This study examines the technical requirements for the base plate and introduces process optimizations for drilling and milling. We have developed a set of effective methods for cutting and grinding drill bits with specific geometric angles to address the challenges of CNC drilling in thin-walled stainless steel. By adjusting the composition of the cutting fluid, we ensure high processing quality and enhance production efficiency.

PART. 01 Introduction

With the rapid development of railway modernization in my country, the quality of vacuum circuit breakers—an essential component of locomotives—has become crucial for ensuring the stability, reliability, and safety of these vehicles. The base plate, which is welded to the vacuum circuit breaker, serves a vital role in its assembly. Since it is installed on the roof of the locomotive, it is critical for maintaining the flatness of the base plate and the positioning of each hole.

Due to various complex environmental factors and the dynamics of high-speed travel, the vacuum circuit breaker must possess sufficient strength and rigidity, an extensive mechanical lifespan, and strong resistance to environmental conditions such as wind, frost, rain, and snow. The BVAC-N99 vacuum circuit breaker is a single-stage AC vacuum circuit breaker designed mainly for connecting and disconnecting the main circuit, and it can also provide overload and short-circuit protection. The quality of processing for the substrate directly impacts the normal operation of the vacuum circuit breaker, which in turn affects the entire locomotive.

The substrate is a key component of the BVAC-N99 series vacuum circuit breaker, which is mounted on the top of the electric locomotive. To meet the requirements for electric traction and withstand harsh working conditions, a 3mm thick cold-rolled steel plate made from 1Cr18Ni9Ti is selected. This material is classified as austenitic stainless steel, known for its good environmental stability and longevity. However, processing thin-walled stainless steel is challenging, making mass production through CNC milling difficult.

PART. 02 Difficulties in Stainless Steel Drilling

Austenitic stainless steel consists of an austenitic structure, specifically 1Cr18Ni9Ti, known for its high toughness and plasticity, which makes it challenging to process. The drills used for machining this material experience significant wear, and the effects of fusion welding ablation can be seen in Figure 1. As a result of the fusion welding, the workpiece has turned black. The following difficulties were encountered during the processing.

– Stainless steel is hardened during machining, resulting in high temperatures, and the tool is prone to wear and ablation.

– Stainless steel has high plasticity, and the chips and tooltips are bonded, making continuous machining difficult and prone to chipping.

– Austenite has poor thermal conductivity, and the local temperature in the cutting area is high, so the chips and tool are prone to bond, forming a built-up edge, accelerating tool wear, and causing drill bit workpiece ablation.

– When drilling thin-walled parts, the drill tip cannot play a centering role when passing through the workpiece, and the main cutting edge squeezes the workpiece, resulting in excessive roundness and position of the hole.

PART.03 Improved solution

3.1 Difficulties in processing substrates

– When drilling thin-walled stainless steel, the stability of the drill bit in centering can be an issue. Once the drill tip penetrates the 3mm thin plate layer, the main cutting edges on both sides of the drill bit continue to cut. As shown in Figure 2, the tip of a standard drill bit passes through the thin plate. Consequently, achieving precise positioning for holes with a circumference of 4×φ14mm is challenging.

– Stainless steel is a highly wear-resistant material that requires processing at low speeds and low feed rates. The processing system may lack sufficient rigidity. When an ordinary drill is clamped too long, it can lead to unstable centering, increasing the risk of drill breakage and making it difficult to guarantee precise dimensions.

– A drill that is overly sharp can cause excessive wear during processing, leading to chipping and making cuts rough and uneven.

– During cutting, tangential stress, plastic deformation, and cutting force are significant, and the poor thermal conductivity of stainless steel leads to increased cutting temperatures. This heat is often concentrated in a narrow area near the tool’s cutting edge, accelerating tool wear.

– Stainless steel is prone to severe work hardening. Austenitic stainless steel, in particular, has an austenitic structure and a high tendency for work hardening, which is typically several times greater than that of ordinary carbon steel. Cutting within the work-hardened zone can significantly reduce the tool’s lifespan.

– Austenitic stainless steel is also prone to sticking to the tool. Due to the toughness of the chips and the high temperatures produced during processing, the strong chips can adhere to the front cutting edge, leading to adhesion and welding issues. This, in turn, affects the surface finish of the machined parts. Additionally, drill tip ablation can cause the center of the chip circle to turn yellow, as illustrated in Figure 3.

– The drill bit wears faster. Stainless steel contains high melting point elements and has high plasticity and high cutting temperature, which makes it easier for chips to stick to the tool. The chips entangled on the drill bit will accelerate its wear. Frequent replacement of the drill bit will affect production efficiency and increase the cost of tool use. Chip sticking to the tool is shown in Figure 4.

3.2 Structure of φ8mm three-pointed six-blade thin-wall drill

The cross-section of the φ8mm three-pointed six-blade thin-wall drill is shown in Figure 5, and the actual object is shown in Figure 6.

3.3 Comparison between φ8mm thin-wall drill and ordinary drill

– The drill bit should not be excessively long, and the processing speed for austenitic stainless steel should be kept low. A φ8mm drill bit lacks sufficient transmission rigidity. The longer the tool holder extends, the greater the runout and unstable centering, which can lead to deviation. This increases the likelihood of the drill bit breaking during operation.

– When standard drills are used on thin plates, the primary wear observed on the drill bit occurs at the back face. As the drill tip penetrates the CNC-machined workpiece, the two main cutting edges may become unstable, resulting in uneven force distribution. This instability can cause the workpiece to be squeezed, leading to plastic deformation of the stainless steel, which results in extrusion drill marks and irregular deformation holes with subpar ovality.

– Ordinary drills produce cutting heat while drilling. The main cause of wear in high-speed steel drills is phase change wear. Stainless steel has poor heat dissipation properties, which can lead to the formation of a high-temperature zone at the drill tip. This heat accumulation can create a built-up edge, causing the cutting edge to weld and bond with the stainless steel, ultimately leading to the bit burning out.

– The inclination angles on both sides of standard drill bits experience the highest torque and cutting force. Additionally, the small back angle of the drill bit reduces the strength at the tip, making it less prone to chipping.

PART.04 Grinding three-point six-edge thin-wall drill

4.1 Geometric angles of φ8mm ordinary drill

The actual φ8mm ordinary drill is shown in Figure 7, and the main geometric angles are shown in Figure 8.

4.2 Advantages of grinding thin-wall drills made of stainless steel

– Reduce the cutting surface in the cutting area, thereby reducing the cutting force.

– Convenient chip discharge, taking away a lot of cutting heat and preventing tool burning.

– Automatic centering and symmetrical and uniform cutting force ensure the position of the positioning hole and product quality.

– Choose the appropriate cutting amount that is suitable for mass production. More than 1,000 pieces can be processed in one grinding.

– The spindle speed (s) is optimal when it ranges from 350 to 450 revolutions per minute (r/min), while the feed rate (f) is most effective at 45 to 55 millimeters per revolution (mm/r). Through calculations, we find that the maximum feed rate per revolution (fzmax) is calculated as fmin/smin = 45/350 = 0.128 mm/r, and the minimum feed rate per revolution (fzmin) is calculated as fmax/smax = 55/450 = 0.122 mm/r. When the feed rate per revolution during drilling is within the range of 0.122 mm/r < fz < 0.128 mm/r, both tool life and efficiency are at their best.

PART.05 Drill bit processing effect after grinding

5.1 Processing plan

– Use a three-point six-blade thin-wall drill to process the φ8mm hole of a 3mm thin plate.

The CNC program for thin-wall drills is as follows:

The CNC program of S450 M3G81 Z-4.2 R2 F50 ordinary drill is as follows. S450 M3G81 Z-7.5 R2 F45 (drill tip out)

The speed of the thin-wall drill is 450r/min, and the feed rate is 50mm/min; the speed of the ordinary drill is 450r/min, and the feed rate is 45mm/min. The drilling depth of a thin-wall drill is 3.3mm less than that of the ordinary drill, and the processing efficiency is nearly 2 times higher. The height of the thin-wall drill tip is relatively flat, and the drill tip depth of an ordinary drill is larger.

– Cutting fluid is essential as it reduces cutting heat, prevents material from sticking, and provides cooling and lubrication. For optimal performance, the recommended ratio of emulsion oil to water is 1:5.

– The drill tip is sharp and self-centering, eliminating the need for additional hole processing. The use of an arc transition significantly minimizes the plastic deformation of chips into a cone shape, preventing them from sticking to the drill tip.

– The drill tip effectively divides and separates the chips. As the chip width decreases and they become smaller, their toughness is reduced. This makes chip removal easier, prevents chips from wrapping around the tool, and aids in heat dissipation.

– When the drill bit is successfully reground, its service life is greatly extended, making it suitable for batch processing. As the drill bit wears, signs of wear can be identified by observing the shape and color of the chips, allowing for timely regrinding of the drill bit.

5.2 Process Improvement

The wear profile and angle of the drill bit are analyzed to improve grinding geometry and extend the life of the drill bit. Continuous innovation and improvement are applied to gradually enhance the various angles of the drill bit. After re-grinding, the drill bit can be used directly without the need to drill into a hole first. This approach effectively ensures the size, roundness, and positional accuracy of the hole. Additionally, it significantly improves the surface roughness to meet the specifications outlined in the drawing. The advantages of the drill bit are as follows:

– The drill tip is sharp and features an automatic centering function.

– It provides effective chip removal, which reduces cutting heat and extends the drill bit’s service life.

– The drill tip is designed with good rigidity, effectively resisting the plastic deformation of the workpiece.

– Chamfered edges on both sides of the drill tip enhance the surface roughness of the hole wall, improve wear resistance, and facilitate chip removal.

– A low-speed and low-feed rate method is employed to address the challenges of drilling austenitic thin-walled stainless steel. This approach not only allows for large-scale processing but also improves product quality and processing efficiency, successfully resolving various processing issues and achieving excellent results.

5.3 Optimization effect

We have successfully saved raw materials, resulting in an increase in the qualified rate from 95% to 99.8%. The grinding process for φ8mm group drills has significantly enhanced processing efficiency, with a single grinding capable of producing over 1,000 holes. By re-grinding φ8mm drill bits and implementing process improvements, we have effectively addressed previous issues related to substandard drilling roundness, hole deformation, and positional accuracy. The results are impressive. After sampling and testing the processed products post-improvement, we found that the deviation in the two φ8mm positioning holes—previously challenging to control—was reduced from 0.2mm to 0.1mm.

PART.06 Drill bit grinding

6.1 Grinding shape and angle of φ8mm drill bit

The basic dimensions of φ8mm holes are shown in Table 1.

The main component substrate in the vacuum circuit breaker is shown in Figure 9. The substrate is used for drilling φ8mm process positioning holes in a 3mm thick austenitic stainless steel sheet.

The actual processing diagram of the drill bit is shown in FIG10, and the specific steps are as follows.

– Grinding the Chisel Edge: Begin by grinding the drill tip to create a new inner straight edge. This CNC machining process should shorten the length of the chisel edge, increase the chisel edge rake angle, and expand the chip space at the drill tip. It is important to maintain the strength of the drill tip while reducing the cutting feed force. There are no strict requirements for the rounding of the grinding wheel corners. For a drill tip with a diameter of 8mm, the tip length should be between 0.15mm and 0.25mm, with an inner edge bevel angle of 60° and a chisel edge bevel angle of 0°.

– Grinding the Main Cutting Edge: Use a standard thin-walled drill to grind an inner concave arc edge. The goal is to create an outer edge and a circular ring of the drill core that nests firmly together, taking advantage of the arc depth of the groove being greater than that of the thin-walled part. Stainless steel is known for its high toughness and the challenge of chip breaking, leading to the formation of thin slices of material that may become trapped in the inner concave arc during drilling. This can result in drill bit breakage due to the deviation from the cutting edge during the next hole drilling. To mitigate this, a 45° chamfered edge should be added to the corner of the main cutting edge near the edge band, serving several purposes:

1. Facilitate chip removal and chip breaking.

2. Enhance the surface quality of the processed hole wall.

3. Create two symmetrical peak angles that aid in centering the drill bit, which improves geometric accuracy and reduces surface roughness.

– Grinding the Drill Tip: Establish a new inner straight edge by grinding the drill tip, which will shorten the chisel edge length, increase the front angle, and enhance chip space at the drill tip. With a chisel edge angle of 0°, the chisel edge of the drill tip, along with the chamfered edges on either side, forms a straight line, reducing cutting force and sharpening the drill tip. An inner edge angle of 60° increases edge strength, making it suitable for processing stainless steel materials known for their toughness and strength.

– Grinding the Arc Edge: Ensure the arc groove transitions smoothly to the edge bands on both sides to facilitate chip removal. Use the edge of the grinding wheel to shape the R2.5mm arc surface. After this, grind the chamfers on both sides of the edge bands once more. When processing tough stainless steel, it is vital to first enhance the strength of the chamfered edge to prevent chipping, followed by a slight reduction of the back angle to maintain sharpness. The chamfered edge and arc edge together facilitate the turning out of strip chips during drilling, aiding chip removal and heat dissipation.

– Grinding the Edge Band: Grind the 45° chamfer edge of the ligament to sharpen the edge, reducing friction between the ligament and the hole wall. This improvement enhances the geometric accuracy of the hole and reduces surface roughness. Additionally, the plastically deformed annular thin-slice chips will no longer become lodged in the arc edge of the drill tip, allowing them to fall off easily and preventing them from sticking to the cutting tool. The chamfer back angle should be set at 7°, with a chamfer width of 0.4mm.

6.2 Specific grinding methods and geometric angles of drill bits

The geometric angles of a three-pointed six-edge thin-wall drill are shown in Figure 11.

It comprises a chisel edge and a chamfered edge, an inner concave arc edge and an inner straight edge which are symmetrically arranged about the drill tip axis and are sequentially connected from the radial outer side to the inner side, and the two inner straight edges are respectively connected to the two ends of the chisel edge. The chamfer angle δ of the chamfered edge is 40°～45°; the height κ from the drill tip to the bottom of the concave arc edge is 1.3～1.5mm; the height μ from the drill tip to the chamfered edge is 0.8～1mm; the transverse edge bevel angle ψ is 0°; the inner straight edge bevel angle τ is 60°～65°; the inner edge vertex angle 2φ of the drill bit is 130°～132°; the outer edge vertex angle 2η is 123°～125°; the drill bit transverse edge inclination angle γ is 89°～90°; the transverse edge length ω is 0.15～0.2mm; the arc edge back angle α is 11°～12°, the arc edge radius χ is 2～3mm; the chamfer edge back angle β is 6°～7°; the chamfer edge width ν is 0.3～0.45mm.

The method of using a drill to drill holes in thin-walled substrates is to set the spindle speed to 350-450r/min in the working state of the drill, and the feed rate during drilling is 0.122-0.128mm/r. The drill directly drills holes on the substrate in a single operation under this working state.

When thin-walled drills of different diameters are used to process workpieces of different materials, the grinding angle, shape and cutting amount will change accordingly. This article uses the grinding angle and cutting edge shape of the φ8mm drill to provide reference and reference for the drilling of austenitic stainless steel.

PART. 07 Conclusion

The geometric grinding method to improve the life of the drill was analyzed through the profile and wear angle of the drill wear position. In actual processing, continuous improvement and innovation were made, and the various angles of the drill grinding were gradually improved. After the drill was re-grinded, it was not necessary to drill the hole again. The drill could be directly used for processing, effectively ensuring the size, roundness, position, and surface roughness of the hole.

With the emulsified cutting fluid of 1:5 and the appropriate cutting amount, nearly 1,000 holes could be processed in one grinding, which solved the problem of drilling austenitic thin-walled stainless steel holes, realized large-scale processing, improved product quality and processing efficiency, and provided effective reference for the same type of thin-walled difficult-to-process materials, and achieved remarkable results.

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