The influence of printing conditions is observed on the average deviation values of the fasteners fabricated with the help of Polyjet Technology. Fasteners were printed at the optimum condition for the printer that were glossy surface finish, heavy support material, 90°-part orientation, high-speed printing, and 25°C ambient temperature. The study compares pitch, depth of thread, thread angle, and thread engagement of the M8 bolt with the standard values.
7.1. Thread Profile Analysis for bolt
Threads are the grooves of a uniform section cut on the internal or external surface of a cylinder in the shape of a helix. The bolt’s strength is incredibly reliant on the thread
Table 5
The Mechanical Properties of Vero Magenta, Cyan, White, and Grey [36]
Mechanical Properties | Vero Magenta / Cyan |
Tensile Strength | 50–65 Mega Pascal (D-638-03) |
Elongation at Break | 10–25% (D-638-05) |
Modulus of Elasticity | 2,000–3,000 Mega Pascal (D-638-04) |
Flexural Strength | 75–110 Mega Pascal (D-790-03) |
Flexural Modulus | 2,200–3,200 MPa (D-790-04) |
Heat Deflection Temperature @ 0.45MPa | 45–50°C (D-648-06) |
Shore Hardness (D) | Scale D 83–86 |
profile. The average observations are presented in FIGURE 12–15. Three regions of the bolt, namely, the greater end, the centre region, and the lower end, are thoroughly observed. Thread angle is defined as the included angle between flanks of the bolt. FIGURE 12 depicts a deviation of approximately 40 in the thread angle for mild steel M8 bolt when measured with a 3D scanner. A deviation of 4.469o and 3.278o is observed for Vero cyan Vero magenta, respectively, as shown in FIGURE 13 and FIGURE 14, respectively. Pitch refers to the space between two consecutive threads. The maximum deviation is observed for the radial distance between root and crest and the minimum deviation for pitch. The depth of thread is the difference between successive bases in a plane normal to the axes. While measuring the depth of thread, a deviation of 0.176 mm for Vero cyan and 0.21 mm for Vero magenta is detected. FIGURE 15 illustrates the thread profile of a Vero white bolt. The thread angle in the thread profile deviates by a maximum of 2.5o, while length of thread engagement deviates by a minimum of 0.8 mm.
7.2. Thread Profile Analysis for nut
A nut is the female counterpart of the bolt [46], consists mainly of a square or hexagonal head with a bore in the middle region along with the internal threads that engage the male threads of the bolt. During effective bolt installation, a nut assists in fastening the component along the bolt's axis. The coefficient of friction between mating threads has a significant impact on the force generated to fasten the component. The threads on the nut are identical to those on the mating bolt. The strength of the bolt is extremely dependent on the mating nut’s thread profile. A thread angle variation of about 3 o for Vero grey nut is observed when compared to ISO standard values as illustrated in FIGURE 16 (a). As shown in FIGURE 16 (b), surface characteristics such as length and width are measured for a completely printed Vero grey nut. A deviation of 0.7 mm for depth of thread and 0.05 mm for length of thread engagement, respectively was observed for Vero grey nut. The mean observations are discussed in FIGURE 17 which compares the pitch, thread angle, depth, and length of thread engagement of Vero cyan bolt, Vero magenta bolt, Vero white bolt, and Vero grey nut with the stainless steel M8 bolt and the ISO dimensions. It can be noted that for Vero grey nuts, the maximum deviation occurs at the thread angle of 3°, while the smallest discrepancy occurs at the pitch of 0.01 mm. The pitch, depth of the thread, and the thread engagement length demonstrate maximum deviation for Vero magenta by 0.06 mm, 0.21 mm, and 0.115 mm, respectively.
The highest discrepancies are observed for the Vero magenta M8 bolt, whereas the lowest is observed for the Vero cyan M8 bolt.
7.3. 3 D Inspection of the bolt
3D scanning is a technique of inspecting precise details of the component. Carl Zeiss COMET L3D2 3D scanner [39] as shown in FIGURE 10 was used to analyse the fastener’s geometric features and compared the gathered dimensions with the ISO standards. Sixteen points are inspected on the printed bolt’s geometric features, and results are averaged in terms of dimensional deviation. The X, Y and Z coordinates are captured using ZEISS colin3D software. Nominal value, actual value and the deviations between the captured and ISO dimensions are presented in FIGURE 18–20. Points are arbitrarily taken in these regions on the periphery of the 3D printed Vero cyan, Vero magenta, and Vero white bolts.
The positive values indicate a larger dimension than the standard value whereas the negative values indicate a smaller dimension than the standard value. These points function as the reference parameters for evaluating the directional deviation. The directional deviations and total deviation values are calculated, and the dimensional inspection is performed along X, Y, and Z directions. The origin is kept at the bolt’s centre for measuring the displacement field, and horizontal and vertical directions are assumed as the positive X and Y-axis, respectively. FIGURE 18 illustrates the deviation trend for the Vero white bolt. The green zone of the tolerance chart indicates that there is little variation than the red zone. The deviations are more towards the lower end of the Vero white bolt. FIGURE 19 depicts that the 3D printed Vero cyan bolts profile slightly deviates from the roots and peaks of the ISO M8 bolt. The major portions of the peaks and roots are missed by Vero magenta. The thread failure of the bolt occurs due to combination of tensile, compressive, shear and torsional stresses. The crack initiates at the thread root and grows until the failure of the bolt. Maximum load is carried by the roots and peaks of the thread. Dimensional deviations, is therefore, the main concern which determines the effective bolt connection. Directional deviations for Vero white, Vero cyan, and Vero magenta M8 bolts are calculated and shown in FIGURE 21 (a), (b), and (c), respectively. The strength of the bolted connection majorly depends on the accuracy of thread profile. FIGURES 22–24 depict a two-dimensional sectional view of the thread profile, highlighting the superimposed peaks and roots of the 3D printed bolt on the ISO-compliant M8 bolt. The deviations in the roots and peeks are more towards the lower end of the bolt.
7.4. 3 D Inspection of the nut
The "fit" is the dimensional clearance between two mating components, and its magnitude affects the mating components' ability to slide and revolve independently. M8 nuts are fabricated with in the IT 06 transition fit grade to determine the tolerance zone for the mating bolt and nut. The mating M8 nut is additively manufactured using Vero grey thermoplastic and fabricated under the similar printing conditions. Internal thread quality is determined using a half3D printed nut, while geometric morphology is determined using a full 3D printed nut. The Carl Zeiss COMET L3D2 3D scanner is used to scan the additively manufactured Vero grey nut [37]. FIGURE 25 illustrates a three-dimensional tolerance chart showing the deviation for the nut. The thread tolerance chart governs the transmission accuracy of the mating parts. The greatest deviation occurs near the nut’s top surface, as shown by the red zone on its thread profile while the smallest deviation occurs towards the nut's threads. The major peaks and crests are missed near the base of the nut. The thread defects among the mating parts initiates failure in the threaded joint. Directional deviations for Vero grey M8 nut are calculated and shown in Fig. 26.
It is perceived that the directional deviations become more prominent as we move towards the lower end of the bolt. They are more along Y-axis for Vero cyan, Vero white bolt and more along the Z-axis for Vero magenta bolt. In case of Vero grey nut the deflection along the Y-axis is more significant. The maximum deviation is ± 0.09 mm in case of cyan bolt, ± 0.12 mm in case of magenta bolt and 0.14 mm in case of Vero white bolt. For the Vero grey nut the maximum deflection is ± 0.1 mm along the Y-axis. Standard deviation for Vero white bolt is higher than Vero cyan and Vero magenta bolts, which can be seen from the scanned data set since the values are unevenly located from the actual values. The standard deviation of Vero grey nuts is lower than that of additively manufactured bolts, since the deviations from the ISO standard values are less scattered.
Finally, Table 6 presents a comparative analysis between various fabrication methods such as photopolymer jetting, SLA, FDM, MJF, MJP, and SLS on the basis of their dimensional accuracies. A range of printing materials such as PLA, ABS, Polyamide12, clear resin, epoxy resin and Vero photopolymer are employed and various printing parameters are presented and compared in terms of their dimensional precision in the fabrication of the manufactured components.SLA reported the highest accuracy of ± 0.05 mm whereas polyamide 12 demonstrated the lowest accuracy of all the presented materials at ± 1.136 mm. An accuracy of ± 0.12 mm is reported with the fabrication of the presented M8 bolts using Vero magenta colour whereas the accuracy was as low as ± 0.1 mm for Vero cyan. It is observed to be more accurate than FDM [6, 11, 20], SLA [13, 21].