A study of contact interface and wear diagnosis for hand taps using
ultrasonic method
C.W. Yao
⇑, C.C. Wu
Department of Mold and Die Engineering, National Kaohsiung University of Applied Sciences, No. 415, Chien-Kung Rd., Kaohsiung City 807, Taiwan, ROC
a r t i c l e
i n f o
Article history:
Received 1 November 2013
Received in revised form 10 March 2014 Accepted 15 April 2014
Available online 3 May 2014 Keywords: Wear diagnosis Ultrasound Contact interface Contact pressure
a b s t r a c t
This work was to study the contact interface between a set of used hand taps and another new one based on the regional scanning of ultrasound. The contact image was a novel disclosure for hand taps contact. The objective of this work was to provide a wear diagnosis by making a comparison of contact area between the used and the new hand taps. The 2D maps showed an apparent change not only in area sizes but also in contact shapes between the used and the new hand taps. The 3D contact images also provided useful information to show the degree of contact.
The contact area between the tap and the workpiece was calculated using an image analysis software package. The range of contact areas varied from 2.49 mm2to 35.31 mm2for the used hand taps and from
1.19 mm2to 28.55 mm2for the new taps, depending on the definition of the contrast ratio. The result
provided another scientific data for users to decide a correct timing for the tool replacement. In addition, maps of reflection coefficient and pressure contour distribution were presented. The range of contact pressure varied from 2.5 Mpa to 4.2 Mpa.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
Tapping is one of the commonly used machining operations for obtaining internal threads in many components. Because there are still many problems encountered in the tapping process, many researches focus on tap breakage, which could be resulted from excessive torque[1,2]. Some problems associated with the tapping process include thread dimensional accuracy, surface roughness and error of thread forms. The cause which leads to these problems is probably the taps wear. Besides the ordinary tool wear due to tapping process, there are many factors which may lead to abnor-mal tool wear, such as wrong tap selection, high tapping speed, insufficient or incorrect lubricant, hole-work hardening, fixation instability and misalignment between taps and workpieces. Because tapping is usually the last process in most production pro-cedures, if any problem occurs in this stage, the manufacturers could suffer great economic losses due to many bad products. Therefore the issue of taps wear cannot be negligible. However, there are few researches found to investigate taps wear. In this work the objectives are to study the contact interface for hand taps and to do a wear diagnosis by mean of the comparison of contact area.
A tap removes material from a pre-drilled hole. The cutting edges at the front of a tap remove material from the workpiece. The chips are pushed forward in front of the tap or drawn up along the flutes, removing the chips from the hole. Basically threads can be produced in two principal methods: hand making or metal cut-ting. In metal cutting, tapping forces have been identified for affecting the cutting process and technological machining perfor-mance. There are different methods to study the cutting force sys-tem in tapping processes. Analytical approaches[3,4]have been applied to predict cutting forces in tapping operations. There also exists experimental work[5]related to thread-tapping processes. In these researches, experimental methods and finite element analysis were used to investigate the material flow and to deter-mine the load level which provides the best surface finishing throughout thread forming process. The result shows that the diameter of the pre-drilled hole has a significant influence on the thread forming torque.
The major machining effect for common steel materials is the high strain hardening, which results from the plastic flow under severe friction conditions. The lubricant selection considerably influences the increase in hardness and it also influences on the taps damage and wear. There are some direct and indirect methods that are usually used to do wear estimation, such as visual inspec-tion, measurements of volumetric changes, online signal (such as cutting force, current and power for drives) monitoring, vibration
http://dx.doi.org/10.1016/j.apacoust.2014.04.015
0003-682X/Ó 2014 Elsevier Ltd. All rights reserved.
⇑Corresponding author. Tel.: +886 7 3814526x5421; fax: +886 7 3835015. E-mail addresses:[email protected],[email protected](C.W. Yao).
Contents lists available atScienceDirect
Applied Acoustics
detection and acoustic emission[6]. In this study the ultrasonic method was used to analyze contact interface and evaluate the degree of wear by the comparison of contact area between a set of used hand taps and a new one.
Different from the usual case that there is only single contact region between two common objects, hand taps have several con-tact points between the tool and the workpiece. It is difficult to apply theoretical models to study the multiple contact regions between the tool and the workpiece. An efficient and convenient method to investigate the contact interface and to measure the area size is to use ultrasonic scanning accompany with signal pro-cessing. A single ultrasonic transducer can perform a regional scan-ning and a point measurement of reflection. A 2D map of the reflection is obtained if the transducer moves back and forth across the interface area. The detailed processes can be found in some Refs.[7–11], both analytically and empirically.
2. Background 2.1. Ultrasonic detection
When ultrasonic wave is traveling through an interface between two objects, a portion of the wave will be transmitted and a portion will be reflected back. The proportion of the wave reflected, known reflection coefficient, depends on the acoustic impedance of the two materials. Acoustic impedance is a material property defined as the product of density and wave speed in a given material. For any two materials, the reflection coefficient, R, as a ratio of the amplitude of the reflected pulse to the amplitude of the incident pulse may be determined by Eq.(1) [12]:
R ¼z1 z2 z1þ z2
ð1Þ
where z is the acoustic impedance of the material and the sub-scripts refer to the two sides of the interface.
For example, the impedance of air, water and steel are 0.0004, 1.51 and 46.02 kg m2
l
s1, respectively, so when a pulse strikes a steel/air interface (z1 z2) it is almost totally reflected and reflection coefficient approaches 100%. If the wave strikes a steel/ steel interface and there is perfect contact, it is fully transmitted (z1= z2) and R approaches zero.The surfaces of a hand tap tool and a steel plate are rough and in real situation the rough surface is composed of individual asperity junctions and air gaps. An ultrasonic wave is transmitted through a rough surface interface where there is asperity-to-asperity contact and reflected where there are small air gaps. So it is possible to per-form a scan of the reflected ultrasound across an interface. The detail of the relationship between the reflection coefficient and the rough surface contact conditions can be found in Refs.[13–16]. 2.2. Spring model of contact surface
For an imperfect interface, a quasi-static spring model[17]can be used to study ultrasonic reflection coefficient to the stiffness of the interface. The interface is represented by a series of flexible springs. The reflection coefficient depends on the stiffness of the interface spring, K, according to
R ¼z1 z2þ i
x
ðz1z2=KÞ z1þ z2þ ix
ðz1z2=KÞð2Þ
where
x
is the angular frequency (2p
f) of the ultrasound wave and z is the acoustic impedance.The stiffness K of an interface is a measure of its ease of closure and is defined as the change in nominal contact pressure, p,
required to cause unit approach of the mean lines of the surfaces [18]. Thus:
K ¼ dp
du ð3Þ
where K is stiffness per unit area, dp is the change in pressure and u is the separation of the mean lines of roughness of the two surfaces. The concept can be used to deduce the contact pressure from the reflected coefficient of ultrasonic pulse.
The stiffness, and hence the reflection coefficient from Eq.(2), varies with contact pressure. Measurement of the reflection coeffi-cient can then give information about the degree of contact at an interface and the distribution of the contact pressure.
2.3. Operation of hand taps
Hand taps come in 3 basic types, usually as a ‘‘set’’, starting a first cut (also called taper), second cut (also called plug) and lastly a bottoming tap. Taper taps are chamfered 7–10 threads at 5° per side to enable easy starting of the threads in the pre-drilled hole. Plug taps have a chamfer length of 3–5 threads and are designed for use after the taper tap. Bottom taps have a very short chamfer length of 1–2 threads for threading the bottom of a blind hole. This tap is always used after, and in conjunction with a taper or plug tap.
Tapping is a process of material removal for making threads and is widely used in the industrial manufacturing. It starts with dril-ling a hole to a specific diameter for the thread size using a tool such as manual mill or drill press. Clean the hole thoroughly by blowing out the chips from the drilling process. Taper tap has the most taper and is design to start the thread in a new hole. Plug tap has a bit less taper and bottoming will cut threads all the way to the bottom of a blind hole. In most situations tapping process is approaching the final manufacturing procedure. Once a failure of tapping such as tap breakage or poor thread quality occurs, it will cause bad products and economic loss. Tap wear is one of the major causes of tap breakage and poor thread quality.
In tapping operations, studying the wear of the hand taps is sig-nificant, because it affects the flow of chips and the cutting force, etc. A critical diagnosis index of tool wear is contact area. However, few experimental methods allow a direct measurement of the con-tact area. In the experiments in this study, ultrasonic detection was used to determine the shapes of the contact patches and then allows measurement of the contact area and contact pressure for the hand taps.
3. Apparatus and experimental procedure 3.1. Test specimens and loading apparatus
Fig. 1shows a sketch of the contact layout. The specimens are placed in a frame, to allow a normal force to be applied, using a hydraulic cylinder. The test specimens consisted of a set of new hand taps and another set of used ones, which were made from high-speed steel (HSS) and had the same diameter of 18 mm. Fig. 2shows photographs of the new and used hand taps. It can be easily visualized that there are apparent and different degrees of wear between the new taps and the used ones. The hand taps and a steel plate were placed in a frame and a hydraulic cylinder was used to normally load the specimens simultaneously. This frame was designed and produced explicitly for contact experi-ments using ultrasonic scanning. It comprises three large stainless steel blocks and four cylindrical supports, in which the contact specimens were mounted. The top block has a specially machined groove that accommodates the steel plate specimen. A hydraulic
cylinder inside this frame applied a normal force on the contact specimens.Fig. 3shows photograph of the specimen contact and the loading apparatus.
3.2. Ultrasonic instrumentation
When the loading frame was assembled, it was placed in a scan-ning device. This device is automatic, enabling the transducer to scan over a given area. A water bath was placed above the contact specimen, with the ultrasonic signal once again focused on the interface. Ultrasonic readings were then taken at prescribed inter-vals across the interface, using the transducer. The dimensions and resolution of the scan varied and were selected according to the specimen geometry and the degree of ultrasonic accuracy required. Fig. 4shows a schematic layout of the ultrasonic instrumenta-tion and scanning procedure. The equipment consisted of an ultra-sonic pulse receiver (UPR), an oscilloscope, a data processing computer and a transducer. The principal part of the ultrasound apparatus is the UPR. This provides a voltage pulse, which excites the transducer to produce an ultrasonic pulse. A 10 MHz focusing
transducer was immersed in the water bath and positioned so that the wave was focused on the interface. The transducer was con-nected to an xy positioning stage, so that it could be scanned across the interface.Fig. 5shows a photograph of the apparatus. The UPR and the oscilloscope were controlled by a purpose-built LABVIEW interface. The software configures the emitted pulses and receives and processes reflected ultrasonic signals, to show the reflection for any given loading.
3.3. Scanning procedure and signal processing
Fig. 4also shows the schematic of the movement of the trans-ducer. It is a 2D scanning process. The ultrasonic transducer was mounted in a water bath and moved back and forth, in order to place it in the most likely region for detection. The transducer was then moved up and down, to focus the ultrasonic signal onto the interface and determine the maximum signal amplitude, as shown on the PC display. Once it had been focused on the surface to be detected, the transducer was moved along the x and y direc-tions, with increments dx and dy, until the entire desired region had been inspected.
A software interface, written in Labview, performed a Fast Fou-rier Transform (FFT) of the signal and performed a series of pro-cessing tasks, to produce useful data. The relationships between reflected signals in the frequency domain were calculated. From the experimental data, the reflection amplitude signals for differ-ent applied forces were obtained. The scanned readings were taken from the original signal and then processed to plot the contact patches, using MATLAB.
4. Results
4.1. Reflection amplitude map
Fig. 6shows the scanning amplitude maps of 2D and 3D, and makes a comparison between a set of used hand taps and another set of brand new ones for normal force of 80 N applied. From these pictures, the different sizes of contact regions between the used and the new hand taps can be clearly identified. From the 2D maps the variation of the contact shapes between the used taps and the new ones are very clear. From the 3D maps the variations of the contact shapes are provided in another point of view to indicate
Fig. 1. Schematic of hand taps/workpiece contact.
this quasi-static study, because the variation of the ultrasonic sig-nal can reflect the degree of ease or difficulty by which the pulses pass through the contact interface, the distribution of contact pres-sure can therefore be disclosed and the result is similar to the map of reflection coefficient, which unveils the information that how many signals are reflected and received by the transducer. Also, the proposed approach can deduce which region of the tap cutter blades has severe wear. In a real tapping process, because the motions of spinning and moving between taps and workpiece have to overcome the friction force of the contact surface, the friction force is proportional to the contact load. The ultrasonic signal read-ing of the contact image can reflect the contact intensity between the contact interfaces.
As mentioned previously in Section 1, the taps wear is an important issue but very few researches were found to pay atten-tion on it. In this work the comparison of contact area was made for wear diagnosis.Figs. 12–14show the obvious increase of con-tact area between the used and the new taps, especially for the BOTTOMING process. Furthermore, the pressure distribution maps have shown inFig. 16. Because of increase of contact area, it is clear to see that the values of maximum pressure for the three used taps are all less than those of the three new ones, respectively. 6. Conclusion
This study used ultrasonic scanning to investigate the contact interfaces for both new and used hand taps. The scanning images showed the change of shapes of the major separate contact patches between the used and the new hand taps. The scanning pictures show that the contact patches are similarly shaped but differently sized between the new and used hand taps. Under the same test condition and procedure such as the contact position and contact loadings, the probable reason for the different contact images could be due to wear in the tapping threads. Furthermore, in this study the different sizes of contact patches between the used and the new hand taps could unveil the regions with severe wear in the major tapping threads.
The method used in this study was ultrasonic detection. Although it is a known method, the application for measuring hand
taps is novel and the contact images have never been disclosed before. The ultrasonic scanning instrumentation included a loading frame, an automatic measuring device and a scanning probe which was coupled to a processing and display unit by a cable. The scan-ning probe included emitting and receiving ultrasonic transducers, which transmitted an acoustic wave in the form of ultrasonic energy into the contact interface being scanned and receive reflected signals from the interface. Data processing allowed the contact images to be obtained using the incident and reflected sig-nals and then the contact areas can be presented.
The data for the contact area of the used and the new hand taps was calculated using an image analysis software package and a comparison was made to provide helpful information. Table 1 shows that the range of contact areas varies from 2.49 mm2 to 35.31 mm2 for the used hand taps and from 1.19 mm2 to 28.55 mm2for the new taps, depending on the definition of the contrast ratio. In the mean time, a wear comparison for the used hand taps was made through the visual inspection, ultrasonic image and processed picture in Figs. 9–11.
By the calculations of using the spring model in the hand taps contact, the contour pictures of the contact pressure were dis-closed. The distribution maps of contact pressure indicate that the pressure is not averagely distributed to the contact regions. It depends on the surface asperities and roughness, which plays an important role in the problems of real contact, contact pressure and wear. According to the comparison map between the new and used taps, it is clear to see the increase of contact area but the decrease of peak pressure for the used hand taps under the same loading as the new hand taps. In this study, the peak contact pressures ranged from 2.5 Mpa for the used TAPER and 4.2 MPa for the new BOTTOMING taps.
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