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Tool failure

The peak von-Mises stress experienced by the tool over one revolution is higher than the configured limit. Often, the peak stress will occur either at the upper end of the cutting flutes where the bending moments are already large but the endmill is weak, or, for long tool stick-out, directly at the interface to the toolholder/collet.

Even if the tool does not break immediately for conditions where this warning is shown, it will probably not last long because the high stresses are likely to initiate small cracks. As the stresses cycle with each revolution, these cracks will grow and soon lead to fatigue failure.

Alleviation: Increase tool diameter or reduce AP, AE or HEX.

Computation of tool stress is based on a representative tool geometry for each flute count. Stresses that are actually encountered by the tool can therefore be both larger and smaller than computed. Tools with large chip space will tend to encounter larger stress, while those with a larger core diameter and less chip space will be stressed less.

High tool stress

Peak stresses are approaching the failure limit or exceed the user-defined limit in the machine limit settings; there is a pronounced risk for tool fatigue when these conditions are maintained for long.

Reduce AP, AE or HEX or increase tool diameter.

Tool overheating

Temperatures are approaching levels where the strength of HSS tools degrades. Should the current set of parameters be maintained, tool wear will likely accelerate very quickly.

Alleviation: Switch to carbide or CBN tool, increase chip thickness [HEX].

Tool RPM too high

The spindle speed [N] is too high for this tool. Ignore this warning if the tool vendor explicitly specifies a higher permitted rotational speed. This warning is most likely to occur for indexed tools with large diameter, as the permitted RPM drops quite rapidly with diameter.

Alleviation: Reduce speed.

Large rake angle

The tool geometry is probably not suitable for the material (steel/titanium/nickel alloy). Prefer a tool with smaller rake angle [GAMF] for high-strength workpiece materials to reduce the risk for edge chipping and extend tool life.

Small edge radius

The tool geometry is probably not suitable for the material (steel/titanium/nickel alloy).

Choose a tool with larger (lapped or honed) cutting edge radius [EDRD] for high-strength materials to avoid edge chipping and extend useful tool life.

BUE risk, low VC

The cutting speed is too low, there is a risk for built-up edges.

Alleviation: Increase spindle speed [N] or tool diameter [DC].


The cutting edge is at risk of ploughing or sliding over the material instead of cutting.

Alleviation: Increase chip-load [HEX], choose a tool with smaller cutting edge radius [EDRD], or switch from conventional to climb milling.

Chip space

The amount of material removed per revolution is too large for the tool geometry; there is not enough space in the flutes for this chip-load.

Ignore this warning if the tool manufacturer recommends the chosen feed/chip-load. Some specialized endmills (one or two flutes, marketed as large chip removal) are ground to allow for very large chip thickness values.

Alleviation: Reduce chip-load [HEX].

Torque limit

The required torque exceeds the capabilities of the spindle as configured in the machine limits (see Edit - Machine limits).

Alleviation: Reduce AP, AE, HEX or decrease tool diameter.

Peak forces

At least once per revolution, the user-defined peak force limit is exceeded.

Alleviation: Reduce AP, AE or feed.

Mean forces

Forces averaged over one revolution exceed user-defined limits. These forces are also responsible for the averaged tool displacement.

Alleviation: Reduce AP, AE or feed.

Unsuitable material

The selected tool is not nominally suitable to mill the current material. Depending on the material, this may or may not be a problem.

Using an endmill designed for aluminum (code N) for milling steel (code P or H), for instance, will produce simulation results that look good, but are often unrealistic: Tools for softer aluminum are usually ground to a very sharp edge and equipped with large gullets to allow for large chip volume. These features compromise the strength of the edges which will therefore be damaged very quickly when used in harder materials. Furthermore, the cutting force simulation for steel assumes that the tool is suitably coated (TiAlN or equivalent), which endmills for aluminum may not be.

Similarly, using a TiAlN-coated endmill that is ground for steel for cutting aluminum is possible, but not advisable. As the aluminum will tend to adhere to this coating much more than to a polished uncoated surface, it may accumulate, clog the gullets or result in built-up edges (BUE). None of these effects are currently simulated.

Alleviation: Choose suitable tool.

Critical screw speed

The chosen feed results in a rotational speed of the ball- or leadscrew that is in excess of the permissible speed at this length. There are two limits for screw speed. The critical speed is a stability limit; reaching the critical speed will cause severe bending oscillations of the screw and thus reduce dimensional accuracy. The second limit applies to ball screws only and is caused by the necessity to pick-up balls for recirculation, which only works reliably up to a certain rotational speed.

Alleviation: When evaluating a drive layout, choose a higher screw pitch to reduce screw speed for the same feed. Otherwise, reduce feed.

Screw load exceeded

The forces encountered by the cutting tool exceed the permissible axial load of the drive screw. At too large axial loads, the screw length between nut and fixed support can buckle, which is likely to cause drive failure.

Alleviation: Increase drive screw diameter or reduce cutting forces.

Servo drive overload

The operation requires driving forces that exceed the capabilities of the configured servo drives. Such conditions could still be feasible for short periods of time if the servo drive permits low-duty-cycle overload. In any case, the present conditions can not be sustained for long. Note that available drive thrust may depend on feed.

Alleviation: Change to drive with higher torque, shorten the gear or reduce cutting forces.


A charring warning is issued when, during the machining of wood, the temperature in the shear zone exceeds a certain level. Depending on the species or product used, the degree of charring at this temperature may be very light (minor discoloration) or more severe (black marks, smoke emission).

Alleviation: Increase chip thickness [HEX].


The mean shear zone temperature is approaching the solidus temperature of the material. It is possible that the quality of machined surfaces will degrade and likely that burrs form at the edges. With some polymers, molten material can clog the gullets and thus interfere with proper chip formation.

Alleviation: Increase chip thickness [HEX].