As we all know, there are two basic types of cutting edge shapes for Schaftfräser: straight shape and helicalshape, among which the helicaltype is more widely applied because it can lead to faster, stabler, and more effective cutting than straight type. You can see the general 2 types of helicaledge in fig.1. Left helicaltype is mainly used for high precision milling, such as processing mobile phone keys , film switch panel, LCD panel, and acrylic lenses. The right helicalmilling cutter conforms to the spindle rotation of miller, and it is convenient to unload at most chucks. Thus, the right helicalend milling cutter is the most widely used and the common right helicalangle is usually 30° ~ 45°.
In our general opinion, The helical angle U of end milling cutter corresponds to the inclination angle λs, but the introduction and discussion of the dip angle is mainly about turning processing. However, there are many differences between milling and turning. It is impossible to apply all knowledge about edge inclination angle to the milling of end mills.
For milling, it is roughly known that a larger spiral angle increases the number of teeth working at same time, reducing impact and increases smoothness during milling, and making the end mill edges sharp. Except for that, what kind of impact will the size of helical angle have on end milling cutters? Tool and edge angles are interrelated and impact for each other. Let’s take a experiment to get some data for further to-know of this subject.
A experiment on groove milling with 2 edge helicalend milling cutter
The experiment was carried out on a vertical machining center. A 2-edge end mill with diameters of φ12mm and different helical angles was used to mill a 12mm× 12mm slot with width × height. The vertical error (ΔX) of both sides of the groove was measured based on the bottom surface of the groove after machining. The influence of spiral angle on the machining accuracy was evaluated by comparing the error values.
The work piece to be processed is carbon steel with hardness of 28HRC. The cutting parameters of each tool in the experiment are as follows: feed speed 50mm /min, cutting speed 29m /min, and cutting depth 12mm. Oil coolant is used. Experimental results are shown in the figure 2.
From the experimental results, we can see that:
( 1) Overcut always occurs on the up milling side of work piece. On the contrary, the missing cut phenomenon always appears on the down milling side, and the maximum point of overcut and missing cut is at the end milling cutter extension farthest. This point accords with the deformation pattern of the tool and the tool extension length during up milling and down milling.
( 2) As the spiral angle of the end milling cutter is less than 30 °, the perpendicularity error value will increase with the increase of the helical angle, whether on the forward milling side or on the reverse milling side.
When the helical angle is greater than 40 °, it will decrease with the increase of the helical angle. Therefore, it can be considered that when the end milling cutter has a smaller helical angle or a larger helical angle, the shape accuracy of its groove milling is high, Although it seems a contradiction.
( 3) From the perspective of machining accuracy, when the helical angle is 0, the cutting edge is straight and the accuracy is the highest. However, from the basic characteristics of the helical angle of the end mill, the end mill will completely present intermittent cutting in this case. This kind of machining with large cutting impact requires high manufacturing accuracy of the tool itself.
A experiment of side milling with helical angle and 4-edge end milling cutter
The side of the workpiece is milled on a vertical machining center with a 4-edge end milling cutter with a helical angle of 30 ° and 55 °. We compare the influence of the change of the two end milling cutters with the cutting width (radial feed) on the machining accuracy. When the diameter of the end milling cutter is 25mm, the 45 # steel with the hardness of 94HRB shall be cut by the straight milling method and dry cutting. The cutting parameters are feed rate of 100 mm/min, cutting speed of 26 mm/min and cutting depth of 38 mm The measured perpendicularity error, flatness error and surface roughness after machining are shown in Figure 3.
It can be seen that when the cutting width is not particularly large, the machining accuracy of the 55 ° large helical angle end mill is higher than that of the 30 ° helical angle end mill. This is consistent with the groove milling experiment results in Figure 2. After analyzing the reasons, it can be considered that this is because when the cutting width is small, the actual rake angle of the large helical angle end mill is large, which makes the edge sharp and reduces the tangential cutting resistance, energy consumption and tool deformation.
Induction of helical angle characteristics
(1) Spiral angle and cutting resistance: tangential cutting resistance decreases with the increase of helical angle, and axial cutting resistance increases with the increase of helical angle.
(2) Spiral angle and rake angle: the increase of helical angle makes the actual rake angle of end milling cutter increase and the edge become sharper.
(3) Spiral angle and machined surface precision: Generally, the perpendicularity and flatness tolerance value of the machined surface increases with the increase of the helical angle, but when the helical angle is greater than 40 °, it decreases with the increase of the helical angle.
(4) Spiral angle and tool life: the wear speed of the circumferential edge belt is basically proportional to the helicalangle.On the other hand, when the helical angle is very small, slight tool wear will also significantly reduce the cutting performance of the tool, cause vibration, and make the tool unable to continue to use. When the helical angle is too large, the rigidity of the tool will become poor, and the service life will be reduced.
(5) Spiral angle and material to be cut: when machining soft materials with low hardness, use large helicalangle to increase the front angle and improve the sharpness of the cutting edge.When machining hard materials with high hardness, small helical angle is used to reduce the front angle and improve the rigidity of the cutting edge
Conclusion about our research on end mills with helical angle
Helical angle is one of the main parameters of helical end milling cutter. The change of Spiral angle has a great impact on the cutting performance of the cutter. With the development of CNC machining technology and flexible manufacturing technology, it is possible to change the size of helical angle in the tool manufacturing process. If we study further the various effects of spiral angle size on the cutting performance of helical end milling cutter, when manufacturing and selecting helical edge end milling cutters, we need to consider the performance and processing accuracy of the processed material, processing efficiency, tool material and tool life, which will undoubtedly play an important role in promoting efficient and high-precision milling.