In automatic fastening lines, fastening failure is the number-one enemy affecting product quality and production efficiency. Statistics from multiple production lines show that 70% to 80% of fastening failures concentrate on two modes: floating (the screw is not fully tightened, with visible gap between screw head and connected surface) and stripping (threads are damaged, preventing effective clamping force). These two failures appear simple, but their root causes often involve multiple process links. This article systematically analyzes from three perspectives: failure mechanism, diagnosis methods, and solutions.
1. Causes and Diagnosis of Floating
Floating refers to a condition after fastening completion where the screw head still has visible gap from the connected surface, failing to achieve design clamping force. Main causes include: insufficient torque setpoint — due to bit torque sensor calibration deviation or low program setting, actual torque falls below requirement; excessive fastening speed — at high speeds, the system reaches torque threshold before the screw head fully contacts the connected surface; hole position anomalies in connected parts — hole deviation, residual chips or burrs inside holes, excessive hole wall roughness increase fastening torque, causing the screw to be insufficiently tightened when set torque is reached; and screw material or size mismatch — such as insufficient screw length, wrong thread specification, etc.
The most direct diagnosis method for floating is visual inspection — using dedicated measurement tools (such as feeler gauges or height gauges) to measure the gap between screw head and connected surface. General industrial standards require gap ≤0.2mm; exceeding this value is judged as floating. A more precise method is torque curve analysis — floating torque curves typically show a 'rapid rise followed by plateau' characteristic, where torque slowly rises or remains basically unchanged after reaching threshold, indicating the screw is 'spinning' in the hole without forming effective clamping. This curve characteristic is distinctly different from normal fastening curves showing 'continuous rise to plateau'.
2. Causes and Diagnosis of Stripping
Stripping refers to thread damage during fastening, preventing the screw from continuing to engage or forming effective clamping force. Main causes include: bit and screw head mismatch — such as using Phillips bits on slotted screws, or bit size too large or too small causing the bit to slip on the screw head; excessive torque — set torque exceeding the limit of screw or connected part, causing thread deformation or damage; insufficient screw material hardness — such as using low-strength screws (below grade 4.8) to fasten high-strength connected parts, easily causing stripping; and excessive fastening speed — stress concentration at high speeds easily causes local thread deformation.
Stripping diagnosis typically uses several methods in combination: listening for sound — stripping produces obvious 'squeaking' or 'rattling' sounds, distinctly different from normal fastening 'click' sounds; observing screw head — stripped screw heads often show obvious wear marks or metal shavings; checking torque curve — stripping torque curves typically show 'peaks' or 'violent fluctuations' in the mid-to-late fastening stage, then sudden drop, indicating threads have been damaged; and disassembly inspection — after removing suspected stripped screws, check threads for obvious local protrusions, peeling, or deformation.
3. Solutions: Eliminating Floating and Stripping at the Root
Solving floating requires attention to several aspects: first, ensure torque sensor accuracy — regular calibration (typically weekly or monthly) using certified calibration tools (such as torque standard instruments) for detection and correction, ensuring actual output torque deviation from set value is within ±1%. Second, optimize fastening speed parameters — adjust fastening speed to appropriate range (typically 50–150rpm), avoiding 'false tightening' caused by excessive speed. For connected part hole position issues, regularly check fixture hole position accuracy to ensure hole deviation ≤0.1mm, and clean holes before fastening (air blow or vacuum) to remove chips and residues.
The key to solving stripping lies in 'matching' — ensuring bit and screw head type and size are perfectly matched. Establish a bit selection cross-reference table, clearly specifying standard bit model and size for each screw specification. Meanwhile, reasonably set torque upper limits based on connected part material and screw strength grade — generally recommend setting target torque at 80% to 85% of screw nominal torque, leaving safety margin. For low-strength connected parts (such as aluminum alloy, plastic), appropriately reduce torque setpoint, or adopt step fastening strategy (low torque pre-locking followed by high torque final locking).
Additionally, equipment error-proofing design is crucial. Modern automatic fastening equipment should have real-time torque curve monitoring capability, able to identify curve anomalies during fastening and immediately stop, preventing further deterioration of stripping. Some high-end equipment also supports 'automatic screw withdrawal' — when detecting stripping trend, automatically reverse rotate to withdraw the screw, avoiding further thread damage. Chisu Automation equipment comes standard with real-time curve monitoring and automatic anomaly shutdown functions, effectively reducing floating and stripping occurrence rates.
4. Prevention First: Establishing Fastening Quality Assurance Systems
Addressing floating and stripping cannot rely solely on 'troubleshooting after problems occur'; more importantly, establishing prevention systems. Recommend starting from several aspects: first, establish incoming screw inspection systems — conduct appearance, size, and thread inspection for each batch of screws, rejecting screws with burrs, chipped corners, or incomplete threads; second, regular maintenance of bits and fixtures — check bit wear, fixture hole position accuracy, positioning pin elasticity, etc.; third, optimize fastening parameter management — establish standard torque, angle, and speed parameter libraries for each product, avoiding human setting errors; fourth, strengthen operator training — ensure operators can recognize early signs of floating and stripping, discovering and handling issues promptly.
In data management, recommend recording and classifying every fastening failure, analyzing distribution patterns of failure causes. For example, if one batch of screws shows significantly higher floating rate than other batches, focus on inspecting that batch's size and thread quality; if one product model frequently shows stripping, check whether its corresponding bit selection and torque setting are reasonable. Through continuous data accumulation and analysis, gradually build a fastening process knowledge base suitable for the enterprise, fundamentally reducing floating and stripping occurrence rates.
Frequently Asked Questions
Q1: What are the most common causes of floating and stripping?
The most common cause of floating is insufficient torque setting and hole position anomalies in connected parts, accounting for over 70% of floating cases. The most common cause of stripping is bit-screw mismatch and excessive torque, accounting for over 60% of stripping cases.
Q2: What is the simplest method to measure floating?
The simplest method is using a feeler gauge to measure the gap between screw head and connected surface. If gap >0.2mm, it is judged as floating. Dedicated floating detection instruments can also be used for torque curve analysis judgment.
Q3: Can stripped screws and connected parts continue to be used?
Not recommended. After stripping, screw threads are damaged and clamping force significantly decreases, unable to meet design requirements. Connected part holes may also be damaged; check hole walls for protrusions or deformation and replace connected parts if necessary.
Q4: How to distinguish floating from stripping through torque curve?
Floating curve characteristic: rapid rise followed by plateau (torque no longer increases). Stripping curve characteristic: peaks or violent fluctuations in mid-to-late stage followed by sudden drop. Normal fastening curve characteristic: continuous rise to plateau and stable maintenance.
Q5: To what extent can automatic fastening equipment error-proofing design prevent floating and stripping?
Modern equipment through real-time curve monitoring, automatic anomaly shutdown, automatic screw withdrawal, and other functions can reduce floating and stripping rates by over 80%. But fundamentally, correct process parameter settings, qualified screw materials, and regular maintenance are still required.
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Author: Chisu Automation Technical Team
Published: August , 2026
Keywords: floating, stripping, automatic screw locking machine, fastening failure, diagnosis, troubleshooting