Box parts is a typical part of mechanical parts, is one of the basic parts of the machine, it will be the machine and components of the shaft, bearings, sets and gears and other parts assembled into a whole, so that it maintains the correct position of each other, and in accordance with a certain transmission relationship coordinated movement, assembled parts of the box, the box with a reference plane installed in the machine, so the box machining quality not only affects the accuracy of its assembly and motion Precision, but also on the working accuracy of the machine, the use of performance and life has a decisive impact.
Formulation of the technology route
The lathe head box requires many surfaces to be machined. Among these, flat surfaces are easier to process accurately than holes. The spindle hole and hole system need high precision. They are the key to overall machining accuracy. So, when arranging process routes, several points must be considered carefully.
1.First face after the hole processing order
First, process the plane to remove unevenness and surface sand. This eliminates defects from the rough surface. It facilitates the distribution of holes on the plane. It also makes scribing and alignment easier. When the boring tool starts boring, a flat surface prevents impact and vibration. This avoids damage to the cutting edge. Therefore, the plane should be processed first.
2.Roughing and finishing stages to be separated
The box structure is complex, with high precision required on main surfaces. Roughing produces large cutting force, clamping force, and heat. These factors affect machining accuracy. If finishing follows immediately, deformation from roughing may not be fully revealed. This deformation can’t be corrected during finishing. As a result, the final accuracy of the box will be affected.
3.Decision of process centralization or decentralization
Box roughing and finishing are usually separated to follow the process decentralization principle. However, in medium and small batch production, it’s better to reduce the number of machine tools and fixtures. Also, minimizing box handling and installation is important. Therefore, roughing and finishing can be done on the same machine. But corresponding process measures must be taken to ensure machining accuracy.
4.Arrange appropriate heat treatment process
Box structure is complex, uneven wall thickness, casting cooling rate is inconsistent, easy to produce internal stress, and the surface is hard, therefore, after casting should be arranged to eliminate the artificial effect of deformation of internal stress reduction.
Selection of positioning reference
Box positioning reference selection, directly related to the box on the various planes and planes, holes and planes, holes and holes between the dimensional accuracy and positional accuracy requirements can be guaranteed.
In the selection of benchmarks, first of all, we must comply with the “benchmark the same” and “benchmark overlap” principle, and at the same time must take into account the size of the production batch, production equipment, especially the selection of fixtures and other factors.
1.Selection of coarse datum
Rough datum selection affects the machining surface of the allowance allocation and do not need to process the surface and the positional accuracy between the machining surface.
According to the principle of coarse datum selection, the spindle hole with the highest machining requirement should be prioritized. Its machining allowance must be uniform to avoid vibration caused by uneven allowance. This helps ensure machining accuracy and surface quality. At the same time, other machining surfaces should also have appropriate allowance.
Secondly, the relative position deviation between the unprocessed surface and the processed inner wall surface of the box should be corrected. This prevents excessive variation in the distance between the machined bearing hole end face and the inner wall. Otherwise, the gear may not align properly with the inner wall during installation.
From this perspective, the inner wall should be chosen as the coarse reference. However, clamping becomes very difficult because the shaft holes and inner cavity form a whole. Therefore, in actual production, a shaft hole farther from the main shaft hole is selected as the coarse reference.
2.Precision benchmark selection
There are two feasible positioning methods for precision benchmark selection of box parts. One is to use the assembly datum as the fine benchmark. The advantage is that the assembly datum includes many designed holes and planes. This allows it to serve as a reliable positioning reference.
So, the positioning datum, design datum, and assembly datum can overlap. This avoids errors caused by datum mismatch. The box can be opened to adjust the tool. It makes processing and measuring hole diameters easier. The disadvantage is for holes in the middle wall. Only hanger-type boring molds can be used. These have poor structural rigidity and high installation errors. The hole system accuracy is low. Loading and unloading are inconvenient. Productivity is also low. This method suits only small and medium batch production.
Another method uses the top surface and two pin holes as fine references. The box mouth faces down. The middle guide support frame is fixed in the fixture seat. This provides high rigidity and guiding accuracy. The hole system positional accuracy is also high. Positioning, clamping, loading, and unloading are convenient. Productivity is high, suitable for batch automatic production. The disadvantage is that the positioning datum does not coincide with the assembly datum. This increases positioning error.
Main surface processing
1.Box plane processing
Box plane roughing and semi-finishing often choose planing and milling.
Single-piece small batch production, with the scribing method, the use of planing and milling processing plane, in the gantry planer can use several tool holders at a time to install the workpiece at the same time processing several planes, economically ensure the positional accuracy of these surfaces.
Considering the high productivity of milling than planing, when mass production, the use of special fixtures in the combination of machine tools for simultaneous machining of a number of surfaces, that is, to ensure the positional accuracy of the plane, but also to improve productivity.
Finishing, in a single piece of small batch production with shovel scraping or fine planing for processing;
A large number of mass production with grinding methods.
2. Spindle support hole processing
Spindle support hole of high precision requirements, surface roughness is small, so the spindle support hole finishing (or finishing) in the other axis hole processing and then carried out individually after the lathe headstock box spindle hole finishing program: precision boring – floating boring; diamond boring – honing; diamond boring – rolling.
Spindle dimensional accuracy, surface roughness by the above spindle hole finishing program in the final process used by the radial “floating” nature of the tool to ensure that the positional accuracy of the hole by the previous process (or step) to be guaranteed.
From the process requirements, fine boring and semi-fine boring should be carried out on different equipment, if the equipment conditions are not enough, should be in the fine boring and semi-fine boring process between the natural aging treatment of the workpiece to ensure the accuracy of the box.
3.Hole system processing
A series of holes on the box with mutual positional accuracy requirements is called hole system.
These holes require high precision, difficult to process, is the key to box processing, which has a parallel hole system and the same axis hole system.
For the parallel hole system, in the processing is mainly to ensure that the parallel holes between the center line and the hole center line and the datum between the dimensional accuracy and parallelism; coaxial hole system is mainly to ensure that the coaxial accuracy of the holes.
3.1 Improving Hole Positioning Accuracy in Small Batch Production of Box-Type Parts
Single piece of small batch production of boxes, in the ordinary boring machine, according to the line in order to find the correct position of the hole for processing, this method of error is large, in order to improve the accuracy, can be used to test the boring method, but this method to find the correct, test cutting, measurement is more time-consuming, low production efficiency.
Rough machining of the box is often used to find the correct method: boring machine boring bar equipped with a micrometer, according to the sample hole to find the correct position of the boring bar, after processing one end of the hole, turn the table 180 °, in the same way to process the other end of the hole on the surface.
3.2 Boring molds are used to process large quantities of holes in boxes
Boring die at both ends of the guide sleeve, can guide the boring bar for processing, in order to ensure the accuracy of the hole distance of the workpiece, the boring bar and the machine tool spindle using a floating connection, the hole distance accuracy depends on the accuracy of the boring die and the boring bar and the guide sleeve with the accuracy and stiffness.
So you can use the precision of the machine tool processing out of high precision workpiece, boring mold can be used in a combination of machine tools for multiple holes at the same time, easy to find the right, high productivity, suitable for batch production, and the box of the same axis of the hole system of the coaxiality of most of the boring mold to ensure that the same axis for the box wall is close to the same axis of the holes, can be used to guide the processing of the same axis of the holes, and vice versa, can be used to boring machine after the pillar of the guide support the boring bar.
Coaxiality error is generated when the spindle feed boring bar due to gravity deflection caused by the coaxiality error of the holes; when the table moves the straightness error of the guide rail leads to the coaxiality error of the holes.
Small batch production, in order to improve the accuracy of sometimes also use the boring mold processing parallel hole system.
3.3 Key points and process requirements for machining high-precision hole systems by the coordinate method
single piece of small batch production in many factories are also widely used in the coordinate method of processing hole system, hole distance accuracy requirements are particularly high, can be used with the vernier precision line size and accurate optical reading device precision coordinate boring machine.
Need to emphasize is the use of the coordinate method of processing hole system, the original hole as well as the determination of the order of boring is very important, in order to ensure that the original hole has a high degree of accuracy and small roughness under the conditions of the two points should be noted:
1) the center distance between the two holes have precision requirements, the two holes should be processed together, otherwise through the many coordinates of the size of the displacement error accumulation is too large, it is difficult to ensure the accuracy of the hole distance.
2) The original hole should be located on the side of the box wall, sequential processing of the holes, the tool can be moved in one direction, avoiding the round-trip movement due to clearance and error.
Conclusion
Box-type parts are critical components in mechanical systems. They integrate shafts, bearings, gears, and other elements. Their machining quality affects the assembly and movement precision of internal parts. It also impacts the machine’s performance, accuracy, and service life.
Throughout the machining process, careful planning and execution are essential to achieve the required precision. Key principles such as “face-first, then holes,” separation of roughing and finishing stages, appropriate arrangement of heat treatment, and the strategic selection of both rough and precision datums ensure high machining accuracy and stability.
Whether adopting traditional scribing and boring techniques or more advanced coordinate and boring mold methods, each approach must be selected based on the production scale, complexity of the hole systems, and required tolerances.
For single-piece and small-batch production, attention must be paid to minimizing cumulative errors. Efficiency must also be optimized. In batch production, boring molds and coordinated machining strategies should be used. These methods significantly enhance productivity and positional accuracy.
Ultimately, the precision machining of box parts requires a balance between technical rigor and practical constraints. By adhering to sound process principles and continuously improving equipment and tooling strategies, manufacturers can ensure that these foundational components meet the demanding requirements of modern machinery.