This paper takes the titanium alloy box parts processed by Elimold as the research object. It explores the deformation control technology and efficient digital processing method of the case titanium alloy parts through the numerical control machining of the box parts in actual production, which provides a good reference case for the company to undertake the CNC machining and manufacturing of other military and civil aircraft titanium alloy box parts in the future.
First, from the perspective of the development direction of aerospace structural parts, the overall structural parts will gradually become mainstream. At the same time, with the increasing complexity of the part structure and the continuous improvement of the geometric accuracy of the parts, high-performance materials have been applied in large quantities. Since titanium alloy materials have excellent properties such as high specific strength, thermal strength, and corrosion resistance, they have been the most widely used in military and civil aircraft structural parts. They are one of the most promising materials at this stage.
The structure of the box body is closed, semi-closed, and open. The incoming materials are all dead forgings, and the allowance is unevenly distributed. This paper selects a typical titanium alloy box part processed by Elimold. Its characteristics are different specifications of the corners in the cavity, deep groove, high dimensional accuracy of the mating surface, and thick sag on the inner and outer surfaces of the edge and the box.
The guide holes of different specifications distributed on the inner wall must be drilled. Since the box-type parts need to be welded into a whole piece after delivery to the United States, they need to be manufactured according to the American technology CNC mold during the processing, and all the ribs or edge strips have stepped on the end faces. The shape allowance needs to be processed with a T-shaped tool. In the research process, Elimold has accumulated valuable experience in CNC machining and manufacturing of titanium alloy box parts, which is of great help in improving the technical level of mechanical manufacturing in enterprises.
(The following technical description, due to the confidentiality of the part design drawing, cannot show the picture)
The box parts selected in this paper are titanium alloy die forgings, which are semi-closed boxes in structure and one of the largest sub-assembly in the closed box welding assembly of the front fuselage of a certain model. The part outline size is 480mm×185mm×1 000mm, the maximum cutting depth is 180mm, the metal removal rate is high, and the processing cycle of parts is long. The welding assembly comprises multiple sub-assemblies because the welding parts generally have to leave a process allowance, which often brings great processing difficulties to machining. After comparing the digital process model and the digital design model after assembly, it is found that each weld part has a process allowance, resulting in a concave step on the side wall of the part. A T-shaped knife can put the concave side wall be processed in place. Due to the low processing efficiency of the T-shaped knife and the large area where it needs to be used for this sub-assembly, it is bound to increase the processing cycle of the part greatly.
The material of the parts is VT6ch, and the corresponding brand in China is TC4 titanium alloy, a difficult-to-machine material widely used in the aviation industry. Its main features are:
① High strength, its value is generally 60 ~ 150kg/mm2, and low specific gravity, only 4.5 × 10-3g/mm3.
②High thermal strength and good thermal stability, under 300～500℃, its strength is about 10 times higher than that of aluminum alloy.
③It has high chemical activity and strong affinity, and it is easy to absorb impurities such as hydrogen, oxygen, nitrogen, and carbon during thermal processing.
④ poor craftsmanship. Because of its poor thermal conductivity and high friction coefficient, it has poor machinability and easy deformation.
⑤ Good corrosion resistance, high corrosion resistance to the atmosphere, seawater, and its vapor, and some acid, alkali, and salt media.
Analysis of processing difficulties
The specific difficulties that Elimold needs to overcome in this project include the following:
- The application of T-shaped tools, the exploration of processing parameters, and processing methods.
Due to their structural characteristics, box-type parts generally require multiple processing stations and multi-directional processing, and there are many difficult-to-machine parts. These difficult-to-machine parts can be processed in place with T-shaped tools.
- Corner treatment.
The specifications of the bottom corners and corners of the parts vary, and the processing and processing methods of the multi-specification corners of deep cavity grooves need further verification.
- Research on multi-station processing technology.
The structure of the box parts is complex, and it often requires multiple stations to ensure the parts are processed in place, and some parts require two or even more stations to ensure the size.
- Tool selection.
Due to its structural characteristics, box-type parts often have large lower tool depths inside and outside the box, which requires special machining tools and is prone to chatter and knife yielding during the machining process, making it difficult to guarantee the size, and requiring multiple smooth knives.
Elimold has broken through the following key technologies in this project.
（1）Use T-shaped tools to achieve machining of difficult-to-machine positions. The digital process model of the box part shows that because the green part of the top of the side wall has a welding allowance, there is a 2mm step difference with the inner wall of the box, so not only the groove of the inner side wall of the box but also the entire inner side wall of the box needs to be machining with a T-shaped knife. The key technology used by the T-shaped tool is explored through the exploration of the actual production process: the first is to verify the tool. Before the tool is used, it is necessary to measure whether the size of the tool matches the design size with a measuring tool. The precise size of the tool is the premise to ensure the size of the part, especially before the part is finished, so the size of the T-shaped tool should be measured again, and the second is the layered processing with a margin on the bottom surface. Since the T-shaped tool has no bottom edge, if there is no margin on the bottom surface during processing, it will cause wear of the parts and the tool, or even damage the tool or cause irreversible damage to the parts. At the same time, when using the T-shaped tool, the layered processing method should be adopted to remove the allowance layer by layer; finally, the tool holder of the T-shaped tool should avoid the parts during the processing to avoid the tool holder and the parts. In the event of a collision, it is necessary to make a tool model strictly according to the tool size during the CATIA programming and VERICUT simulation process and then detect the position of the tool and the tool holder during the machining process to ensure that the tool holder can avoid the part and not collide with it.
（2）The processing method of multi-standard corners.
Box-type parts will have corners of multiple specifications, and with the change of the depth of different grooves, the depth of the corners is also different. The corners of deep grooves have high requirements on the cutting depth of the tool. If the tool length is too long, chattering and yielding will occur during processing, and it is impossible to ensure that the corners are processed in place. If the corners after rough milling are not processed for finishing, they will often lead to tool damage.
Aiming at this problem, the methods of layered clearing and plunging corners are adopted to solve this problem.
First, use the short knife to process the part that can be machined in place, and then replace the long knife to perform layered cleaning and corner treatment on the places that were not processed by the previous short knife. Note that the depth of the layered layer should consider the processing efficiency and effect and integrate the previous processing. The tool diameter, corner diameter, and corner depth are considered, and the corners are cleared in layers.
Another more efficient method of clearing corners is plunging. Plunge milling uses the tool’s bottom edge to mill the corners, which is more efficient than the layered method. For the parts with large residual corners, it is often necessary to plunge multiple cutters, and the specific plunging cutters must be comprehensively considered. The size of the previous machining tool diameter and corner diameter. In addition, the axial force of the tool during plunging is large, which is not suitable for plunging deep corners. Because deep corner plunging requires a longer tool, and the tool is subjected to a long time during the plunging process, the stressed part is easy to concentrate locally on, which in turn causes the tool to break during the plunging process. Therefore, plunge milling is often not used for corner processing in the corner processing of deep pockets. After the corner is cleaned, the precision milling can ensure that the CNC machining of the corner is in place and no conventional supplementary processing is required.
Due to the characteristics of its structure, box-type parts often require multi-station processing to manufacture parts. Box-type parts must be processed in four directions, and some dimensions must be guaranteed in more than two stations. Due to the accumulation of errors, the thickness tolerance is not easy to guarantee. In the process of processing, it is necessary to avoid the influence of the accumulation of errors caused by multiple re-clamping on the wall thickness size. This part is processed by a three-coordinate vertical machine tool, and the processing in all directions is realized by using the positioning of different stations of the milling clamp. Try positioning directly on the machine tool to reduce the time and cost of applying special tooling for manufacturing.
（4）Tool selection and cutting parameters.
The feature of the box is that the groove cavity is very deep, and the welding allowance is left in the digital process model, which greatly limits the choice of tools in processing. Special tools are used, and long and short tools are alternately processed. Due to the influence of the inner surface of the part cavity, it is not suitable to choose a tool with an excessively large diameter. Roughing, semi-finishing, and finishing are mostly selected with a tool diameter of 40mm. Due to the box’s deep inner shape, the tool should be matched with length and length when roughing the inner shape. When the depth of cut is shallow, the tool with the short edge and the long edge is used to improve the tool’s rigidity so that the processing line speed can be increased as much as possible, thereby improving the processing efficiency.
The selection of cutting parameters needs to be considered comprehensively. Different cutting parameters are selected when machining different part materials for different tool materials. In addition, for different processing purposes, the selected cutting parameters are also different. Therefore, the upper limit of the cutting parameters should be selected as much as possible for roughing to remove most of the allowance. Selecting the appropriate processing parameters is necessary to ensure the part’s size.
For side cutting, the cutting depth and width can be converted according to the method of equal cross-sectional area. Select larger parameters for roughing and smaller parameters for finishing. Since the titanium alloy material’s deformation coefficient is small, the feed per tooth of the tool should not be too large. Otherwise, it will aggravate the tool’s wear and reduce its life. Generally, f z=0.13～0.20mm/z is selected as the feed per tooth for roughing, f z=0.10～0.13mm/z for semi-finishing, and f z=0.06～0.06～0.13mm/z for finishing. 0.10mm/z. In the finishing process, the feed shortage per tooth is mainly compensated by the increase in the cutting speed, and finally, the finishing of the surface is achieved. In addition, the linear speed of the tool for cutting titanium alloy materials is generally selected as v c = 20 ~ 60m/min.
In the roughing process, the axial depth of the cut is strictly limited, and the maximum depth of each layer is 10mm to avoid excessive cutting heat caused by the excessive cutting depth and leading to the deformation of parts. In addition, the cutting fluid in the cutting process must be fully supplied, and the chips must be cleaned in time. Otherwise, it will harm the tool’s cutting performance, and in severe cases, the surface quality of the machined surface will be affected.
In addition, due to the concave steps on the side wall of the part, the corner angle of R12mm, and the groove depth of 11mm, the T-shaped tool cannot be machined in place. Only the carbide tool with a large overhang can be machined in place, so it is necessary to apply a longer tool shank, and the length of the tool shank is 320mm.
Through the exploration and formulation of the above process plan by Elimold, a deformation control technology and high-efficiency CNC machining method for box-type titanium alloy parts has been obtained. Stable and achieved remarkable results, realizing Elimold’s breakthrough in aviation manufacturing technology, its application scope, and implementation scale are at the top level in the same industry, for Elimold to undertake CNC machining of titanium alloy box parts for other military and civil aircraft in the future Manufacturing operations provide a favorable reference.
The material of the box parts described in this article is titanium alloy, which is difficult to process, and the structure of the parts is complex, and it is difficult to process. According to the machining difficulties of parts, the engineers of Elimold Company analyzed and formulated technological measures such as the difficult machining positions of T-shaped tools, the treatment of various specifications of corners, and the multi-station machining of parts, and verified them in actual machining. The processing method of box-like parts has accumulated rich experience for the company to process such parts in the future.