According to the processing characteristics of PTFE gasket, through the sharpening of cutting tools and the rational use of processing methods, the dimensional accuracy error and

 

The shape accuracy error ensures the surface roughness, enables the product to guarantee the quality and quantity, and provides experience for the gasket processing in the future.

 

1. Introduction

 

In the field of aerospace, PTFE gaskets are widely used as seals in valve connections to prevent leakage and strengthen sealing efficiency in gas and liquid media. These pads

 

The plate includes flat gaskets, V-shaped gaskets and ball valve gaskets, with a diameter of 5~600mm, a thickness of 0.3~4mm, a dimensional tolerance of 0.05~0.1mm, and a surface roughness Ra

 

1.6 μm, with different specifications and sizes, small quantity and fast change of dimensional accuracy, is not suitable for molding, and it is difficult to meet the production task in time.

 

The shim can be machined as soon as the drawing arrives, with short cycle, low cost, energy saving and consumption reduction.

 

PTFE gasket using PTFE bar and plate material processing, due to the nature of the raw material, its shape size is different, easy to produce dimensional accuracy error and shape precision

 

Degree error, in order to eliminate the resulting error, according to its characteristics, with different methods to solve.

 

 

Figure 1: PTFE gasket Source: Network

 

2. Technical difficulties and cutting parameters

 

2.1 material properties

 

Because Dankai PTFE has good chemical stability, corrosion resistance, sealing, flame retardancy, high lubrication and non-stick, electrical insulation and good anti-aging endurance

 

It can be used for a long time in any kind of chemical medium at a temperature of -180~250. The rigidity, hardness and strength of PTFE are relatively small, while those of PTFE are relatively small.

 

Large linear expansion, its coefficient is (10~12) × 10-5(1/), and the smaller thermal conductivity, only 0.27W/ (m · K), the elastic modulus is very low, when the cutting feed is too large or

 

When the speed is too fast, the large machining friction and the cutting force applied to the workpiece by the tool will generate a large amount of heat, which will eventually lead to a large deformation of the product.

The processing accuracy of the product.

 

Grinding of 2.2 Cutting Tools

 

In the processing of the gasket, because the thickness is relatively thin, the choice of the cutter is the most critical, the carbide turning tool is blunt, easy to generate heat, if the cutting amount is too large, the cutting speed is relatively

 

High, the cutting temperature rises, the material softens will produce sticky knife, and the burr is relatively large. The high-speed steel turning tool has good sharpening performance, which can make the cutting edge of the tool sharp and easy

 

Turning. If the sharpening of the chute of the cutter roll is not suitable, chip removal will not be smooth, or even wound onto the workpiece and bar, resulting in potential safety hazards, or the surface of the gasket will vibrate.

affect the surface roughness.

 

 

Figure 2: PTFE gasket Source: Network

 

According to the characteristics of Dankai PTFE, for gaskets processed with bar stock, the size of the rake angle of the cutter directly affects the cutting effect, and the larger rake angle can reduce the cutting deformation and reduce

 

Cutting temperature, reduce the tool edge blunt radius, make the blade sharp, improve the quality of processing. However, if the rake angle is too large, it will weaken the strength of the tool, the heat dissipation conditions will become worse, and the cutting temperature will be reversed.

 

And increase, so that the tool durability and parts processing quality decline. Through the cutting test, the rake angle should be sharpened γ = 10 ° ~ 15 °, and the length is the difference between the major diameter and the minor diameter of the gasket.

 

More than 1/2 of the 3mm circle said chip flute [I. e. L =(D-d) /23 (mm) ].

 

In this way, the cutting chips are smoothly discharged backward and will not be wound on the bar and the workpiece. Then grind the tip arc with a radius of 0.1. If the tip arc is too large, it will increase.

 

Cutting force, the gasket surface will produce vibration pattern. When sharpening the rear angle of the cutter, the smoothness of the flank surface is very important, which affects the surface roughness of the gasket. The grinding wheel should be corrected before sharpening.

 

Stable, there can be no bounce phenomenon, otherwise the blade sharpened out of the micro view is serrated, ups and downs.

 

After dressing the grinding wheel, the grinding angle α = 6 ° ~ 8 ° can reduce the friction between the tool flank and the workpiece and reduce the heat. For gaskets processed from sheet metal, especially those with a diameter greater

 

For gaskets above 300mm, the rake angle of the cutter should be sharpened γ =-3 ° ~-5 °, and the length is 3~5mm longer than the thickness of the gasket, so that the rigidity of the cutter will not be affected. If cutting a large area

 

Sheet metal, the use of cutting tools, due to the relatively large rake angle, the influence of centrifugal force is prone to tensile fracture, and produce a knife, sheet metal tearing off.

 

The outer round knife and the inner hole knife should have a larger rake angle γ = 10 ° ~ 15 °, which can reduce the cutting deformation, reduce the generation of cutting heat, reduce the temperature, and take the larger value α = 15 ° ~ 30 ° for the rear angle, which can reduce

 

The friction between the tool and the surface of the workpiece can reduce the blunt radius of the cutting edge, the cutting edge is sharp, easy to cut into the workpiece, and improve the processing quality.

 

 

Figure 3: PTFE gasket Source: Network

 

Selection of 2.3 cutting parameters

 

Due to the low thermal conductivity and large thermal expansion coefficient of PTFE, high-speed steel turning tools are used. Due to technical requirements, coolant cannot be used for cooling during processing, and the conditions are limited.

 

There is a cold air cooling system, under the existing conditions, the cutting speed can not be too high, can be reasonably selected according to the diameter of the workpiece. The selection range is cutting speed 70 ~

 

225 mm/min, feed 0.05~0.25mm/r, cutting depth of 0.12~3mm is appropriate. When the outer circle and the inner hole are roughly turned, a larger cutting amount of 0.25mm is selected, which is deeper.

 

The cutting depth is 3mm, which can shorten the processing time. When finishing turning, 0.05mm can be selected to ensure the surface roughness, and the cutting depth is 0.12~0.3mm. The effect ratio

better.

 

3 Processing methods

 

For the gasket processed with bar material, in order to reduce the dimensional error of the gasket due to the influence of the clamping force, an open tire is first made of 45# steel, and the diameter of the inner hole is smaller than that of the bar material.

 

The diameter is 1mm, and a 2mm wide opening is opened on the outer circle of the tire to increase the contact area of the round bar material and reduce the deformation of the raw material affected by the clamping force. First test run one

 

Pieces, cut off at one time, can't be divided into coarse and fine cars, otherwise it will be deformed due to uneven stress, and the amount of cutting is 0.05mm. Wait for an hour or so after cutting

 

Process, let the gasket be the same as room temperature, and measure the size of the gasket. Because the machined gasket is relatively soft and not easy to measure, it can be machined with a pass with the same dimensional tolerance as the inner hole of the gasket.

 

Stop gauge, put the gasket on the general gauge to measure the size, which can not only ensure the size, but also facilitate the measurement.

 

For gaskets processed with sheet metal, the diameter is relatively large, and the processing can be completed with a cutter. During processing, metal plugs are used to tighten both sides of the sheet metal without slipping. Select

 

When the plug is in the direction of the tailstock, it should be about 10mm smaller than the inner diameter of the gasket, not too small, otherwise the sheet metal processing position is not tightly attached, forming a knife. due to the large diameter, the rotation speed is 70

 

~ 90r/min, the amount of cutting is 0.05mm. in order to prevent the phenomenon of knife binding, which affects the unusable surface of the workpiece, take a clean brush and gently place it above the cutter position during cutting.

Hold against the outer end face of the shim until it falls off.

 

4 Conclusion

 

Through the use of reasonable processing methods for PTFE bars and sheets, various errors caused by various factors encountered in the processing make a large number of pads of different specifications.

 

In the production of the sheet, the influence of various factors on the size accuracy, shape accuracy and surface roughness of the lathe processing gasket is solved, so that the quality of the product is stable, the cost is low,

 

The product quality is improved, and it has certain reference significance for processing PTFE gasket.

 

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