CNC milling defects: seven symptoms and 33 causes, ranked for checking
Pick what the part shows, answer a few quick questions and get the causes in three levels, each with a check and an action. Two calculations, tool stickout and thermal expansion of the part, give a real number.
Opens without an account 3 times a day. A free account removes the limit.
- 7 symptomsfrom chatter to burr
- 33 causeswith a check and an action for each
- 4 originsmachine, tool, workholding, program
- 2 calculatorstool stickout and thermal expansion
What the CNC milling troubleshooting tool shows
CNC milling defects is a free online tool from EduFacturing that turns a symptom on the part (for example chatter, the vibration of the cutter) into ranked causes to check. It has seven symptoms and 33 causes, each from one of four origins: the machine, the tool, the workholding or the program.
The ranking is heuristic, not measured statistics: each cause starts with a weight from how common it is and how cheap it is to check, and your answers change it. The result is three levels, with no percentage.
The diagnosis and both calculations run in the browser and what you enter does not leave the page. The tool does not give speeds and feeds: cutting regimes have a separate tool.
Milling chatter causes, burrs and burnt edges
Starts from the symptom
Seven symptoms: chatter 5 causes, burnt edge 5, size out of tolerance 5, broken tool 5, unusual noise 4, step marks 4 and burr 5.
Shows where each cause comes from
Every cause is from the machine, the tool, the workholding or the program. The tiles show how many causes come from each origin, because who has to act depends on it.
Asks in order to rank
2 or 3 quick questions per symptom, for example what happens when the spindle speed changes by 10 to 20%. The answers move causes between the three levels.
Works out stickout and expansion
For chatter it works out the ratio of stickout to tool diameter, and for a size out of tolerance the thermal expansion of the part.
Why does an end mill break: who uses this and when
A defect is fixed faster when the cheapest thing is checked first.
The cutter chatters or the edge is burnt
The operator picks the symptom, answers the questions and checks the highest-scoring cause first, before calling service.
A burr, step marks, a broken cutter
The programmer sees when the cause is in the program and is fixed by editing the CAM or the G-code.
The size is out of tolerance and nothing is broken
The calculator shows how much expansion from warming changes the size, before the machine is blamed.
How the causes of CNC milling problems are ranked
The diagnosis is a points system, and the two calculations are physics formulas. They are two different kinds of number and the tool keeps them apart.
Starting weight
Each cause starts with a number of points from how common it is in practice and how cheap it is to check.
Answers
Each answer adds or removes points. For chatter, if changing the spindle speed makes it vanish sharply, the resonant speed gains 5 points.
Levels
7 or more points is “Check first”, 3 to 6 is “Check this too”, under 3 is “Less likely”. There is no percentage.
| Applies to | Formula | Meaning |
|---|---|---|
| Tool stickout | r = stickout / diameter | up to 3 is low, under 5 is medium, 5 and above is high |
| Deflection vs baseline | k = (r / 3)³ | how many times more the tool deflects than at a stickout of 3 diameters; at 45 mm and a 10 mm diameter k is about 3.4 |
| Thermal expansion | ΔL = L · α · ΔT | the difference between a warm and a cooled part; 150 mm of aluminium and 15 °C give 0.0531 mm |
| Coefficient α | aluminium 6061: 23.6; mild steel: 11.7; stainless 304: 17.3; POM: 110; hardened tool steel: 11.0 | in 10⁻⁶ per °C |
CNC milling troubleshooting without made-up percentages
The heuristic is called a heuristic
Three levels instead of a probability. In its limits the tool says the EDi Brain core has no rules for milling defects.
Every cause has a check and an action
33 causes, and each one says how to confirm it and what to do, not just a name.
The calculations are physics with a source
The coefficients for the metals come from the ASM Handbook and Engineering ToolBox. Tool makers advise a stickout under 3 to 4 diameters for stability and above 5 to 6 for a high risk; the tool puts its limits at 3 and 5.
The tool does not see the part, the machine or the cutter. It orders your own observations: the more precisely you answer, the more useful the ranking, and without answers the causes are ranked only by typical frequency.
It does not give speeds, feeds or depth of cut, and it does not replace measuring with an indicator and calipers. When the core check is switched on, the page shows under the list how many core rules name milling, but for defect diagnosis there are none.
Chatter, burrs and size errors: questions
Why does my end mill chatter?
Start by changing the spindle speed by 10 to 20% at the same feed per tooth. Lowering the feed usually does not help and is often worse: a lower feed increases each tooth's contact time.
Why does an end mill break?
One cause is too long a stickout: above 5 to 6 diameters the risk of breakage rises sharply. For a broken tool the tool lists 5 causes, each with an origin, a check and an action.
How far can a cutter stick out of the holder?
Under 3 to 4 diameters is the general guidance. Above 5 to 6 the risk rises sharply, because deflection grows with the cube of the length: doubling the stickout gives 8 times the deflection.
Why is the size off when nothing is broken?
A warm part is larger: 150 mm of aluminium warmed by 15 °C is about 0.05 mm larger than once cooled. For critical sizes, measure after cooling.
Does a burr always mean a worn tool?
No. A burr only on the exit side of the cut is a support problem: there is no support behind the edge. An equal burr on entry and exit is more likely from a worn edge.
Is the CNC milling defects tool free?
Yes. Without an account it opens 3 times a day, and with a free account there is no limit. Pro is not needed.
The CNC milling defect tool is free. Pro opens the rest
Pro costs 15 EUR a month and opens 45 Pro tools.
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Find the cause of the defect
Opens without an account 3 times a day. A free account removes the limit.