Designing Multi-Diameter Internal Features with Stepped Core Pins
Key Takeaway: Stepped core pins form multi-diameter holes in a single molding operation — eliminating secondary machining. Design each step with length ≥ 1.5× diameter and transition radius ≥ 0.3 mm to prevent stress concentration and pin breakage.
Common Multi-Diameter Applications
| Application | Internal Geometry | Step Type | Typical Dimensions |
|---|---|---|---|
| Counterbore screw hole | ø5 bore + ø8 counterbore | One-step | Step ratio 1:1.6 |
| Connector housing | ø1.5 contact + ø3 pocket + ø5 entry | Two-step | Step ratio 1:2:3.3 |
| Threaded insert pocket | ø4 insert bore + ø6 knurl recess | One-step | Step ratio 1:1.5 |
| Stepped bearing bore | ø8 bearing + ø10 shoulder + ø12 entry | Two-step | Step ratio 1:1.25:1.5 |
Step Transition Design Rules
- Step length ≥ 1.5× step diameter — Ensures adequate steel cross-section. Shorter steps fracture under cyclic injection pressure.
- Transition radius ≥ 0.3 mm — Sharp 90° corners at step transitions create stress concentrations that reduce pin life by 3–5×. A 0.3–0.5 mm radius at each step dramatically improves fatigue life.
- Step diameter ratio ≤ 2:1 — The smaller diameter should be at least 50% of the larger diameter. Ratios exceeding 2:1 create very thin cross-sections that are prone to breakage.
- Concentricity specification — All steps must maintain concentricity within 0.01 mm TIR. This is critical for connector housings where multiple contacts must align precisely.
According to ScienceDirect's stress concentration reference, the stress concentration factor at a sharp shoulder step transition can exceed 3.0, meaning the actual stress is 3× the nominal stress. Adding a 0.5 mm radius reduces this factor to approximately 1.5.
One-Step vs Two-Step Selection
| Feature | One-Step | Two-Step |
|---|---|---|
| Diameter sections | 2 (small ø + large ø) | 3 (small ø + mid ø + large ø) |
| Typical applications | Counterbore, simple pocket | Connector housing, multi-section bore |
| Manufacturing complexity | Standard CNC turning | Multi-axis or staged CNC turning |
| Concentricity control | Easier (single transition) | More challenging (two transitions) |
| Cost | 1.0× (reference) | 1.3–1.5× |
| Available models | 14 product series | 6 product series |
Design Checklist for Multi-Diameter Features
- ☐ Define all diameter sections and their lengths
- ☐ Verify step length ≥ 1.5× step diameter at each transition
- ☐ Specify transition radius (0.3 mm minimum, 0.5 mm preferred)
- ☐ Check step diameter ratio does not exceed 2:1
- ☐ Specify concentricity requirement (TIR) between steps
- ☐ Determine if gas venting is needed (depth/ø > 3:1 at any section)
- ☐ Select one-step or two-step core pin based on number of diameter sections
Frequently Asked Questions
When should I use a one-step vs two-step core pin?+
One-step for two diameter sections (counterbore, pocket). Two-step for three sections (connector housings, multi-section bores). Each step adds a diameter transition that must maintain concentricity.
What is the minimum step length for a stepped core pin?+
At least 1.5× the step diameter. For ø3 mm step, minimum 4.5 mm length. Shorter steps create stress concentrations that dramatically reduce pin life.
Can I combine stepped geometry with gas release?+
Yes. Gas release stepped core pins combine multi-diameter geometry with flat-sided or SVC venting — common solution for deep connector housings with trapped air.
Stepped Core Pin Products
Need Custom Stepped Core Pins?
Send your part drawing with internal feature dimensions — we manufacture custom step configurations with concentricity ≤ 0.01 mm TIR.
✓ Custom step configurations✓ MOQ 1 piece✓ Concentricity certified