The process covers a wide range of applications: from the machining of large free-form surfaces in tool and mold making and in the production of components for energy generation to highly automated mass production. What many of these applications have in common is the growing cost pressure. Machinists have to reconcile high demands on the surface quality and dimensional accuracy of a workpiece with high cost and time pressure. The choice of the right milling tool plays a central role here. But to fully exploit the performance of an indexable insert milling cutter, the process must also be right. From many years of experience in Technical Sales at Walter, I know how errors can creep in, particularly in the area of basic process parameters, which have a negative impact on the efficiency and reliability of the process.

Analyze the process holistically
If processes are stable and the results are economically sound, nobody in the company normally takes a closer look at the individual process steps or parameters. They only become the focus of attention when, for example, output drops, surface quality or dimensional accuracy deteriorates or price pressure increases. Attempts are then often made to achieve the desired result by making selective changes, for example by altering the cutting values or changing the tool. This can work or at least solve the acute problem in the production process. However, sustainable optimization success is more likely to be achieved by taking a holistic view of the process.

The key questions here are

•     Which material is being processed? Which material properties have a decisive influence on the milling process?
•     How high is the tensile strength of the material?
•     How should the machining conditions be assessed? The decisive factors here are the component stress and the projection length of the tool. They influence whether and to what extent the milling cutter vibrates during machining, which in turn influences the surface quality of the area to be machined or the service life of the milling cutter.
•     What is the aim of process optimization? Should the machining time per workpiece be reduced, the tool life increased or the process reliability improve

Adjusting the tooth feed
The tooth feed depends on the properties of the material to be machined and those of the milling tool. Manufacturers usually specify the optimum range here. Many users use a relatively low feed rate. However, there is usually some scope to increase the feed rate, which can increase the number of workpieces produced per tool, as the milling cutter effectively covers a shorter distance on the milled surface. Whether and how the tooth feed can be increased depends on conditions such as the setting angle, the engagement ratio of the tool, the projection length of the tool, the component clamping and the material to be machined.

Important: Does the generated chip thickness (h) match the tool geometry and the material? If the chip thickness is too low, this has a negative impact on wear and the service life of the indexable inserts. If the chip thickness is too high, the cutting edge will break.

Cutting speed and wear behavior
Probably the most important wear-promoting factor in milling is the constant alternating thermal load on the indexable inserts when the milling cutter enters and exits the cutting process. The change from heat generation to cooling leads to cracks along the cutting edge. As a result, chipping occurs on the cutting edge, the cause of which lies in the formation of cracks. However, the chipping can also be misinterpreted, as it can be blamed on a cutting material that is too wear-resistant, for example.

The choice of cutting speed therefore plays a major role, especially with small engagement ratios. If the engagement ratio during milling is small, i.e. only a very small radial engagement (ae) is used, then the cutting speed (vc) must be increased in order to reduce the thermal interaction between hot and cold. This in turn reduces the formation of cracks on the cutting edge and prevents premature tool wear. As a rule of thumb: increase vc with a low ae/Dc ratio.

Choose the right milling cutter position
The correct choice of cutter position depends on the entry contact of the cutting edge with the workpiece. This can be influenced by the operator specifying a certain milling width (ae) in relation to the cutter diameter (Dc). If the milling width is half the milling cutter diameter, the cutting edge hits the workpiece with the maximum possible chip thickness. The entry contact is equivalent to an impact. The cutting edge is thus subjected to very high loads, which can very quickly lead to cutting edge breakage. It is advantageous if a ratio of ae > 2/3 Dc or ae < 1/3 Dc is selected.

Selecting the correct milling cutter position
The correct choice of milling cutter position depends on the entry contact of the cutting edge with the workpiece. This can be influenced by the operator specifying a certain milling width (ae) in relation to the cutter diameter (Dc). If the milling width is half the milling cutter diameter, the cutting edge hits the workpiece with the maximum possible chip thickness. The entry contact is equivalent to an impact. The cutting edge is thus subjected to very high loads, which can very quickly lead to cutting edge breakage. It is advantageous if a ratio of ae > 2/3 Dc or ae < 1/3 Dc is selected.

Choose the right milling strategy
If the surface to be machined during face milling is larger than the milling cutter diameter used, the best strategy for milling surfaces is to machine in several paths with spiral movement from the outside to the inside. This strategy results in only one entry of the milling cutter into the workpiece. The milling cutter is always under pressure when the cutting direction changes in a radius movement. There is only one exit. The radius in the corners and at the entry should be approx. ¼ to ½ of the cutter diameter. This achieves a uniform load on the milling cutter by avoiding constant entries and exits, which occur in monodirectional line milling, for example. Compared to monodirectional line milling, this spiral strategy generally also achieves a machining time saving of at least 30 percent without changing the cutting parameters.

The milling cutter position when entering the workpiece determines both the chip formation and the tool life. The entry and exit points are the most sensitive machining operations and reduce the service life of the milling cutter. To increase this, the entry operation should always be carried out in a quarter-circle movement. The negative effect of the entry can be reduced by a so-called “roll-in” entry with a quarter-circle radius in climb milling. The “roll-in” entry depends on the surface of the workpiece. If the surface of the workpiece is soft, the “roll-in” should take place in climb milling. If the surface of the workpiece is hard, roll-in in the opposite direction is the better choice and more favorable for the service life of the tool.

If spiral milling is not possible because the surface to be machined is smaller than the milling cutter diameter, the milling cutter position should be offset in the middle to ensure a predominantly synchronous run.

Synchronized or up-cut milling?
From what has been said so far, it is probably already clear that climb milling is actually the best milling strategy. The tool is guided with the direction of rotation. The chip thickness decreases from the beginning of the cutting edge entry until it reaches zero at the exit and the chip separates. This prevents the cutting edge from rubbing and grinding against the surface before engagement. The resulting forces pull the workpiece towards the milling cutter, keeping the cutting edge in engagement.

In up-cut milling, the tool is guided against the direction of rotation. Here, the chip thickness increases sharply from the beginning of the cutting edge entry. Significantly stronger mechanical and thermal forces act on the tool, which significantly reduces the tool life. However, there are applications in which up-cut milling is the better method. These include the machining of workpieces with hard material edges, the machining of thin, highly vibrating workpieces or when the tool itself has a long overhang.

Peter Kalenbach, Product Manager for WDE-MS rotary indexable insert tools at Walter Deutschland GmbH, will be happy to answer any questions you may have on the subject of indexable insert milling at peter.kalenbach@walter-tools.com.

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