The motor of the power tool has a pinion (composed of shaft insulation, silicon steel sheet, copper wire winding, fan, commutator, etc.), and the pinion plays an important role in the process of converting electrical energy into mechanical energy. At present, the processing of small modulus gears for electric tools usually adopts the following processes according to different grades of products: (1) Low-end product materials: 45 steel, 40Cr or 42CrMo; Process: hobbing teeth and bearing end high-frequency quenching (tooth Hardened) grinding bearing gear; advantages: simple process, low equipment and manufacturing costs. Disadvantages: low gear precision, high noise and poor impact resistance.
(2) Mid-end product material: 40Cr or 42CrMo; Process 1: hobbing tooth and bearing end high-power ultra-high frequency quenching (tooth), hardened layer 025-05mm; advantages: small deformation of teeth, there is a certain Impact resistance. Disadvantages: Quenching equipment is expensive and the process requirements are high. Process 2: The overall tempering is to the hard tooth hobbing in the 38-42HRC grinding bearing gear. Advantages: high precision of the teeth, no heat treatment deformation, and good comprehensive mechanical properties. Disadvantages: The hobbing tool is more expensive, the process cost is high, and the wear resistance is poor.
(3) High-end product materials: 20CrNiMo or 20CrMnT process: hobbing carburizing and quenching (tooth hardened layer thickness 04-08mm, leaving allowance 015-025mm) grinding center hole grinding bearing gear hard surface scraping. Advantages: high gear precision and best overall mechanical performance. Disadvantages: Hobbing tools are more expensive and the process cost is high.
Since the number of teeth of the motor tooth shaft is small, the modulus is small, the diameter is small, and the helix angle is large (generally greater than 15), in the high frequency quenching, not only the deformation of the tooth portion but also the center hole is deformed, so the high frequency quenching is performed. It is difficult to ensure the coaxiality of the bearing gear and the teeth in the future. At the same time, due to the metallographic transformation during quenching (the volume of martensite is large), heat treatment deformation is unavoidable, which causes the accuracy of the gear to decrease. The noise and life of the finished product are also affected to varying degrees. In addition, due to the high equipment cost, equipment status and manufacturing cost of domestic suppliers, especially the stability of high-frequency quenching heat treatment equipment and processes is difficult to guarantee, since 1999, in order to reduce the noise and life of the products, Our company basically uses the latter two methods.
This paper analyzes the process and characteristics of these two methods with examples. The hobbing of the medium hard tooth surface is exemplified by an electric drill. The material of the gear parts in the electric drill is 40Cr or 42CrMo. The heat treatment process is usually: quenching and tempering HRC39-42. The hobbing equipment is generally made by ordinary hobbing machine (such as Ningjiang Machine Tool Factory), and the manufacturer does not need to increase equipment investment. The tool for machining gears is made of cobalt-tungsten carbide and its domestic grade is YG8 (ISO standard is K20 or K30).
Due to the development of material technology, the current foreign tool manufacturers have adopted ultra-fine grain materials (grain diameter less than 05m). Such tools have good wear resistance and high impact resistance, and the tool durability is greatly improved; The tool uses a TiN coating with a thickness of about 2.5m, which greatly improves the wear resistance of the tool.
The tool uses a 0 front angle for easy re-grinding. The power tool product requires 7-stage gear to drive, so it is better to use AA-level hob. If the power tool product requires 8-stage gears, the A-stage hob can meet the processing requirements.
The gear processing route is: fine car tempering treatment grinding center hole grinding hobbing. The process analysis is as follows: (1) Since both the grinding and the hobbing are centered, the machining reference has no cumulative error, and the hobbing is the finished product, which fundamentally avoids the deformation caused by the heat treatment. (2) Since the motor shaft is the first-stage transmission, although the rotation speed is high, the torque torsion is not large, and the general modulus is about 0.5-0.8 mm, and one-time hobbing can be used. (3) Considering that the rigidity of the entire cutting system includes not only the rigidity of the machine tool and the tool but also the rigidity of the workpiece, when the shaft diameter is small, the V-shaped auxiliary support needs to be added to increase the rigidity of the workpiece. This is of great help in ensuring the quality of the hobbing and increasing the service life of the tool.
(4) The axial runout and radial runout of the toolholder should be strictly controlled within 0.01mm.
(5) Ensure adequate lubrication throughout the cutting process.
(6) Determine the reasonable boring cycle by process test to avoid the tool's service life and processing quality affected by excessive wear.
(7) When the tool is recoated, the correct deplating process should be selected. The general chemical deplating will cause the loss of cobalt, which makes the cemented carbide very brittle and greatly affects the durability of the tool.
(8) If the tool is magnetized after regrind, it must be demagnetized.
For the company's gear map produced by the medium hard tooth surface hobbing process, the test report is as shown.
Gear map gear test report produced by hard tooth surface hobbing process 2.2 Hard tooth surface scraping process Take the cylindrical gear of cutting machine as an example. The gear part material in the cutting machine is 20CrNiMo, and the heat treatment technology usually used is carburizing or carbonitriding. The processing route is: hard car carburizing or carbonitriding grinding center hole grinding bearing hard tooth surface scraping.
It can be machined by ordinary machine when rough rolling, and HAMAIN60 machine can be used for hard surface scraping.
Since the processing characteristic of the hard tooth scraping is that only the tooth-shaped portion of the gear is micro-cut after hardening, the top edge portion of the hob which is easy to wear does not participate in the cutting, and only the side cutting edge participates in the cutting. Therefore, in order to avoid the top edge of the carbide hob participating in the cutting during scraping, the coarse hob must be cut to the full tooth depth and subjected to an appropriate amount of undercut treatment. The coarse hob has a lobed angle and increases the tooth height of the hob by 01-02m (as shown). The common normal line of the coarse roll generally reserves a margin of 015-03mm.
Schematic diagram of coarse hob with lobes: choice of tool material: TiN coated high speed steel hob with a hob accuracy of Class A; hard alloy as material, domestic grade YG8 (ISO standard K20 or K30) At the same time, the tool is coated with a TiAlN coating of about 2.5m thick, and a zero rake angle is also used in the small modulus hob to facilitate re-grinding of the tool with an accuracy of AA. The process analysis is as follows: (1) The coarse roll mainly selects a special tool to avoid the top edge of the hob when scraping, and the other requirements are the same as ordinary hobs.
(2) Since the thin layer is cut from the tooth table with high hardness during scraping, the number of teeth meshed by the left and right edges during cutting changes, resulting in inconsistent cutting amounts of the left and right cutting edges. It causes periodic fluctuations in the axial cutting force, so use a machine with high precision and rigidity for scraping (recommended HAMAIN60 machine).
(3) In order to ensure the rigidity of the whole cutting system, the front end bearing gear must be used for positioning and clamping, so that the cantilever of the workpiece is reduced as much as possible to increase the rigidity.
(4) The axial and radial runout of the scraper blade must be strictly controlled within 0.005 mm.
(5) Because dry cutting is used, it is necessary to determine a reasonable boring cycle by process test to avoid excessive wear and even chipping of expensive tools.
(6) Select the correct deplating process when the tool is recoated, otherwise it will have a very serious impact on the durability of the tool.
Since the power tool is small in size and extremely high in speed, the gear ratio is large and the number of gear teeth of the driving gear is small. In order to increase the coincidence coefficient and improve the smoothness and load carrying capacity of the transmission, a helix angle greater than 15 is often used. To improve its noise and axial contact area at high rotational speeds, toothed and toothed trims are typically used.
The toothed trim is often in the form of a double pressure angle with the hob profile as shown. The trimming hob reduces the tooth tip of the gear by about 002 mm compared with the conventional one, which avoids the interference that is easy to occur at high speed, and thus greatly improves the noise.
The tooth direction trimming is mainly used for the electric hammer motor shaft. It mainly relies on the machine tool holder to generate displacement in the radial direction while feeding the tool, so that the tooth thickness of the machining gear is variable in the axial direction (both thin and medium thickness is about 002mm) Therefore, the meshing area of ​​the tooth direction is ensured to fall in the middle of the effective tooth length, so as to improve the bearing capacity of the tooth.
Profile of the toothed edge hob Cutting power tools are generally larger (generally greater than 1500W), so their large gears are often machined to ensure sufficient strength. The large gear processing route is the inner hole of the finishing hobbing and quenching mill. The material of the large gear is generally quenched by 40Cr or 42CrMo. Since the inner hole is deformed after quenching, in order to ensure accuracy, the inner hole needs to be ground. Since the top cutting process is adopted for hobbing, the tooth shape and the tooth top garden are cut out at the same time, which ensures the concentricity of the tooth shape and the tooth top. Therefore, it is only necessary to position and clamp the tooth top after grinding and grind the inner hole. The tooth jump is ensured, and the grinding of the inner hole by the toothed positioning is avoided (the positioning fixture of this process is very troublesome to manufacture), which greatly simplifies the process and reduces the manufacturing cost.
Since the wall of the disc-shaped tooth is thick, the amount of deformation during heat treatment is small, generally no hard-tooth scraping is required (except for special precision requirements), and the power of the drill-type tool is small, and the powder metallurgy gear is often used to reduce the cost. No cutting is required.
Measurement and Control of Gears There are two methods for measuring gears for power tools: one is to use a gear comprehensive tester (the manufacturer has Tokyo Precision, Harmony, etc.). The instrument includes almost all gear measurement requirements, and can measure the error items such as the circumference, the base joint, the cumulative runout, and the tooth profile. Because the measurement project is complete and targeted in the process analysis, it can be effectively comprehensive. QC. The disadvantage is that the test is slower.
The second is the gear meshing instrument. The instrument measures the radial integrated error and the radial cumulative integrated error of a tooth by meshing with a measured gear through a standard gear. The instrument has high measurement efficiency and is often used for on-site rapid inspection. This method can also be required for full inspection by manufacturers.
Conclusion Gear transmission plays an important role in the transmission system of power tools. The use of hard-toothed gears can improve their load-bearing capacity and achieve miniaturization. Therefore, it is of practical significance to study the processing methods and processes of small-modulus gears for power tools.
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