In the precision machining phase of turning operations, the last thing operators want is to discard parts due to poor surface finish.
Factories also need to consider many factors to enhance surface smoothness and meet customer requirements.
Choosing the right tooling and cutting parameters is essential, but so is adopting a comprehensive approach to produce superior surface finishes.
I. Adopting a Comprehensive Approach
It’s crucial to consider all the stages, from roughing to finishing, as they’re interconnected and interdependent. For example, before initiating rough cuts, allowance for semi-finishing and finishing processes should be contemplated.
To ensure a high-quality surface finish, operators need to remove the appropriate amount of material during the roughing and semi-finishing stages. This way, the finishing process can be executed with minimal errors. Any misstep could lead to subpar surface quality.
Roughing removes most of the initial material, allowing finishing tools to operate without undue stress. Balancing the roughing allowance also prevents premature wear of the finishing tools. Many factories prefer aggressive feed rates during roughing, which might result in larger burrs on the part walls, making them difficult to break.
If these burrs are discovered during the finishing operations, they can be exceptionally hard, leading to shortened tool life. Roughing tools should programmatically remove these burrs, ensuring a suitable surface for the finishing stage.
If you observe a regularly operating machine tool or soft steel turning, achieving an optimal surface finish early on is essential. When researching hardened steel components, the surface finish of the rough-turned part before heat treatment will significantly influence the final post-treatment finish.
II. Selecting Appropriate Feed and Speed
During finishing, you’ll employ higher surface feeds, which means faster speeds but slower feed rates. Also, your cutting depth is generally reduced. However, it’s equally important to align the feed rate with the desired surface finish. A feed rate that’s too slow can lead to excessive tool friction and premature wear, resulting in a suboptimal finish.
A faster cutting speed slightly elevates the temperature, yielding a better surface finish. It also prevents the material from sticking to the tool’s top or surface. Compared to roughing applications, operators should moderately increase the speed but not excessively, lest it produces adverse effects. If there’s tool buildup on the tool’s flank, the feed should be increased.
In contrast to roughing operations, many brands tend to decrease the speed, a common mistake in finishing. Increasing speed is essential for obtaining a high-quality surface finish.
Excessive tolerances in tool holders reduce the contact area between the insert (e.g., WNMG insert) and the holder, causing holder movement. This can lead to micro-vibrations, negatively impacting the surface finish.
Identifying the correct cutting depth promotes stability during turning. A too-shallow depth means the tool’s nose radius exerts all its force radially onto the part, causing vibrations and adversely affecting the finish.
It’s also important not to opt for an excessive cutting depth, as most material should be removed during roughing and semi-finishing stages. Typically, you’d want a light cutting depth paired with a slower feed rate.
III. Choosing the Right Branded Cutting Tools
Using branded cutting tools can yield better cutting results. Another consideration is discussing new applications with tool manufacturers, which can help determine which blade will achieve high-quality surface finishes in turning operations. The machining conditions and the material of the part dictate which type of blade is appropriate, but some general characteristics can be recommended during the precision machining stage.
As long as the geometry of the part allows, a larger radius is typically preferred during precision machining. A larger radius helps smooth out the material more efficiently, almost like a squeegee. By using a larger tip radius, you can slightly increase the feed rate while still maintaining a high surface quality. However, in thin-wall applications, a smaller tip radius reduces the radial cutting force, which might lead to deflection and vibration, negatively impacting the surface finish.
The shape of the blade has a significant impact on initial chip formation and surface finish.
IV. The Role of Finishing Blades in Machining
Using branded cutting tools can lead to superior cutting results. Another advantage is the ability to discuss new applications with blade manufacturers, assisting in determining which blade will produce high-quality surface finishes in turning processes. The machining conditions and the material of the component decide the suitable blade type. However, during the finishing stage, certain general characteristics can be suggested.
Provided the part’s geometric shape allows, a larger radius is often the top choice in finishing operations. This larger radius aids in effectively smoothing the material, acting much like a wiper. With a larger tool tip radius, you can slightly up the feed rate while maintaining a high-quality surface. However, in applications with thin walls, a smaller tool tip radius reduces the radial cutting force, potentially leading to deflections and vibrations that can mar the surface finish.
The blade’s shape plays a vital role in the initial formation of chips and the resulting surface finish.
V. Choosing the Right Chip Breaker
The selection of an appropriate chip breaker is pivotal. When the cutting tool engages, the top surface of the tool directly correlates with the material processing and chip area. Therefore, if you’re achieving a shallow cutting depth at a lower feed rate, the chip breaker’s appearance will differ from when cutting deeper at a higher feed rate. It’s essential to choose the right chip breaker for the material, as consistent chip removal is crucial for maintaining a uniform and fine surface finish, especially across multiple parts.
VI. Thinner Coatings Are Preferable
One overlooked aspect during this process is how the grade of the cutting tool impacts surface finish, particularly in relation to its coating.
When discussing Physical Vapor Deposition (PVD) versus Chemical Vapor Deposition (CVD) coatings, CVD coatings tend to be much thicker than PVD ones. Thicker coatings, compared to their thinner counterparts, consistently face greater challenges in producing a high-quality surface finish. Due to its adhesive nature, PVD yields a better surface finish than CVD. PVD-coated tools have a complete coating on all surfaces, while CVD might reduce coating at the microscopic geometry level, altering the shape of that geometry.
VII Controlling Chip Formation
For most turning operations, it’s recommended to direct high-pressure coolant straight at the cutting edge. This aids in clearing chips from the cutting zone. Chip control is vital for maintaining a high-quality finish. Clearing chips prevents the tool from coming into contact with them again, which can potentially damage the tool’s edge. It also stops chips from curling around the tool and moving across the workpiece surface, possibly resulting in scratches or blemishes during polishing.
“Coolant helps in keeping both the part and the tool cool, allowing for faster cutting speeds. If your equipment doesn’t support high-pressure coolant, conventional or internal coolant methods are still optimal choices.”
Coolants aren’t recommended for all applications. For turning hardened materials – anything beyond HRC50, avoid using coolant with ceramic tools as it tends to thermally shock the tool, which can lead to tool breakage. However, for softer materials, coolants can be used with ceramic tools.
Chip control is indispensable, as we need to dissipate heat from the chips. Yet, you also need a substantial cutting area. If you reduce the cutting area, the mass removing heat from the cutting zone diminishes, and you’ll begin to notice chemical wear, flank wear, and crater wear effects on the tool. Controlling chips becomes a real challenge when considering surface polishing. That’s why selecting the right geometry and maintaining appropriate cutting parameters for a given application is imperative.
VIII. Rigidity is Crucial
Many experts concur that the toolholder and the fixture play significant roles in achieving a high-quality surface finish. If the fixture lacks rigidity, it can lead to vibrations, potentially compromising the finish. It’s equally vital to ensure the toolholder has the shortest possible overhang to maintain its rigidity. Both the workpiece and the tool need ample support to prevent vibrations during precision machining.
One detail often overlooked is how the cutting insert is placed in its holder. The design of the holder can be pivotal. An excessively open clamping slot can reduce the contact area between the insert and the slot (like with WNMG inserts), introducing movement into the holder. This can lead to micro-vibrations, adversely affecting the surface finish.
Any movement on the tool can make maintaining part dimensional tolerances challenging. The holder should match the insert’s dimensional tolerances and be kept in optimal condition. There shouldn’t be any wear or deformations, as even the slightest movement can lead to undesirable outcomes.
IX. Employing Good Machining Practices
The best approach to achieve a good surface finish begins with selecting the right insert manufacturer and adhering to their recommendations; it’s an excellent starting point. Adjustments can be made during test cuts, but these guidelines best suit seasoned operators. Thus, regular professional knowledge accumulation is essential.
For precision machining, choosing inserts with a positive rake angle is preferable. A positive angle helps create a sharp cutting edge to shear through the material. For roughing stages, a negative rake angle is advisable as it applies more force behind the cutting edge, removing more material and providing a better foundation for the finishing stages.
Another consideration is the direction of the force. In the final stages, apply as much force as possible along the part’s axial direction, providing the desired stability. Choosing a tool with a near 0° entry angle will give you more force axially, but it’s also crucial to increase the tool’s clearance angle to attain a high-quality surface finish.
In machining, the tangential force plays a pivotal role. This force, the sum of axial and radial forces, is considered constant during turning. If operators increase the axial force, it reduces the effect of the radial force, enabling them to maintain tighter tolerances and minimize micro-vibrations due to reduced inherent instability. This isn’t always a primary concern during the roughing and semi-finishing stages.
Lastly, the direction of the tool’s cut is crucial. Ensure that operational forces are directed towards well-supported sections of the part. Machining away from the support will result in vibrations, affecting tool life, and undoubtedly, your surface finish will be compromised.