Fixed vs Adjustable Support: The Core Trade-Off
Fixed rest pads are cheaper, harder (can be hardened to 62+ HRC), and require zero maintenance. Height adjusting pins cost more, have moving parts, and need periodic inspection. So why would you ever choose adjustable? Because fixture downtime during changeover costs far more than the price difference.
Cost Comparison
| Factor | Fixed Rest Pads | Height Adjusting Pins |
|---|---|---|
| Unit Cost | $5-15 per pad | $20-60 per pin |
| Changeover Time | 15-25 min (swap pads) | 1-3 min (adjust thread) |
| Accuracy | ±0.005 mm (ground surface) | ±0.05 mm (thread resolution) |
| Maintenance | None | Clean threads quarterly |
| Variant Handling | Requires pad sets per variant | One pin covers all variants |
When Fixed Rest Pads Win
- Single-product fixture: No changeover needed → fixed pads are simpler and more accurate
- Datum surface precision <±0.01 mm required: Ground hardened pads provide better surface accuracy than threaded adjustment
- High clamping forces (>10 kN): Fixed pads distribute load better than threaded pins
- Dirty environments: Threaded pins collect chips and debris that impede adjustment
When Height Adjusting Pins Win
- Multi-variant fixtures (3+ variants): Changeover time savings accumulate rapidly. At 3 changeovers/shift × $50/min labor × 15 min saved = $2,250/shift savings
- Incoming part variation >±0.5 mm: Cast/forged parts with rough datum surfaces need per-part height adjustment
- Prototype/small-batch production: New part dimensions every week — fixed pads require new pads each time
- Vertical CNC setups: Gravity assists in supporting the workpiece — lower clamping force requirements make threaded adjustment viable
Break-Even Analysis
Height adjusting pins pay for themselves when:
(Cost_pins - Cost_pads) < (Time_saved_per_changeover × Labor_rate × Number_of_changeovers)
Example: 8 pins at $40 each vs 8 pads at $10 each → $240 difference. Time saved: 12 min/changeover × $1/min labor = $12/changeover. Break-even: 240 / 12 = 20 changeovers. At 2 changeovers/day, the pins pay for themselves in 10 working days.