Chip thickness could well be the most important cutting condition for any machining operation.

If the machinist uses the same machining conditions and cutting data as those that the designer of the cutting-edge had in mind when he was designing the tool; he can optimize the cutting behaviour of the cutting tool much more accurate.

The rewards will be legion, it starts with having a sustainable machining process delivering up to expectations, and it ends with satisfied customers, and everything in between like being productive, work cost-efficient, deliver high quality, avoid waste ...

Patrick de Vos is Seco’s Global Expert Educator and Advisor on Applied Machining Physics and Sustainable Production Economics. Patrick discusses in this essay the important role of the chip thickness (hm) as a key metric in machining operations in general and in milling operations in particular.

The term “chip thickness” is defined in the basic modelling of general machining. It is the thickness, measured perpendicularly to the cutting-edge, of the material to be machined, and just before it is machined. ?

Figure 1: chip thickness (h), width of cut (b) and their relation to feed (f) and depth of cut (ap).

Important is to realise that feed relates to the movement of the tool along the workpiece, hence having direct consequences to the machined surface, whereas chip thickness is referring to the cutting edge and as such is a metric for the behaviour and performance of the cutting edge.

A similar relation exists between the width of cut (b) and the cutting depth but is in the context of this essay not relevant to discuss.

In milling operations, the chip thickness is determined in the same way. But as milling is a ‘two-dimensional’ operation, it is important that the definition of chip thickness is applied correctly.

In a typical milling operation, the thickness of the material located just in front of the cutting-edge changes constantly as the position of the cutting-edge is not fixed but is moving circular (as the milling cutter is rotating). The result of this is that with a certain feed the chip thickness changes constantly.

Figure 2: Chip thickness in milling changes constantly, even with a constant feed.

Therefore ‘average chip thickness’ (hm) was introduced. This is the thickness of a “theoretical” rectangular chip with the same surface area and length as the actual chip.

Figure 3: Definition of average chip thickness in milling

The relationship between the average chip thickness, the milling method or strategy, the feed per tooth and the entering angle must be defined. To be able to do that we need a connecting factor. This connection factor is the ae/Dc ratio, whereby Dc is the diameter of the cutter and ae is the radial cutting depth.

Figure 4: Average chip thickness and the ae/Dc ratio

But why is the average chip thickness so important in a milling application? All research into the behaviour and possibilities of the different cutting-edge geometries is largely based on the used (or desired) average chip thickness. All the machining process variables, such as cutting temperature, cutting forces, chip formation and evacuation, tool life, cutting-edge wear and vibrations are strongly affected by the relationship between the cutting-edge geometry and the average chip thickness.

If the milling operator works with the same cutting conditions as the designer of the cutting-edge, he can optimize and predict the cutting behaviour. And because the same average chip thickness with different operations results in different feeds per tooth, he can maximize the operation’s productivity.

Which practical instruments are available to help the machining expert to calculate with this?

First, the ISO-name of the insert. In code position 10 the options about the geometric cutting conditions (cutting depth and feed) are given. E.g., Seco uses a code in which the letter represents the degree of difficulty of the operation and the number represents the average chip thickness under normal operating conditions in traditional steel materials. For example, M14 means an operation under normal operation circumstances with an average chip thickness of 0.14 mm in standard steel.

Figure 5: In code position 10 (M14) in the ISO name the preferred average chip thickness is indicated.

Another tool is the cutting-edge graph (see also figure 5), in which the degree of difficulty of the operation is marked on the vertical axis and the correct chip thickness on the horizontal axis. With this graph it is possible both to select a specific cutting-edge geometry and to solve problems during the operation. After assessing the degree of difficulty, the right cutting-edge geometry and corresponding average chip thickness can be determined.

The ISO code and the cutting-edge graph help to determine which average chip thickness should be used for a specific cutting-edge in a specific operation. It is important to convert this value into a feed per tooth (table feed). Figure 6 shows the equation for calculus.

Figure 6: Calculus of feed.

The equation shown in figure 6 is rather complex and asks for some time-consuming calculus. There are however several more practical instruments available.

Conversion charts can be used that show the relation between the different variables in the equation.

Figure 7: Conversion table for calculus of feed.

Most useful is a conversion graph. This shows the influencing factors when determining the feed for a desired average chip thickness by means of correction factors. The cutting method is considered (off centre or central milling), the ae/Dc ratio and the entering angle (and indirectly the strength of the cutting-edge used) of the cutter. Based on two correction factors, is defined the feed per tooth suitable for a specific average chip thickness. Correction factor C1 considers the ae/Dc ratio during the operation and the cutting method used. Correction factor C2 considers the entering angle of the cutter. Several safety factors have been included in the graph to ensure that the maximum chip thickness, that can be cut by the cutting-edge at a certain moment, is not exceeded.

Figure 8: Conversion graph for calculus of feed.

An example to illustrate. A 100 mm cutter with an entering angle of 90° is used for face milling of a surface of 20 mm wide. The milling method used is central milling. The average chip thickness is known (see code position 10 in the name of the insert): 0.14 mm. In the chart we see that for an ae/Dc ratio of 20/100 (=20%) C1 has a value of 1; for an entering angle of 90°, C2 also equals 1. This means that the feed to be used equals 0.14 x 1 x 1 = 0.14 mm/tooth. If the user decides to carry out the operation with a 45° cutter the feed will be 0.14 x 1 x 1.4 = 0.20 mm/tooth. Using the off-centre milling method, the feed would be 0.14 x 2 x 1.4 = 0.40 mm/tooth. This means an increase of the initial feed rate by 186% with the same average chip thickness, or in other words, with the same cutting-edge load, cutting temperature and tool life.

?Conclusion

To optimize the use of milling tools one needs to know the right average chip thickness and be able to convert it in a practical manner into the right cutting conditions, in this case the feed per tooth. The instruments described here are essential for this. The term average chip thickness and the different ways it can be used to increase the productivity of milling operations are discussed in detail in different technical publications.

The average chip thickness could well be the most important cutting condition for milling tools that are used in for example difficult materials (hard materials, super alloys …) or in specific technological approaches, such as high-speed or high-feed milling.

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