In order to obtain the highest possible surface quality in large mold processing, we need to select the right machine and configure the necessary tools. Only in this way can we reduce the subsequent processing time of the EDM and the manual grinding time of the fitter.
With the rapid development of the automobile industry, the demand for various large-scale, high-decoration molds for automobiles is rapidly increasing. For example, automotive dashboards, designers are constantly increasing the size and complexity of such mold products. As a result, the production of various large-scale molds has become a new area where many processing companies seek opportunities for global competition. However, it is not an easy task to enter this field. Under the lagging processing technology, the machine tool feeding speed is slow, the operator needs to carry out several debugging and clamping of the workpiece, and it also requires several hours of manual polishing work, so the processing cycle is very long. Therefore, we must seek more advanced processing technology to make large-scale molds as simple and easy to process as small molds.
one. Major problems in large-scale mold processing
1. Huge size and weight
When processing large molds, how to deal with its own huge size and weight is a major challenge for processing companies. The processing of large-scale molds often requires a large amount of labor, special equipment, and multiple debugging setups, and the machining accuracy is also affected by many potential factors and is not easy to guarantee.
2. Acquisition costs
The biggest cost directly associated with processing and producing various types of large-size molds is the cost of purchasing the machine tool. Machines that can produce large molds are quite expensive, especially in complex process arrangements, where multiple machines are needed to complete the entire process from roughing to finishing. This high initial investment cost is also the biggest obstacle for many companies to enter this market. From this, we can see that if a large tool can be rough-machined and finished on a suitable machine tool, or even just one test fixture, many problems will be solved and the machining accuracy can be guaranteed.
II. Large mold processing center
Considering the general machining accuracy of large-scale machine tools and the problems of commissioning and clamping, we must explore some of the design features necessary for large-scale mold machining centers.
1 cast iron bed structure, machine tool spindle with heat dissipation
Cast iron materials have high rigidity and heat dissipation characteristics and are therefore the most stable materials for the manufacture of machine tool structural parts. For any machine tool used to mill large parts, it is first necessary to have a very strong cast iron structure and equipped with a spindle with heat dissipation.
In terms of the spindle of the machine tool, it must use built-in cooling technology to cool the spindle from outside the bearing to ensure that the spindle itself will not be burned out or suffer from loss of precision due to thermal expansion during prolonged machining. These factors are very important because the processing of large molds takes a long time and at the same time it increases the heat and stress of the mold under heavy cutting conditions. Therefore, the structural components of the machine must have good rigidity and heat dissipation characteristics, which is the prerequisite for processing large-scale high-quality molds. Therefore, it is necessary to limit the vibration of the machine tool during the machining process to a maximum extent and quickly spread the heat generated during the machining process. Choosing the right machine tool and tool can achieve a win-win result in both cost and cycle time.
2. Thermal stabilization technology
Due to the long processing time, the influence of ambient temperature must also be considered. For example, when a large mold is machined on an ordinary machine tool, when the ambient temperature changes by 10°C, a temperature change of 6°C occurs in the column of the machine tool, causing a 0.07mm change in the parallelism of the spindle angle plate. Therefore, the design of the machine tool must take into account the effects of the ambient temperature and avoid the influence of the ambient temperature on the precision of the machined part.
For a large-scale mold machining center that can move quickly, the spindle speed of a large mold processing machine tool should be at least 20000 r/min, and the metal cutting speed should be 762 to 20,000 mm/min.
Accuracy control is always carried through at all stages of mold processing. If it is necessary to realize rough machining and finishing of a large mold on a machining center, then the positioning accuracy and repeatability of the machine tool must be strictly controlled. Large-scale mold-dedicated machining centers generally have a positioning accuracy of ±1.5 μm and repeat positioning accuracy of ±1 μm. At the same time, its pitch accuracy should be kept within 5μm.
High-precision surface machining.
5. Feedback resolution
The machine's own feedback resolution is very important for detecting the accuracy of machined parts. With a standard 1 μm feedback resolution, the results usually obtained are not ideal. If the resolution can reach 0.05μm, there is almost no flaw in the finishing results. Furthermore, the machining quality of the part surface can be further improved by controlling the machine resolution, scale feedback, and small-pitch ball screws.
Machining centers used in large-scale mold machining centers must meet the requirements for roughing, semi-finishing, and high-quality finishing. As a reference standard, the quality of surface machining that can be achieved should be controlled at the 2 μm level. Usually, the finishing of the mold closing surface and parting line is very important, but under the traditional process, many mold manufacturers have to use hand-polished way to make up for the lack of tool processing accuracy. Because of the high cost of large-scale processing machine tools, it is obviously not practical to purchase multi-function machine tools for this process.
The horizontal machining center whose spindle angle can be changed with the workpiece.
In addition, a reasonable spindle design must be able to maximize the tool's useful life, allowing it to continue working with low vibration and low temperature rise during the processing cycle. For example, when processing automotive dashboard molds in large mold machining centers, such as the use of 16mmCBN inserts finishing tools, the processing speed can reach 8m/min, the service life of more than 30h, the surface quality can be controlled at 0.336 ~ 3.2μm. It can be seen that taking into account the increase in tool costs when processing large-scale molds, the use of specially designed large-scale mold processing machines not only can prolong the service life of the tools, but also can save a large amount of tooling costs for processing each mold.
7. Movable multi-axis machining head
Due to the limitations of the size and weight of the mold, it usually takes a long time to clamp the workpiece. Therefore, the use of 3-axis linkage machining center not only reduces the number of workpiece mounting and installation, but also does not affect the machining accuracy of the machine, thus greatly improving the production capacity of the workshop processing large molds.
The movable multi-axis machining head can be used to process large-scale molds with particularly complex structures. The machining heads designed in a variable geometry can allow 3-axis simultaneous machining. The machining cavity can be deep-drawing only by clamping workpieces at one time. Dies and cooling holes, as well as machining many other geometrically complex parts. For example, when the spindle is tilted at the optimum angle, the proximity of the machining head to the milling machining point can be increased, so that machining of the oblique hole can be accomplished using a multi-axis machining head.
In addition, since the multi-axis machining head machined the surface of the workpiece, the radius of the tool was used instead of the tool's cutting edge, so the surface roughness was improved.
8. Chip Management
Metal cutting will produce a large amount of chips. If it cannot be eliminated in time, it will inevitably lead to secondary cutting, as well as temperature rise of the structural parts of the machine tool or the surface of the workpiece. The large mold processing center usually has 18 evacuation holes below the worktable to reliably remove chips regardless of the position of the table. There are four built-in hinged chip conveyors on the machine that discharge the chips to the front of the machine at a high speed.
9. High pressure coolant
High pressure coolant plays a very important role in the processing of large molds. For example, when drilling a slant hole using a 2+3-axis machining method, a coolant pressure of 1000 psi (1 psi = 6890 Pa) is required to effectively remove the chips and achieve a more accurate cutting. If there is no such high-pressure coolant, additional machine tools will be needed to machine the inclined hole, which will require secondary loading, reducing the machining accuracy and increasing the cycle cost. Based on the above analysis, it can be seen that achieving simple machining of a large mold requires the machine to have more and better functions. The new MCC2516VG3 axis horizontal machining center developed by Makino has a spindle speed of up to 15000r/min and uses the "shaft cooling" and "bearing internal pressure lubrication" functions to ensure that the spindle and its associated bearings can be cooled in a timely and effective manner. . In addition, the spindle can be moved not only along the transverse X-axis, the vertical Y-axis, and the front-rear Z-axis, but also with the A-axis and C-axis. Thanks to its two indexing functions, it not only reduces the need for adjustments, but also cuts complex workpieces such as bumpers, dashboards and car headlight lenses.