How to choose the correct screw: 7 technical parameters to check
The choice of the appropriate screw is not limited to simply screwing into the component: the goal is to ensure solid and secure tightening over time, avoiding errors that could cause defects or damage to the materials.
The choice should not be based on habits, but on well-defined technical parameters and taking into account both the material and the environmental conditions in which it will be applied.
In most applications, the critical issues do not arise from the quality of the component, but from incomplete or missing information on the correct use of the fastening systems that can be used.
This guide illustrates the 7 technical parameters that, if verified, lead to the choice of the correct screw in order to optimize the fastening.
Preliminary technical definitions
- The “female” thread created by the screw in the material during screwing.
- Tightening torque: This is the force to be applied to the fastener to screw it in and ensure tightening. If this is greater than the one the screw can withstand, it can cause it to break.
- Cam-out: insert slipping from the recess. It causes damage to both the insert and the recess, making it difficult to finish tightening or remove the screw.
- Gripp: Friction/adhesion locking between surfaces. It is affected by inadequate finishes, lack of lubrication, incorrect hole diameters, and incorrect screw selection for that application.
Operational table: the 7 parameters to define to understand how to choose the correct screw
| # | Parameter | Technical question | Data to collect | Problem |
|---|---|---|---|---|
| 1 | Material application | What does it screw into? (wood, panels, iron, plastic, pvc, aluminium…) | Type and its mechanical characteristics | Inadequate grip and/or damaged material |
| 2 | Thread | Which type is suitable for fixing? | Compatibility between material and thread | Poor reliability and tightening, risk of screw breakage, tightening torque too high |
| 3 | Screw material and/or surface finish | Used inside or outside? Are there any aggressive agents? | Placement and environmental conditions | Screw corrosion and risk of breakage |
| 4 | Dimensions | What diameter and length are needed? | Thickness of the components to be assembled and mechanical strength of the joint | Problems in tightening and poor reliability over time |
| 5 | Tip | Does it have to perform special functions? | Hole present or not; compatibility with pilot hole and safety/size constraints | Difficulty of engagement, screw swaying and damage to the piece |
| 6 | Head | What mechanical and/or aesthetic function should it perform? | Size constraints; support surface; possible chamfer; aesthetic requirements | Material damage, poor fastening tightness, low tightening torque |
| 7 | Recess | Screwing mode? Type of insert? | Manual or automatic screwing; bit/insert size | Difficulty screwing, recess/insert damage and cam-out risk |
Technical FAQ
Is there a specific screw for plastic materials?
Yes. For plastic materials, there are dedicated screws, with thread and tip geometries designed to create a stable female screw in the polymer, reducing the risk of breaking, deformation, and loss of tightening over time. The choice depends on the type of plastic (thermoplastic, thermosetting or pvc), the thickness and the function of the joint.
What is the difference between self-tapping and wood/chipboard screw?
A self-tapping screw is designed to create the thread in the material (often metal, sheet metal, or specific materials) without pre-tapping, with geometries optimized for that type of material.
The wood or chipboard screw, on the other hand, is designed to work on wood fibers and panels (chipboard, MDF, etc.), with a thread profile and tip designed to penetrate and fasten without splitting the material, while withstanding high tightening torques, as is required for hardwoods.
What is a self-forming or trilobular screw?
It is a screw that creates the female screw by controlled deformation of the material, rather than by cutting. It has a three-lobed cross-section: during screwing, it progressively deforms the material and forms the thread, reducing chips and processing such as threading, when the application allows.
What is a self-drilling screw used for?
It is used when drilling and screwing is necessary in a single operation. The tip is designed to drill the support (typically sheet metal or metal profiles) and, immediately after, start the threading, reducing production steps and installation times.
Are there different types of thread?
Yes. There are metric threads, wood and panel threads, sheet metal/metal threads, plastic threads and self-forming threads. They change profile, pitch and geometry because they have to generate/engage the female screw in materials with different mechanical behavior.
What is the function of the partial thread in wood screws?
The partial thread serves to improve the clamping between two components: the unthreaded portion under the head allows the screw to “pull” the second component toward the first, promoting proper assembly and reducing play, especially in thicker materials.