Why per-part is the only unit that works
Fixture suppliers quote in total cost; production engineers think in cycle time; finance thinks in payback. The unit that reconciles all three is cost per part. Once the saving is expressed per part, the annual volume turns it into money and the fixture price turns it into a payback period. Nothing else is needed to make the decision.
Line 1 — Load and unload time
Seconds saved per cycle × cycles per year ÷ 3,600 × machine hour rate. Include the operator's time only if the operator is the constraint; on a machine that runs unattended, load time matters because it eats into spindle hours, which is the same as machine time.
Example: 45 s saved, 8,000 cycles, 180 CNY/h → 100 h → 18,000 CNY/year.
Line 2 — Scrap and rework
Scrap rate reduction × annual volume × part cost at the point of scrap. Be honest about the base rate: measure it for a month, do not estimate it. On a part worth 300 CNY with 1.5 percent scrap falling to 0.4 percent at 8,000 parts, the saving is 1.1% × 8,000 × 300 = 26,400 CNY/year — larger than the time saving, and usually ignored in the quote.
Line 3 — Tool life
Better workholding reduces vibration, which extends tool life. Express it as a percentage change in cost per edge, not as a claim about vibration. If tooling costs 12 CNY per part and edge cost drops 15 percent, the saving is 1.80 CNY per part, or 14,400 CNY/year at 8,000 parts.
Line 4 — Unattended capacity
A fixture that allows lights-out running multiplies output without a second machine. Value it as the machine hours released: if the fixture enables two extra hours of unattended cutting per night, 240 nights a year, that is 480 machine hours — 86,400 CNY a year at the same rate. This line is where a fixture decision becomes a capacity decision, and it is usually the largest of the four.
Line 5 — The cost side, honestly counted
| Cost element | Typical share | Notes |
|---|---|---|
| Fixture design and build | 60–70% | Multi-station and hydraulic fixtures at the top of the range |
| Hydraulic power unit or air preparation | 10–20% | Often forgotten in the first quote |
| Integration and trial cut | 5–10% | Programme changes, clamp sequence commissioning |
| Maintenance over five years | 8–15% | Seals, wear pads, locating pin replacement |
| Changeover and storage | 5% | Fixture must be findable and re-installable |
Putting it together
For the example above: total annual saving 18,000 + 26,400 + 14,400 = 58,800 CNY on a 42,000 CNY fixture, ignoring unattended capacity. Payback is 8.6 months. Add unattended capacity and it is under four. That is a clear yes.
Now change one number: annual volume 1,200 instead of 8,000. Load-time saving falls to 2,700 CNY, scrap to 3,960, tool life to 2,160 — total 8,820 CNY against the same 42,000 CNY fixture. Payback is nearly five years, and the answer is a manual fixture at 9,000 CNY instead.
The batch-size thresholds
- Under 500 parts/year — no custom fixture pays. Use standard vises, soft jaws and a machined sub-plate.
- 500–2,000 — manual dedicated fixture, or modular with custom jaws. Payback usually closes on scrap reduction alone.
- 2,000–10,000 — pneumatic or hydraulic dedicated fixture starts to win, particularly where several clamps are needed.
- Above 10,000 — multi-station or tombstone with automatic clamping, and the unattended-capacity line becomes the dominant one.
What we do with the model
Liuzhou Dongben Machinery Equipment Co., Ltd. runs this five-line calculation on every fixture enquiry and states the payback period in the quotation. When the model says a fixture will not pay back inside the batch size, we quote a cheaper fixture or recommend that the part stays on soft jaws.