Scissors are a common tool in our daily lives. Their principle is actually quite simple - they are levers. By gripping the handle and applying force, the power is transmitted through the pivot to the blades, completing the cutting action. However, in actual operation, whether a pair of scissors is "sharp" or "comfortable to use" can mostly only be judged by experience and feel.

We attach tiny sensors called "strain gauges" onto the scissor blades. When the scissors are subjected to force, the blades undergo extremely small deformations, which are then converted into electrical signals. This allows the forces acting on the scissors during operation to be recorded in real-time and quantitatively.

This principle is not only applicable to ordinary scissors. For example, medical instruments such as surgical forceps and bone rongeurs also need to clarify the relationship between "how much force is applied by the hand" and "how much force is exerted at the jaw" in order to optimize design, improve feel, or be applied in the field of intelligent robotics. In industry, force monitoring of tools like wire crimping pliers and rebar cutters is related to product quality and operational safety. This measurement method can be applied to everything from determining tool wear to ergonomic evaluation.

This solution uses the CMCU-08A multi-channel strain gauge transmitter launched by RunesKee for demonstration. Taking an ordinary pair of scissors as the object, six strain gauges are attached to the surface of the blades (three on each blade) to perform real-time measurement and analysis of the forces during the cutting process. The measurement accuracy is 24-bit AD, and the minimum measurement resolution can reach 0.1με ‌. More importantly, it can be connected to a computer with a single USB cable for synchronous data acquisition, data saving, and using the data screen recording playback function. It can also be paired with an RS485 wireless transmission kit to achieve wireless data transmission, eliminating the constraint of cables, which offers high flexibility for the scissor operation measurement environment. We hope that the method established by this solution can also provide some reference for similar applications such as medical instruments, industrial tools, and even tool health monitoring.

2. Measurement Solution
2.1 Composition of the Measurement System
The measurement system consists of the following parts:


2.2 Strain Gauge Attachment Locations
The selection of strain gauge attachment locations is a critical step in the measurement. The approach adopted this time: attach the strain gauges to the surface of the two scissor blades, selecting three measurement points to analyze the deformation of each part of the scissors during operation separately. Before attachment, the bonding surface needs to be sanded (if the surface is smooth and flat, this step can be skipped) and cleaned to remove the oxide layer and oil stains, ensuring close contact between the strain gauge and the blade surface. After attachment, it is necessary to apply pressure with a finger for curing, and then wait for the adhesive to fully cure before proceeding with the experiment.

According to the requirements of the experiment, this is a short-term experiment with an ambient temperature of about 28~30°C. Ordinary 502 adhesive glue was used for testing, with finger pressing for curing for one minute. For ultra-short-term experiments, the test can be conducted after leaving it for more than 20 minutes. It is recommended to leave it to stand for 1 hour (full curing).

If conducting long-term monitoring experiments, professional glue must be used. Generally, there is room-temperature curing AB glue, and more professionally, H610 epoxy resin AB glue.

3. Experiment Process
3.1 Experiment Preparation

1. Select a standard pair of scissors, clean the upper surface of the blade arms, and attach the strain gauges at the predetermined positions.
2. Connect the strain gauge lead wires to the sensor interface of the CMCU-08A acquisition board.
3. Connect the acquisition board to the computer via a USB cable, open the debugging software to set the strain gauge parameters, and then start data acquisition. Additionally, a data analysis software is also provided.

3.2 Experiment Design
Experiment 1: Comparative Measurement of Empty Cutting and Actual Cutting
By comparing the strain signal differences between "empty cutting" (cutting nothing) and "actual cutting," the force used purely for material fracture is isolated. Specific operation: First, perform empty cutting with the same pair of scissors and record the strain signal (mainly reflecting the blade arm's own movement and pivot friction). Then, cut the standard test material and record the strain signal during actual cutting. The difference between the two is the effective force used to cut the material, which serves as a quantitative indicator for evaluating scissors sharpness.

Experiment 2: Comparison of Shear Forces for Different Materials
Select objects of different materials and thicknesses such as paper, thick cardboard, PVC plastic sheet, and cables. Conduct shear experiments separately, recording the strain-time curve during the cutting of each material. By performing shear experiments on different samples, materials with different mechanical properties can be distinguished. (Since it is a manual experiment, the cutting force and angle are difficult to control, so the data may vary. However, this does not affect the principle of this shear experiment.)

Extended Experiment: Trend of Force Variation in Repeated Cutting
Continuously cut the same standard material multiple times (e.g., 50 or 100 times), recording the peak strain for each cut. Observe the trend of the peak force as the number of cuts increases - if the force continues to increase, it indicates gradual wear of the blade edge.

Alternatively, after cutting sticky tape multiple times, the adhesive residue sticks to the blade edge, making it increasingly dull. The required cutting force will also increase. Through these data phenomena, abnormal conditions of the blade edge can be identified, allowing for timely cleaning.

3.3 Data Acquisition
The AD chip sampling rate of the CMCU-08A collector supports 10Hz, 40Hz, 640Hz, and 1.28KHz (The chip sampling frequency is not the serial port acquisition rate), with a communication baud rate of 115200 (adjustable). When communicating with a computer, due to limitations of the computer's serial port environment, the fastest acquisition interval can reach 10ms per sample. If connected to an industrial control device, the fastest acquisition interval can reach 5ms per sample.

For transient processes such as scissor cutting, the aim is to capture the stress peak and dynamic changes at the moment of cutting. The experimental setup used in the literature "Force Sensing Surgical Scissor Blades using Fibre Bragg Grating Sensors" has also been verified to accurately distinguish homogeneous tissue samples, demonstrating the potential of this method in material identification.

4. Application Value
4.1 Comprehensive Evaluation of Scissor Performance
This solution measures the force on the scissor arm using strain gauges, serving the evaluation of scissor performance at three levels simultaneously. For sharpness quantification, the pure shear force required to cut through the material is calculated from the difference in strain signals between "empty cutting" and "actual cutting." This value directly reflects the sharpness of the blade edge and can serve as an objective standard for factory inspection, replacing subjective "trial cutting." For material identification, different materials exhibit distinct force-time curve characteristics during the cutting process, which can be used to analyze the mechanical properties of the cut material. For wear monitoring, by repeatedly cutting the same material and tracking changes in peak force, when the peak force exceeds a certain percentage of the initial value, it can be determined that sharpening or replacement is needed, enabling predictive maintenance.

4.2 Medical Device Field (Surgical Forceps/Needle Holders/Bone Rongeurs)
Surgical forceps, needle holders, bone rongeurs, and other instruments share the same mechanical structure as scissors - centered on a pivot shaft, with handle input and jaw output. By attaching strain gauges near the jaws, the interaction force between the instrument and the tissue can be measured. During the development phase, comparing the measured jaw force with the handle force allows for evaluating the instrument's transmission efficiency and tactile feedback. Taking bone rongeurs as an example, measuring the force on the jaws when breaking bone ensures that the doctor can obtain sufficient clamping force with minimal hand effort, avoiding slippage during surgery (insufficient force) or tissue damage from clamping (excessive force). This method can also be used for comparative evaluation of similar products from different brands, as well as for assessing performance degradation of the same instrument after repeated use.

4.3 Industrial Crimping and Cutting Tools (Wire Harness Crimping Pliers/Cable Cutters)
In aerospace and automotive wiring harness factories, crimping pliers and cable cutters are used at high frequency daily, and their working quality directly determines product reliability. The force on the jaw of the crimping pliers directly reflects the tightness of the crimped terminal - too little force can lead to excessive contact resistance (a risk of overheating), while too much force may crush the core wire. By using strain gauges to monitor the peak force of each crimp, and prompting mold replacement when the peak force continuously decreases, batch quality incidents can be prevented. The force monitoring on the blade edge of cable cutters is used to determine whether the blade has become dull or whether the cable has been completely cut, avoiding burrs or core wire damage caused by incomplete cutting. This method is also applicable for assessing operator fatigue - if a worker's handle force on the same tool is consistently high, it indicates issues with operating posture or tool maintenance.

4.4 Agricultural and Horticultural Pruning Tools (Pole Pruners/Heavy-Duty Fruit Shears)
Pole pruners achieve remote operation through pulley systems or linkage mechanisms, with the core design principle being the force-saving ratio. By attaching strain gauges to the base of the blade to measure the force during branch cutting, the actual force-saving ratio can be directly verified against the design target. If the measured cutting force is higher than the theoretical value, it indicates excessive friction loss in the pulleys or jamming in the linkage hinges, requiring design optimization. In practical use, fruit farmers and gardeners often rely on feel to judge whether shears need sharpening. This solution provides a quantitative basis - if the peak force for cutting the same type of branch increases by a certain amount compared to initial use, the blade has become dull and needs sharpening. For large orchards, this means maintenance guidance can be provided remotely without disassembly.

4.5 Robot Flexible Grippers
When industrial grippers or bionic hands grasp precision parts (such as eggs or chips), precise control of the gripping force is required. By attaching strain gauges to the fingertips or gripper arms, the gripping force on the object can be measured in real time. This measurement principle is the same as that for measuring force on scissors - scissors involve "human holding the handle → blade outputting cutting force," while grippers involve "actuator inputting torque → fingertip outputting gripping force." By monitoring changes in gripping force in real time, the controller can dynamically adjust the drive current, achieving "compliant control" and preventing the object from being crushed.

5. Application Prospects
5.1 Smart Surgical Instruments
With the increasing popularity of minimally invasive surgery and robotic surgery, the demand for smart surgical instruments with force sensing capabilities is growing. The strain gauges used in this solution are small in size and low in cost, and are expected to serve as force-sensing components for surgical scissors, forceps, and other instruments, providing surgeons with real-time force feedback and enhancing surgical safety.

5.2 Health Monitoring of In-Service Cutting Tools
The tool force monitoring system based on strain measurement can be integrated with the IoT platform to achieve automatic warning and life prediction of tool wear. When the shear force continues to increase and exceeds the set threshold, the system automatically prompts tool replacement, which is especially suitable for manufacturing production lines that use molds and tools in batches.

5.3 Mechanical Properties Testing of Materials
Using sensorized blades to measure the force-time curve during material shearing provides a simple, low-cost new method for fracture toughness testing of biological tissues, soft materials, etc., and can serve as a supplementary solution to traditional material testing machines.

5.4 Ergonomics and Labor Safety
Through long-term accumulation of tool operation force data, a database of operation forces for different types of work can be established, providing data support for the formulation of labor safety standards and the design of labor-saving tools.

6. Summary
This paper proposes a set of experimental schemes for stress measurement of scissor arms based on strain gauges and the CMCU-08A strain acquisition unit. By pasting strain gauges on the surface of scissor arms and calibrating the relationship between strain and force, real-time measurement and quantitative analysis of the force on the blade during shearing are achieved.

For the first measurement of a particular style or material of scissors, for the sake of rigor, it is recommended for large scissors to paste strain gauges in sections and first measure whether the force in each area is consistent. If the measurement results are satisfactory, only one strain gauge at one location can be selected for use, eliminating the need for multiple strain gauges. For small scissors, generally one strain gauge is sufficient, pasted on a limited area of the blade or arm.

The core value of this scheme lies in providing a quantitative means for evaluating scissor sharpness, identifying material mechanical properties, and monitoring tool wear. The measurement approach can be extended to fields such as medical devices, industrial crimping and cutting tools, agricultural and forestry pruning tools, and robot grippers, offering technical references for the performance evaluation and condition monitoring of related tools.

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