Redefining Reliability in Stainless Steels and Titanium Alloys
Kazuteru Takai | OSG Corporation Applications Engineer
With stainless steel being a durable and corrosion resistant material, it is used in a wide variety of applications and industries. Stainless steel’s excellent wear resistance property, however, makes it a challenging material to machine. In order to improve the workability of this valuable material, OSG has developed a new coolant-through carbide drill series – the WDO-SUS, to help manufacturers maximize their shops’ potential and performance. Titanium alloy, which shares many similar characteristics as stainless steel, can also be effectively machined with the new WDO-SUS.
Why is stainless steel so difficult to machine?
Four problems commonly associated with the machining of stainless steel include work hardening, poor thermal conductivity, welding, and the elongation of cut chips. The WDO-SUS was developed with specific features in response to these challenges.
Work hardening is a phenomenon where the strengthening of a metal occurs due to plastic deformation. Such strengthening is caused by the dislocation movements within the crystal structure of the material. Non-brittle metals with high melting points, such as stainless steel and titanium alloy, are prone to this condition. The key to reducing plastic deformation is by minimizing cutting force against the workpiece. To do so, maintaining a sharp cutting edge is of utmost importance. Taking note of this key feature, OSG’s WDO-SUS has employed a sharp cutting edge with a minimum chamfer width to suppress cutting forces.
Poor Thermal Conductivity
Materials with poor thermal conductivity have difficulties dispersing heat generated by machining. The rise in overall cutting heat temperature can cause the tool material and the coating on it to oxidize, accelerating tool wear. In order to suppress heat generation, the sharpness of the WDO-SUS’ cutting edge has been enhanced, and the margin area was reduced to a minimum. Furthermore, in order to quickly disperse machining heat, the WDO-SUS has employed a new oil hole shape to facilitate three times the coolant flow velocity in comparison to other conventional oil hole shapes. With the new coolant hole design, the total flow volume has been increased by 1.3 times versus conventional products, which can drastically improve chip evacuation.
Stainless steel has a high affinity with carbide materials, making it highly susceptible to welding, which can damage the cutting tool. To prevent this condition from occurring, the WDO-SUS drill has adopted OSG’s patented WXL coating, which has high adhesive strength to the tool material and a low coefficient of friction.
Elongation of Cut Chips
A unique characteristic of stainless steel is its high tensile strength. Therefore, when machining stainless steel, the chips emitted have a high tendency to elongate, causing them to clog during evacuation. With drilling, it is very difficult to analyze the actual machining condition within the hole. To overcome this challenge, OSG has turned to computer aided engineering (CAE) technology, which is commonly used for simulation, validation and optimization of products and manufacturing. With CAE, OSG was able to more accurately examine the environment within the hole in order to develop the ideal tool geometry tailored for creating compact chips in stainless steel.
Figure 1 illustrates the tool life difference between OSG’s WDO-SUS drill and a conventional product when machining SUS 304. The WDO-SUS drill outperformed the competitordrill with twice the durability.
Figure 1a. Stainless steel cutting data
Figure 1b. Margin wear comparison after machining SUS 304
Moreover, the WDO-SUS was able to excel in titanium alloy in addition to stainless steel, as shown in figure 2. The WDO-SUS was able to machine over 2,000 holes in titanium alloy while the competitor tool chipped and failed during the initial stage.
Figure 2. Titanium alloy cutting data
OSG’s new WDO-SUS series has adopted a tool geometry that emphasizes sharpness to reduce work hardening, thereby prolonging tool life for post-processing including reaming and tapping. Its new flute form encourages the creation of small cutting chips, which is essential for trouble-free chip evacuation. Furthermore, the WDO-SUS has employed a unique oil hole design for diameter sizes above 6 mm to suppress heat generation and to facilitate smooth chip evacuation. With the addition of OSG’s patented WXL coating, which has strong adhesion strength, high resistance against welding can be achieved. Utilizing OSG’s latest cutting tool technology, the WDO-SUS series is capable of drilling stainless steel and titanium alloy with predictable and consistent tool life, making efficient machining of difficult-to-machine materials a reality.
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