Processing of cylindrical gear parts with 40Cr straight teeth
This gear is a standard spur gear with modulus m = 3.5 mm, number of teeth z = 63, tooth angle a = 20 & ordm. Since it is a gear of an aircraft steam turbine, its processing accuracy is required to be high.
Forged blanks are chosen because steam turbine gears require a hard tooth surface and a sturdy tooth center. 40Cr is used.
(1) Main technical requirements
- 1) The accuracy level is set to the first acceptable group with 6 levels of accuracy and the detected item pitch cumulative error ΔFp. The second permissible group is level 5 accuracy, detection item tooth profile error Δff and base section deviation Δfpb. The third acceptable group is level 5 accuracy, which is the tooth orientation error ΔFβ for the detected item. The tooth thickness deviation Wk is measured by measuring the length of the normal. Tooth thickness deviation code M, tooth thickness deviation code P. (In the accuracy level display, the deviation of the tooth thickness limit is used to control the backlash, which is indicated by the code MP).
- 2) The reference depth of the blank reference plane is the accuracy of IT6. There is a requirement for circular runout of the end face with the inner hole shaft at both ends.
- 3) Surface roughness Ra The reference hole was 0.8 μm, both end faces were 1.6 μm, the tooth surface was 0.8 μm, and the tooth tip cylindrical surface was 3.2 μm.
(2) Forged blanks are selected to improve the mechanical properties of the material.
Free forging is used for low volume production and mold forging is used for mass production.
(3) The choice of the main surface treatment method for gears has a higher level of accuracy.
The main surface finishing methods are as follows.
- Reference hole: Grinding
- End face: Grinding
- Tooth surface: Hobbing-Surface quenching-Grinding
- General process for machining aircraft turbine spur gears:Document number for process card product type parts and components
Product name Gear part name Total page
Number Process Content Equipment Quantity Measurement Time Quota Remarks
- 1, cutting machine 1
- 2, rough car end face, inner hole and chamfered vertical car
- 3, blank inspection non-destructive flaw detector
- 4, rough car mouth, outer chamfer and surface lathe
- 5, heat treatment (tempering) box furnace
- 6, fine car inner holes and end face lathe
- 7. Excavation: Vertical excavation
- 8. Large end face grinding: surface grinding machine
- 9, Reaming: Drilling machine
- 10, pull key: slot broach tool
- 11. Intermediate inspection: caliper and angle ruler
- 12. Make a factory standard: Bench Bench
- 13, rough hobbing: hobbing
- 14. Fine hobbing: Hobbing
- 15. Tooth tip treatment: Milling machine
- 16. Cleaning: Cleaning machine
- 17. Mid-term test
- 18, heat treatment (surface hardening): box furnace
- 19. Finely grind inner holes: inner grinding machine
- 20, cleaning: cleaning machine
- 21. Mid-term test
- 22, Matching: Inspection machine
- 23, Grinding: Grinding machine
- 24, Cleaning: Cleaning machine
- 25, Matching: Inspection machine
- 26. Write the pairing number
- 27, Cleaning: Cleaning machine
- 28. Final test
Design calibration approval
The gearing process generally goes through the following steps: blanking heat treatment, tooth blanking, tooth profile processing, tooth edge machining, tooth surface heat treatment, precision benchmark correction and tooth profile finishing, etc.
The first stage of Cnc Machining is the first stage in which the blank is first machined. Since the transmission accuracy of the gear is mainly determined by the tooth profile accuracy and the uniformity of the tooth pitch distribution, this is directly related to the accuracy of the positioning reference (hole and end face) used when cutting the tooth. Therefore, this stage is mainly to prepare the detailed criteria for processing the tooth shape of the next stage, and the accuracy of the inner hole and the end face of the tooth basically meets the specified technical requirements. In addition to machining the benchmark at this stage, the treatment of small surfaces other than tooth contours should be done as late as possible at this stage.
The second step is the machining of the gear tooth shape.
For gears that do not require quenching, this stage is usually the final machining stage of the gear. After this stage, gears that fully meet the requirements of the drawing must be machined. For gears that require hardening, the precision of the tooth profile required to meet the final finish of the tooth profile must be machined at this stage, so machining at this stage is important to ensure the machining accuracy of the gear. It is a stage.
Particular attention is required.
Tooth tip treatment:
- A) Inverted circle
- B) Inverted tip
- C) Chamfer
The cnc machining method of the gear end of the gear is as follows. There are four methods: inverted circle, inverted point, chamfered edge, and deburring. Chamfering, chamfering, and gear after chamfering. It is easy to engage when moving along the axis. Chamfering the tooth surface is the most widely used. Figure 2 shows the chamfering principle of a finger milling cutter. When rounded, the gear rotates slowly and the finger cutter reciprocates linearly along the gear axis while rotating at high speed. Each time the gear rotates one tooth, the milling cutter reciprocates once and the two ends move relative to complete the rounding of the tooth ends. Before tooth quenching, it is necessary to adjust the treatment of the tooth edges.
The third stage of processing is the heat treatment stage. At this stage, the quenching process of the tooth surface is mainly performed to achieve the specific hardness requirements of the tooth surface.
The final stage of machining is the finishing stage of the tooth profile.
The purpose of this stage is to correct the deformation of the tooth profile caused by gear quenching, further improve the accuracy of the tooth profile, reduce the surface roughness and achieve the final accuracy requirement. At this stage, the positioning reference (holes and end faces) is trimmed first, and after quenching, the holes and end faces inside the gear are deformed. If the hole shape is directly finished using such a hole and the end face after quenching, it is difficult to achieve the accuracy of the gear. The toothed finish by positioning the trimmed reference plane ensures accurate and reliable positioning and a relatively uniform balance distribution to achieve the finishing objectives. It is usually necessary to prepare the tooth tip treatment after hobbing (inserting) the tooth and before quenching the tooth.
The base hole of the gear will be deformed after quenching. The base holes need to be modified to ensure the quality of the tooth profile.
Cylindrical hole gear modifications can be used for push holes or grinding holes, which are highly productive and are often used for uncured gears. Grinding accuracy is high, but productivity is low. For gears with high internal hardness and high hardness after quenching, or gears with large inner diameter and thin thickness, it is recommended to use grinding holes.
When grinding a hole, the gear index circle is usually centered, so the radial runout of the gearing after grinding the hole is small, which is beneficial for later grinding or honing. To increase productivity, some factories have achieved good results by replacing the grinding holes with diamond boring.
Keywords: gear parts processing, 40Cr gear, turbine gear, gear grinding, tooth edge processing, tooth edge inverted circle, gear quenching
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