Due to high temperature, high pressure, and high-speed rotation conditions required for automotive motors in actual operation, to prevent damage to core components such as spindle and blades caused by enormous centrifugal force resulting from equipment vibration and dynamic imbalance, as well as thermal expansion and contraction effect caused by temperature cycles, machining process must not only accurately ensure consistency of key dimensional parameters, but also control dimensional tolerances such as coaxiality and roundness to meet technical requirements for high-reliability operation of motor.
Shape and dimensions of die-cast aluminum alloy motor housing studied are shown in Figure 1. Its forming requires 11 processes: ① Die casting → ② Sawing gate solidified material → ③ Heat treatment → ④ Grinding → ⑤ First CNC turning (hereinafter referred to as "CNC Turning 1") → ⑥ First CNC milling (hereinafter referred to as "CNC Milling 1") → ⑦ Shot blasting → ⑧ Oil spraying → ⑨ Second CNC turning (hereinafter referred to as "CNC Turning 2") → ⑩ Second CNC milling (hereinafter referred to as "Milling Turning 1") → ⑪ Packaging. Following focuses on solving processing difficulties of four processes of CNC turning one (process ⑤), CNC milling one (process ⑥), CNC turning two (process ⑨), and CNC milling two (process ⑩) of parts.

 

Figure 1 Motor housing
Before machining, non-fitting parts of parts are pre-treated by die casting, sawing gate solidified material, heat treatment to relieve stress, and deburring. Inner cavity fitting holes, external positioning planes and side holes are all reserved with a machining allowance of 0.5 mm. Subsequent machining needs to complete machining of 83 dimensions. Surface of parts includes die casting features such as conical surfaces, reinforcing ribs, cylindrical surfaces with different outer diameters and flange edges. During machining process, it is necessary to avoid collisions and clamping. Assembly holes of each step in inner cavity need to be processed by multiple processes such as rough turning, fine turning and fine milling. Coaxiality of inner hole is required to be ≤0.02 mm. Requirements for machining dimension accuracy, coaxiality, roundness and surface roughness are high. For parts with multiple side holes and assembly holes and significant differences in hole characteristics, there are two core requirements during processing: ① Process engineers need to plan a reasonable processing route based on actual equipment configuration in workshop; ② Operators need to have a high level of professional skills to ensure stability of part processing process and ensure that final product meets quality standards.

After pre-treatment motor housing is machined, it still needs to be cleaned, shot blasted, and sprayed with oil. Machining process needs to be completed before above processes, then finishing process is carried out to meet design size requirements. During processing, dimensional errors caused by multiple clamping and shot blasting and oil spraying processes need to be considered, and damage to oiled surface during clamping process needs to be avoided. Main processing difficulties of such parts are as follows.

A total of 83 dimensions of part require machining, including holes, threads, fillets, chamfers, lengths, etc., as shown in Figure 1. Four high-precision fitting dimensions require close control: Inner hole one image: coaxiality tolerance is 0.03 mm, hole-shaft positioning datum B; Inner hole two image: coaxiality tolerance is 0.05 mm; Inner hole three image: roundness tolerance is 0.01 mm; Inner hole four image: coaxiality tolerance is 0.03 mm, hole-shaft positioning datum A. During die casting, inner cavity has a pre-reserved thickness of 0.5 mm. Before machining, it undergoes four processes: die casting (process ①), sawing gate solidified material (process ②), heat treatment (process ③), and grinding (process ④). After machining, it undergoes two processes: shot blasting (process ⑦) and oil spraying (process ⑧). CNC milling process requires positioning using inner hole, while shot blasting and oil spraying processes affect machining dimensions. To ensure dimensional accuracy, finishing process must be performed after above processes, and clamping process must avoid damaging already oiled surface.

Bottom surface of part has 7 M5*15 mm threaded holes; top surface has 2 M4*10 mm threaded holes; front surface has 7 threaded holes: 4 M4*12 mm, 2 M4*10 mm, and 1 M6*10 mm; back surface has 2 M8 through threaded holes. As mating holes, aforementioned threaded holes have requirements for perpendicularity and position. Part already has pre-drilled holes, machining process requires hole enlargement and thread machining. Multi-faceted machining must be completed in a single clamping operation, clamping process must avoid damaging already oiled surface and internal cavities.

Four machining processes involve six forming and surface treatment processes, requiring repeated clamping and positioning. This multi-process, interconnected machining mode poses a challenge to positioning accuracy control of motor housing. From a process logic perspective, before CNC turning one, there are four processes: die casting, sawing gate solidification material, heat treatment, and grinding. CNC turning one and CNC milling one are continuous machining processes. After CNC milling one, there are two surface treatment processes: shot blasting and oil spraying, before proceeding to CNC turning two and CNC milling two. Each process requires a new positioning datum due to different clamping positions, and continuity of datum transfer is easily disrupted. There is a risk of error accumulation during datum conversion of four machining processes. In this process, each datum transformation introduces positioning errors, errors from previous processes are passed down and superimposed on subsequent processes through datum. Therefore, while meeting positioning requirements, a unified machining datum is needed throughout the entire machining process.

Processes ⑤ (CNC Turning 1) and ⑨ (CNC Turning 2) require a single clamping operation to complete turning of 62 dimensions, including internal holes, fillets, depths, and end faces. Since intermediate processes affect final dimensions, same fixture is used for transitional dimensions in CNC Turning 1 and for final dimensions in CNC Turning 2. Taking image of internal hole 1 as an example, die-casting dimension in process ① is blank size image, process ⑤ (CNC Turning 1) is machined to transitional size image, and process ⑨ (CNC Turning 2) is machined to final size image, gradually expanding machining to design requirements.
Based on requirements for coaxiality of inner hole and clamping force analysis of thin-walled parts, and considering characteristics of CNC lathes, fixture shown in Figure 2 was designed. This fixture's elastic clamping structure uses three planes of motor housing part for positioning. Positioning is achieved through cooperation with fixture body, tightening and loosening are achieved through push-pull action of a cylinder. After part is locked, inner hole image shown in Figure 1 is used as machining datum A, and inner hole image shown in Figure 1 is used as machining datum B. These two machining datums serve as a common datum. Coaxiality error of inner hole turning center line of same fixture relative to datums A-B is within 0.03 mm. Subsequent machining processes use machining datums A and B from this process as machining datums to reduce error propagation. Simultaneously, inter-process detection feedback and compensation algorithms are used to dynamically correct machining parameters, controlling cumulative errors within tolerance zone to ensure that final assembly accuracy meets requirements for efficient and stable motor operation.

 

Figure 2 CNC Lathe Fixture for Processes ⑤ and ⑨
1. Motor Housing Part 2. Elastic Sleeve 3. Fixture Body 4. Transition Plate 5. Push Rod 6. Socket Head Screw 7. Ejector Block 8. Socket Head Screw
As shown in Figure 2, this lathe fixture mainly consists of four parts: spring sleeve 2, fixture body 3, transition plate 4, and ejector block 7. Parallel heat dissipation fins on the side of motor housing part are fitted with oblique side of elastic sleeve 2, as shown in Figure 3, eliminating four degrees of freedom in XY directions. Outer stepped plane of heat dissipation fins is fitted with upper plane of elastic sleeve 2, eliminating Z-direction degree of freedom. A 2 mm elastic gap is reserved on elastic sleeve 2 using wire cutting. Elastic sleeve 2 and fixture body 3 cooperate to press part along axis, causing elastic sleeve 2 and 30° conical surface of fixture body 3 to fit together. With application of axial force, convex locking block of elastic sleeve 2 fits with groove of heat dissipation fin of part 1, part reaches positioning and locking state. During clamping, operator holds part in hand, presses it evenly, and makes left end face fully contact step surface of fixture. After ensuring that part is parallel to fixture, it is inserted, pressed and locked. After clamping, center line of fixture and lathe are on same axis, and coaxiality is ≤0.03 mm. Push rod 5 is connected to machine tool cylinder. After part is turned, pneumatic foot switch is pressed to start cylinder. Push rod 5 drives top block 7 to push fixture body 3, so that part is released. After part is taken out, above steps are repeated to complete batch processing of inner hole of part. During processing, an inner micrometer, a height gauge and a full inspection bearing precision hole gauge are used to check to ensure dimensional accuracy of part and to achieve effective control of processing quality. On-site processing and inspection are shown in Figure 4.

 

Figure 3 Part clamping

 

Figure 4 On-site machining and inspection

Process ⑥ mainly processes machining plane shown in Figure 5. Front plane is assembly plane of motor, and height error requirement with shaft core is ±0.05 mm, serving as reference plane for process ⑩ CNC milling 2; two rear small planes are correction planes for process ⑩ CNC milling 2. This fixture is designed to meet needs of mass production.

 

Figure 5 CNC milling fixture for process ⑥
1. Motor housing part 2. Clamping block 3. Positioning sleeve 4. Locking screw 5. Socket head cap screw 6. T-block 7. Worktable
As shown in Figure 6, positioning sleeve 3 is designed with two right-angled inclined surfaces diagonally according to shape of part, eliminating four degrees of freedom in XY direction. Outer step plane fits to eliminate degree of freedom in Z direction. After part is pressed in, positioning is achieved. Clamping block 2 cooperates with reference hole in image, and motor housing part is locked by locking screw 4. Positioning sleeve 3 is locked to worktable 7 by hexagonal screws 5 and T-block 6. Part 1 is locked to positioning sleeve 3 by clamping block 2 and locking screw 4. During clamping, press part evenly by hand to ensure that positioning bevel and outer step end face are in complete contact with fixture positioning surface. Place clamping block and locking screw, and tighten with a fixed torque electric screwdriver. Then start program to machine side plane of part.

 

Figure 6. Clamping for Operation ⑥
1. Motor housing part 2. Clamping block 3. Positioning sleeve 4. Locking screw

Based on analysis of machining difficulty 2, operation ⑩ requires machining 7 M5*15 mm threaded holes on bottom surface, 2 M4*10 mm threaded holes on top surface, 4 M4*12 mm, 2 M4*10 mm, and 1 M6*10 mm (7 in total) threaded holes on front surface, and 2 M8 through threaded holes on back surface, totaling 18 threaded holes on 4 surfaces. To complete above machining operations in a single setup, a multi-axis machining center is required. Considering company's actual equipment, a four-axis machining center was selected, and fixture shown in Figure 5 was designed. The entire fixture is mounted on a four-axis rotary table, and machining of four angular surfaces is completed by rotating table, as shown in Figure 7.

 

Figure 7 Process ⑩ CNC Milling Fixture
1. Upper Pressure Block 2. Motor Housing Part 3. Lower Positioning Block 4. Locking Screw 5. Correction Block 6. Base Plate 7. Support Block 8. Plate Bridge
As shown in Figure 7, lower positioning block 3 is locked to base plate 6 by pins and screws. Part is positioned by engaging lower positioning block 3 through holes already machined in previous process, eliminating four degrees of freedom in XY directions. Upper pressure block 2 engages with holes in part image. Correction block 5 is flush with product's correction plane, eliminating Z-rotational degree of freedom. Locking screw 4 clamps part, eliminating Z-direction degree of freedom, thus meeting part's machining requirements. During clamping, part is pre-fixed using locking screw 4, allowing space for rotational adjustment around Z-axis. Left hand pushes correction block 5 to align it with correction plane of part, correcting X-axis direction and maintaining pressure. Right hand uses a torque wrench to turn locking screw 4 to secure part, completing clamping process. Through rotation of four-axis rotary table, enlargement and threading of 18 threaded holes in four directions are completed, enabling batch processing of multi-directional holes on part.