Working principle

Principle of Work of the CNC Machine Probe

A CNC machine probe finds the real position of a part or a tool by touching it and reading the machine's axis coordinates at that instant. This page explains the mechanism, where it helps, and the conditions where probing gives you nothing useful.

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Touch triggerSkip and vector modesCalibration routine±0.005 mm

CNC machine probe checking a machined engine part on a 5-axis machine

On this page

1
How a CNC machine probe turns contact into coordinates
2
Measuring modes: edge find, skip, vector, and 3D scanning
3
Calibration is what sets the real accuracy of a CNC machine probe
4
What probing actually buys you on the shop floor
5
When a CNC machine probe will not help
6
Choosing a probing method for the task

Mechanism

How a CNC machine probe turns contact into coordinates

The probe is a measuring head that sits in the spindle taper or tool turret and is treated as a normal tool by the control. Inside the head, the stylus rests on three contact points loaded by a spring. Touch the stylus tip against a surface, the ball deflects a few micrometres, one contact opens, and the head sends a trigger pulse to the machine's skip input. The control latches the current X, Y, Z positions of the axes at that pulse.

That latch is the whole trick. The machine does not measure distance; it records where the axes were when the signal arrived. The measured position therefore equals a machine coordinate, and everything that follows depends on knowing what that coordinate means in the part's frame.

Trigger repeatability of a good head is around ±1 μm unidirectional. The number that matters for your part is the system number: head repeatability, stylus bending, machine positioning error, and calibration error added together.

Stylus length eats accuracy. A 50 mm stylus tip takes roughly twice the force of a 20 mm stylus for the same deflection, and that force bends the shank. Long styli still work, but you must calibrate with the same stylus you measure with.

1
Contact, not proximityTouch trigger heads deliver one pulse per touch, at one point.
2
Skip inputThe control's high-speed input that freezes axis position.
3
Stylus ballRuby is standard; silicon nitride for aluminium-heavy work.

Motion modes

Measuring modes: edge find, skip, vector, and 3D scanning

Edge find is the old method: run the probe slowly toward the surface, stop on contact, and back off. It works on any control with a skip input, but cycle time is long because feed must stay low, often 10–50 mm/min during the touch.

Skip mode speeds this up. The probe moves toward the surface at a fast feed, typically 500–3,000 mm/min, and the control stops the move the moment the skip input fires. No dwell, no reversal search. Repeatability depends on how fast the control can latch the signal, which is why probing feeds and deceleration settings must be tuned per machine.

Vector mode adds a two-touch routine in one direction. The first touch is fast, the second is slow, usually 5–20 mm/min, and the reported value is the second one. That removes most of the pre-travel error of the first contact. On a well-set machine this gets you to the ±2 μm range on the surface.

Scanning heads are a different animal. They stay in contact and deflect continuously, feeding thousands of points per second. Use them for form work such as blade profiles or free-form surfaces. For checking a pocket position or a bore centre, a touch trigger head is faster and cheaper.

Calibration

Calibration is what sets the real accuracy of a CNC machine probe

Before any measurement means anything, the control must know the effective tip position of the stylus: the ball centre expressed in spindle coordinates. That is done with a calibrated ring gauge or a precision sphere, usually Ø50 mm or Ø100 mm, mounted on the table.

The routine touches the artefact at several points around the ring or sphere. Each touch gives a machine coordinate on a known geometry, and the control solves for the ball radius offset and the stylus runout. On most controls this takes 3–8 minutes with a ring gauge, longer for a full 3D sphere calibration.

Calibration is not permanent. A tool change crash, a hot spindle after a long roughing cycle, or a stylus that has been bumped all shift the offset. Recalibrate at the start of a tight job, after any alarm, and after any spindle warm-up that changes the Z datum.

Thermal drift is the quiet killer. A spindle that grows 15 μm over a two-hour cycle moves the probe tip with it. Probing early and cutting late is the common error; either probe close to the cut, or re-datum between operations.

1
Ring gaugeFast 2D calibration for X, Y and ball radius.
2
Precision sphereFull 3D calibration, including stylus tilt.
3
Recalibrate after crashesAssume runout changed until proven otherwise.

Application

What probing actually buys you on the shop floor

The largest gain is setup time. Instead of indicating a vise or a fixture by hand with a dial test indicator, the probe finds the datum in a fixed routine. A setup that took 20 minutes drops to 2–3 minutes, and the result does not depend on who is on shift.

The second gain is locating a feature that already exists. A casting or a forged blank arrives with 1–2 mm of stock variation. Probe the boss or the machined pad, work out the offset, and shift the program origin. Without this, you either leave extra stock or risk a thin wall.

In-process probing checks a critical dimension between operations. If a bore is trending oversize, the control can apply tool wear compensation before the next part. This is common on automotive and medical work where a batch of 500 parts must hold a tolerance band.

On-machine verification replaces some CMM time. It does not replace the CMM for a final inspection report, because the machine's own geometry error is part of the measurement. Use it to catch a problem early, not to sign off a part.

Limits

When a CNC machine probe will not help

Deep bores and narrow slots are a problem. A Ø2 mm stylus ball on a 100 mm shank cannot reach the bottom of a Ø6 mm hole that is 80 mm deep without shank contact. Long, thin styli deflect under the touch force and report a position that is not the surface.

Soft materials mislead you. On unaged aluminium, plastics, or rubber, the stylus ball indents the surface and the trigger fires early or late depending on approach speed. Touching a POM part at 500 mm/min gives a different number than touching it at 20 mm/min.

Very rough or interrupted surfaces are unreliable. A sand-cast skin, a thermal spray coating, or a surface with Ra above 3.2 μm can trigger on a burr rather than the nominal surface. If the datum face is saw-cut, face it first, then probe.

Finally, probing cannot fix a machine that is out of square. If X and Y are not perpendicular to better than a few micrometres over the travel, the probe will measure that error faithfully and pass it into your part. Check the machine geometry first.

Selection guide

Choosing a probing method for the task

Match the method to what you need to know, not to what the machine already has.

Task	Best method	Typical feed	Why
Find vise or fixture datum	Skip mode, single touch	500–1,000 mm/min	Speed matters; ±10 μm is enough
Measure a bore centre	Vector mode, four touches	5–20 mm/min on second touch	Removes pre-travel error
Locate a cast boss	Skip mode, one point per axis	300–800 mm/min	Blank variation is 1–2 mm
Check a critical diameter	Vector mode, two-point or three-point	10–20 mm/min	Needs a repeatable number
Scan a free-form surface	Scanning head, continuous	200–2,000 mm/min	Thousands of points, form data
Deep narrow bore, Ø6 × 80 mm	Not recommended	—	Stylus shank contact gives false triggers
Soft plastic datum	Low feed, then CMM check	5–10 mm/min	Indentation shifts the trigger point
Final inspection report	CMM off the machine	—	Machine geometry error is included

Use the probe to set up and to catch drift, not to replace the CMM

If you need fast, repeatable setup and early warning on a trending dimension, a touch trigger probe on a calibrated machine is the right tool. If you need a certified final dimension or a full form report, take the part off the machine and measure it on a CMM.

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FAQs

Frequently asked questions

What accuracy can I expect from a CNC machine probe on a good machine?

Repeatability of the head alone is around ±1 μm. The system number on a well-maintained machining centre, after ring gauge calibration, usually lands between ±3 μm and ±10 μm.

The spread comes from machine positioning error, spindle thermal drift, and stylus deflection. If those are not controlled, a tighter head will not save the measurement.

How often should the probe be calibrated?

Calibrate at the start of a job that holds tight tolerances, after any alarm or crash, and after a long spindle warm-up that changes Z.

For general work, once per shift is a common rule. Store the calibration data with the stylus serial number so a swapped stylus does not inherit the wrong offset.

Can probing damage the part surface?

The touch force is small, typically a few newtons, and a ruby ball leaves no visible mark on steel, aluminium, or titanium at normal feeds.

Soft plastics, polished optical surfaces, and thin coatings can show a witness mark. Lower the approach feed to 5–10 mm/min and, where possible, probe a non-cosmetic face.

Why do I get a different number on the machine than on the CMM?

The machine measures in its own coordinate frame, which carries squareness, straightness, and thermal errors. The CMM measures in a frame set by its own calibration.

A 10–20 μm gap between the two is normal on a working machine. If the gap is larger, check the probe calibration first, then the machine geometry.

Does probing add cycle time?

A simple datum find adds 10–30 seconds. A four-point bore measurement adds 1–2 minutes depending on feed and clearance moves.

That cost is usually paid back by removing a manual setup and by catching a trend before a whole batch is scrap.

What stylus material should I use?

Ruby is the default for steel, aluminium, and most plastics. It is hard and chemically stable.

For heavy aluminium work, silicon nitride reduces the material build-up that ruby can pick up. For cast iron and abrasive surfaces, a tungsten carbide ball wears more slowly.