The machining noise control of large size extruded aluminum utilized in high-speed trains

The machining noise intensity was measured on a 21 m long hollow extruded aluminum floor section with corrugated cross section during the machining operation. The machining experiments were performed in horizontal direction and set the cutters movement perpendicular to the aluminum extrusion, namely that the cutter heads would knock with the reinforcement ribs frequently. The sound samples were taken at the position of 3 m distance and 0.3 m high from the machining line by a single acoustic pressure sensor. Fig. 1 is the sketch map and scene photo of the experiments designed. The noise level was assessed by A-weighted sound pressure level under various machining technology adjustments of cutter tool, machining feed rate, cutting way, and sealing the openings with sound-absorbing materials or not.

2.2. Procedures and results

2.2.1. Experiments of cutter tool

The aim of the experiments of cutter tool is to study the noise effect of various kinds of structure and size were tested under the machining parameters of 8000 r/min rotation speed, 1000 mm/min feed rate, cutting along milling, 20 mm transverse cutting depth and 28 mm vertical cutting depth. Fig. 2(a) and 2(b) is the photographs of monolithic milling cutters with head diameter Ø32 mm and Ø40 mm, and Fig. 2(c) and 2(d) is the photographs of assembled milling cutters with head diameter Ø72 mm and Ø32 mm respectively.

Fig. 2photographs of four kinds of cutters

a) Ø32 mm monolithic milling cutter

b) Ø40 mm monolithic milling cutter

c) Ø72 mm assembled milling cutter

d) Ø32 mm assembled milling cutter

Fig. 3 is the A-weighted sound pressure level contrast between Ø32 mm monolithic and assembled cutters, Ø32 mm and Ø72 mm assembled cutters, as well as Ø32 mm and Ø40 mm monolithic cutters. The sharp peaks come from the cutter’s knocking with reinforcement ribs during the machining. Obvious noise intensity diversity was observed in Fig. 3, which indicated that monolithic cutters were superior to assembled, and thick diameter was superior to slim diameter. An exception exists in the first half time of Fig. 3(c), and author think that it could be from some non-steady state factors such as no clamping cutter.

Fig. 3The A-weighted sound pressure level contrast of various cutters

a) Ø32 mm monolithic and assembled cutters

b) Ø32 mm and Ø72 mm assembled cutters

c) Ø32 mm and Ø40 mm monolithic cutters

Fig. 4The A-weighted sound pressure level contrast among the feed rates of 1000 mm/min, 800 mm/min and 600 mm/min

2.2.2. Experiments of machining feed rate

Three kinds of machining feed rates, 1000 mm/min, 800 mm/min and 600 mm/min, were tested under the machining parameters of Ø40 mm monolithic cutter, 8000 r/min rotation speed, cutting along milling, 20 mm transverse cutting depth and 28 mm vertical cutting depth. Fig. 4 is the A-weighted sound pressure level contrast among the feed rates of 1000 mm/min, 800 mm/min and 600 mm/min. It can easily be shown that a low feed rate present a weak noise level, but too low feed rate will seriously affect the machining efficiency, which is not what the producers want to see.

2.2.3. Experiments of cutting direction

Two kinds of cutting way, cutting along milling and reverse milling as sketched in Fig. 5, were tested by using Ø32 mm monolithic cutter, Ø40 mm monolithic cutter, Ø32 mm assembled cutter and Ø72 mm assembled cutter. The other machining parameters are 8000 r/min rotation speed, 1000 mm/min feed rate, 20 mm transverse cutting depth and 28 mm vertical cutting depth.

Fig. 5The sketch maps of cutting directions

a) Cutting along milling

b) Cutting reverse milling

Fig. 6The A-weighted sound pressure level contrast of various cutting directions

a) Ø32 mm monolithic cutter

b) Ø40 mm monolithic cutter

c) Ø32 mm assembled cutter

d) Ø72 mm assembled cutter

Fig. 6(a) is the A-weighted sound pressure level contrast between cutting along milling and reverse milling using Ø32 mm monolithic cutter, and Fig. 6(b)-(d) is Ø40 mm monolithic cutter, Ø32 mm assembled cutter and Ø72 mm assembled cutter respectively. It can be concluded that cutting along milling is good for noise control compared with reverse milling, no matter what kind of cutter. Furthermore, the effect of noise control is more notable when the cutter diameter is thicker.

2.2.4. Experiments of sealing the openings

In order to study the noise reduction, all the openings and ports on the extruded aluminum surface and cross section were sealed for blocking the sound transmission with sound-absorbing materials as shown in Fig. 7. The machining parameters are Ø32 mm assembled cutter, 8000 r/min rotation speed, 1000 mm/min feed rate, cutting along milling, 20 mm transverse cutting depth and 28 mm vertical cutting depth.

Fig. 7the photographs of sealing the openings and ports on the extruded aluminum

a) Ports on cross section

b) Openings on surface

Fig. 8Wavelet transforms of machining sound before a) and after b) pasting sound-absorbing materials

a)

b)

Fig. 9The A-weighted sound pressure level contrast of before and after pasting sound-absorbing materials with different zooms

a) Global region

b) Local region

Measurements were conducted before and after pasting sound-absorbing materials, and Fig. 8 shows the sound pressure level in the range of 20-5000 Hz frequency band. Note that the reduction in sound pressure level was even distinct for the noise after pasting sound-absorbing materials on openings and ports as shown in Fig. 8(b) when compared to that before as shown in Fig. 8(a).

Fig. 9 shows the results of the A-weighted sound pressure level contrast with different zooms. An overall more than 5 dB noise reduction was observed by this means, which is an extremely simple and effective way of controlling the noise.