Troubleshooting guide

Why CNC Machine Used: Understanding the Process and Fixing Common Faults

This page explains why CNC machine used in modern machining, then walks through the faults engineers see most often: drifting tolerances, chatter, poor finish and short tool life. Written for machinists, process engineers and buyers who need to trace a symptom back to a cause and act on it.

Get Instant Quote Talk to an Engineer

±0.005 mm tolerance127 CNC machines100% inspection3-5 day delivery

On this page

1
7 CNC Faults: Symptom, Likely Cause, Action
2
Why CNC Machine Used: The Mechanism Behind the Accuracy
3
When CNC Replaces Casting, Forging or Manual Work
4
Why Setup and Datum Choice Decide the Outcome
5
Why Material Choice Changes the Cutting Data
6
Step-by-Step Fault Tracing on a CNC Job

Symptom to action

7 CNC Faults: Symptom, Likely Cause, Action

Use this table to narrow the fault before you touch a single offset.

Symptom	Likely cause	Action
Dimensions drift over a run	Thermal growth in spindle and ball screw	Warm up 20-30 min; re-check first part after 2 h
Chatter marks on a wall	Tool overhang too long, low radial engagement	Shorten holder; raise radial depth to 25-40% of Ø
Ra worse than 1.6 μm	Wrong feed per tooth or dull insert	Set 0.05-0.15 mm/tooth; index insert at 0.2 mm wear
Tool breaks on the first pass	Runout above 0.02 mm TIR	Indicate holder; replace collet; re-touch length
Hole position off by 0.03 mm	Fixture clamp pushed the part	Reduce clamp torque; add a support under the web
Burrs on a 6061 edge	Feed too low, edge radius worn	Raise feed 20%; add a 0.3 mm chamfer pass
Surface step between two setups	Datum shift between 3-axis ops	Move to 5-axis single setup, or pin the second op

The Short Version

Most CNC faults come from setup, tooling or thermal drift, not from the machine itself. Fix those three and the process holds ±0.005 mm.

Get Instant Quote Talk to an Engineer

Basics

Why CNC Machine Used: The Mechanism Behind the Accuracy

The core reason why CNC machine used is simple: a program moves the tool, not a hand. A CAM file defines every X, Y, Z and rotary position, so the same G-code produces the same geometry on the first part and the five-hundredth. Manual turning and milling depend on the operator reading a dial and reacting. CNC removes that reaction from the loop.

That matters most when the drawing carries tight limits. On our 16 simultaneous 5-axis centers we hold ±0.005 mm on production parts, with the machine geometry and thermal compensation doing the heavy lifting. A skilled operator can still hit a tight bore on a manual lathe, but not repeat it across a 500-piece run without constant gauging.

CNC also cuts shapes that manual machines cannot reach. Undercuts, blended 3D surfaces, deep pockets with small corner radii and angled ports all need coordinated motion on several axes at once. A 5-axis center tilts the tool or the table so the cutter stays normal to the surface, which keeps the step-over even and the finish predictable.

1
RepeatabilityThe program, not the operator, sets the cut.
2
Complex geometry3D surfaces and undercuts need coordinated axes.
3
TraceabilityTool offsets and inspection data stay with the job.

Cost logic

When CNC Replaces Casting, Forging or Manual Work

CNC is a subtractive route: you start with a billet or plate and cut material away. That makes it the right choice when quantities are low, when the design is still moving, or when the part needs a tolerance that a casting cannot hold. Tooling for a die casting or a forging costs far more than a machining program, so the break-even sits at high volume.

For prototypes and bridge builds, machining wins on speed. We can start production within 24 hours of a released file and ship parts in 3-5 days. A die-casting tool takes weeks before the first sample exists. Engineers use CNC to validate fit and function, then move to casting once the design is frozen and the annual volume justifies the tool.

There is a limit. A part with a deep internal cavity, a thin closed wall or a hollow section that no cutter can reach is a poor fit for milling. Those go to casting, additive or a welded assembly. Knowing when to stop designing for the spindle is as important as knowing how to program it.

1
Pick CNCLow to mid volume, tight tolerance, design still changing.
2
Pick castingHigh volume, wall thickness above 3 mm, simple cavity.
3
Pick additiveInternal channels or lattice that no tool can reach.

Setup

Why Setup and Datum Choice Decide the Outcome

Most tolerance failures trace back to the setup, not the machine. If the part is located on a rough surface, the datum moves between the first and second operation. If the vise crushes a thin wall, the part springs back after unclamping and the measured size changes. Both look like a machine fault, and neither is one.

The fix is to locate on a machined surface and to control clamping force. For a thin-walled housing, support the web with a jack or a custom soft jaw so the wall cannot deflect. On a 5-axis center, one setup keeps every feature in the same coordinate frame, which removes the datum shift entirely and is a strong reason why CNC machine used for parts with tight hole-to-hole relationships.

Thermal drift is the other quiet variable. A spindle that has run for ten minutes is not the same size as one that has run for two hours. On a ±0.005 mm job, warm up for 20-30 minutes, cut a test part, measure it, then re-check the same feature after two hours of running to confirm the offset is still valid.

1
DatumLocate on a machined face, never on raw stock.
2
ClampEnough force to hold, not enough to distort.
3
Warm-up20-30 min before the first tight-tolerance cut.

Material

Why Material Choice Changes the Cutting Data

Aluminium 6061 and 7075 machine fast and forgiving. Surface speed runs high, feeds are generous, and Ra 0.8-1.6 μm is easy with a sharp carbide cutter. 7075 is stronger but more notch-sensitive, so avoid a dwell in the cut and keep the tool moving. Both are common in aerospace brackets, EV housings and robot arms.

Stainless 304 and 316L work-harden. If the feed per tooth drops too low, the edge rubs instead of cutting and the next pass meets a harder skin. Keep the chip load up, use a coated insert, and never let the tool sit in the cut without advancing. Titanium TC4 (Ti-6Al-4V) is worse: heat stays at the edge, so flood coolant and a rigid setup are not optional.

Plastics behave the opposite way. POM and PEEK cut cleanly at high speed with sharp, polished flutes, but they expand with heat and can melt at the edge. ABS and PC need sharp tools and air blast rather than flood coolant. In every case, the material sets the starting feed and speed, and the finish and tolerance follow from there.

1
AluminiumHigh speed, high feed, easy finish.
2
StainlessKeep chip load up to avoid work hardening.
3
PlasticSharp flutes, air blast, watch thermal growth.

Procedure

Step-by-Step Fault Tracing on a CNC Job

Work through these in order. Stop as soon as the symptom disappears.

1
Measure the first part against the drawingRecord actual values, not pass or fail. A trend of 0.005 mm per hour points to thermal drift; a sudden 0.03 mm jump points to a setup or tool change.
2
Check tool runout at the holderIndicate the flute at 10 mm from the tip. Keep TIR under 0.02 mm for finishing. Replace the collet if it is worn, and re-touch the tool length after any change.
3
Re-cut the suspect feature at reduced feedDrop feed per tooth by 30% and radial depth to 20% of tool Ø. If the finish improves, the fault is cutting data, not the machine.
4
Re-clamp and re-measureIf the size changes after unclamping, the part was distorted in the vise. Add support under thin walls and reduce clamp torque by half.
5
Confirm the datum on a second setupTouch off the same face used in operation one. A shift above 0.01 mm means the fixture moved; pin or dowel the location before re-cutting.
6
Run a warm-up cycle and re-checkLet the spindle idle at 6,000-8,000 rpm for 20-30 minutes, then cut a test part. If the offset holds over two hours, the job is stable.
7
Log the fix and the offsetWrite the corrected tool offset and the clamp torque into the setup sheet. The next run starts from a known point instead of a guess.

FAQs

Common Questions About CNC Machining

Why use CNC instead of manual machining?

A manual machine depends on the operator reading a dial and reacting. CNC follows a fixed program, so the geometry repeats across a run without constant gauging.

For a one-off repair, a manual lathe can be faster because there is no programming. For anything with tight limits or more than a handful of parts, CNC is the lower-risk route.

What tolerance can CNC actually hold in production?

On our 5-axis centers, production parts are quoted at ±0.005 mm. That figure assumes a rigid setup, a warm machine and a measurement method that matches the tolerance.

Hold a tighter band on a single feature when the design needs it, but expect the cost and lead time to rise. Very tight limits on many features at once usually mean a different process.

Why does finish change between the first and last part?

Tool wear is the usual answer. An insert that starts sharp develops a wear land, and the edge rubs rather than shears. Index or replace the tool at roughly 0.2 mm of flank wear.

Thermal growth also shifts the effective depth of cut. Re-check the offset after two hours of running and correct it before the finish drifts out of spec.

When is 5-axis worth the extra cost?

Use 5-axis when a part has features on several faces, deep pockets with short tools, or blended 3D surfaces. One setup removes datum shift between operations.

Skip it for simple prismatic parts. A 3-axis machine with a good fixture will be cheaper and just as accurate for a flat plate with through-holes.

Can CNC machines cut different materials on the same job?

Yes, but not in the same setup without care. Aluminium and stainless need different feeds, speeds, coolants and sometimes different tooling.

Contamination matters too. Aluminium chips left on a fixture can damage a stainless surface. Clean the machine and the fixture between material changes.

What causes chatter and how do I stop it?

Chatter comes from a lack of rigidity somewhere in the loop: tool, holder, fixture or machine. Shorten the tool overhang first, then stiffen the setup.

If the marks persist, raise the radial depth of cut to 25-40% of the tool diameter. Too light a pass lets the edge rub and regenerate the vibration.