BIZOMAT MAKEOS · Materials
Cantilever Snap-Fit Design (Plastics)
Will this snap-fit finger survive assembly, and what forces does it need?
Assembly strain of a straight or tapered cantilever snap-fit finger by the handbook formula, the deflection force and the assembly (insertion) force from the lead-in angle and friction.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Deflection during assembly (undercut depth) | mm | How far the finger tip must deflect to pass the undercut. |
| Finger thickness at the root | mm | Thickness of the finger where it joins the part. |
| Finger length | mm | Length from the root to the hook. |
| Finger width | mm | Width of the finger. |
| Taper factor K (1 = straight) | Factor from the handbook chart for fingers that taper to half thickness (about 1.6–1.7); 1 for a straight finger. | |
| Secant modulus at the strain | MPa | Secant modulus of the plastic at the expected strain (lower than the initial modulus). |
| Permissible dynamic strain | % | Permissible short-term strain for the material from the resin supplier (e.g. about 2–4% for many unfilled thermoplastics, less for filled grades). |
| Friction coefficient | Friction between the mating materials. | |
| Lead-in angle | ° | Angle of the lead-in ramp on the hook. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Deflection during assembly (undercut depth)
- What it is
- How far the tip of the finger must bend sideways to pass over the mating undercut during assembly, normally the depth of the hook or undercut.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.2 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Strain and assembly force rise in direct proportion to the deflection.
- Get it right
- Include any tolerance stack that makes the undercut deeper than nominal.
What this means: Finger thickness at the root
- What it is
- The thickness of the finger in the direction it bends, measured at the root where it joins the main part and where strain is highest.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.6 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Strain rises in proportion to thickness and deflection force with its cube.
- Get it right
- Measure in the bending direction, not across the finger width.
What this means: Finger length
- What it is
- The length of the bending part of the finger, from its root at the main part to the hook.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Strain falls with the square of length, so a longer finger strains much less.
- Get it right
- Measure from the root, not from the outside of the wall it springs from.
What this means: Finger width
- What it is
- The width of the finger across the direction of bending.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Deflection and assembly force rise in direct proportion to width; strain is unaffected.
What this means: Taper factor K (1 = straight)
- What it is
- The handbook factor for a finger that tapers in thickness from root to tip, read from its taper chart. A straight finger of constant thickness has a factor of one.
- Units and format
- Enter a plain number with no unit. The form accepts 0.5 to 3. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Strain is divided by this factor, so a tapered finger strains less.
- Get it right
- Use the chart for the actual taper ratio; the factor only applies to fingers that thin towards the tip.
What this means: Secant modulus at the strain
- What it is
- The secant modulus of the plastic at the expected strain: the stress divided by strain at that point on the stress-strain curve, from the resin supplier’s data.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 100000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2300 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Deflection and assembly force rise in direct proportion; strain is unaffected.
- Get it right
- Use the secant modulus, which is lower than the initial (tensile) modulus quoted on data sheets.
What this means: Permissible dynamic strain
- What it is
- The largest short-term strain the plastic can take once during assembly without damage, from the resin supplier’s design data for the grade.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0.1% to 20%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2.5% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Repeated assembly needs a lower limit than a single snap; check the supplier’s guidance.
What this means: Friction coefficient
- What it is
- The coefficient of friction between the hook ramp and the mating part as they slide during assembly.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 2. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.4 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher friction raises the assembly force; too high with a steep ramp locks the finger.
- Get it right
- Plastic-on-plastic friction varies with material pair and surface; use supplier data for the pair.
What this means: Lead-in angle
- What it is
- The angle of the ramp on the hook that pushes the finger aside during assembly, measured from the direction of insertion.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 1° to 80°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A steeper ramp raises the assembly force sharply.
- Get it right
- Measure the lead-in ramp, not the retaining face on the back of the hook.
Method and formulas
- Strain ε = 3Yh ÷ (2L²K) (K = 1 for a straight finger).
- Deflection force P = b h² E ε ÷ (6L).
- Assembly force W = P (μ + tan α) ÷ (1 − μ tan α).
- Example: the default finger strains 1.28% and needs about 6.3852 N to assemble.
Limitations
- Elastic beam theory with a rigid root; real roots flex, which lowers strain but also holding force.
- Sharp corners at the root multiply the strain; always add a root radius.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Design snap-fit latches in moulded plastic parts
- Check a finger against the material’s strain limit
- Estimate insertion and deflection forces
Who it is for
- Product designers
- Plastics engineers
- Design engineers
What you get
- Assembly strain
- Deflection force
- Assembly force
Interpreting the result
- If the strain is too high, lengthen or thin the finger, or taper it.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 593–605. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Join parts without stripping them
In Printed Mechanisms That Run
What it covers: Choose between inserts, nut traps, self-tapping screws and snap fits, limit screw torque so plastic does not crush or creep, and test joints that must be reassembled.
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