Introduction
Stainless steel has high hardness and wear resistance, making it difficult to drill. Choosing the ideal drill bits for cutting stainless steel is beneficial for enhancing the cutting performance. This guide will help you choose the most suitable type of drill bit for stainless steel to enhance the tool life and surface finishes.
What are Drill Bits for Stainless Steel?
Features of Drill Bits for Stainless Steel
Drill bits for stainless steel are typically made from high-hardness materials to protect tools from the frictional heat. They feature better heat resistance to maintain sharpness under continuous machining. Their advanced geometry design and durable coatings can significantly decrease the tool friction and heat accumulation in diverse application scenarios.
Why is it Crucial to Use the Correct Drill Bit for Stainless Steel?
If you choose the wrong drill bit for stainless steel, you may face common problems like oversized holes, excessive heat accumulation, and fast tool wear. Choosing the correct drill bits can ensure the consistent depths and tight tolerances of holes and avoid excessive heat to extend the tool life.
How to Choose the Right Drill Bits for Stainless Steel?
Step 1: Confirm the Types of Drill Bits for Stainless Steel
- Cobalt Drill Bits for Stainless Steel
Cobalt is the best drill bit material for stainless steel because of its optimal durability and cost-effectiveness for machining stainless steel. Cobalt drill bits are outstanding with their exceptional hot-hardness, which means they can maintain their ultra-sharp cutting edges even when the temperature rises rapidly in low-speed drilling. They can deal with interrupted cuts because of the high rigidity of cobalt.
- Solid Carbide Drill Bits for Stainless Steel
If you use solid carbide drill bits for stainless steel correctly, they will deliver outstanding drilling accuracy with their superior edge retention, high rigidity, and wear resistance. However, the stable setup is essential for solid carbide because it is more brittle. When you need to use a CNC machine to machine harder stainless steel in mass production with high dimensional accuracy, solid carbide drill bits will be your ideal choice.
- HSS Drill Bits for Stainless Steel
Standard HSS drill bits are not suitable for machining stainless steel unless you use the coated HSS drill bits, which improve their durability. Use them occasionally when you need to machine soft stainless steel sheets at a lower cost. Although HSS drill bits have higher toughness, they are lacking in the hot-hardness required for drilling stainless steel.
Step 2: Key Criteria to Consider
- Hardness
The hardness of the drill bits must be greatly higher than that of the materials required to be cut. The drill bits must be rigid enough to ensure the ultra-sharp cutting edges with improved wear-resistance, enabling their stable cutting of work-hardening surfaces without tool deformation. For example, cobalt and solid carbide feature high hardness to maintain strong cutting forces.
- Thermal Stability
Since stainless steel has poor heat conductivity, drill bits for stainless steel should have the capabilities to maintain consistent hardness without softening even under high-temperature machining. Both cobalt and solid carbide drill bits feature exceptional thermal stability. Cobalt drill bits have superior hot-hardness to enable their sharpness, while solid carbide drill bits have outstanding stability even under extremely high temperatures.
- Toughness
The higher hardness of drill bits can resist the tool deformation, while the improved toughness is essential for setups without stability to protect drill bits from tool chipping and breakage. Drill bits for stainless steel require enough toughness to resist the tool vibration and excessive feed rates. Cobalt drill bits feature exceptional toughness, while solid carbide drill bits have enhanced hardness. In general, cobalt drill bits outperform carbide drill bits because they can keep a balance between rigidity and toughness.
- Brittleness
The brittleness will greatly affect the risk of tool breakage. Although solid carbide drill bits feature exceptional cutting performance, they are more brittle. Choosing carbide drill bits only when the CNC environment is rigid. However, cobalt drill bits and HSS drill bits demonstrate higher toughness to minimize the risk of tool breakage.
- Edge Retention
Drill bits should remain sharp in continuous drilling. The edge retention of cobalt and carbide drill bits is outstanding, and they can drill more holes after resharpening and tool changes. Solid carbide drill bits have optimal edge retention, while HSS cutting edges are prone to becoming dull. However, advanced coatings can significantly enhance the edge retention.
- Wear Resistance
In long-time drilling, machining stainless steel will cause abrasive wear, so that the tool service life can be determined by its wear resistance. Solid carbide drill bits feature more wear resistance than cobalt drill bits, which enables their outstanding capabilities to resist abrasive wear, while coated cobalt drill bits can deliver consistent cutting stability.
- Stability
The stability can greatly decrease the heat accumulation, tool vibration, and deflection, which also extends the tool service life. The stable geometry design can minimize the risk of tool chatter and the creation of poor-quality holes. The stable drill bit can also ensure the straightness of holes and less tool wear. Typically, stub-length drill bits can deliver higher drilling stability than standard-length drill bits.
The drill bit size will directly affect the cutting load and thermal generation in machining stainless steel. Choosing the appropriate drill bit size can ensure accuracy. Accurate drill sizing is essential when machining fluid passages in a stainless steel hydraulic manifold. Drill bits with smaller diameters heat up faster, which means higher accuracy and rigidity are required. Additionally, although drill bits with large diameters feature a thicker core and rigid cutting edges, they require superior cooling and improved torque.
- Through-Coolant Design
Drill bits with through-coolant design are extensively applied to CNC machining and deep-hole drilling of stainless steel. They can significantly improve the chip evacuation and lower the cutting temperature to avoid the heat buildup, which is beneficial for long-time machining of stainless steel. High-performance solid carbide drill bits are typically equipped with internal coolant channels to deliver efficient cooling, which is ideal for deep-hole drilling.
Step 3: Consider Drill Bit Geometry
The point angle is at the cutting tip of the drill bit, which controls the centring capabilities and the penetration. The 135° split-point is perfect for drill bits for stainless steel due to their capabilities of self-centring and decreasing thrust. The 118° split-point is suitable for machining softer stainless steel because of slower penetration and improved thrust.
- Helix Angle
The helix angle of the drill bit is the angle between the cutting edge and the tool axis, which is commonly between 20° and 40° to influence the rigidity of the cutting edges of drill bits for stainless steel. The helix angle between 28° and 35° is the ideal choice for drill bits to machine stainless steel with the smooth chip flow. The helix angle between 15° and 25° is typically applied to machine stainless steel with higher hardness and viscosity. The high helix, which is higher than 35°, is crucial for machining softer stainless steel with more efficient chip evacuation.
- Flute Design
The types of flutes of drill bits for stainless steel are versatile to decide the chip shape, chip, and coolant flow. The variable-helix flute is ideal for deep-hole drilling of stainless steel, enabling less chip clogging and tool vibration. The standard flute, featuring the simpler structure and the thicker core, is suitable for general-purpose drilling of larger holes. In addition, through-coolant flutes are perfect for drilling deep holes in mass production. The coolants are beneficial for decreasing the cutting temperature and boosting the chip flow.
- Core Thickness
The higher core thickness of drill bits for stainless steel will cause less tool deflection and higher toughness. However, the excessively thick core will decrease the chip evacuation. Typically, the core thickness is 0.3 to 0.5 times the bit diameter. Drill bits for stainless steel feature the web-thinning design to decrease the cutting forces with consistent rigidity. When the rigidity of drill bits for stainless steel is the crucial factor, you should choose the thicker core.
- Chip Evacuation
Chips are long and stringy in machining stainless steel, which may cause excessive heat and extra cutting. Hence, you should apply the wider flutes and appropriate helix angles to produce the shorter chips. Additionally, the variable helix should be used to eliminate the chip harmonics. In deep-hole drilling, complete chip-clearing cycles and peck drilling are required. To remove the chips in a timely manner, the drill bits for stainless steel should feature internal or external coolant channels.
- Margin Design
The narrow margin design of drill bits for stainless steel can cause less friction and heat generation, making it suitable for being applied to stainless steel to improve the cutting. Although the multi-margin design will enhance the dimensional accuracy with improved surface finishes, it can also cause more tool friction, so that high-quality coolants are required. The web design is perfect for machining workpieces with smaller diameters and wear-prone materials to decrease the torque and heat generation.
Step 4: Select Suitable Coatings
- TiN
If you choose TiN, you will achieve improved lubrication and wear resistance. Although it is perfect for being applied to cobalt drill bits, it is not suitable for application scenarios under high temperatures.
- TiAlN / AlTiN
Both TiAIN and AITiN have outstanding heat resistance in high-speed machining of stainless steel. They can effectively form the aluminium oxide layer to protect drill bits from heat generation, especially in dry-cutting.
- TiCN
TiCN has more rigidity and wear resistance than TiN, enabling its superior performance in machining hardened stainless steel with higher precision. TiCN is commonly used for drill bits for stainless steel with its excellent lubricity.
- AlCrN
AICrN is perfect for drill bits for stainless steel with its excellent capabilities in resisting oxidation and heat generation. It can help tools withstand extremely high cutting forces without breaking, enabling their wide use in CNC industries.
Step 5: Choose the Correct Cutting Parameters
- Proper Cutting Speed
The appropriate SFM can effectively protect drill bits for stainless steel from work hardening. The cutting speed of machining stainless steel should be lowered to avoid excessive heat buildup. The cutting speed varies based on the bit material. Slow to medium speeds are suitable for cobalt drill bits, while higher cutting speeds should be strictly controlled for carbide drill bits.
- Correct Feed Rates
Feed rates of drill bits for stainless steel should be steady to maintain smooth cutting. Too light feed rates can improve the smooth surface finishes with higher temperature, while excessive feed rates may cause the tool to break. You should keep the stable chip load to ensure the cutting operations are smooth and clean.
- Proper Lubrication & Cooling
Proper lubrication is crucial for drill bits to machine stainless steel. Cutting oil and coolant floods can effectively control the cutting temperature to ensure the service life of the cutting edges. Improper lubrication will enhance the risk of tool failure of drill bits for stainless steel.
Common Problems
- Tool Slipping
If tool slipping happens, the improper point design and less tool rigidity may be the reason. The split point should be modified to 135°, and the optimized fixturing can handle the problem.
- Oversized Holes
Tool chatter, less rigid core, and dull cutting edges can all be the reasons for oversized holes. You can use a more rigid drill bit or choose the carbide drill bits for stainless steel to enhance the dimensional accuracy.
Heat buildup may be caused by higher cutting speed, worn edges, improper lubrication, and cooling. It’s effective for you to decrease the cutting speed or apply the proper coolant to avoid work hardening and extend the tool’s lifespan.
Conclusion
Choosing the appropriate drill bits for stainless steel depends on tooling configurations like the bit material and the geometry, with proper cutting parameters. Follow the guide, and you will know how to correctly choose the suitable drill bits for stainless steel. At JINOO, you can also find proper drill bits to improve your machining with improved precision and extended tool life.
FAQ
What drill bit is perfect for machining stainless steel?
Cobalt and solid carbide drill bits are perfect for machining stainless steel due to their exceptional heat resistance, high rigidity, and edge retention, which means they are crucial for machining stainless steel.
Can I use standard HSS drill bits for machining stainless steel?
Regular HSS drill bits are not recommended for machining stainless steel. Stainless steel will harden in drilling, which means it’s difficult for HSS drill bits to penetrate stainless steel. It’s easy for them to become dull and generate too much heat.
Why is it easy for drill bits to become dull in machining stainless steel?
Stainless steel has poor thermal conductivity, and it’s easy for stainless steel to work harden under frictional heat. High pressure and excessive heat will be generated to lead to the rapid wear of improper drill bits.
What is the most suitable point angle of drill bits to machine stainless steel?
The split-point angle can be modified to 135° to decrease the cutting load and thermal generation. It can also help reduce the risk of tool slipping in drilling operations.
Is lubrication required when drilling stainless steel?
Of course. In drilling stainless steel, proper lubrication and cutting oil should be applied to protect drill bits for stainless steel from work hardening and ensure less heat generation with longer tool life.
