Evolution of chip thickness model in the applicati

2022-08-07
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Shangao tools: evolution of chip thickness model in milling applications

the results of metal cutting process depend on many factors. One of the most important but least known factors is the "chip thickness" cut by the cutting tool. Basically, the chip thickness refers to the thickness measurement of undeformed material at right angles to the cutting edge. Chip thickness is closely related to certain factors, such as the forces that affect the tool and workpiece. Too much chip thickness will lead to edge collapse and fracture of the cutting edge, while too little chip thickness will lead to rapid wear of the cutting edge

by determining and controlling chip thickness, manufacturers can maximize the productivity and efficiency of metal cutting, customize the cutting process for specific workpiece materials, and control costs. Because the importance of chip thickness is not fully recognized, many manufacturers' cutting tools are either overloaded or underutilized, which will have a negative impact on tool life and productivity

in this case, we can use some mathematical models to understand the functional significance of chip thickness. At first, the chip thickness model was a simple equation used to calculate the chips generated in stable turning. Later, it became more and more complex and took into account a variety of variables in intermittent milling environment

chip thickness model in milling applications

in continuous turning, the chip thickness will remain unchanged. However, in milling, the chip thickness will change as the cutting edge intermittently enters and leaves the workpiece

in order to simplify the understanding of milling chip thickness, about 40 years ago, metal cutting researchers proposed the concept of "average chip thickness". Their formula mathematically creates theoretical chips with uniform average thickness. The average chip thickness model is helpful to better understand and control the milling process

when determining the average chip thickness, the radial meshing amount of the tool in the workpiece, as well as the groove type of the cutting edge, the main deviation angle of the tool and the feed rate must be taken into account. By adjusting the feed rate, the machinist can control the chip thickness

the engagement angle between the tool and the workpiece and the spindle stops rotating at the same time can vary from small to large according to the tool diameter and cutting depth. The smaller the radial engagement, the thinner the chip produced. With the increase of radial engagement, when the engagement is 50% of the tool diameter, the chip thickness will reach the maximum. When the radial engagement exceeds 50%, the chip will start thinning again

the cutting edge treatment of the cutting edge will also affect the chip thickness. The chip thickness must generally not be less than the radius of the cutting edge. For example, when the radius of the cutting edge is 60 μ M, the feed rate needs to be adjusted to produce at least 60 μ M. When the feed rate is too low, the cutting edge will rub against the workpiece and cannot cut the workpiece material

the cutting edge treatment of milling cutter often increases the strength of the edge by increasing the fillet radius of the edge, so as to reduce the risk of edge collapse and edge damage. Such edge treatment includes grinding, chamfering and chamfering. These edges allow for more aggressive feed rates when milling difficult to machine materials or rough surfaces. The goal is to form chips behind the cutting edge and thus avoid accelerated wear or fracture of the cutting edge due to concentrated pressure and influence. Adjusting the feed rate can change the chip forming position and control the chip thickness. Increasing the feed rate causes the chips to become thicker, and decreasing the feed rate causes thinner chips

the main deflection angle of the tool directly affects the chip thickness. When the main deflection angle of the tool is 90 degrees (like a square shoulder milling cutter), the chip thickness is 100% feed. However, when the main deflection angle of the tool is 45 degrees, the chip thickness is 70% of the feed rate, because the chip is formed on a longer cutting edge. Reducing the angle of the cutting edge will cause the chip to become thinner, and the feed rate must be increased to maintain the required chip thickness

application of the average chip thickness equation

the average chip thickness equation takes into account the cutting edge angle and radial engagement of the tool. Figure 3 shows the application of this equation in the side milling and center milling of safety belts in China in 1989 in blue and red respectively. In the main figure, the radial meshing amount of the tool is compared with the diameter of the tool, and the comparison result is expressed as ae/dc ratio. The small figure in the corner of Figure 1 shows the influence of the main deflection angle of the tool

this figure shows the situation when the average chip thickness formula is not completely valid. When side milling, the radial cutting width is very small compared with the tool diameter, and this formula is not applicable (see dotted line). In center milling, when more than 50% of the tool diameter is involved in cutting, the red line indicates that the feed rate needs to be increased. This is contrary to practical experience (when the engagement of the tool increases, the feed rate usually decreases. Therefore, the average chip thickness model is most useful when the radial engagement is greater than 20% to 25% of the tool diameter and less than 50% to 75% of the tool diameter.

the average chip thickness model is based on geometric factors and simplifies complex situations. Decades of application have shown that using the average chip thickness model in the tool life equation can yield an error of ± 15 %Within. Such high accuracy is sufficient to meet the requirements of power and torque calculation and processing of a variety of conventional workpiece materials. In addition, the time and effort spent in solving the average chip thickness equation manually is also within a reasonable range

however, when applications require higher accuracy or milling difficult materials, models containing other factors need to be used

equivalent chip thickness

Swedish researcher s Ren h gglund has developed a more comprehensive model. This model provides a measurement method called equivalent chip thickness, which can predict the tool life with an error within ± 2%. In the model shown in Figure 4, the Yellow arc represents the thickness change of the actual chip produced by the milling cutter. The orange bar shows the average chip thickness method and is an expanded version of the yellow picture. The blue bar represents the equivalent chip thickness. A major difference between them is that the equivalent chip thickness model takes into account the time spent by the cutting edge in cutting. This is very important because the meshing amount of the tool in the workpiece is not fixed, and the time spent by the cutting edge in cutting and applying the microphase composite technology to form a fireproof isolation film on the surface of each organic particle is also different, and the chip thickness will also change

the equivalent chip thickness model also takes into account the influence of the tool tip arc radius on the chip thickness. The model adopts a concept originally developed for turning by Swedish Engineer ragnarwox é n in the early 1930s. WOx é n's formula can calculate the theoretical chip thickness of the tool tip, thoroughly analyze the arc radius of the tool tip, and can use the rectangle to explain the chip area

by calculating the chip thickness, manufacturers can avoid problems caused by the chip thickness being less than a specific minimum level or greater than a specific maximum level. When the radial engagement increases relative to the diameter of the tool, the feed rate must be reduced to maintain the same chip thickness. This ensures that the maximum chip thickness does not become too large, which can lead to a shorter tool life and eventual fracture

on the other hand, when working strain hardened materials (such as superalloys and titanium alloys), it is particularly important to form thicker chips than the lowest level. When the cutting edge produces too thin chips, it will cause the later cutting edge to cut out the work hardened area. Cutting the resulting layer of strain hardened material will accelerate tool wear and may reduce tool life by two-thirds

many workshops will adopt the same strategy as machining hardened steel to process materials prone to work hardening, and use smaller cutting depth and feed rate. As a result, the parameters used in milling cutter operation usually can not reach a large enough chip thickness, and the milling quality will not be satisfactory. Selecting a conventional milling strategy or a forward milling strategy (see note) also affects chip thickness and the processing of work hardened materials

controlling chip thickness is a key factor for successful milling. To make full use of the concept of chip thickness, it is necessary to first calculate the equivalent chip thickness, and then determine the lower and upper limits of chip thickness

because the complex equivalent chip thickness model contains a set of variables, it takes much more time and effort to solve the equation than the simplified average chip thickness model. It is neither cost-effective nor time-consuming to perform these calculations manually in the production environment

however, the computer software program can be used to calculate the processing parameters (such as the program provided by shangao). The user can input the data and calculate the result of the equation in a few seconds. This optimizes the milling process to improve productivity and profitability

chip thickness and milling technology

when the concept of equivalent chip thickness is adopted, the formation mode of chips should be considered. During milling, chips are formed in two different ways, depending on the direction of tool rotation relative to the direction of workpiece movement. The two methods are conventional milling (up) and forward milling (down). In traditional milling, the rotation direction of the tool is opposite to the feed direction of the workpiece. In forward milling, the rotation direction of the tool is the same as the feed direction of the workpiece

in conventional milling, the cutting edge enters the workpiece with zero cutting depth. The chip thickness is the minimum at the beginning of formation and the maximum at the end. On the contrary, the chip thickness formed during forward milling reaches the maximum at the beginning, and then gradually decreases to the minimum

when the traditional method is used for milling, the cutting edge will rub the workpiece before cutting, and the heat absorption capacity of thin chips is poor. Both of these conditions will lead to strain hardening of workpiece surface and shortening of tool life. The chip will fall in front of the tool, so it may be cut again and cause the surface roughness to decrease. In horizontal milling, the upward cutting force may lift the workpiece, so complex workpiece clamps are required

the reason why down milling is popular is due to many reasons. The cutting edge does not need friction when entering the workpiece for cutting, so it maximizes the tool life and reduces the heat generated. The required power of the machine tool is smaller, the chips fall behind the tool, which can avoid cutting again to the greatest extent, improve the surface roughness and prolong the tool life. The cutting action produces a downward force, which helps to stabilize the workpiece and simplify the fixture. When machining materials such as superalloys, stainless steels and titanium alloys, the initial chip thickness can achieve heat dissipation and minimize the strain hardening of the workpiece surface

however, the downward force generated during forward milling may cause the machine table to recoil, especially on older and/or manual milling equipment. The recoil will affect the accuracy and cause the tool to break due to the increased chip load. Therefore, when using unstable machine tools and workpieces, traditional milling methods may be required

when milling castings, forgings and case hardened materials, traditional milling methods can also be used preferentially. This is because traditional methods start cutting below the hardened or rough surface of the material

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