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Fig. 2.32. High Speed Machined Unitized Speedbrake Source: The Boeing Company

Fig. 2.32. High Speed Machined Unitized Speedbrake Source: The Boeing Company

Chatter appears as a series of uniform continuous series of marks on the work-piece surface, while part vibration causes deeper marks that are more randomly distributed. Chatter occurs when the impact frequency of the cutter begins to vibrate near its natural frequency. Part vibration occurs in two slightly different ways depending on whether a web or floor is being cut or a rib or flange is being machined. Webs or floors excited by the cutting process can start to vibrate at their natural frequency giving the appearance of a bouncing motion. Ribs and flanges vibrate by forced resonant vibration in which the natural frequencies of the ribs match the natural frequency of the cutter.

Fig. 2.33. High Speed Machined Unitized Bulkheads Source: The Boeing Company
Fig. 2.34. High Speed Machined Unitized Substructure Source: The Boeing Company

Matching the cutter impact frequency to the workpiece vibration frequency produces a stable non-chattering cut and a smooth surface. As shown in Fig. 2.35, if the teeth are hitting the workpiece at one frequency and the cutter is vibrating at another frequency, the cutter produces chatter because the teeth will hit the workpiece at different points in the vibration. On the other hand, if the teeth hit at the same rate the cutter is vibrating, chatter is eliminated and a smooth surface finish results. Chatter can be detected by using a microphone along with a data acquisition system during machining. The system records the data and chatter detection software uses a Fast Fourier Transform to produce a plot like the one shown in Fig. 2.36. The extra spike indicates the chatter frequency.

Cutter Impact Period

Unstable Chattering Cut

Cutter Impact Period

Unstable Chattering Cut

Vibration

Vibration

Good Surface Finish

Period

Stable Non-Chattering Cut

Fig. 2.35. Stable and Unstable Cutting

Period

Stable Non-Chattering Cut

Fig. 2.35. Stable and Unstable Cutting

If the tool system has a chatter frequency of 2000 Hz for a two flute cutter, then the optimum spindle speed would be 2000 Hz x 60s/min/2 teeth = 60000rpm. If the spindle only has a maximum speed of 40000rpm, then another region of stability and optimum speed can be determined by dividing the optimum spindle speed by an integer. For example, in our case, 60000rpm/2 = 30000rpm. To find the optimum spindle speed for highest metal removal rate (MRR) possible, gradually increase the depth of cut until a new chatter limit is encountered and repeat the process of measuring the chatter frequency and then calculating the best spindle speed. Machining can be conducted safely at or below this speed. Reducing spindle speed is another effective method for controlling chatter and improving part quality by damping out vibrations. It should be noted that this assumes that vibration due to the workpiece has been eliminated by proper

Good Surface Finish

Tool Impact

Frequency

Tool Impact

Frequency

Frequency (Hz)

Fig. 2.36. Frequency Response to Detect Chatter

Tool Impact , Frequency Harmonics

Frequency (Hz)

Fig. 2.36. Frequency Response to Detect Chatter workpiece fixturing and proper tool path programming.20 Machine tool and spindle builders that supply high speed machining equipment have the capability to help in establishing high speed machining parameters.

2.9.2 Chemical Milling

Shallow pockets are sometimes chemically milled into aluminum skins for weight reduction. The process is used mainly for parts having large surface areas requiring small amounts of metal removal. Rubber maskant is applied to the areas where no metal removal is desired. In practice, the maskant is placed over the entire skin and allowed to dry. The maskant is then scribed according to a pattern and the maskant removed from the areas to be milled. The part is then placed in a tank containing sodium hydroxide heated to 195 ± 5° F with small amounts of triethanolamine to improve the surface finish. The etchant rate is in the range of 0.0008-0.0012 in./min. Depths greater than 0.125 in. are generally not cost competitive with conventional machining, and the surface finish starts to degrade. After etching, the part is washed in fresh water and the maskant is stripped.

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