19-05-2014, 11:53 AM
Study on Ultrasonic Vibration Assisted Tapping of Titanium
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Abstract
Tapping is an important process in assembly of aircraft structures because on an average one
millions of tapped holes are made on an aircraft structure. However, sudden breakage of tap is the
most undesirable event frequently encountered during the tapping process. In particular, this can
mostly occur when small diameter internal threads are made in a difficult-to-cut material like
titanium. In the past, research’s on tapping is mainly of the experimental type. The experimentally
analysis shows that adding controlled vibration in a tapping process reduces the maximum torque
significantly. This report present a detailed account of ultrasonic vibration assisted tapping of
titanium and its alloy is presented. The report also presents the effect of processing parameters on
the performance of tapping process and its comparison with conventional tapping process.
Introduction
Internal thread tapping is one of the most demanding machining operations in today’s
aircraft and automobile industries. It requires a large number of tapped holes with high accuracy,
large depths and small diameters. Because by using bolts and properly tapped holes effectively
fasten two or more components in assembly. But it is observed that, there are number of
difficulties in tapping process. They are 1) frequent breakage of taps, 2) removing broke taps from
finished parts and, 3) chip jamming, because of the excessive torque. As tapping operation is the
almost final operation performed on the workpiece, the tap breakage at this stage is extremely
costly. It creates a large down time.
Zhang et al. [1] found that during tapping on the titanium alloy a relief face friction of the
tap is very large due to large spring back from material. Cao et al. [2] developed a mechanistic
model for accurately predict the tapping torque and axial force generated during the chip
formation with various machining conditions and cutting fluids. Zhang et al. [3] experimentally
investigated introducing controlled vibration during tapping process to reduce the frictional
torque. Kuo [4] divides the vibration assisted tapping process into six stages based on torque and
force distribution. Kuo [5] explain experimentally the effect of various process parameters on
vibration assisted tapping process. Yang et al. [6] use the Tuguchi method to optimize the
vibration tapping process parameters in order to achieve the lowest maximum torque.
Conventional tapping process
Tapping is a relatively little documented, most complex and least understood cutting
operation as compare to other machining operation like milling, turning, etc. It is a process of
cutting internal threads inside the round blind holes with the tap. Tapping is achieved by hand
tapping using a set of taps or using machine like HMC, VMC, etc. Now-days, hand tapping is
replaced by machine tapping to achieve mass production and high accuracy. It uses generally
three types of the taps. They are 1) bottoming tap, 2) plug tap and, 3) taper tap. In bottoming taps
very less tap chamfer is used and in the second pass threads at bottom side of blind holes are
generated. In plug taps, 3-5 teeth have tapper and it is most commonly used. In tapper taps, very
gradual cutting action takes place and is generally used for the difficult-to-cut materials like
titanium.
Summary
In this chapter, a cutting model and force of conventional tapping process are explained.
In convectional tapping process, tool breakage is a major issue with titanium and its alloy. The
main reason for this breakage is huge frictional torque developed because of a large elastic
recovery from the finished titanium surfaces.
Conclusion Remarks
Following conclusions are derived based on the study
Introduction of controlled vibration in tapping process is the most effective and efficient
solution to reduce frequent breakage of tap bits during the conventional tapping on
difficult-to-cut material like titanium and its alloy.
By adding controlled vibration in conventional tapping process, it experimentally
observed that thrust force distribution remain same in both the processes but torque
distribution is significantly reduced, when vibration is applied.
For smaller size tapping in titanium alloy, adding the ultrasonic vibration is more
beneficial in terms of greater degree of tapping torque reduction then for large size
tapping.
Tapping torque is the lowest when vibration assisted tapping is performed at the resonance
frequency and, with a cutting fluid.
The optimum vibration amplitude in a vibration assisted tapping depends on the tap size
and reduce with tap size.