The only equation that matters
Cost per part on a single-station fixture is (cut + load) × rate. On an N-station fixture it is (N × cut + load) × rate ÷ N, which simplifies to (cut + load ÷ N) × rate.
Subtract one from the other and the saving per part is:
saving = load × (1 − 1/N) × rate
For two stations that is half the load time; for four, three quarters. It converges on the whole load time and stops there. No number of stations beats hiding the entire load — which is exactly what a pallet changer does for free.
The worked number
Six-minute cut, two-minute manual load, machine rate 180 CNY per hour (3 CNY per minute):
| Stations | Minutes per part | Cost per part | Saving vs single | Parts to repay 18,000 CNY |
|---|---|---|---|---|
| 1 | 8.0 | 24.00 | — | — |
| 2 | 7.0 | 21.00 | 3.00 | 6,000 |
| 4 | 6.5 | 19.50 | 4.50 | 4,000 (plus two more station costs) |
At 12,000 parts a year, the second station repays in about six months. At 1,500 parts a year it never repays, and the honest advice is to spend the money on a better vise and a faster load instead.
Where the saving disappears
- Long cuts. At a 30-minute cut, a 2-minute load is 6% of the cycle. Halving it saves 3%. The fixture money is better spent on a second machine hour or on reducing the cut itself.
- High mix. Multi-station fixtures are part-family specific. If the family changes every quarter, the amortisation restarts each time and the fourth station is always the one you did not need.
- Per-part probing or in-process gauging. If every part must be probed, the constraint is the probing, not the load. Adding stations multiplies probe cycles and gains nothing.
- One station drifting. A multi-station fixture multiplies a repeatability problem. If station three runs 0.03 mm low, you now scrap in batches of N rather than one at a time — and the batch is discovered later.
- A pallet changer already fitted. With two pallets the operator loads off-machine. A second station on the same plate only adds weight, clamping variation and chip traps.
The repeatability tax nobody quotes for
Every added station is another set of locators that must agree with the first within the part tolerance. On a ±0.05 mm part with four stations, that is four locator sets each qualified to ±0.01 mm, and a first article that proves all four rather than one.
The practical procedure: qualify each station separately with a test bar, then run one part on each and compare the results. If the spread exceeds a third of the part tolerance, fix the fixture before chasing cycle time — otherwise the cycle saving is funding a scrap rate.
Clamping, chips and reach change with station count
- Hydraulics. Clamp force divides across stations unless the circuit is sized for simultaneous actuation. Four stations clamping at once need four times the flow at the same actuation time — see the sizing note before ordering the power unit.
- Chips. On a horizontal, stations at the bottom of a tombstone collect chips that the coolant never reaches. Add flush ports, or accept that the bottom station is the one that scratches parts.
- Reach. Stations three and four are usually loaded over the top of the fixture. Check the operator's reach envelope, not only the tool's clearance. A station nobody can load cleanly is a station that runs out of tolerance.
When the second station pays
- The part family is stable for twelve months or more.
- Cut time under ten minutes per part.
- Loading takes more than 45 seconds and is done by hand at the machine.
- No pallet changer on the machine.
- Tolerance ±0.05 mm or looser, or the fixture can be qualified station by station.
Five out of five, and the arithmetic above applies. Fewer than four, and the fixture is being bought for the drawing rather than for the parts.