Start from the number, not the technology
Workholding converts operator time into machine time. The only question that matters at the start is how many seconds per cycle you are trying to remove, and how many cycles a year you run. Everything else — force, pressure, circuit cost, maintenance — follows from that.
| Attribute | Manual | Pneumatic | Hydraulic |
|---|---|---|---|
| Load and unload time per cycle | 30–90 s | 8–20 s | 5–12 s |
| Clamping force per clamp | Operator-dependent, 2–8 kN | 1–6 kN | 10–60 kN |
| Force repeatability | Poor — varies with the operator | Good | Excellent |
| Shop services required | None | Clean dry air, 6 bar | Hydraulic power unit, 50–200 bar |
| Typical fixture cost index | 1.0 | 1.6–2.2 | 2.5–4.0 |
| Main failure mode | Under-clamping, part movement mid-cut | Air leaks, seal wear, pressure droop | Oil leaks, seal failure, thermal drift in long cycles |
When manual is the correct answer
Manual clamping wins in three situations: low volume, geometry that needs the operator to feel the part seat, and any job where the fixture must be portable between machines. For a run of a few hundred parts, a well-designed manual fixture with a torque wrench and a documented tightening sequence will hold tolerance as well as a hydraulic one — the loss is time, not accuracy.
The failure mode to design out is under-clamping on the last part of a shift. Specify a torque value, put the wrench on the fixture, and record it in the set-up sheet.
When pneumatic is the sweet spot
Pneumatic clamping sits in the middle and is often overlooked. Air is fast, the components are cheap and available everywhere, and force is adequate for aluminium parts, thin-wall components and any operation where cutting force is modest. A pneumatic fixture typically costs 60 to 70 percent of a hydraulic equivalent and needs no power unit.
Its limits are real: air is compressible, so force droops when a tool grabs; and air quality determines seal life. If your shop air is wet or oily, budget for a filter-regulator-lubricator at the fixture and plan on annual seal replacement.
When hydraulic earns its cost
Hydraulics are for heavy cuts, hard materials, and any part with enough cutting force to move it. They are also the answer whenever you need many clamps closing in a defined sequence — five clamps on a thin-wall housing, for example, closing first on the datum and last on the flexible wall. Sequencing is done with the hydraulic circuit, and it is the single strongest reason to pay for oil rather than air.
The costs people forget: the power unit, its floor space, its heat, and its noise. A 1.5 kW unit running all day adds both electricity and a maintenance item. On a machine that is already fully loaded, that is usually acceptable; on a lightly loaded machine, it rarely is.
The payback arithmetic
Take the seconds saved per cycle, multiply by cycles per year, divide by 3,600 to get hours, and multiply by your machine hour rate. That is the annual saving. Compare it with the fixture cost difference and require payback inside eighteen months.
Worked example: 45 seconds saved per cycle, 8,000 cycles a year, 180 CNY per machine hour. That is 100 hours, or 18,000 CNY a year. A hydraulic fixture costing 30,000 CNY more than the manual one pays back in twenty months — marginal. If the same job also removes one scrapped part per thousand, the case flips.
Hybrid arrangements that solve both problems
Two combinations are worth knowing. First, hydraulic base with manual fine adjustment: the base clamps automatically and the operator sets a support or a jack by hand, which is common on castings. Second, pneumatic clamping with hydraulic clamping cylinders on the critical clamp only — cheap speed for most of the fixture, high force where the cut is heaviest.
Liuzhou Dongben Machinery Equipment Co., Ltd. designs and builds all three types in Liuzhou, Guangxi, and quotes the clamping method as a recommendation with the payback arithmetic attached, not as an upgrade to be sold.