The clinch allowance for semi-tubular rivets is the amount of rivet shank that extends beyond the material thickness and is needed to form the rivet's head, typically ranging from 50% to 70% of the rivet's shank diameter. This allowance is critical because too little results in a loose or incomplete clinch, while too much can prevent a tight clamping action. The specific clinch allowance is determined by the rivet's diameter and application, and it is added to the total material thickness to calculate the required rivet length.
How Clinch Allowance is Used
- Determine Total Material Thickness: Measure the combined thickness of all the parts that will be riveted together.
- Select a Rivet Diameter: Choose a semi-tubular rivet with an appropriate shank diameter for your application.
- Find the Suggested Clinch Allowance: Consult a manufacturer's chart to find the suggested clinch allowance for that specific rivet diameter.
- Calculate Total Rivet Length: Add the total material thickness to the suggested clinch allowance.
- Choose the Rivet: Select a rivet with a length that is equal to or slightly less than the calculated total rivet length.
General Rule of Thumb
- Clinch allowance ≈ 0.5 × rivet diameter (D)
- In other words, the rivet should project beyond the material stack by about ½ the shank diameter before setting.
Example:
- A 1/8" (0.125") diameter semi-tubular rivet → clinch allowance ≈ 0.062" (1/16").
This extra length ensures there’s enough material for the tubular end to roll over and form a proper clinch without cracking or leaving the rivet loose.
Clinch Allowance Chart for Semi-Tubular Rivets
| Rivet Diameter (D) | Recommended Clinch Allowance (Stick-Out) |
|---|---|
| 1/16" (0.062") | ~1/32" (0.031") |
| 3/32" (0.094") | ~1/16" (0.047") |
| 1/8" (0.125") | ~1/16" (0.062") |
| 5/32" (0.156") | ~5/64" (0.078") |
| 3/16" (0.187") | ~3/32" (0.094") |
| 1/4" (0.250") | ~1/8" (0.125") |
| 5/16" (0.312") | ~5/32" (0.156") |
| 3/8" (0.375") | ~3/16" (0.187") |
Factors Affecting Clinch Allowance
- Rivet Diameter: The recommended clinch allowance varies with the rivet's shank diameter.
- Material Hardness: Harder materials require more force to form, which can influence the required clinch.
- Application: The specific application and desired tightness of the joint can also dictate the required clinch allowance.
