Effect of the helix angle on a cutting edge of the milling cuter in the milling of aluminum alloy AlZn5.5MgCu

* Dr hab. inż. Jan Burek, prof. PRz (jburek@prz.edu.pl)https://orcid.org/0000-0003-2664-5248 Katedra Technik Wytwarzania i Automatyzacji, Wydział Budowy Maszyn i Lotnictwa, Politechnika Rzeszowska im. Ignacego Łukasiewicza, Polska Dr inż. Marcin Płodzień (plodzienprz.edu.pl) https://orcid.org/0000-0001-8369-3604Katedra Technik Wytwarzania i Automatyzacji, Wydział Budowy Maszyn i Lotnictwa, Politechnika Rzeszowska im. Ignacego Łukasiewicza, Polska Mgr inż. Artur Szajna (a.szajna@prz.edu.pl) https://orcid.org/0000-0002-3820-7272Katedra Technik Wytwarzania i Automatyzacji, Wydział Budowy Maszyn i Lotnictwa, Politechnika Rzeszowska im. Ignacego Łukasiewicza, Polska Mgr inż. Jarosław Tymczyszyn (j.tymczyszyn@prz.edu.pl),https://orcid.org/0000-0003-2972-5112Katedra Technik Wytwarzania i Automatyzacji, Wydział Budowy Maszyn i Lotnictwa, Politechnika Rzeszowska im. Ignacego Łukasiewicza, Polska


INTRODUCTION
AlZn5.5MgCu aluminum alloys are commonly used in the aviation industry -most often for hull structural elements (frames). Currently, these constructions are usually made of one block, which means that even 90% of the material must be removed during the cutting process [1]. Therefore, machining of such elements is carried out by the method of HPC (high performance cutting). High values of cutting speed and feedrates as well as depth of cut in HPC machining, as compared to conventional machining, require proper macro-and microgeometry of the tool [2]. In the HPC milling process, the basic problems are the shape of chips and the way they are removed, which depends, among others, on cutting edge geometry. Due to the fact that the milling cutter works mainly with a cylindrical surface, an important parameter of the milling macro-geometry is the value of the cutting edge inclination angle.
Manufacturers offer special geometry tools for machining the aluminum alloys. Differences in geometry result, among others, from the asymmetrical distribution of cutting edges and from the change in the inclination angle λ of the helical line of the cutting edge. Due to different heights of the helix of the cutting edges, chips with a variable cross-section are formed, which results in a change in the components of cutting force (feed Ff, normal to feed FfN, axial Fa) [4]. The cutting force components are of great importance, especially when machining the thin-walled elements, such as frames in aircraft structures. The force component perpendicular to the surface being machined (force normal to feed FfN) can cause elastic or plastic deformation of the wall. These deformations result from the pressure exerted on the machined surface by the cutting edge of the tool.
Next, the influence of different inclination angles λ of the cutting edge of the milling cutter on the course of the milling process is presented, depending on the change of cutting parameters.

Test conditions
The study used samples made of aluminum alloy AlZn5.5MgCu. Milling was carried out on the vertical milling center DMC 635 V DECKEL MAHO from DMG ( fig. 1). The cutting force components were measured using a Kistler force meter type 9121 and a 5019A charge amplifier, and the surface roughness measurements were made using a MarSurf M300 profilograph. Solid carbide, three-flute end mills with a diameter of d = 10 mm, differing in the cutting edge inclination angle were used for the tests:

Results
In the first test sample, constant technological parameters were adopted: cutting depth ap = 10 mm, cutting speed vc = 200 m/min and cutting width ae = 3 mm. The feed per tooth was changed fz = 0.04; 0.06; 0.08; 0.1 mm/tooth. Studied relationships of the influence of feed per tooth fz on the components of cutting forces for the tested tools are shown in fig. 3.

Fig. 5. Components of cutting force depending on the cutting speed vc for a milling cutter: a) with constant helix angle λ = 45°, b) with a variable helix angle λ = 45°÷25°
In the second test sample, constant technological parameters were adopted: cutting depth ap = 10 mm, cutting speed vc = 200 m/min, feed per tooth fz = 0.04 mm/tooth. The cutting width was changed to ae = 2; 4; 6; 8 mm. The obtained dependencies of the influence of cutting width ae on the cutting force components are shown in fig. 4.
Analyzing the obtained results, a change in the direction of the feed force Ff was observed for a milling cutter with a constant angle λ. The force Ff direction is changed when the milling cutter works with a cutting width ae> 50% • d. The feed force Ff with a cutting width ae = 80% • d is approximately 200 N. The normal to feed FfN and axial Fa component force for both cutters show an increase in value during increasing the cutting width ae.
In the third test sample, constant technological parameters were adopted: cutting depth ap = 10 mm, feed per tooth fz = 0.04 mm/tooth and cutting width ae = 3 mm. The cutting speed vc = 100 was changed to 200; 300; 400 m/min. Results of the dependence of the influence of cutting speed vc on the components of cutting forces for the tested tools are shown in fig. 5.
As can be seen, there is a decrease in components of cutting force as the cutting speed vc increases. The normal to feed FfN for a milling cutter with a constant angle λ is approximately 300÷370 N, and for a milling cutter with a variable angle λ -approximately 140 N. The other components of cutting force, i.e. Ff and Fa, have similar values (60÷100 N).
Results of the influence of the angle of the cutting edge of the milling cutter on the surface roughness parameter Ra are shown in fig. 6.
After surface machining with a constant λ cutter, the Ra parameter was twice as low as compared to a variable λ cutter. An increase in the Ra parameter as a function of the cutting width can be seen for a cutter with a variable angle λ. The Ra parameter for a surface shaped with a constant λ angle cutter remains in the whole range at approximately 0.18÷0.25 μm. The increase in the cutting width ae to the value of 80% of the tool diameter with a variable angle λ caused the Ra parameter to increase to 1.12 μm.
For both milling cutters used, the surface roughness Ra tends to increase over the entire tested range of fz, ae and vc parameters. Maximum value of Ra = 1.44 μm was achieved at cutting speed vc = 400 m/min when machining with a variable angle cutter λ.

Summary
Based on the research, it can be concluded that the use of tools with a variable angle of inclination of the cutting edge λ significantly reduces the component of the normal to feed force FfN, however, it leads to a greater surface roughness compared to the surface roughness after machining with a constant angle cutter λ. For this reason, milling cutters with a variable cutting angle λ are suitable for high-performance roughing of aluminum, carried out at maximum cutting speeds and feed. In contrast, milling cutters with a constant cutting edge inclination angle λ should be used for finishing.