Urea gear pump plastic gear precision cutting fixture design

Starting from the analysis of urea gear pump gear processing technology, based on the principle of precision cutting, designed a set of gear shaping cutting fixture, and introduced the work process of the fixture. Detailed description of the gear precision cutting fixture positioning, cutting and other structural design, and analyze the main components of the fixture. Fixture by the use of cutting gear surface flat, smooth, burr-free, no cracking, fixture service life is high.

Urea pump gear introduction and processing technology

Urea gear pump for external gear pump, he is relying on the active gear driven by the rotation of the driven gear inlet and outlet volume changes to achieve the suction and discharge of aqueous urea solution of hydraulic pumps, as shown in Figure 1.

Gears are the core components of external gear pumps, gear materials usually need to choose good wear resistance, high deformation resistance, commonly used materials are steel, cast iron and non-metallic materials, etc., to meet the application of traditional gear pumps. But urea pumps and traditional gear pumps are different, because the urea aqueous solution is corrosive, so the urea pump gears require corrosion resistance. Compared with metal gears, plastic gears have the advantages of low cost, light weight, corrosion resistance, good self-lubrication, low transmission noise.

Therefore, this paper gear selection PEEK (polyether ether ketone) plastic, PEEK is a semi-crystalline polymer, not only has a high temperature resistance, wear resistance, chemical corrosion resistance and comprehensive mechanical properties and other characteristics, but also has a low water absorption, high toughness and impact resistance.

Plastic gear manufacturing process is divided into non-cutting and cutting processing.

(1) non-cutting processing, such as injection molding, molding, etc., applicable to large quantities, low dimensional accuracy of plastic gears.

(2) cutting, usually by the plastic gear by turning, milling, gear insertion or hobbing and other processing, to achieve the dimensional accuracy and surface roughness requirements, but the efficiency is low.

Urea pump gear shown in Figure 2, gear relative to the shaft of the circular runout of 0.03, gear surface roughness of Ra0.8, high precision requirements; in order to meet the precision requirements of the plastic gear, this paper combines the advantages and disadvantages of non-cutting machining and cutting, design a set of plastic gear precision cutting fixture.

Plastic cutting principle and cutting fixture structure

Principle

Plastic cutting process refers to the cutting motion, so that the tool from the workpiece to remove excess plastic layer, the formation of chips and the process of the process of the machined surface, plastic cutting process is extrusion process. As shown in Figure 3, when the use of front angle γ0 and rear angle α0 cutter cutting plastic workpiece, the pressure of the plastic began to produce elastic deformation, followed by plastic deformation; when the pressure reaches the shear strength of the plastic, it is along the slip surface OA in the OMA area, the cutter continues to move, prompting the deformation of plastic near the neighboring blade, until the edge of the blade fracture, the formation of chips. The plastic gear processing in this paper belongs to the molding method of processing gears, the cutting tool is machined into the shape of a gear, and the blank plastic gear is cut and molded in one precision cutting.

Structure

The plastic gear precision cutting fixture can be used for small punch cutting, cutting fixture structure shown in Figure 4, the workpiece to be processed clamped under the lower pressure slider and the limit block between the positioning, through the cutting block up and down the sliding of the workpiece for cutting.

For the processing of high precision requirements of plastic parts, roughing and finishing should be carried out separately. That is, rough milling according to the principle of efficiency first, as far as possible in the permissible range of deformation to choose the processing parameters that can improve productivity, but must ensure that leave 0.1 mm or so of the machining allowance for finishing. In this paper, before precision cutting of gears, is the first through the mold injection molded into a gear blank, gear shaft as an insert was injected together, the overall external dimensions of the blank gear than the drawings of a single side larger than 0.1 mm. in the cutting of gears to be positioned first, due to the gear pump gears with the rotation of the gear shaft, in order to ensure that the gear shape and positional tolerance of the precision requirements to gear shafts as a reference point for positioning. Positioning process as follows.

(1)As shown in Figure 5, the positioning plate and limit blocks processed into gear-shaped, both clearance fit, the gear shaft end of the first into the limit block of the bore, half of the gear is embedded in the positioning plate gear groove.

(2)As shown in Fig. 6, the machine is pressed down, the other end of the gear shaft is inserted into the bore of the lower pressure slider, and at the same time, the lower pressure slider pushes against the gear under the action of the spring to carry out secondary positioning.

(1)After the workpiece positioning for cutting, cutting process as shown in the following steps: 1.

As shown in Fig. 7, the cutting block starts to cut the gear as the machine is pressed down.

(2)As shown in Figure 8, the machine tool continues to press down, driven by the positioning plate down, limit blocks and gears together with the top of the cutting block into the cutting hole, then the cutting is completed, cutting waste falls into the chip recycling tank.

The design of the main working parts of the fixture

Cutting block

Cutting block structure shown in Figure 9, gear processing accuracy is mainly determined by the cutting block, if the cutting edge wear, or gear hole inner wall roughness, etc., will make the gear workpiece surface roughness, and even cause gear cracking, so the cutting block is required to have a high hardness and roughness of the cutting surface is low; at the same time, the hardness of thermoplastic material is low, the need for the cutting force is small, so it should be easy to choose to grind out the sharp cutting edge of tool material, such as after the cutting edge of the tool material, the cutting edge of the tool material, such as after the cutting hole, the cutting edge of the tool material. Cutting tool materials, such as hardened carbon tool steel, high-speed steel, alloy tool steel and tungsten-cobalt carbide, need for precision machining can be selected diamond turning tool. The cutting block of this fixture is made of tungsten steel (alloy tool steel).

As the cutting block in the downward pressure slider and limit block sliding, along the gear a whole circle to do cutting, so the cutting block after the angle α0 = 0 is more appropriate. Cutting block front angle γ0 size directly affects the cutting effect, the size of the choice should take into account the type of plastic, tool materials and processing nature of the three factors. The size of the front angle is mainly to solve the cutter head of the solidity and sharpness of the contradiction. Therefore, first of all, according to the hardness of the processing material to choose the front angle. The hardness of the material to be processed is high, the front angle takes a small value, and vice versa takes a large value. Secondly, according to the nature of the process to consider the size of the front angle, roughing the front angle to take a small value, finishing the front angle should take a large value. Front angle is generally selected between -5 ° ~ 25 °. In this paper, in order to ensure the accuracy of the gear and the strength of the cutting block, choose the cutting block front angle γ0 = 0.

Cutting block of the gear hole with a slow walking wire processing, processing surface roughness range between Ra0.2-0.3μm, shape and size according to the requirements of the workpiece drawings; in addition to the cutting block cutting plane can be used in the process of grinding machine finishing, machining surface roughness can also reach Ra0.2-0.3μm; by the above processing methods for processing of the cutting block, the cutting edge is sharper; cutting block cutting the surface of the workpiece is The surface of the workpiece cut by the cutting block is flat and smooth.

Downward sliding block and limit block

Downward pressure slider and positioning block shown in Figure 10, mainly for positioning purposes is to locate the workpiece to match the cutting block cutting, due to withstand extrusion and impact, so there is a certain degree of hardness and abrasion resistance, can be selected from Cr12MoV manufacturing, quenching hardness HRC58-62; downward slider and limit the block with quenched Cr12MoV for the slow-feeding wire processing, machined into a gear-shaped, and the base of the interference fit, and then welded firmly and ground weld points, and then welded firmly. Then weld firmly and grind the welding point. Cutting, gear shaft ends were put into the limit block and the lower pressure slider bore, the size of the gear shaft is Φ4.5, in order to ensure the concentricity of the gear relative to the gear shaft, the size of the bore than the gear shaft on one side of the larger 0.05mm; in order to facilitate the placement of the gear and the gear shaft into the lower pressure slider, limit block and lower down the block of the bore port to do the beveled chamfer of 0.1mm × 0.5mm.

Precision cutting clearance

As shown in Figure 11, the limit block holds up the blank gear, the cutting block slides from top to bottom, the limit block needs to penetrate into the cutting block for complete cutting, if the limit block gear-shaped external dimensions are too small, cutting will cause the plastic gear collapse angle, cracking; from the references, it can be seen that the stamping and cutting double-sided clearance of the convex-concave die should be less than 0.01mm, but from the cutting fixture manufacturing and assembly precision and service life considerations, the requirements are too high. According to the production practice, it can be seen. According to production practice can be known, as long as the cutting block and limit the block between the single side of the side clearance does not exceed 0.02mm can ensure the accuracy of the gear and the workpiece is not collapsed, no cracking.

As shown in Figure 12, the cutting process, the cutting block in the lower pressure slider for sliding, cutting is completed, the lower pressure slider will be cutting block gear holes in the workpiece for the top out of the dislodgement, in order to ensure that the cutting block and the lower pressure slider sliding smoothly between the two, the gap between the two is about a single side of 0.1mm; Similarly, as shown in Figure 9, in order to ensure that the limiting block and the positioning plate between the sliding smoothly between the two gaps for the same single side of 0.1mm. 0.1mm.

Conclusion

This paper uses the principle of plastic cutting, combined with the characteristics of plastic gears, designed a pair of plastic gear precision cutting fixture. The gear has been mass production, and since the use of the fixture, the cutting gear surface is smooth and no collapse, no cracking, and the round runout with the gear is less than 0.03mm, to meet the technical requirements of urea gear pumps. Production results show that the precision cutting fixture structure principle is correct, parts design and material selection is reasonable, fixture using high stability and high efficiency.