Troubleshooting, Accuracy and Safety

A method for finding problems

• Most problems come from a short list: a wrong zero or offset, a loose tool or part, a worn tool, and too little or too much feed. Check those before anything exotic.
• Listen and look. A good cut sounds steady; squeal or a rhythmic growl means chatter or rubbing; a pop means a chipped edge. The chips tell you about the speed and feed: see the chip figure on Machining Metals.
• Cut a test part in scrap, measure it, and fix one cause at a time.

Chatter and vibration

Chatter is a self-excited vibration between the tool and the work. Each tooth cuts a wavy surface left by the one before, which changes the chip thickness and the force, which feeds the vibration. It shows up as a loud tone, a rippled finish (waves on the wall), and short tool life. It is controlled by stiffness, damping, the depth of cut and the spindle speed.

lobe peak speeds: n = 60 × f_n ÷ (z × k)    k = 1, 2, 3, … (f_n the natural frequency of the tool and holder, z the number of flutes)

Fix	How it helps	Cost
Shorten the tool (stick-out)	Stiffness rises as the cube of the length	Free: use a shorter tool or a deeper holder
Use a larger diameter tool	Stiffness rises as the fourth power of the diameter	Needs access
Change the spindle speed by 10–20 %	Moves to a stable lobe	Free
Reduce the radial or axial depth	Lowers the force	Slower cut; use adaptive paths
Raise or lower the feed per tooth	A thicker chip often damps vibration; a thin chip rubs	Free
Use a variable-pitch or variable-helix tool	Breaks up the regular excitation	A better tool
Fewer flutes	Lower tooth passing frequency; room for chips	Free
Clamp better	Stiffer part: use a thicker support, more clamps, or tape under thin parts	Time
Hold the tool better	Shrink-fit or hydraulic holder; clean collets	Tooling
Climb milling with a light finish	A thin chip at exit reduces force	Free

Tool breakage and wear

Symptom	Likely cause	Try
Tool snaps at the start	Wrong Z zero or offset, plunging into stock, wrong tool in the spindle	Re-check offsets; air-cut; ramp or helix in
Tool snaps in a slot	Chips packed in the flutes, too deep for the diameter, feed too high	Reduce depth, clear chips (air, mist), use fewer flutes, peck or adaptive
Tool snaps in a corner	Sudden full engagement at an inside corner	Use a larger corner radius or slow the feed in corners; adaptive paths
Small tool snaps repeatedly	Too little feed (rubbing and heat), run-out, too much depth	Increase the feed per tooth to the maker’s value; check run-out; reduce depth
Tool wears out quickly	Too fast, too little feed, wrong coating, hard material, no coolant	Lower the speed, raise the feed, change the coating, cool the cut
Chipped edge	Vibration, run-out, hitting a clamp, hard spots	Reduce the stick-out, check the set-up
Tool pulls out of the holder	Under-tightened collet, dirty collet, worn collet, oil on the shank	Clean and torque; use a correct collet size; replace worn parts
Insert breaks (lathe)	Too heavy a cut, interrupted cut, wrong grade, long overhang	Reduce the depth or feed, choose a tougher grade, shorten the holder

Replace tools by the clock or by a wear limit rather than waiting for failure. For carbide a flank wear land of about 0.3 mm is the usual limit (ISO 3685); a tool that sounds different, leaves a different finish or raises the spindle load is already worn.

Size, taper and position errors

• Tool deflection. The tool is pushed away from the cut and leaves the wall short of the programmed size, more at the bottom of a deep wall. A 6 mm carbide mill with a 30 mm stick-out and 50 N of force deflects about 11.8 µm; with an 18 mm stick-out only 2.5 µm. The cure is a shorter tool, a lighter cut and a spring pass (repeat the finishing pass with no extra depth).
• Taper in a milled wall is a sign of deflection; in a turned part it is a sign of the work being pushed away (see Designing for Turning) or of the tailstock being out of line.
• Tool wear makes walls oversize on the outside and undersize inside; apply wear offsets.
• Thermal growth. A 200 mm aluminum part warmed by 15 °C grows 71 µm; the same steel part grows 35 µm. The machine also warms: run it for 10–20 minutes before a precision cut. Let the part cool before the final measurement.
• Wrong size in every part by the same amount: a tool or work offset is wrong, or the tool diameter in the program differs from the real one; correct the offset.
• Size drifts during a run: tool wear, heating, or a loosening collet or chuck.
• Position errors (holes in the wrong place, a part that is not square): lost steps, backlash, loose belts, a part that moved, or a wrong work offset. Cut a test pattern and measure.
• Parts differ between the first and last cut of a sheet: the part moved, or the vacuum or tape released.

Surface finish problems

Symptom	Cause	Try
Wavy surface, chatter marks	Vibration	See the chatter fixes above
Tool marks, steps between passes	Step-over too large, ball nose scallop	Reduce the stepover; see CAM and Toolpaths
Fuzzy, torn surface (wood, composites)	Dull tool, wrong bit	Sharp compression or downcut bit; backing board
Melted, smeared plastic	Too slow a feed, heat	Raise the feed; use a single-flute bit and air
Burnt, dark marks (wood)	Rubbing, dull bit, low feed	Raise the feed, lower the speed, change the bit
Smeared or galled metal	Built-up edge, low speed, dull tool	Increase the speed, sharper tool, lubricant
Burrs on edges	Dull tool, exit side, soft metal	Sharp tool, climb cut, a light final pass, deburr or chamfer
Dull, rough turned surface	Feed too high for the nose radius, long overhang	Reduce the feed; see the finish chart on the turning page
Witness line where the tool started	Straight plunge, tool stopped at the start point	Use a tangent arc lead-in and a continuous finishing pass

Troubleshooting table

Symptom	Likely cause	Try
Part is the wrong size	Steps per mm wrong, belt slip, kerf or tool offset not applied	Cut a test square and measure; correct the steps per mm; apply tool compensation
Circles are oval or do not close	X and Y scaling differ, backlash, loose belts	Calibrate each axis, tighten belts, adjust anti-backlash
Lost position part way through	Stepper stalled (feed or acceleration too high), loose coupling	Lower feed and acceleration, check couplings, increase driver current within the motor rating
Machine alarms or limit errors	Work zero set outside the travel, homing not done, switch noise	Home first, check the work offset, shield and route the limit switch cables
Spindle stalls or slows	Cut too heavy, dull tool, low torque at low speed	Reduce the depth, raise the speed, sharpen or replace the tool
Unexpected moves or crashes	Wrong program, wrong units, wrong post, wrong zero	Simulate, air-cut, single-block, and verify the zero
Part moves in the vise or on the bed	Hold too weak, chips under the part, climb cut	Clean the seat, clamp harder or more, use a stop; see Workholding
Chips pack the slot	Cut too deep, no air or coolant, too many flutes	Clear the chips, use fewer flutes, peck, or shallower passes
Drilled holes oversize or off position	Drill wanders, dull point, bent drill	Spot-drill first, sharp drill, pecking, check run-out
Taps break	Drill too small, hole too shallow, wrong tap, chips packing	Correct the tap drill, drill deeper, use a spiral-flute tap or thread mill

Accuracy and the error budget

The size of a machined feature is the programmed size plus a list of small errors: backlash and positioning, tool deflection, thermal growth, set-up and zero, and measurement. They can be added worst-case, or in root-sum-square (RSS) if they are independent. Seeing the biggest term shows where to work.

Error source (example values)	Size
Machine positioning and backlash	0.020 mm
Tool deflection	0.012 mm
Thermal growth	0.035 mm
Set-up and zero	0.020 mm
Measurement	0.010 mm
Worst-case total	0.097 mm
RSS total	0.048 mm

• In this example the RSS total of 0.048 mm is about ±0.048 mm, which would hold a ±0.05 mm tolerance but not ±0.025 mm. Thermal growth is the biggest term, so the first improvement is to let the machine and part reach a steady temperature before the finishing cut.
• The way to improve accuracy is to find the biggest term and shrink it: warm up the machine, probe the part, use a stiffer tool, take a spring pass, and measure at a controlled temperature.
• A tolerance tighter than the sum of these errors is a gamble. Either loosen it or add a finishing process (grinding, honing, reaming). See Tolerances by Process and Surface Finish.
• Machine calibration. Check squareness, backlash and scale with a precision square, a dial indicator and gauge blocks; a ball-bar test checks circular accuracy.

Inspection and measurement

Tool	Typical resolution	Good for
Steel rule, tape	0.5 mm	Rough checks
Calipers (digital)	0.01 mm (accuracy ±0.02 mm)	Everyday sizes; do not use for tolerances under ±0.05 mm
Micrometer	0.001–0.01 mm	Diameters, thickness; the standard for ±0.01 mm
Dial or test indicator	0.001–0.01 mm	Run-out, flatness, tramming, set-up
Bore gauge	0.001 mm	Holes and bores
Go/no-go pin gauges, thread gauges	pass or fail	Holes and threads in production
Surface plate and height gauge	0.01 mm	Heights and flatness
CMM and optical systems	0.002 mm	Complex parts and geometric tolerances
Surface roughness tester or comparator	Ra	Finish

• Use a gauge with a resolution of about one-tenth of the tolerance band. For a total tolerance of 0.10 mm (±0.05) a gauge reading to 0.01 mm is the minimum; calipers at 0.02 mm accuracy are marginal.
• Measure at room temperature, with parts and gauges soaked to the same temperature. Do not measure a part that has just been cut.
• First article. Measure the first part against the drawing before running the batch; record the results.
• Burrs and chips give false readings: deburr and wipe the part.
• Apply the tolerance rules in the drawing: see Fits & Tolerances and Inspection and Tolerance Stack-up.

Safety

• Stay in control. Know where the emergency stop is and be able to reach it without leaning over the work. Never leave a running machine unattended, and be especially careful with a router cutting dusty material.
• Eye and ear protection for all milling, turning and routing, and a mask or dust extraction for wood, MDF, plastics and composites. Chips and broken cutters are ejected at speed.
• No gloves or loose clothing near rotating cutters, chucks and bars. Tie back long hair; remove rings, watches and necklaces. Short sleeves or rolled sleeves.
• Guards and doors closed while the machine runs. Never defeat an interlock. Use a safe shield or an enclosure for a hobby machine.
• Chip handling. Chips are razor sharp and hot: use a brush or chip hook, never bare hands. Do not blow chips with compressed air towards yourself or others (they become projectiles), and never use air on your skin or clothing.
• Lathes. Take the chuck key out before starting. Do not hold a rag or emery cloth on a rotating part. Support long stock that protrudes from the spindle; set a spindle speed limit.
• Fire. Chips and dust can ignite: keep the area clear, have a suitable extinguisher at hand (and a Class D one if you cut magnesium or titanium), and keep oily rags in a metal bin.
• Electrical. Lock out and tag out the machine before maintenance or changing a tool in a way that exposes you to the spindle.
• Lifting. Use proper lifting for heavy stock, vises and chucks. A dropped chuck or plate is a serious hazard.
• Coolant and dust. See Machining Metals and Machining Plastics, Wood and Composites for materials that make hazardous fumes and dust.
• Lasers. Laser safety is covered on Laser Cutting.
• Fatigue and distraction. Do not run a machine when tired, rushed or distracted. Most accidents happen in a hurry.

See also Workshop Safety. Follow your machine maker’s safety instructions and local rules; this page is a summary, not a safety manual.

Before every run

Check	Why
Eye and ear protection on; hair, sleeves and jewelry secure	Chips, noise, entanglement
The right tool in the spindle, tightened to torque	A loose tool is thrown
The part held, seated and clamps clear of the path	Crashes and flying parts
Work zero and tool offsets set and checked	The most common cause of a crash
Program checked in simulation and the right post used	Wrong units or moves
Air cut at a raised Z, in single-block mode, with feed override low	Shows errors safely
Door closed, extraction and coolant on, emergency stop reachable	Safe running
Fire extinguisher at hand; clean machine and floor	Fires and slips
Stand to the side and keep a hand near the stop for the first cut	Quick reaction

Related

• CNC Machining: the overview of this guide.
• Milling: Operations and Tooling: tools and chip load, the fixes for chatter and breakage.
• Workholding and Fixturing: holding parts without movement.
• Workshop Safety: PPE, fire, fumes and electrical safety.
• Fits & Tolerances: what the process can hold.

FAQ

Why is my machine chattering?

The tool is too long or the part is not stiff enough, the cut is too deep, or the spindle speed is at an unstable point. Shorten the tool, reduce the depth, or change the speed by 10–20 %; see the chatter section.

Why do my small end mills keep breaking?

Small tools break from rubbing (too little feed), packed chips, run-out, or too much depth. Raise the feed per tooth to the maker’s value, clear the chips with air, reduce the depth, and check run-out.

Why are my parts the wrong size?

Check three things: the work and tool offsets, the steps per mm (a test square will tell you), and temperature and deflection. Measure at room temperature and take a spring pass.

Can I use compressed air to clear chips?

Only with the machine door closed or a guard in place, wearing eye protection, and never aimed at people. Air can drive chips into slides and your eyes.

Is it safe to leave a CNC machine running?

No, not for routers cutting wood or plastic, or any machine with a fire risk. Use an alarm and a fire-suppression system if you must leave a production machine, and never leave a hobby machine alone.

References

• Oberg, E. et al., Machinery’s Handbook, 31st ed., Industrial Press: tool wear, machining accuracy, measurement, safety.
• Altintas, Y., Manufacturing Automation: Metal Cutting Mechanics, Machine Tool Vibrations, and CNC Design, Cambridge University Press (chatter and stability lobes).
• ISO 3685, Tool-life testing with single-point turning tools (wear criteria).
• OSHA 1910.212 and 1910.213 (machine guarding), HSE (UK) guidance on safe use of lathes and milling machines.
• Kalpakjian, S. and Schmid, S., Manufacturing Engineering and Technology, Pearson.

Last reviewed: October 2026. Cutting data are starting points: machines, tools and materials differ, so test on scrap and follow the tool maker’s data. Follow your machine maker’s safety instructions and your local rules.