Milling tools wear with extended use. Millalyzer contains a tool life estimation that predicts how long an endmill will remain useful, in terms of
- the total length of the cut;
- cutting time (minutes);
- volume of material removed.
Wear life prediction is available for most metals, but not for other materials at this time. That means that tool life predictions for machining wood are based on tool fatigue only, which will in most cases be overly optimistic.
Calculations for HRSA, titanium and stainless steels (such as EN 1.4301 / AISI 304) assume that flood cooling with a suitable emulsion is used, as tool life would be extremely short otherwise. Flood cooling (or rather lubrication) will also improve tool life in aluminium, as it drastically reduces friction between tool surface and stock material. Effectively employed minimum quantity lubrication (MQL) can have a similar effect when milling aluminium alloys, but not for steel.
The underlying model used to predict tool life is empirical and attempts to account for tool wear and material fatigue only. That means that factors which can drastically reduce life such as chip re-cutting or clogging of flutes due to insufficient chip evacuation, are not considered. The following machining parameters have a strong influence on wear:
- Feed per tooth [FZ] will increase tool life expressed in total length of cut. This is because the amount of wear caused by each tooth engagement is typically less than the increase in feed. This effect is especially pronounced in brass and aluminium but present in all metals.
- Cutting speed negatively affects tool life once a certain limit is exceeded. This limit may increase with lubrication. The decrease is more or less linear for brass, aluminum and steel (ISO P) but much more radical for titanium and stainless steel.
- Lower radial engagement [AE] increases tool life expressed in cutting length and time. Beyond a certain point it may however reduce the amount of material removed within that tool life, all else being equal.
- Axial engagement has no impact on tool life expressed as length or time, because the same wear occurs along the entire exposed tool edge. This means that the volume of material removed during the useful life of the tools scales directly with axial depth of cut.
Changes to improve tool life
Any of the following changes leads to an increase in tool life (due to wear):
- increase AP and reduce AE;
- increase feed and reduce AE;
- use lubrication for aluminium, titanium and HRSA;
- limit cutting speed to moderate values.
In the case of HSS tools, high cutting speed is much more detrimental than for carbide endmills or inserts.
The prediction methods assumes good-quality tools that are designed for the workpiece material. With a nondescript general-purpose tool that is advertised as being suitable for all materials, tool life will be much shorter than predicted here.
In reality, tools with somewhat less rake, larger cutting edge radius or facetted inserts can have a longer life because the exposed cutting edge is stronger and hence less prone to breakage. When milling aluminium, the opposite can be the case as a blunt edge is more susceptible to material buildup (BUE) that has a very detrimental effect on tool life. These details are not yet accounted for.