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What errors may occur during CNC machining and CNC machining​?

 CNC machining and numerical control machining are a method to control the movement of machine tools and the production process of machining through computer digitization and information digitization. It is an intelligent numerical control device developed as an economical, high-speed, reliable, multi-functional, intelligent, and open structure. CNC machining is also an important indicator that can measure the continuous level and comprehensive ability of a safety technology, as well as the degree of modernization of related science and technology capabilities, especially in aviation, biology, medical and other high-tech cultural industries, and it is also a powerful indicator. reflect. So, what errors may occur during CNC machining and CNC machining? Let us understand together:   The use of approximate machining motion or approximate tool contours causes errors in the CNC principle of machining. The reason why it is called machining principle error is because of the error in machining

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Multipurpose axis CNC lathe process design

Abstract: We designed a multi-axis type shaft component. The shaft has an arc, a groove for machining, a screw undercut, a screw, a taper screw, an inner hole, and an inner arc. The material is 45 steel. This multi-axis design uses the “coarse turning” lathe method. That is, the two end faces, the outer circle, the screw, the outer cone, the groove, the arc and the inner diameter are divided into seven steps of roughing and finishing, respectively.

This design is mainly related to the analysis of Cnc Machining processes and the Machining of specific part drawings. First, the NC machining technology is briefly introduced, and NC machining analysis is performed according to the parts drawing.

  • First, the amount of cutting and other related factors are used to determine the need for five tools based on machining process, part material, tool, handle, and part profile characteristics.
  • Next, create a part graphic. The parts are multi-axis and the outer shell consists of straight lines, undercuts, arcs and threads. A tapered hole and an inner arc are drilled inside the left edge of the part. During the machining process, the work piece needs to be drilled and then rotated into an inner arc.
  • Third, when setting up a drilling tool, you first need to refer to the reference point. Correct the position of the tool, match the position of the tool with the replacement point of the tool, determine the programmed coordinate system and the origin of programming, and program the NC machining program. Finally, we used programming simulation software to simulate and process the axis components.

Keywords: numerical control programming, machining plan, cutting amount, machining program

Analysis of parts Machining technology

1.1 Analysis of part drawing Machining

Part structure analysis:

As can be seen from the figure, the component consists of a cylindrical surface, an inner hole, an inner tapered surface, an arc surface, a groove, a screw, and the like. The contours of the turning parts of the part are more complex and require double-edged machining. The Machining accuracy and surface quality of the parts are very high. An important radial machined part of the part is the spiral 54-0.0390mm cylindrical part (surface roughness R pump = 1.6μm). R45 connects the arc, Ф40-0.0330 cylindrical section, Ф22 + 0.021 0 and the inner hole radius of R25. These parts meet the CNC Machined size requirements and the contour description is clear and complete. The part material is No. 45 steel, and the blank is φ60mm * 110mm.

1.2 Part Technical Requirements Analysis

When programming in small batch production conditions, it is not possible to modify the plane with abrasive cloths and ladders. This is a requirement for precision aircraft. According to GB1804-M, without specified tolerance, heat treatment, quenching and tempering, HRC25-35 does not mean rough part, surface roughness is Ra6.3, embryo size is mm60mm * 110mm.

1.3 Part blanks and material analysis

(1) Material analysis

When machining shaft parts, the cutting force between the tool and the workpiece is large. The workability, strength, hardness, plasticity, cold cutting and mechanical properties of the workpiece material are all related to the workpiece material. Therefore, 45 steel was selected as the shaft material. The chemical composition of 45 steel is C0.42% to 0.50%, Si0.17% to 0.37%, Mn0.50 to 0.80%, P≤0.035%, S≤0.035%, Cr≤0.25%, N≤0.25%. contains. Cu ≤ 0.25%. Cold working 45 steel hardness, hot rolled steel, indentation diameter 3.9 or more, Brinell hardness 241HB or more, annealed steel indentation diameter 4.4 or more, Brinell hardness 187HB or more .. Mechanical properties of 45 steel: δs ≧ 35Mpa, δb ≧ 600Mpa, ≧ ≧ 40%, Ak ≧ 47J. Relative machining of 45 steel, which is an economical and rational machining requirement for carbide tool 1.0, high speed steel tool 1.0, 45 steel, is also rational, 45 steel is mainly steam turbines, compressors, pump parts, gears. Widely used in the manufacture of shaft piston pins and other parts. Based on the above data, it is suitable for machining this shaft.

(2) Blank analysis

Blanks for shaft components include bars, forgings and castings. Forged products: Suitable for high-strength and simple-shaped parts. Large parts are generally die-cast forged due to equipment restrictions. Optional mold forging for small and medium-sized parts; rigid parts with complex shapes are not suitable for free forging and are suitable for blanks with complex shapes. Forged blanks for steel parts have higher mechanical properties than steel and cast steel. Forging is used according to the structural shape and external dimensions of the shaft. Forged parts blanks, forged, bar saws, forged billets Φ60X110mm, forged steel rods to obtain a uniform fiber structure, their mechanical properties need to be improved, the proportion of parts and billets increased, and material consumption reduced.

2.4 Selection of component Machining equipment

2.4.1 Machine type

CNC lathes can automatically complete machining of inner and outer cylindrical surfaces, conical surfaces, circular surfaces, etc.

Rotating parts such as shafts and discs can perform operations such as grooving, drilling, and expansion. An economical CNC lathe can be selected according to the technical requirements of the part. It is a semi-closed loop servo system, usually in the form of a stepper motor. These lathes are easy to build and relatively inexpensive. This type of lathe is equipped with a 3-axis automatic centering chuck, a common tailstock, or a CNC hydraulic tailstock for rotating long shaft parts. Horizontal CNC lathes are selected according to the requirements of the spindle configuration. High precision CNC lathes, parts dimensional precision workability requirements can be automatically made by manual linear and arc interpolation, rigid CNC lathes, high manufacturing precision and precision toolset, manual and accurate. You can compensate. For rotating bodies whose contour shape is difficult to control, it is possible to handle surfaces and sizes that are particularly difficult. This allows the tapered conical surface and the inner and outer cylindrical screws to be easily rotated in order to maintain machining accuracy and increase production efficiency. Therefore, it is very advantageous for Machining.

Selection of functional and economical type of some waste, selection SSCK20 / 500 CNC lathes are produced according to the selection such as swivel structure, size and precision multi-axis member.

2.4.2 Machine accuracy

In addition to hardened steels with machining accuracy of IT7 to IT8 and Ra 0.8 to Ra 1.6 μm, common metals can be completed in two steps: roughing and finishing. Machining accuracy is IT5 to IT6 grade, Ra0.2 to Ra0.63 μm In addition to general hardened metal steel, precision CNC lathes can also be used. Roughing → Semi-finishing → Finishing → Finishing. Machining accuracy is higher than general metals other than hardened steel such as IT5 and Ra <0.08 μm, and precision CNC lathes capable of roughing, semi-finishing, finishing and precision turning can be used. For hard-to-rotate materials, such as hardened steel, roughing and semi-finishing methods can be used prior to quenching and grinding is performed after quenching. Therefore, according to the requirements of the parts drawing, the accuracy of SSCK20 / 500 CNC lathe is suitable for multi-axis accuracy.

Position the workpiece and jig

2.5.1 Determination of clamp method

A 3-jaw automatic centering chuck is used to clamp the rough outer circle of the part and determine the appropriate length of the part extension (taking into account the extreme distance of the machine). Since both ends of the part need to be machined, two clamp positions need to be considered, considering that a 54mm x 15mm left step can be used for the clamp. Therefore, first machine the right edge, clamp the M22 Ph3P1.5 threaded head, machine a 54mmx15mm step, 22 inner holes and R25 inner arc.

2.5.2 Positioning criteria

Workpiece positioning and reference must match design criteria. It is best to select “one side and two pins” as the positioning reference for workpieces with cassettes, and the positioning reference should be carefully placed on the Cnc Machine. The workpiece is a solid shaft with a 30 degree taper at the tip. Since the length of the shaft is not very long, the right end face of the work and the outer circle of 48 are the reference for positioning. To clamp the workpiece, use three common chuck chucks.

The center of the right end face of the work is the origin of the work coordinates. The tool point is at (100.100). Since the entire surface of the part must be machined, the outer circle and one end face must be used as rough references and cross-referenced. Follow the principle of “baseline contingency”. When machining the right edge, select the outer surface on the left side of the blank, and when machining the left edge, select the outer circular surface on the right side to indicate that the positioning reference is the centerline of the axis. ..

Cutting tool selection and cutting ability

2.6.1 Tool selection

The Machining of this part is

  • (1) A 35-degree cemented carbide left deviation cutter is used for rough turning and fine turning of the outer and flat ends.;Choosing to order will prevent the rake face of the accessory from interfering with the contour of the workpiece, but the auxiliary deflection angle should not be too small. Select Kr = 35.
  • (2) Slot cutter,
  • (3) The thread is selected with a male thread cutting tool with a carbide of 60 degrees, and the radius of the tip is smaller than the minimum fillet radius of the contour, and re = 0.15 to 0.2 mm.
  • (4) Drill the left hole with a 22 mm drill
  • (5) Internal turning tool.

Tool selection is one of the important contents of CNC machining design. Proper selection of tools not only affects the efficiency of machining of machines, but also the quality of machining. Tool selection usually takes into account the Machining power of the machine, the Machining content, and the material of the workpiece.

Compared to traditional lathe methods, CNC turning requires more tools. It requires high precision, good rigidity, high durability, as well as dimensional stability, ease of installation and adjustment. This requires creating CNC machining tools with new high quality materials and optimizing the tool parameters.

Selection of cutting amount

The amount of cutting includes spindle speed, cutting depth, and feed rate. Depending on the machining method, it may be necessary to select different cutting amounts. The principle of cutting amount selection is to ensure part accuracy and surface roughness, maximize tool cutting performance, ensure reasonable tool life, maximize mechanical efficiency and maximize production efficiency. , Is to reduce costs.

(1) Determine the spindle speed

The spindle speed should be selected based on the permissible cutting speed and the diameter of the workpiece (or tool). The tool material is cemented carbide steel, the roughing speed is 500 rpm and the finishing speed is 1000 rpm. Considering the small cutting force of a thin thread, the thread is 400R / min. Due to the low rigidity of the bore, crude oil trucks are used at 600 rpm and are relatively easy to meet Machining requirements. The cutter is larger and safer at 350r / min.

(2) Feed rate (feed rate) F selection F (mm / r, mm / min)

Feed rate is an important parameter for CNC machine cuttings. It is mainly selected based on the machining schedule and surface roughness requirements of the part and the material properties of the tool and workpiece. Maximum feed rate. Limited by the rigidity of the machine and the performance of the feed system. In general, roughing carts use higher feed rates to quickly remove coarse material. The finish takes into account the principles of surface roughness and part accuracy. Since no high quality workpiece surface is required, the feed rate is then the strength and stiffness of the machine feed mechanism, the strength and stiffness of the toolbar, the material of the tool, the size of the tool holder and workpiece, and the depth of cut selected. Is based on. During finishing, the feed rate is selected based on surface roughness requirements, tool and workpiece materials, and other factors. The feed rate Vf can be calculated according to the equation Vf = f × n, where f represents the feed rate per revolution and generally requires a roughness of 0.3-0.8 mm / r; finishing wheels are usually 0.1. ~ 0.3mm / Cutting usually requires 0.05 ~ 0.2mm / r.

(3) Determining cutting depth

The cutting depth is determined by the stiffness of the machine, workpiece and tool. If rigidity is allowed, the cutting depth must be as equal as possible to the machining allowance (excluding the finish turning amount), which can reduce the number of passes and increase production efficiency. A small amount of finishing allowance may remain to ensure the quality of the machined surface.

In short, the specific value of the amount of cut should be determined as well based on machine performance, relevant manuals and hands-on experience. At the same time, the spindle speed, depth of cut and feed rate can be adjusted to each other to form the optimum amount of cutting. The cutting amount requires different cutting amounts for different machining methods. Choosing the right amount of cutting has a significant impact on the surface quality, accuracy and machining efficiency of the part.

Cutting amount selection method

Ensure a high metal removal rate and required tool life during roughing.

When the turning process is completed, it is required that the Machining accuracy and surface roughness are high, the Machining margin is small, and the surface roughness is uniform. The focus should be on how to ensure Machining accuracy and how to improve Machining efficiency based on this. Therefore, turning requires smaller (not too small) cutting depths and feed rates, and high performance tool materials and reasonable geometric parameters are used to maximize cutting speeds.

2.10 How to guarantee machining accuracy

In order to secure and improve machining accuracy, it is necessary to take effective technical measures such as appropriate error correction and error correction according to the main factors of manufacturing error, and directly control the influence of machining error.

2.10.1 Tool radius selection

  • (1) If the tool radius R is larger than the corner radius of the workpiece, the tool radius R cannot be machined.
  • (2) If the tool is small, many cutting processes cannot be performed (tool rigidity is poor).

2.10.2 Use the right cutting fluid

  • (1) Cutting fluid is mainly used to reduce friction during cutting and lower the cutting temperature. The rational use of cutting fluid plays an important role in improving tool durability, machined surface quality and machining accuracy.
  • (2) Water-insoluble cutting fluid: Cutting oil, solid lubricant, and insoluble cutting fluid mainly play a role in lubrication.
  • (3) Water-soluble cutting fluid: Aqueous solution, emulsion, and water-soluble cutting fluid have good cooling effect and cleaning effect. Therefore, this design uses an aqueous solution for cooling.

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