In order to optimize the structure of the fixture in actual production, to eliminate the influence of over-positioning on the accuracy of the workpiece and to reduce the production cost, the fixture design for gear shaping of shaft parts was optimized. The practical results prove that the method is feasible.


1 Preface
In machining, fixtures are often used to position and clamp workpieces. In mass production, in order to shorten the process time, improve the production efficiency, and at the same time reduce the labor intensity of the operator, various special jigs are widely used. Our YKH5132H CNC gear shaping machine is a high-efficiency CNC machine tool specially designed for automobile, reducer and military industry. Some users reflected that when using the machine tool to process a kind of automobile transmission gear shaft (see Figure 1), the machining accuracy often appears to be super poor. After communication with the user, it was found that the problem of the user's incoming blank. In the deflection meter to the workpiece on the top and bottom of the top hole positioning table, the workpiece is clamped part of the accuracy exceeds the poor, but because the upper process can not be improved, the incoming material accuracy can not be improved, so we can only think from the process of this order, that is, the gear inserting fixture to optimize the design. At present, because of the problem of poor accuracy, the scrap rate is high during batch processing on the existing fixture, which affects the product quality. For this reason, the customer asked our company to optimize the existing fixture. In response to this situation, the existing fixture was improved, which greatly reduced the scrap rate and ensured normal production.

 
Figure 1 Automotive transmission gear shaft



2 Problem Description

In order to solve the problem of high scrap rate of auto transmission gear shaft processed by YKH5132H CNC gear shaping machine, firstly, we checked our own equipment and inspected the bore of the collet chuck, lower center, upper center of the tailstock and the tool with a percentage table, and no problem was found. When the spring chuck is pulled down to clamp the outer circle of the workpiece, the pulling force of the downward hydraulic cylinder is too large, so the spring chuck will deform the workpiece and the lower center hole of the workpiece is forced to be pulled out of alignment, so the upper and lower center holes are not on the same axis.

If the tooth insertion processing is carried out at this time, after the workpiece exits the fixture, the workpiece itself deforms and resets, and when the upper and lower top holes are inspected for tooth processing accuracy, the inspection reference is not consistent with the processing reference state, which causes the accuracy of the inspection to exceed the standard and produces scrap.



3 original fixture structure analysis and optimization requirements

In the original fixture, the lower tie bar is connected with the hydraulic cylinder of the machine tool, and after the hydraulic cylinder starts working, the lower tie bar moves downward, driving the connected sleeve and upper tie bar, and the upper tie bar drives the spring chuck downward through the transition sleeve. The tapered surface of the spring chuck and the tapered surface of the tapered sleeve cooperate to produce a pressure perpendicular to the tapered surface of the tapered sleeve, and likewise, the tapered sleeve produces a radial compression reaction force on the spring chuck, which in turn compresses the spring chuck and clamps the outer circle of the workpiece.

The disadvantage of the original fixture structure (see Figure 2) is that since the taper sleeve itself is coaxial with the outer surface, the collet plays the role of both positioning and clamping during the contraction process, and the fixture and the tailstock have formed the positioning of the upper and lower tops, so the fixture structure is an over-positioning structure. This structure requires high machining accuracy for the positioning and clamping parts of the workpiece itself, which can easily cause machining accuracy to exceed the standard and make the part scrap, which is also the case in practice. Considering the above problems, the fixture structure must be optimized and the clamping and positioning method must be improved.

 
Figure 2 Original fixture structure
1 - lower center 2 - upper center 3 - taper sleeve 4 - collet chuck 5 - connection sleeve 6 - upper tie bar 7 - joint 8 -Lower tie bar



4 New fixture structure scheme
Considering that the top hole of the shaft part is a common machining reference for finishing, the optimized design of the fixture structure needs to remove the positioning function of the collet chuck and only retain the clamping function. The original taper sleeve is changed into a split structure, so that the taper part and the lower center positioning hole part are separated. And keep the clearance between the outer circle of the improved taper sleeve and the inner hole of the chip stopper cover about 0.2mm. In this way, the improved taper sleeve is actually floating (hereinafter called floating taper sleeve) and has the effect of adaptive centering. When the spring-loaded chuck moves downward, the chuck head is shrunk while being influenced by the geometric tolerance of the floating taper sleeve and the workpiece outer circle, the chuck head will adaptively produce a corresponding offset according to the deviation of the workpiece outer circle relative to the top hole, while clamping the workpiece outer circle to achieve the purpose of clamping only and not positioning. The structure of the new fixture is shown in Figure 3.

 
Figure 3 New fixture structure
1-Connecting sleeve 2-Clip stopper 3-Floating taper sleeve 4-Spring chuck 5-Lower center

The fixture design should avoid over-positioning as much as possible, except in high precision requirements or special machining. In this case, the original fixture not only has high manufacturing cost, but also requires high accuracy for the workpiece itself, which in effect increases the machining cost of the workpiece and brings unnecessary machining hazards.



5 Conclusion

In the original fixture design, although the principle of datum coincidence and datum unification is adhered to, and over-positioning is designed for the purpose of high-precision machining, the design does not take into account the problem that the workpiece's own accuracy cannot meet the over-positioning requirement through practical testing, and over-positioning plays a negative role instead. By optimizing the original fixture, the over-positioning was eliminated and the positioning and clamping functions were separated to ensure the quality of the workpiece, while the fixture was easier to make. At present, the improved fixture has been used in the mass production of transmission gear shafts. Practice shows that the accuracy of the parts machined with the improved fixture is stable and the scrap rate is greatly reduced compared with that before the improvement, which improves product quality and ensures normal production.