Research on processing technology of monel thin wall parts

As an ultra-thin wall shell structure, the overall dimension of the cold screen φ 20mm × 14mm, with a wall thickness of about 0.2mm. The part is composed of multiple step circles, two deep inner ring grooves, conical surfaces and two bosses. The diaphragm in the middle of the inner ring groove is only 0.25mm thick. The structure is very complex. At the same time, it requires high precision and is very difficult to process. After repeated analysis, a unique solution is found.

Part analysis

The grade is NCu30-4-2-1 Monel bar. It is an alloy made of metal nickel as the matrix and added with copper, silicon, iron, manganese and other elements. The silicon content is more than 4%, which makes this alloy more ductile, stronger and harder than ordinary Monel alloy. It is a kind of difficult machining material.

Figure 1 shows the cold screen part model, which is a typical thin-walled structure. Because the smallest part of the inner hole is only φ 6 + 0.10 / + 0.02mm, and the maximum position of the outer circle is φ 20.70 / – 0.21mm, no suitable pipe, can only be used φ 45mm monel bar is processed with large machining allowance, completely hollowed out inside, and the material removal rate is more than 99.7%. The average wall thickness of the whole part is 0.25mm, and the wall thickness of the thinnest part is only 0.15mm. There are two deep inner ring grooves with a width of 4mm and a conical surface, which are easy to deform during machining.

Chemical composition of NCu30-4-2-1

Grade Cu
Si
Ni
Mn
Fe
Mg
C
Al
S
NCu30-4-2-1 29.0-31.0 3.9-4.2 Allowance 0.8-1.5 1.5-2.5 ≤0.10 ≤0.20 ≤0.30 ≤0.20

Mechanical properties of NCu30-4-2-1
Grade Rockwell hardness/HRC Tensile strength
Rm/(N/mm2)
Elongation
A(%)
NCu30-4-2-1 ≥38 ≥1200 ≥6

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Figure 1 cold screen part model

Main processing elements and technical requirements

The main structure and dimensions of the parts are shown in Figure 2. The elements requiring high accuracy are 0.15 + 0.15 / – 0.05mm, 0.25 + 0 / – 0.05mm φ 12.7+0.05/0mm、 φ 6+0.10/+0.02mm、R9+0/-0.09mm、 φ 20 + 0.10 / + 0.05mm and 20220108050618 67858 - Research on processing technology of monel thin wall parts.
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Fig. 2 main dimensions of parts

Main technical difficulties

  • ① The parts are of ultra-thin wall shell structure, the thinnest wall thickness is only about 0.15mm, and the rigidity is poor. When the cutting force is slightly large during final finishing, the parts will be bent, deformed or even broken. It is easy to chatter and vibrate the tool during machining, which affects the machining accuracy and surface quality.
  • ② The rigidity of the part is very poor and easy to deform after being stressed. Therefore, how to effectively control the internal stress deformation, clamping deformation, cutting force deformation and cutting thermal deformation in the machining process is the key to ensure the accuracy of the part.
  • ③ The deep inner ring groove on the part is difficult to remove chips, and the minimum part of the inner hole of the part is only φ 6 + 0.10 / + 0.02mm, φ 2 inner cavity of 12.8mm is deep, 13.2mm furthest from the machined end face, and the tool overhang is long, which is easy to vibrate during machining. The inner cavity is deep, the material toughness is good, the chip breaking performance of the strip chip is poor, and it is difficult to wrap it into a ball for chip removal. When finishing the inner groove contour, normal tools or ordinary grinding tools cannot be processed by conventional methods.
  • ④ φ 6 + 0.10 / + 0.02mm through hole and φ 12.7mm and 0.3mm deep hole is located in the cutting section and cannot be completed in the first process. It must be processed in different processes. At this time, the parts have been basically processed and formed, the wall thickness is only 0.15mm, the rigidity is poor, and there is not only no suitable clamping surface, but also easy to clamp deformation. Therefore, the appropriate clamping design is very key.
  • ⑤ Materials are difficult to cut. The material is nickel base alloy with high strength and good toughness. When cutting this metal, the tool wears fast, the cutting vibration is large, and it is not easy to cut.

Process design

The part structure is complex, the machining allowance is large, and the part precision is high. During normal machining, the process of rough machining, aging stress relief and finishing machining is usually selected. However, the wall of the part is thin and difficult to clamp. After comprehensive consideration, the process flow is designed based on the principle of concentrating the process as much as possible, reducing the number of processes, optimizing the geometric angle of the tool and cutting parameters to reduce the deformation of the part.

Processing technology of monel thin wall parts

The processing process route is: 0 blanking → 5 counting cars → 10 counting cars → 15 wire cutting → 20 inspection → 25 sand blasting → 30 blackening.
Process 5: process as many structural elements as possible in one process, except on the cutting section φ 12.7mm, 0.3mm deep hole step and two bosses on the large end face, the rest are guaranteed in this process and cut from the blank bar after processing. Procedure 10: turn the head to cut the section and process the surface φ 12.7mm, 0.3mm deep hole step, and because the tool overhang is too long, it is arranged for boring in this process φ 6 + 0.10 + 0.02mm hole. Procedure 15: machining two bosses on the large end face by wire cutting. Finally, sand blasting and surface blackening treatment.

Deep groove and thin wall machining

In order to reduce part deformation and meet accuracy requirements, the first process needs to subdivide work steps to complete rough and finish machining. The thickness of the inner ring groove is only 0.15mm, and one and a half finishing is added here.
When removing the allowance in rough machining, because the inner hole structure is more complex and the tool overhang is too long, the horn part at the opening is machined first, the allowance is removed by drilling center hole, drilling and boring, and then the external allowance is removed to process the shape of the part. Ordinary machining methods cannot complete the cutting of two complete 90 ° deep grooves. Therefore, according to the subdivision of work steps, the allowance shall be removed in layers first, and each layer shall be cut more than the upper layer, so as to remove the lower allowance as much as possible. In order to avoid extrusion deformation of parts due to chip extrusion, chip removal shall be suspended after each layer of cutting. Then semi finish machining shall be carried out once according to the contour of the inner ring groove to ensure that the finishing allowance is consistent.
During finish machining, first finish machining the outer circle of the end face to ensure the length dimension, and then select the inner cone and overall dimension of the inner hole that is relatively easy to ensure. Finally, because the depth and width of the two internal grooves have exceeded the processing range of the normal grooving cutter, and the part wall is extremely thin, if the cutter is not sharp enough, it is extremely easy to produce extrusion deformation, and the margin cannot be removed at the 90 ° angle. Therefore, two self grinding tools with different handles are used to remove the allowance from both ends for finishing, so as to ensure deep groove root cleaning. The specific processing method and tool selection are shown in Table 1.
During machining, the cutting fluid shall be poured fully to avoid part deformation caused by cutting heat.
Table 1 work step process of the first process of cold panel processing
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Select the appropriate tool

Most structural elements of parts can be completed by turning. Due to its difficult cutting characteristics such as high material strength and good toughness, and the complexity of its structure, alloy tools are selected except drilling. In particular, the parts are of ultra-thin wall structure with extremely poor rigidity. It is necessary to select the appropriate geometric angle of the tool to minimize the cutting force of the parts and control the direction of the cutting force. The general principle is to grind the blade, increase the front angle and back angle, and increase the sharpness of the tool. In order to ensure that the deformation caused by chip extrusion does not occur in the cutting process, peck cutting method is used, and chip removal is suspended in time. Avoid cutting and crushing parts and tools. During finishing, grind the cutting edge obliquely to reduce the contact area between the cutting edge and parts. Two knives are selected for the inner groove to process the left and right sides respectively.
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Fig. 3 special tool

Select appropriate cutting parameters

The rigidity of parts is very poor and easy to deform after being stressed. How to reduce the deformation caused by cutting force in the machining process, the selection of cutting tools and cutting parameters is particularly important. After several trial machining, the cutting parameters are finally determined. Rough machining profile parameters: rotating speed s = 700r/min, feed speed VF = 35mm / min; Finish machining profile parameters: rotating speed s = 800r/min, feed speed VF = 64mm / min; Rough machining conical surface parameters: rotating speed s = 700r/min, feed speed VF = 9mm / min; Finish machining conical surface parameters: rotating speed s = 700r/min, feed speed VF = 42mm / min; Parameters of rough machining inner ring groove: rotating speed s = 1000r/min, feed speed VF = 10mm / min; Parameters of semi finishing inner groove: rotating speed s = 800r/min, feed speed VF = 40mm / min; Finish machining inner groove parameters: rotating speed s = 700r/min, feed speed VF = 35mm / min.

Tooling design

NC lathe tooling

After the first process is cut off, there are still φ 6 + 0.10 / + 0.02mm through hole and φ The structural elements of 12.7mm and 0.3mm deep holes do not meet the requirements and need to be turned around. However, the parts are of typical thin-wall structure and have deep grooves inside. Conventional plugs, bushings and other fixtures cannot be used for clamping to prevent deformation. Therefore, special tooling is designed to meet the requirements.
Prevent clamping deformation by changing the direction of clamping force, which is consistent with the parts φ 8.6MM inner hole matching, nut pressing and end face processing to ensure the total length and φ 12.7mm hole size. Change the direction of clamping force by pressing the end face with nuts to avoid clamping deformation of thin-walled parts. The specific clamping is shown in Figure 4.
20220108053143 52310 - Research on processing technology of monel thin wall parts
(a)
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(b)
Figure 4 Procedure 10 – schematic diagram of tooling and clamping

Wire cutting tooling

In order to ensure that the thin wall will not deform under the influence of cutting force as much as possible, after the NC turning process, the other dimensional elements shall be selected for wire cutting. The main processing content of process wire cutting is to cut off the outer part of r9mm circle. As the material of the part is monel rod, it is not magnetic and cannot be positioned by adsorption. At the same time, the part has become a thin-walled structure after processing by NC lathe, and ordinary clamping methods such as vise can not be used for clamping and positioning. Therefore, special tooling is designed to meet the requirements.
Prevent clamping deformation by changing the direction of clamping force, cooperate with the inner cone of the part, and use the inner cone to locate the part to avoid clamping deformation of thin-wall part. The specific process content and clamping are shown in Figure 5.
20220108053225 42744 - Research on processing technology of monel thin wall parts
(a)
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(b)

Fig. 5 Procedure 15 – schematic diagram of tooling and clamping

Source: China Pipe Sleeve Manufacturer – Yaang Pipe Industry (www.pipelinedubai.com)

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