Discussion on Dimension Standard and Control Precision of Cemented Carbide Rods with Helical Coolant Holes

Discussion on Dimension Standard and Control Precision of Cemented Carbide Rods with Helical Coolant Holes

• Admin
• 2024/12/15

ABSTRACT

The cemented carbide rods with helical coolant holes were the important products researched by powder metallurgy industry for high added value due to its dimensional accuracy and molding difficulty. In this paper, based on the key indicators as pitch, concentricity, hole diameter and hole interval, the dimension and tolerance standard of rods was discussed. The influencing factors of the dimensional accuracy were analyzed. The processing technology of improving the dimensional accuracy of rods with helical coolant hole was studied. The results show that the pitch of internal and external consistency pruducing by die curing technology is better than by electrodynamatic regulation. The effect of extruding speed on pitch is stronger than pressure. The concentricity is more scientific than hole position deviation. The stability of hole interval is related to the mechanical equilibrium.The hole angle accuracy of rods with three helical coolant holes can reach 120°±4° through the improvement of the core rod seat.

The development and application of high-speed cutting technology depends on the advancement of machine tool and tool technology, in which the progress of tool materials plays a decisive role. Deep hole processing is widely used in various industries, such as the gun industry, deep hole processing of gun barrels; in the automotive industry, vehicle shock absorber barrels, hydraulic cylinders, engine water holes, piston pins, crankshaft oil Processing of holes; in the aerospace industry, precision high-strength catheters for rocket launchers, precision tubes for flow and pressure measurement of aviation instruments; in the nuclear industry, processing of stainless steel tubes in nuclear power plants, and in the oil drilling industry, oil logging instruments and many more.

Deep hole drilling conditions are relatively poor, it is carried out in a closed or semi-closed state, therefore, the cutting situation of the tool cannot be directly observed, the cutting heat is not easily dissipated, the chip removal is difficult, the rigidity of the process system is poor, and the cutting effect is not ideal . Deep hole drilling has special requirements on technology, tools and equipment. Carbide twist drills with internal cooling spiral holes can improve the efficiency of deep hole machining, drill difficult materials, and meet the needs of automatic machining lines and machining centers. The internal cooling process is to transfer the coolant to the cutting edge under high pressure through the internal spiral channel.

The function of such holes is twofold. Due to the rapid delivery of the coolant, the high temperature in the process is quickly reduced; at the same time, because the internal holes directly reach the processing front, the mechanical debris can be quickly and cleanly removed , To promote the improvement of the processed surface. Therefore, drilling and milling tools with internal cooling channels can achieve obvious practical results. Its application not only extends its own service life, but also shortens the reorganization time of mechanical equipment.

Carbide spiral hole rods due to the high dimensional accuracy requirements and difficulty in forming, its preparation technology was once the bottleneck of China’s powder metallurgy industry. Foreign manufacturers have launched mature products in the mid-1990s, monopolizing the domestic market for more than 20 years. In the past five years, Chinese manufacturers have also developed spiral hole rods, breaking the monopoly of foreign products. However, there is still a certain gap between the dimensional control accuracy of China’s domestic spiral hole rods and well-known foreign manufacturers, which needs further optimization and improvement. Moreover, due to the high added value of spiral hole rods, all manufacturers are strictly confidential and users are scattered, so there is no domestic and international standards for spiral hole rods.

Based on the application of spiral orifice rods, this paper discusses the dimensions and tolerance standards of spiral orifice rods, analyzes the influencing factors of spiral orifice rods’ dimensional accuracy, conducts technological research to improve the accuracy of spiral orifice rods, and obtains relatively good results.

1 Spiral Hole Rod Pitch (Helix Angle)

1.1 Helix Angle Classification and Accuracy

The spiral angle of the spiral hole rod is mainly divided into four categories: 15°, 30°, 40°, and 45°, and there are many other non-standard spiral angles required by users. The measurement of the helix angle in practical applications is very troublesome, and it is generally converted by measuring the pitch using the formula (1).

P tgθ=πD (1)

In the formula, P is the pitch, D is the diameter, and θ is the helix angle.

It can be seen from formula (1) that under the condition of the same diameter D, the larger the spiral angle θ, the smaller the pitch P, the more difficult to make a large spiral angle; under the same spiral angle θ, the smaller the diameter D, the more the pitch P Small, small diameter spiral hole rods are more difficult to manufacture; under the same pitch P, the smaller the diameter D, the smaller the spiral angle θ; the same rod, the radial position of the hole is different, the spiral angle is different.

The higher the accuracy of the spiral angle control of the spiral hole rod, the better, but there are objective influencing factors in actual production, which make the spiral angle of the spiral hole rod fluctuate. Although spiral hole rods have been introduced abroad 20 years ago, there are no relevant international, national, or industry standards regarding the spiral angle and internal hole size of spiral hole rods.

After more than 20 years of development, spiral hole rods are widely accepted by the market with a spiral angle accuracy of ≤±0.5°, such as the well-known German company K screw rod. However, there are also overall accuracy of the spiral angle ≤±1.0°, ±1.5°, and the requirement of the spiral angle accuracy ≤±0.5° can be achieved through technical classification, such as Luxembourg C Company. The main reason for the difference between the two is the preparation technology.

Spiral hole rods are mainly used for processing into internally cooled drill bits. The spiral angle accuracy ≤±0.5° can meet the requirements of ≤8D slotting multiple diameters within a certain range. For drills above 8D, it is generally recommended to process with a more precise pitch. Avoid perforation.

1.2 Preparation Technology of Spiral Holes

The key to the manufacturing technology of carbide rods with cooling spiral holes is the forming technology of spiral holes. Due to the characteristics of the spiral hole, it can only be prepared by the extrusion molding process.

The preparation technology of cooling holes in cemented carbide has been studied for decades, and can be summarized into two categories: one is the axial movement of the bar billet and the rotation of the core to form the spiral holes. This technology is too difficult to be Mass production. The other is that the core is not actively rotated, and the axial movement of the bar blank plus the rotary motion drives the core to rotate to form a spiral hole. The latter is the technology currently used successfully abroad. The power of the axial movement of the billet is derived from the plunger movement of the extruder. The source of power for the rotary motion of the bar billet is that the motor drives the bar billet to rotate, which is extruded by a laser speedometer, combined with the mechanical system of the rotating moving mold and the fixed mold, as shown in Figure 1(a), referred to as “electric adjustment method” “(Technology 1#).

Fig.1 Schematic diagrams of forming principles of rods with helical holes: (a) electrodynamic regulation; (b) mold solidification

During extrusion molding, the pitch of technology 1# is flexibly adjustable, but the mechanical system requires extremely high-speed accuracy. The fluctuation of the pressure of the extruder causes the fluctuation of speed, and the key dimensions such as the pitch of the bar and the pitch of holes often fluctuate. And if the mechanical system of the mold is unstable or does not coordinate with the laser speedometer, it will cause stalling and jamming. Therefore, it has to be shut down for maintenance, and mass production is difficult to carry out.

Another source of power for the rotary motion of the billet is the frictional resistance of the extrusion nozzle against the billet. The literature introduces a very practical extrusion die with internal threads, as shown in Figure 1(b), referred to as “mold curing method” (technology 2#). During extrusion, the bar blank produces axial movement, and due to the resistance of the thread groove, the bar blank rotates, forming a spiral hole. This preparation technology makes the extruded double-hole spiral bar material not very high in the precision of the extrusion equipment, and due to the curing effect of the mold, the dimensions of the screw pitch and the hole spacing of the extruded bar are stable, which can meet the requirements of deep hole machining tools. Requirements, and the process equipment is simple to operate, suitable for mass production.

1.3 Consistency of Internal and External Pitch and Accuracy of External Pitch

Due to the cumbersome internal pitch measurement steps, no matter which process, for the convenience of the measurement, at least one helical thread that rotates synchronously will be attached to the surface of the green compact during the extrusion of the spiral hole rod. P, see Figure 2. Due to the high precision requirements of the screw hole rod pitch, the parameters of the green compact must be controlled during the extrusion molding process. The green compact measures the external pitch to judge whether it is qualified, and the actual product application is the internal pitch, so the consistency of the internal and external pitch Is very important.

Fig.2 Sketch map of rod with two helical coolant holes

The internal and external pitches of the spiral hole rods prepared by the two techniques are compared and analyzed. The results are shown in Table 1.

TechnicalOuter pitch of each section within one rodAverage of outer pitchOuter pitch rangeInner pitchDifference between outer and inner pitch

1#	67.24, 67.13, 67.72, 67.05, 65.92	67.01	1.80	67.27	-0.26
2#-1	64.87, 65.39, 65.24, 64.48, 63.85	64.76	1.54	64.70	0.06
2#-2	65.73, 66.11, 66.30, 66.24, 66.29	66.13	0.569	66.22	-0.09

Table 1 Pitch of rods with helical holes prepared by different process

Technology 2#-2 in Table 1 is the technology after optimizing and upgrading the mold of 2#-1. It can be seen from Table 1:

• The difference between the internal and external pitch of the spiral hole rod of technology 2# is smaller than that of the technical 1# product, which shows that the internal and external consistency of the spiral hole rod pitch produced by the mold curing method of technical route 2# is better. Therefore, the inner pitch can be replaced by the outer pitch to facilitate product inspection and process quality control.
• The outer pitch of the technology 2#-2 product is less than that of technology 1# and technology 2#-1, which is about half of them. It shows that the optimized technology 2#-2 single spiral hole rod has better pitch accuracy and has greater advantages in the application of deep hole drilling.

1.4 Effect of Extrusion Process on Screw Pitch
Despite adopting technology 2#-mold curing, the extrusion process still has a direct impact on the pitch. To further investigate the impact of the extrusion process on the pitch, the main consideration is the hardness of the extrudate and the extrusion speed. The results are shown in Table 2.

TypeTEP /kNExtrusion speed /(mm/min)Pitch /mm

Ф14.3	18.17	4	99.8~101
Ф14.3	18.17	5	102.5~103.5
Ф14.3	12.78	4	100.8~101.2
Ф14.3	12.78	5	101.8~102.5

Table 2 Effect of extrusion process on pitch

The hardness of the extrusion material is mainly related to the characteristics of the extrusion molding agent and the amount of addition, ambient temperature and humidity. Generally, the test pressure is taken, and the test pressure (TEP) is measured to measure the hardness of the extruded material.

It can be seen from Table 2 (1) With the same TEP value, the extrusion speed is increased by 1 mm/min, and the pitch of the wet billet is extremely poor by 3.7 mm. (2) At the same speed, TEP is reduced by 5.39 kN, and the pitch difference is 1.7 mm. It shows that the extrusion speed has a greater influence on the pitch. However, both of them have an effect on the pitch. Only by stably controlling the extrusion process parameters can the pitch accuracy be further improved.

1.5 Influence of Mandrel on Pitch
The spiral hole rod is rotated and formed during the extrusion process, and the resistance of the rod blank directly affects the pitch. This article uses the same set of molds, from not equipped with core rods (solid rods with 0 core rods) to equipped with 2, 3, 4 core rods of different numbers. table 3.

Number of core pinsPitch /mm

0	84
2	80
3	78
4	77

Table 3 Effect of core pin on pitch

From the data in Table 3, it can be seen that the more the number of core rods, the greater the resistance to the rotation of the bar billet, the greater the frictional force on the bar billet, and the greater the distortion, so the smaller the pitch.

2 Characterization of Screw Hole Hole Position

The hole position degree is an important indicator to measure the technical level of spiral hole rod preparation, second only to the screw pitch. There are two ways to express the hole position degree, that is, the concentricity deviates from the hole.

2.1 Detection of Concentricity and Hole Deviation

As shown in Figure 3, the concentricity is the distance between the center of the circle of the two inner holes, that is, the center of the TK circle and the center of the bar, expressed by the letter eccentricity; the hole deviation is the measured hole center and the other The distance between the hole center and the centerline of the bar center is indicated by the letter a.

Fig.3 Eccentricity (e) and hole deviation (a)

2.2 Discussion on Concentricity and Hole Deviation

This paper believes that there is a blind area with a hole deviation to indicate the hole position degree, see Figure 4, that is, when both holes are on the central axis and eccentric, the hole deviation is 0 mm, but the hole position degree is actually unqualified. Concentricity means that there is no blind spot in the hole position, so it is more scientific than the hole deviation.

2.3 Optimization and Improvement of Concentricity of Spiral Hole Bar

Before 2017, the concentricity of China’s spiral hole rods was benchmarked to the internationally renowned company K Company, as shown in Table 4. In 2017, by optimizing the mold design, the “five-level principle” was strictly implemented in the process of a mold assembly and extrusion molding, that is, “core rod “Mould-mould-fixture-fixture-boat” must be aligned and flat with each other, and the concentricity of the product should be improved. For the benchmark international famous company C, see Table 4.

Diametere(ZCC) Before 2017e(ZCC) After 2017e(K)e(C)

3.3	≤0.08	≤0.04	≤0.08	≤0.04
3.4~4.3	≤0.10	≤0.05	≤0.10	≤0.05
4.4~8.3	≤0.15	≤0.10	≤0.15	≤0.10
8.4~10.3	≤0.20	≤0.15	≤0.20	≤0.15
10.4~14.3	≤0.37	≤0.18	≤0.37	≤0.18
14.4~35.3	≤0.50	≤0.20	≤0.50	≤0.20

Table 4 The improved concentricity of rods with helical holes

3 Diameter and Pitch of Spiral Hole Rod

3.1 Reference Standard for Aperture and Hole Spacing

Spiral hole rods can be made into standard twist drills, step drills, deep hole drills to meet the requirements of different drilling processes. Different applications have different requirements for the size of the hole diameter d and the hole spacing TK, so the spiral hole rod model is complicated. Based on market demand and product conditions of different manufacturers, Tables 5 and 6 are recommended as reference standards for the diameter and spacing of spiral hole rods to provide technical support for the specification of spiral hole rod models.

Table 5 The hole diameter deviation of rods withTable 6 The hole interval deviation of rods withFig.5 Hole-space precision controlling of rods with helical holes

3.2 Stability of Hole Spacing

In the process of process testing, this paper found that during the extrusion process, the hole spacing of large diameter (>20.3 mm) spiral hole rods is unstable, and there is a regular change trend, as shown in Figure 5.

There are many factors affecting the stability of the pitch of the screw hole rod, such as the structure of the core rod seat, the size of the core rod, the uniformity of the extrusion material, and the stability of the extrusion equipment. We have studied the structure of the core rod seat. It can be seen from Figure 5 that using the original mandrel seat, the hole pitch of the extruded wet billet continuously decreased from 17.5 mm to 12.5 mm, far exceeding the hole pitch tolerance of 1.0 mm, which may be due to the pressure of the billet during the extrusion process Caused by uneven power.

When the spiral hole rod is extruded, the compacted mixture in the material cylinder of the extruder travels forward under the push of the plunger. Due to the compression of the mold cavity, 3-4 strands are recombined into one. The 2-3 mandrels in the cavity form the inner holes and the hole spacing.

The material close to the mold wall is affected by friction, and the flow speed is slower and the compact is denser, while the intermediate material flows faster and the compact density is smaller. Therefore, the intermediate material is easily compressed and deformed by the surrounding material, causing the hole spacing. Stable, and the larger the diameter, the more obvious.

This paper designs the core rod base on the principle of hydrodynamic balance, see Figure 6. Obtained obvious results through the test, and then rationally matched with the shunt plate to control the change of the hole spacing in the range of 0.4 mm to meet the product needs, see Figure 5.

Fig.6 Sketch map of new core holder

4 Accuracy of Hole Angle of Triple Helix Hole Rod

The three-spiral hole rod has an additional inner hole than the double-spiral hole rod, as shown in Figure 7. It is difficult to produce a three-hole 120° uniform distribution in a limited space. The key technology lies in the design and manufacture of the core rod seat. This article designs and manufactures a three-hole mandrel seat with a uniform distribution of 120°, and develops a new product of three-spiral rods with typical models of 30° and 40° from φ6.3 mm to φ25.3 mm. 120°±4°.

Fig.7 Sketch map of rods with three helical coolant holes

Conclusion

• Carbide spiral hole rods have a variety of models and high dimensional accuracy requirements. Based on market demand and product conditions of different manufacturers, this article proposes reference standards for the pitch (helix angle), concentricity, aperture, and hole spacing of spiral hole rods. Provide technical support for the specification of spiral hole rod models.
• The preparation technology of spiral hole rod is mainly divided into two types: electric adjustment method (technology 1#) and mold curing method (technology 2#). The spiral hole rod pitch produced by the mold curing method has better internal and external consistency, so the external pitch can be used instead of the internal pitch to facilitate product testing and process quality control.
• The internal resistance of the spiral hole rod during extrusion molding affects the pitch of the product. The greater the resistance, the smaller the pitch. The extrusion speed has a greater influence on the pitch than the extrusion pressure.
• The concentricity of the spiral hole rod is a more scientific measure of the position accuracy of the inner hole than the hole deviation. There is a blind zone in the hole deviation.
• It is difficult to control the stability of the hole spacing of the large diameter spiral hole rod. The core rod seat is designed from the principle of hydrodynamic balance, and the change of the hole spacing is controlled within the range of 0.4 mm.
• Through the improvement of the core rod base, the accuracy of the hole angle of the triple helix hole rod reaches 120°±4°.

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