Research on milling process of fir tree shaped ten

2022-08-11
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Research on milling process of fir tree shaped dovetail groove of wheel disc

Abstract: through the research and trial production of milling process of fir tree shaped dovetail groove of an engine fan wheel disc, an integral cemented carbide forming milling cutter is designed, and a correct and reliable process method, pecking milling, is created, which breaks through the technical obstacles in milling the dovetail groove of such complex parts, and successfully solves the problems of tool vibration in processing and low surface roughness after processing, It provides a scientific basis for similar tenon and groove milling

key words: fir tree shaped long tenon; Milling process; Solid carbide forming milling cutter; The traditional machining method of gouging fir tree shaped mortise groove of aeroengine wheel disc is broaching process. Its tooling, tool design and manufacturing are relatively complex and expensive, and the production preparation time is long, which is not suitable for single piece trial production

in order to meet the requirements of rapid trial production of Aeroengine Parts, master the milling technology of fir tree shaped tenon groove of wheel disc. In this paper, aiming at the problems of severe ultra sonic displacement sensor re cutting and serious vibration of the cutter, edge collapse of the milling cutter surface and low machining surface roughness in the process of milling tenon and groove of a machine disc, it is proposed to use the integral cemented carbide forming milling cutter for semi finishing and finishing, and to improve the rigidity of the cutter by reducing the overhang length of the shank and increasing the effective diameter of the shank diameter and the cutter center as much as possible, and to allocate the milling allowance reasonably at the same time, Adopt pecking milling and other technological measures to solve the problems of serious knife and vibration and low machining surface roughness, improve the quality of parts and ensure the smooth trial production of a machine

1 technical points and process analysis

1.1 technical points

an engine fan disc is made of martensitic stainless steel, and its structural diagram is shown in Figure 1. 22 fir tree shaped tenons and grooves are evenly distributed on the rim of the part, and the length of the tenon and groove along the axial direction is 140mm. The radial groove depth is 53mm, the narrowest part of the groove width is only 12.3mm, and the maximum depth width ratio is 4.3

Figure 1 Schematic diagram of the first stage disc structure

the design accuracy of the disc tenon and groove is high, the surface profile of the two working edges is required to be no more than 0.02mm, the surface profile of the non working edge is no more than 0.05mm, the parallelism of the two working surfaces on the same side of the tenon and groove along the full length is no more than 0.0lmm, and the difference between the two characteristic sizes of N1 and N2 along the full length is no more than 0.025mm. The circumferential position of the tongue and groove shall not be greater than 0.15ram, and the included angle between the symmetrical plane of the tongue and groove and the axis shall be 9., As shown in Figure 2

1.2 forming and processing technology of fir tree shaped mortise and groove of wheel disc the most commonly used forming and processing technology of mortise and groove of aeroengine wheel disc is broaching method, which can be divided into straight groove, dovetail shape and fir tree shape mortise and groove according to different groove shapes. According to different types of broaching machines, it can be divided into high-speed broaching and ordinary broaching

a few parts use milling to process the tenon, but the length of the tenon is short, the depth width ratio is small, and the requirements for groove shape and roughness are not high

the circular tenon along the circumference adopts the turning process

Figure 2 Schematic diagram of tenon size and tolerance zone

1.3 broaching tenon forming processing technology

broaching is a processing technology method that uses group cutters to process and form in steps with allowances. The broached tenon and groove has good consistency of forming size, high precision and stable quality, and is suitable for mass production

the profile of fir tree tenon is relatively complex. The axial length of this part is long, the depth of the tenon is deep, and the maximum depth width ratio is 4.3. According to the calculation of 6m stroke of high-speed broach, if broach is dispatched, the length of high-speed broach should be (20 ~ 30) m, and 4 ~ 5 stations need to be arranged. The production preparation cycle is long, the tool design and manufacturing are difficult, the cost is high, and it is time-consuming and laborious

in addition, the mortise depth of the part is 53mm, and the diameter of the fixture dividing plate must be smaller than the bottom diameter of the mortise. Broaching the tenon and groove of this kind of parts on the indexing disc with a diameter of more than 200 mm should not be too fast, otherwise the rigidity of the parts is not good, but the broaching speed of martensitic stainless steel parts should not be too slow, which is easy to tear the surface of the parts

therefore, consider whether other process methods can be used to replace broaching methods

1.4 milling tenon forming process

compared with broaching process, milling process has low cost, short cycle, and simple design and manufacture of tooling and cutting tools

but the consistency of the formed size of the milled products is not good, and the quality is relatively unstable; The rigidity of the cutting tool is not as good as that of the broach, and the wear is faster; The roughness of the machined surface of the parts is also slightly poor

therefore, the milling process is suitable for mortise and groove or single piece parts with simple groove shape and low precision

2 technical scheme and guarantee measures

combined with the structural characteristics of the part and the requirements of single piece production, it is determined to process the part by milling

2.1 process difficulties

1) the rigidity of the milling cutter is not as rigid as the broach, the vibration of the cutter is serious during processing, and the milling cutter face has broken edges

2) tool wear is fast, and the consistency of mortise forming size is not good

3) the roughness of the milled surface is low

2.2 process method

the first step is to check the profile of the precision milling cutter through the projection of the enlarged drawing of the mortise groove type, and record the starting position of the profile and the offset distance of the profile

the second step is to machine mortise (straight groove) on the test block with a length of 140mm

step 3: according to the starting position of the projection of the milling cutter profile, project the test block to check the offset distance of the tenon and groove profile

step 4: project the milled milling cutter again and check the tool wear

step 5: carry out metallographic examination on the processed tenon and groove. Check the deformation layer, pulling, peeling and cracks on the machined surface of the test block

2.3 milling allowance allocation

allocation principle: large allowance milling removes materials, medium allowance milling eliminates machining stress, and small allowance milling modifies the contour surface

considering that the contour of the working surface of the tenon groove type is required to be no more than 0.02mm. The designed number of cutting tools is 1 set of 5, 3 rough machining, 1 semi finishing and 1 finishing. The cutting tool material is solid cemented carbide, which is divided into straight shank cylindrical spiral milling cutter and straight shank profile milling cutter. The precision milling cutter directly ensures the final profile of the tenon and groove

2.4 milling steps of milling cutter

milling of milling cutter is divided into the following five steps, as shown in Figure 3

1) rough mill the upper end of the tenon and groove with a straight shank solid carbide rod milling cutter to remove most of the allowance at the upper end

2) rough mill the lower end of the tenon and groove with a straight shank solid carbide rod milling cutter, and remove most of the allowance at the lower end

3) rough milling the contour surface of tenon and groove with straight shank solid carbide forming rough milling cutter

4) semi finish milling the contour surface of tenon and groove with straight shank integral carbide forming semi finish milling cutter

5) finish milling with straight shank solid carbide forming finish milling cutter + modify the contour surface of tenon and groove

Figure 3 Schematic diagram of milling steps

2.5 determination of cutting parameters

according to the machining characteristics of martensitic stainless steel and combined with the previous experience of machining similar materials, determine reasonable machining parameters, and constantly verify, improve and correct in time when machining test blocks and test pieces. Refer to table 1 for the cutting parameters selected in the actual machining

2.6 test conclusion of test block

the projection result of tenon and groove after machining is consistent with that of finish milling cutter; Through metallographic examination, the deformation layer, pulling and peeling of the machined surface of the test block are qualified, and no cracks are found on the surface

2.7 solution to the problem of tool vibration

when machining the test plate, it is found that the tool vibration is serious, the milling cutter surface is broken, and the machined surface roughness is extremely poor

the reasons are analyzed in the following two aspects:

1) the diameter of the outer circle of the fixture positioning lip is larger, the length of the milling cutter hanging out of the shank is longer, and the strength of the milling cutter is reduced, causing the vibration of the cutter

2) the depth of the mortise and groove of the part is relatively deep, the cutting length is too long, and the allowance is too large, which intensifies the wear of the tool, increases the cutting resistance, and the strength at the minimum size of the tool surface is too poor, which has the hidden danger of tool breakage

in view of the reasons analyzed, the following measures are taken:

on the basis of the original fixture, reduce the size of the outer circle of the positioning lip, so as to achieve the purpose of reducing the hanging length of the tool handle and increasing the rigidity of the tool

in addition, in addition to the four milling cutters originally designed, a straight shank integral cemented carbide forming semi precision milling cutter is added, which divides the original finishing allowance into two parts, leaving only a small part of the allowance for the finishing cutter. This will help to reduce the deformation of parts, reduce the cutting force and tool wear, improve the machining quality of parts' tenon surface, and solve the problem of tool vibration

2.8 ensure the size and accuracy of the groove

the overall wettability of wood flour is good when processing the test block. The milled mortise is a straight groove, and there is no problem of cutting during processing. When machining the test plate, because the milled tenon is the same as the official part, there is a serious phenomenon of knife yield in the machining, and the roughness of the machined surface of the part is low

generally, the technological method of milling mortise and groove is to use milling cutter to form from top to next time. There is no problem milling straight grooves and grooves with low precision in this processing method, but for inclined grooves and grooves with high precision, it is difficult to guarantee the size and roughness of the milled mortise and groove. Because when milling is used to process the tenon and groove, the tenon and groove profile is guaranteed by the forming milling cutter at one time. During milling, the processing state of the left and right sides of the tenon and groove is different. There are problems on both sides of the tool, such as the rotation direction of the tool is opposite to the cutting direction, the stress direction of the processing surface is inconsistent, and the cutting heat and chips are not easy to be discharged, resulting in poor surface roughness of the parts and the appearance of groove processing traces, The roughness on both sides of the machined surface of the part is different

in addition, when processing the tenon groove, both the inlet and outlet are cut unilaterally, and the serious vibration of the knife will lead to the deviation of the angle of the tenon groove. The longer the length, the more serious the phenomenon of cutting

in view of these situations, pecking milling method is adopted when milling with forming milling cutter, as shown in Figure 4. That is, after milling 5mm, back off 1mm, and then continue milling 5mm, back off 1mm 3 Accurate gauge distance measurement and recording device cycle processing method. This can eliminate iron filings in time and reduce surface scratches; Effectively eliminate knife yielding; Reduce tool wear and improve tool life

Figure 4 Schematic diagram of pecking and milling method

3 conclusion

milling the fir tree shaped long tenon groove of the fan disc, through the use of appropriate technological measures and quality control means, the trial production task of the fan disc of a certain machine was finally completed, and valuable experience was accumulated for the processing of this kind of parts

the design requirements of fir tree tenon and groove can be ensured by adopting milling method and using straight shank integral cemented carbide forming fine milling cutter. Especially under the condition of single piece production with short product delivery cycle and insufficient production preparation time, the milling of fir tree tenon is an economical and feasible method. However, the process control requirements are high and the production efficiency is low, so it is not suitable for mass production

the profile of the forming milling cutter depends on the contour of the cutter to ensure the profile of the part, and the wear of the cutter should be considered in the design. The manufacturing requirements of integral cemented carbide forming milling cutter are high, and it is difficult to repair and grind after tool wear. From the perspective of actual machining, the shovel back structure should be added in the design of the tool, so as to increase the number of grinding times, microcomputer control and other experimental machines, and prolong the service life of the tool

References:

[1] Wang Xiankui. Machining process manual [m]. Beijing: Machinery Industry Press, 2007.

[2] Feng Dao. Practical manual of machining process and technical standards for mechanical parts [m]. Anhui culture audio visual publishing house, 2003. (end)

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