Machine systems

How Hydraulics on a CNC Machine Work

This page explains how hydraulics on a CNC machine work, from the pump and relief valve to the clamps that hold your part. It is written for engineers and buyers who need to judge whether a hydraulic circuit, or a hydraulic workholding setup, fits a given part and tolerance.

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Relief valve settingsClamp pressure limitsOil temperatureCylinder sizing

How hydraulics on a CNC machine work, with hydraulic clamping on a 5-axis machined auto part

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Key takeaways

One fluid, many jobsA single pump feeds tool changers, clamps, tailstocks and pallet locks through separate valves.

Pressure clamps, flow movesThe relief valve sets the maximum force. Flow controls set how fast the cylinder strokes.

Heat is the silent problemOil above roughly 60 °C thins out, internal leakage rises, and clamp force drifts.

Clean oil outlasts cheap oilA 10 μm filter and a sealed reservoir do more for repeatability than a bigger pump.

Not every job needs itThin plates and small batches often run better on pneumatic or mechanical clamping.

Mechanism

How Hydraulics on a CNC Machine Work Inside the Circuit

Oil is incompressible enough that pressure applied at the pump arrives at the cylinder almost unchanged. That is the whole trick behind how hydraulics on a CNC machine work. A 5.5 kW pump pushing 70 bar into a Ø63 mm bore cylinder produces roughly 2.2 t of force, and the machine builder can dial that force with one relief valve screw instead of swapping springs.

A typical circuit runs from reservoir to pump, through a check valve, into an accumulator, then out to a manifold that splits flow to each function. Every branch has its own directional valve and pressure or flow control. The return line passes through a filter and an oil cooler before it drops back into the tank.

The reservoir does more than store oil. It lets air separate out, sheds heat through its walls, and gives contamination somewhere to settle. A tank sized at two to three times the pump flow per minute keeps oil temperature stable on a machine running two shifts.

Pressure and flow do different jobs, and mixing them up causes most setup mistakes. The relief valve caps force. A flow control valve caps speed. If a cylinder slams, the fix is a flow control, not a lower relief setting.

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PumpFixed-displacement gear or piston pump, most often 5–15 kW on a machining center.
2
Relief valveSets system ceiling, commonly 50–100 bar on machine tool circuits.
3
AccumulatorBladder type, holds pressure for clamp release during a power drop.
4
FilterReturn-line element at 10 μm nominal; change when differential pressure climbs.

Where it is used

Which Machine Functions Depend on Hydraulics

Tool changers are the most common hydraulic load on a machining center. The clamp that grips the tool holder needs a short, hard stroke and a repeatable holding force. A 40-taper spindle typically wants 8–12 kN of drawbar force, and hydraulic clamping holds that number far more consistently than a stack of disc springs that has been cycled 200,000 times.

Workholding is the second big consumer. Hydraulic vises, expanding mandrels, swing clamps and zero-point pallet receivers all convert oil pressure into clamping force. On a 4,000 × 400 × 150 mm travel machine, a hydraulic tombstone lets one operator lock four or six parts at once without walking around the table.

Tailstocks, steady rests, chuck actuators and bar feeders on turning centers run on the same circuit. So do pallet changers and some counterbalance systems on heavy vertical mills.

The pattern is consistent. Hydraulics win wherever the machine needs high force in a small package, short stroke, and a force that stays the same across thousands of cycles.

1
Drawbar clampingHolds tool holders at 8–12 kN with repeated accuracy.
2
Hydraulic visesLock 2–6 parts per station with one pressure setting.
3
Zero-point receiversRepeat pallet location to within a few micrometres.
4
Steady restsSupport long shafts on turning centers without marring the surface.

Where it hurts

When Hydraulics on a CNC Machine Cause Problems

Hydraulics are not free. A leak at 70 bar makes a mess, and on a machine cutting medical or aerospace parts a single drop of oil on a workpiece means a rejected part and a cleaned fixture. Seals wear. Hoses harden. Fluid has to be tested and replaced on a schedule rather than when someone remembers.

Heat is the other cost. On a machine running continuously, oil temperature climbs until the cooler can shed the same power the pump puts in. Above roughly 60 °C, viscosity drops, internal leakage rises, and a clamp that held 10 kN in the morning holds 8.5 kN by afternoon. That shift shows up as chatter on thin walls.

For light work the trade is poor. A 1 mm aluminium bracket held in a hydraulic vise gains nothing over a manual vise, and the setup takes longer. Small batches of one to ten parts rarely justify the plumbing.

There is also a safety side. An accumulator stores energy after the pump stops. Anyone opening a hydraulic line before bleeding pressure is opening a loaded system.

1
Leak riskOil on a machined surface can scrap a part outright.
2
Thermal driftClamp force falls as oil temperature rises past 60 °C.
3
Setup timePlumbing and purging a new fixture takes hours, not minutes.
4
Stored energyAlways bleed the accumulator before breaking a connection.

Setup

Step by Step: Setting Up and Checking a Hydraulic Circuit

1
1. Confirm the required clamp forceCalculate force from the cut. For a roughing pass at 2 mm depth, 0.15 mm/tooth feed in 6061, a 3:1 safety factor over cutting force is a normal starting point. Do not size the cylinder on the vise catalog alone.
2
2. Size the cylinder and pick the pressureForce equals pressure times piston area. A Ø63 mm bore at 70 bar gives about 2.2 t. Stay between 50 and 100 bar so the relief valve has room to work without stressing seals.
3
3. Set the relief valve firstBring the pump up with the valve backed off, then raise pressure slowly to the target and lock the jam nut. Never set pressure with a part clamped and the cut running.
4
4. Set flow controls per functionMeter-in on clamping, meter-out on release, so the part is not thrown. Aim for a 0.3–0.6 s clamp stroke and a 0.5–1.0 s release. Slower is safer on heavy plates.
5
5. Purge air from the circuitCycle each function 10–15 times at low pressure with the return line open. Trapped air makes the clamp feel spongy and delays pressure build-up by several seconds.
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6. Verify clamp force with a pressure gaugeCompare the gauge reading at the cylinder port against the calculated value. A 10% shortfall usually means internal leakage past a worn seal or a partly closed valve.
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7. Check oil temperature after one hourRun the machine under normal load for 60 minutes, then read the tank thermometer. Anything above 60 °C needs more cooler capacity or a larger reservoir.
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8. Log the baselineRecord pressure, temperature, filter differential and clamp stroke time. Compare against these numbers every 500 hours instead of waiting for a failure.

Selection

Hydraulic vs Pneumatic vs Mechanical Clamping

Use this when choosing how to hold a part on a CNC machine.

Criterion	Hydraulic	Pneumatic	Mechanical
Typical clamp force	1–20 t per cylinder	0.1–1 t per cylinder	Depends on screw or cam
Pressure range	50–100 bar	5–7 bar	None
Force repeatability	High, holds across shifts	Medium, varies with air supply	Drops as operator torque varies
Footprint	Compact for the force	Bulky for the force	Bulky and manual
Best for	Heavy cuts, multi-part fixtures	Light parts, fast cycle	One-offs, thin plates
Maintenance load	Seals, oil, filters	Filters, water traps	Threads, cams, springs
Heat sensitivity	Yes, above 60 °C	Low	None

The short verdict

Use hydraulics when you need high, repeatable force in a small space and the machine runs long enough to justify the plumbing. Stay with pneumatic or mechanical clamping for light parts and short runs.

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FAQs

Frequently Asked Questions

What oil pressure does a CNC hydraulic system normally run at?

Machine tool circuits are usually set between 50 and 100 bar. Clamping and drawbar functions sit in the middle of that band.

Some high-force presses and special fixtures go higher, but then the seals, hoses and fittings must be rated for it.

How often should the hydraulic oil be changed?

Most builders specify 2,000 to 4,000 operating hours, or once a year, whichever comes first. Contamination drives the real interval.

Pull a sample every 500 hours and check particle count and water content. Change the return filter whenever differential pressure doubles from the baseline.

Why does clamp force drop during a long run?

The usual cause is oil temperature. As the circuit warms past 60 °C, viscosity falls and internal leakage past the spool valves and cylinder seals increases.

Check the cooler and reservoir level first. If temperature is normal, the pump or a seal is worn and the circuit is bypassing internally.

Can hydraulic clamping hold thin-walled parts without distortion?

Yes, but only with pressure control and a soft contact. Expanding mandrels and swing clamps with pressure-reducing valves let you clamp at 10–20 bar instead of full line pressure.

For walls under 2 mm, a low-melt or vacuum fixture often beats hydraulics because the load is spread over an area rather than concentrated at a few contact points.

Is a hydraulic system safe to service in-house?

Pressure lines and accumulators store energy, so a proper lockout and bleed procedure is required before any fitting is opened.

If your team does not have a documented procedure and a pressure gauge at the manifold, bring in the machine builder or a qualified hydraulic technician.

Does hydraulic workholding improve part tolerance?

It improves consistency, which often shows up as tighter tolerance in production. A clamp that holds the same force on part one and part 5,000 removes one variable from the process.

The achievable tolerance still depends on the machine, the tooling and the material. We hold ±0.005 mm on machined features when the setup and process are controlled.

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