The sensor-integrated tap holder is based on the Softsynchro 3 from the company Emuge Franken. It is a purely mechanical system for compensating synchronization errors allowing the tapping tool to move along the longitudinal axis, a so-called floating tap holder. The functional principle of the floating tap holder shown in Section 4.1 allows the detection as well as the quantification of synchronization errors by integrating the axial length compensation sensor introduced in Section 4.3. The second sensor concept for uncertainty detection is the close-to-tool vibration sensor introduced in Section 4.2. The principle of a direct sensor attachment on the tapping tool allows the measurement of vibrations close to the tool center point, thus avoiding the influence by other mechanical components of the tapping tool holder such as the mechanical interface of the linear guidance for length compensation. Furthermore, this sensor concept makes the sensor-integrated tap holder more sensitive, which is essential for small tapping tool diameters. The data acquisition as well as the wireless data transmission is done by a telemetry unit which is shown in Section 4.4.
Floating tap holder
The Softsynchro 3 shown in Fig. 2 consists of four components: (1) an A63 type hollow shank taper, (2) a cylindrical attachment rigidly connected with the hollow shank taper, (3) a movable piston, (4) the tapping tool. The piston (3) is aligned with the z-axis by two rolling elements (5) which are offset by 180∘, thus enabling a degree of freedom for the tap in both z-axis directions. The rolling elements (5) also prevent the rotation of the piston (3) about the z-axis, thus allowing the transmission of torque from the motor spindle to the tapping tool.
The stiffness in z-axis direction is determined by the two pairs of polymer spring elements (6), which are also offset by 180∘. The floating tap holder allows a maximum stroke of the tapping tool of ± 1.5 mm.
Sensor for measuring close-to-tool vibrations
For measuring the close-to-tool vibrations, a sensor concept has been developed based on capacitive Micro-Electro-Mechanical System (MEMS) accelerometer sensors. The MEMS sensors used for this design provide a wide frequency bandwidth from DC up to 11 kHz with a sensitivity of \(20 {\frac {\text {mV}}{\mathrm {g}}}\) and a measurement range of ± 100 g. These specifications allow structural dynamic measurements such as tool vibrations. The developed close-to-tool vibration sensor on basis of the two MEMS accelerometers enables a direct measurement of the radial vibrations ax and ay in two perpendicular directions of the tapping tool as illustrated in Fig. 3b.
The close-to-tool vibration sensor shown in Fig. 3a consists of a housing (6) which carries the printed circuit board (5) with two uniaxial MEMS accelerometers (3 and 4). A sleeve (2) with an external thread is attached on the tapping tool (1) by an adhesive. This allows to screw the housing (6) on the sleeve (2) to establish a rigid connection between the tapping tool (1) and the housing. This type of sensor application was deliberately chosen to reuse the vibration sensor in case of tapping tool breakage.
Sensor for measuring the length compensation
The sensor that measures the axial length compensation of the floating tap holder is depicted in Fig. 4a. The sensor element consists of an outer (1) and inner (2) ring, which are connected by two webs (3), thus functioning as a ring spring element.
The spring element is connected with a spacer ring (5) which is connected on the bottom side (9, Fig. 2) of the cylindrical attachment (2, Fig. 2) with an adhesive. Figure 4b shows the integration of the sensor into the floating tap holder, whereas Fig. 2 position (7) shows the sensor in the integrated state. In case of axial length compensation, the piston (3, Fig. 2) moves according to Fig. 2) in negative z-axis direction and presses the inner ring (2) of the sensor. The resulting stress within the two webs (3) yielding a strain state, which is measured by the full-bridge strain gauge (4). The screw (6), which is located at the bottom side (8, Fig. 2) of the piston (3, Fig. 2), enables the adjustment of the sensors pretension.
Since the axial length compensation sensor measures the strain, we will now discuss the conversion of measured strain to the length compensation, thus displacement. Each of the polymer spring elements (6, Fig. 2) is connected to the piston (3, Fig. 2) with a single screw only shown in Fig. 5. This allows the modelling of polymer ring elements cp and the sensor cs by simple spring elements as shown in Fig. 5, whereby damping effects are neglected. Considering linear elastic deformation of the sensor element, Hooke’s law and first order beam theory can be utilized to gain the necessary equations.
By inserting the Hooke’s law σ = E𝜖 into Eq. 2, which describes the beam stress σ due to torque load ML,
$$ \sigma(y,z) = \frac{M_{\mathrm{L}}(y)}{I_{\mathrm{x}}(y)}z=\left.\frac{M_{\mathrm{L}}(y)}{I_{\mathrm{x}}(y)}\frac{t}{2}\right|_{z=\frac{t}{2}} $$
(2)
yields to Eq. 3, which describes the relation of strain 𝜖 and torque load ML.
$$ \epsilon E = \frac{M_{\mathrm{L}}(y)}{I_{\mathrm{x}}(y)}\frac{t}{2} $$
(3)
Furthermore, the torque load ML(y) can be substituted by ylFPiston, where yl is the distance between strain gauges and the point of force application of the piston FPiston along the y-axis as shown in Fig. 6.
Since the two springs cp and cs, shown in Fig. 5, are connected in parallel, the piston force can be expressed by Eq. 4,
$$ F_{\text{Piston}} = [2c_{\mathrm{p}}+c_{\mathrm{s}}] w $$
(4)
where w is the deflection in z-axis direction, thus the length compensation. By considering Eqs. 4 in Eq. 3, the equation for converting measured strain 𝜖 into length compensation w expressed by Eq. 5 can be gathered.
$$ w(\epsilon)=\frac{2 E I_{\mathrm{x}}(y_{\mathrm{l}})}{[2c_{\mathrm{p}}+c_{\mathrm{s}}] y_{\mathrm{l}} t}\epsilon $$
(5)
with E as Young’s modulus, Ix as second area moment of inertia around the x-axis and t as constant thickness of the sensor webs in z-axis direction.
Since the stiffness of the polymer ring elements cp has not yet been determined, the amplified bridge voltage of the full-bridge strain gauge was used for signal analysis of the evaluation experiments in Section 6.
Rotating telemetry unit
A telemetry unit which is rotating with the tool holder was developed for signal processing, data acquisition and data transmission or data storage. As shown in Fig. 7 the telemetry unit is divided into two sections, where the first section is the signal preprocessing stage for the strain gauges (length compensation sensor) and the MEMS accelerometers (tool vibration sensor).
The second section includes a data acquisition stage using a 24 bit high speed four channel simultaneous analog-to-digital converter (ADC) as well as a dual core embedded system running a real time operating system. The embedded system receives the bit stream of the samples from the ADC, whereby the sampling rate can be configured up to f = 52.7kHz. Depending on desired sample rate fS, digitized samples will be transferred directly via WiFi or stored on a microSD card by the embedded system.
For attachment of the telemetry unit on the sensor-integrated floating tap holder as well as for battery integration a plastic carrier (1) shown in Fig. 8 was 3D printed.