What the shoulder is for
Three common jobs: locating a plastic housing without cracking it, carrying the load of a moving part (a drawer wheel, a hinge), and acting as a pivot with a controlled clearance. Each one puts a different requirement on the shoulder diameter and its concentricity with the shank.
Formed vs machined
A turned shoulder is cut, so its grain is severed at the transition and its diameter drifts with tool wear. A formed shoulder is displaced from the same billet, so the grain runs continuously through the step — which is why formed steps survive fatigue better. The diameter is set by the die, so batch one and batch ten are the same.
Where formed shoulders lose
If the shoulder needs a very sharp internal corner, or a tolerance tighter than the die can hold, turning wins. Cold heading also struggles when the diameter ratio between shank and shoulder is extreme.
Checking concentricity
Concentricity between shank and shoulder is what decides whether the part binds. It is measured on a 2D projector against the drawing, and it is the first thing to ask a supplier for on a sample report.
Choosing the route
| Requirement | Formed shoulder | Turned shoulder |
|---|---|---|
| High volume | Lower cost per piece | Cost scales with time |
| Fatigue life | Better — continuous grain | Grain severed at the step |
| Tolerance | ±0.02 mm typical | ±0.01 mm achievable |
| Sharp internal corner | Difficult | Easy |
Ask for the concentricity value, not just the diameter tolerance. A part can be in tolerance and still bind.