The following is just one of several blog posts relevant to High Efficiency Milling. To achieve a full understanding of this popular machining method, view any of the additional HEM posts below!
Defining Tool Wear
Tool wear is the breakdown and gradual failure of a cutting tool due to regular operation. Every tool will experience tool wear at some point in its life. Excessive wear will show inconsistencies and have unwanted effects on your workpiece, so it is important to avoid tool wear in order to achieve optimal end mill performance. Tool wear can also lead to failure, which in turn can lead to serious damage, rework, and scrapped parts.
To prolong tool life, identifying and mitigating the various signs of tool wear is key. Both thermal and mechanical stresses cause tool wear, with heat and abrasion being the major culprits. Learning how to identify the most common types of tool wear and what causes them can help machinists remedy issues quickly and extend tool longevity.
The wear land is a pattern of uniform abrasion on the cutting edge of the tool, caused by mechanical abrasion from the workpiece. This dulls the cutting edge of a tool, and can even alter dimensions such as the tool diameter. At higher speeds, excessive heat becomes more of an issue, causing more damage to the cutting edge, especially when an appropriate tool coating is not used.
If the wear land becomes excessive or causes premature tool failure, reducing the cutting speed and optimizing coolant usage can help. High Efficiency Milling (HEM) toolpaths can help reduce wear by spreading the work done by the tool over its entire length of cut. This prevents localized wear and will prolong tool life by using the entire cutting edge available.
Chipping can be easily identified by a nicked or flaked edge on the cutting tool, or by examining the surface finish of a part. A poor surface finish can often indicate that a tool has experienced some sort of chipping, which can lead to eventual catastrophic tool failure if it is not caught.
Chipping is typically caused by excessive loads and shock-loading during operation, but it can also be caused by thermal cracking, another type of tool wear which is explored in further detail below. To counter chipping, ensure the milling operation is completely free of vibration and chatter. Taking a look at the speeds and feeds can also help. Interrupted cuts and repeated part entry can also have a negative impact on a tool. Reducing feed rates for these situations can mitigate the risk of chipping.
Thermal cracking is often identified by cracks in the tool perpendicular to the cutting edge. Cracks form slowly, but they can lead to both chipping and premature tool failure.
Thermal cracking, as its name suggests, is caused by extreme temperature fluctuations during milling. Adding a proper coating to an end mill is beneficial in providing heat resistance and reduced abrasion on a tool. HEM toolpaths provide excellent protection against thermal cracking, as these toolpaths spread the heat across the cutting edge of the tool, reducing the overall temperature and preventing serious fluctuations in heat.
Fracture is the complete loss of tool usage due to sudden breakage, often as a result of improper speeds and feeds, an incorrect coating, or an inappropriate depth of cut. Tool holder issues or loose work holding can also cause a fracture, as can inconsistencies in workpiece material properties.
Adjusting the speeds, feeds, and depth of cut and checking the setup for rigidity will help to reduce fracturing. Optimizing coolant usage can also be helpful to avoid hot spots in materials which can dull a cutting edge and cause a fracture. HEM toolpaths prevent fracture by offering a more consistent load on a tool. Shock loading is reduced, causing less stress on a tool, which lessens the likelihood of breakage and increases tool life.
It is important to monitor tools and keep them in good, working condition to avoid downtime and save money. Wear is caused by both thermal and mechanical forces, which can be mitigated by running with appropriate running parameters and HEM toolpaths to spread wear over the entire length of cut. While every tool will eventually experience some sort of tool wear, the effects can be delayed by paying close attention to speeds and feeds and depth of cut. Preemptive action should be taken to correct issues before they cause complete tool failure.