Detecting CNC tool crashes from shock events

A plant knows every time a tool crashes by mounting a vibration sensor on each CNC and recording every shock above a set threshold in g. A crash hits the machine far harder than cutting or a tool change does, so the threshold catches it whether or not anyone saw it. Each event carries the time, the machine and its peak value.

This guide is for maintenance leads and manufacturing engineers who find crash damage after the fact. It covers what a crash looks like as a signal, how to set the threshold and what to do with each alert.

What counts as a CNC tool crash?#

Most crashes start with a number, not a machine fault. A setup operator types a tool length offset of 4.125 as 41.25, and the next rapid move drives the tool into the vise.

Haas describes exactly that case: an incorrect number typed into the offset column, sometimes from a missing decimal point, with a scrapped part or a broken tool as the result.^[1] Its controls have carried a setting since 2001 that warns when an offset changes by more than a set amount.^[1]

What does a crash cost?#

A crash costs the tool, the part and often the spindle, and the spindle damage can stay hidden. A patent for a CNC control's collision detector puts it plainly: “When a spindle collision occurs, the machine tool may fail at the time.”^[2]

The same document describes the harder case. No problem may be found at the time of the collision, but a failure is found after a certain time period, through abnormal noises from the spindle or a decrease in machining accuracy.^[2]

The unreported crash is the expensive one. For example, a second-shift operator on a Haas VF-2 breaks an end mill, swaps it and restarts the job. Three weeks later the spindle runs hot, and nobody can say what changed.

What a crash looks like as a signal#

A crash is a single spike of acceleration, many times higher than anything the machine does in normal work, lasting a fraction of a second. Cutting looks different: a band of vibration that rises and falls with every cycle.

Peak g is the highest acceleration in the event, where 1 g is the acceleration of gravity. The glossary entry for a shock event uses the same terms. Two numbers tell a crash from normal work: how high the peak goes, and whether it repeats.

The figure shows why a threshold works. Roughing and the tool change stay well under 50 g, and the crash at 148 g is six times the highest normal peak.

Mount the sensor on the spindle housing or the head casting, where the shock arrives first. Every bolted joint between the tool and the sensor softens the peak, and the CNC mounting guide covers the spots that work.

How do you set the shock threshold?#

You set the shock threshold by starting high, reviewing every event and lowering it in steps until routine work starts to cross it. Then you settle one step above that point.

Both mistakes are costly. A threshold set low reports a collision during normal machining, and one set high fails to detect a collision that has actually occurred.^[2]

1. Start at the top of the range

   Set the threshold at the sensor's maximum alert level, so only severe impacts fire. Leave it for two weeks of normal production.

2. Review every event

   For each event, ask the operator on shift what happened and write the answer next to the peak value. A threshold is only as good as the review behind it.

3. Lower it in steps

   Drop the threshold by 10 to 20 g and run another week. Each step down catches smaller impacts, such as tools that break in the cut.

4. Stop when routine work crosses it

   When events start arriving from normal work, such as every tool change or every pass of one tool, you've gone one step too far. Raise the threshold between the highest routine peak and the smallest real event.

5. Recheck after a process change

   A new fixture, a heavier roughing tool or a sensor moved to another spot changes the peaks. Review a week of events after each change.

What to do after a shock alert#

After a shock alert, keep the machine stopped, inspect it, log the cause and open a work order before the next part runs. A restart without a check is how a crash becomes a spindle failure three weeks later.

Then record the incident in three places:

• The stop reason. Classify the downtime as a tool crash, with the cause in the note: "Wrong offset, tool broke on part." A reason code list with one crash code and a required note keeps causes comparable.
• The work order. Open one for the spindle check and any repair, so the inspection is done and signed off rather than promised. Opening work orders from downtime covers the handoff.
• The fix. Change the procedure that allowed it: an offset check on the setup sheet, a prove-out at reduced rapid or a second sign-off on new programs.

Turn shock events into crash forensics#

Crash forensics is matching each shock event's time stamp to the job, program and shift that were running, so every crash has a cause and an owner. The event gives you the second it happened and how hard it hit. The plant's own records give you the rest.

For example, a month of events on six mills might show five crashes. Matched to the job records, four came in the first hour after a setup, and three of those ran the same revised program.

Review the log once a month, grouped three ways:

• By cause: wrong offsets point to the setup sheet; broken tools point to tool life or feeds.
• By program or job: a program with two crashes needs a prove-out, not a reminder.
• By shift and hour: crashes clustered after setups or at shift start point to handover and training.

The aim is a fix, not a culprit. A record that exists for every crash, reported or not, lets an operator say "it happened" without having to.

Record every shock with IoTFlows#

IoTFlows records shocks with the sensor that already tracks uptime on each CNC: a SenseAi, or a SenseAi Embedded on the spindle motor where coolant reaches. Both detect shocks well past 200 g.

The Abnormal Shock Detection rule fires above a threshold you set from 20 to 100 g, 100 g by default. Each event can email, push, text or post to chat, join the machine's event log with its value in g, and open a Maintain work order, as the shock rule settings describe.

RNPSA, a mining equipment manufacturer in Matehuala, Mexico, put SenseAi sensors on its critical machines. Its story reports zero silent failures (no undetected tool crashes or abnormal shocks) and 100% incident forensics, six months after deployment.

When is a spindle-load monitor worth adding?#

A spindle-load monitor in the control is worth adding when you want the machine to stop itself, or to catch overload that builds slowly. A shock threshold records the impact; the control acts on load.

Haas controls, for example, let the operator set a load limit for each tool. Past it, the control raises an alarm, enters feed hold or beeps, as Setting 84 selects, which can help prevent machine damage through tool failures.^[3]

A threshold also misses impacts below it. A 3 mm end mill that snaps may never reach 45 g, and neither will a tool that wears until it rubs.

Shock detection doesn't predict a failure; it records an impact the moment it happens. For gradual wear, such as a bearing damaged by an old crash, pair it with condition-based maintenance. The sensor and PLC comparison sets out what each approach sees.