How to Improve Surface Finish in Turning

 

During the finishing stage of turning operations, the last thing an operator wants is to scrap a part due to poor surface finish. Manufacturers must consider various factors to improve surface finish and meet customer requirements. While using the right insert and cutting parameters is essential, taking a holistic approach to producing better surface quality is equally important.

 

▲ Turning Operations

 

I Taking a Holistic Approach

 

It is crucial to consider all steps from roughing to finishing, as they are interconnected, related, and interdependent. For instance, you must consider the allowance left for finishing and semi-finishing before starting roughing operations.

 

To ensure a high-quality surface finish, the operator needs to remove an appropriate amount of material during the roughing and semi-finishing stages so that the finishing process can proceed smoothly and with minimal errors. Any issues during these stages can lead to poor surface quality.

 

Roughing removes most of the material, reducing the stress on the finishing tool. Properly matching the roughing allowance also prevents premature damage to the finishing tool. Many manufacturers prefer to use more aggressive feed parameters during roughing, which may cause large burrs on the part walls, making them difficult to break.

 

▲ Remove an Appropriate Amount of Material

 

If these burrs are found during finishing operations, they can be very hard (resulting in built-up edge), leading to a significantly shortened insert life. The roughing tool should be programmed to remove notch burrs and provide a good machined surface for the finishing stage.

 

▲ Built-up Edge

 

If you observe a normally operating machine or turning operation on soft steel, the operator needs to achieve a good enough surface finish at the early stages to attain the necessary surface quality. If you start analyzing hardened steel components, the surface finish during rough rolling before heat treatment will greatly influence the final surface quality after heat treatment.

 

 

II Selecting the Appropriate Feed and Speed

 

During finishing, you will use a higher surface feed, leading to a faster speed and a lower feed rate. Additionally, the depth of cut will generally be smaller. However, it is equally important to ensure that the feed rate aligns with the desired surface finish. If the feed rate is too slow, it can cause excessive friction and premature wear on the insert, resulting in poor surface quality.

 

A faster cutting speed helps to slightly increase the temperature, which can improve surface finish. It also prevents the material from adhering to the top or surface of the tool. Compared to roughing applications, the operator should slightly increase the speed, but not too much, as this could have the opposite effect. If built-up edge appears on the side of the insert, the feed rate should be increased.

 

Many brands tend to lower the speed compared to roughing applications, which is a common mistake in finishing operations. Increasing the speed is essential for achieving a high-quality surface finish.

 

▲ A tool holder slot with excessive dimensional tolerance can reduce the contact area between the insert (such as a WNMG insert) and the slot, leading to movement in the slot. This can cause micro-vibrations, which negatively impact surface finish.

 

Determining the correct cutting depth will help stabilize the turning process. Too shallow a cutting depth can cause the tool's nose radius to apply all the force radially onto the part, which can lead to vibrations and negatively affect the surface finish.

 

It is also important to avoid using too large a cutting depth, as most of the material should be removed during the roughing and intermediate machining stages. Typically, you would want a light cutting depth and a lower feed rate.

 

 

III Selecting the Appropriate Brand Tools

 

▲ Branded Inserts

 

Using branded inserts can help achieve better cutting performance. Additionally, discussing new applications with insert manufacturers can assist in identifying which inserts will produce high-quality surface finishes during turning operations, thereby allowing for the selection of the appropriate insert. The machining conditions and material of the part determine which type of insert is suitable, but some general characteristics can be recommended during the finishing stage.

 

As long as the part geometry permits, larger radii are generally preferred during finishing. A larger radius helps smooth the material more effectively, almost like a wiper. With a larger insert radius, you can slightly increase the feed rate while still maintaining a high surface quality. However, in thin-wall applications, a smaller insert radius reduces radial cutting forces, which can prevent deflection and vibration, thereby avoiding negative impacts on surface finish.

 

The shape of the insert has a significant impact on initial chip formation and surface finish.

 

 

IV The Role of Wiper Inserts in Machining

 

Using branded inserts can help achieve better cutting performance. Additionally, discussing new applications with insert manufacturers can assist in identifying which inserts will produce high-quality surface finishes during turning operations, thereby allowing for the selection of the appropriate insert. The machining conditions and material of the part determine which type of insert is suitable, but some general characteristics can be recommended during the finishing stage.

 

As long as the part geometry permits, larger radii are generally preferred during finishing. A larger radius helps smooth the material more effectively, almost like a wiper. With a larger insert radius, you can slightly increase the feed rate while still maintaining a high surface quality. However, in thin-wall applications, a smaller insert radius reduces radial cutting forces, which can prevent deflection and vibration, thereby avoiding negative impacts on surface finish.

 

The shape of the insert has a significant impact on initial chip formation and surface finish.

 

 

V Selecting the Appropriate Chipbreaker

 

Choosing the appropriate chipbreaker is also crucial. When the insert engages in cutting, the top surface of the insert directly interacts with the material being machined and the chip area. Therefore, if you are using a lower feed rate to achieve a shallow depth of cut, the chipbreaker will appear different compared to when using a higher feed rate to achieve a deeper depth of cut. You need to select the appropriate chipbreaker for the material, as proper chipbreaking is essential for maintaining consistent and good surface finish, especially across multiple parts.

 

 

VI Thinner Coatings are Better

 

One often overlooked aspect of this process is how the grade of the insert affects surface finish, especially concerning coatings.

 

When comparing Physical Vapor Deposition (PVD) with Chemical Vapor Deposition (CVD) coatings, CVD coatings are often much thicker than PVD coatings. Thicker coatings face greater challenges in producing high-quality surface finishes, consistently so. Due to the adhesive nature of the coating, PVD provides a better surface finish than CVD coatings. PVD inserts have a complete coating on all surfaces, whereas CVD can reduce the coating on micro geometries, thereby altering the shape of the micro geometries.

 

 

VII How to Control Chips

 

▲ High-pressure Coolant Directly at The Cutting Edge

 

For most turning operations, it is recommended to direct high-pressure coolant directly at the cutting edge. This helps to clear chips out of the cutting area. Chip control is essential for maintaining a high-quality finish. Removing chips prevents the tool from re-engaging with them, which could damage the tool's cutting edge. It also prevents chips from curling around the tool and moving across the workpiece surface, which could cause scratches or blemishes during finishing.

 

"Coolant helps keep both the part and the tool cool, allowing you to cut at faster speeds. If high-pressure coolant is not an option, conventional or internal cooling is the best alternative."

 

Coolant is not recommended for all applications. For turning hardened materials-anything above HRC50-avoid using coolant with ceramic inserts, as it tends to cause thermal shock to the tool, which could lead to tool breakage. However, if the material is softer, coolant can be used with ceramic inserts.

 

Chip control is crucial because, in this process, we need to dissipate heat through the chips. However, you also need a sufficiently large cutting area. If the cutting area is reduced, the mass of material removing heat from the cutting zone will decrease, leading to chemical wear, flank wear, and crater wear on the insert. When considering surface finishing, chip control becomes a real challenge. This is why it's essential to choose the correct geometry and maintain proper cutting parameters for a given application.

 

 

VIII Rigidity is Also Important

 

Many agree that the toolholder and the fixture play a significant role in achieving high-quality surface finishes. If the fixture lacks rigidity, it will cause vibration, which can affect the finish. It is equally important to ensure that the tool holder has the shortest possible overhang to help maintain its rigidity. Both the workpiece and the tool should be adequately supported to eliminate vibration during finishing.

 

One aspect that many may not consider is how the insert is mounted in the tool holder. The design of the tool holder can play a critical role. A tool holder slot that is too open reduces the contact area between the insert and the tool holder slot (such as with WNMG inserts), introducing movement into the slot. This can lead to micro-vibrations that negatively impact surface finish.

 

Movement in the tool also makes it difficult to maintain dimensional tolerances of the part. The tool holder should match the insert's dimensional tolerance and remain in good condition. There should be no wear or deformation, as even the slightest movement can result in adverse consequences.

 

 

IX Adhere to Good Machining Practices

 

The best way to achieve a good surface finish is to start with the recommendations from the insert manufacturer and follow their guidelines. This is a good starting point. Adjustments can be made during test cuts, but these recommendations are best suited for operators with extensive experience, so it's essential to accumulate professional knowledge regularly.

 

Selecting inserts with a positive rake angle is preferred for finishing operations. A positive rake angle helps create a sharp cutting edge to shear the material. For roughing, a negative rake angle may be recommended, as it will exert more force behind the cutting edge to remove more material, providing a better starting point for finishing.

 

Another factor to consider is directional force. In the final stages, you want to apply as much force axially along the part as possible, as this will provide the stability you need. Choosing an insert with an approach angle close to 0° will allow you to exert more force axially, but you also need to increase the insert's back clearance to achieve a high-quality surface finish.

 

Tangential force is an important factor in machining. Tangential force, which combines axial and radial forces, can be considered a constant in turning operations. If operators increase axial force, they will reduce the impact of radial force, allowing them to maintain better tolerances and reduce micro-vibrations by minimizing natural instability. This may not be a crucial consideration during the roughing and semi-finishing stages of the process.

 

Finally, note that the direction of the cutting force is also important. You must ensure that the operational force is directed into the well-supported part of the workpiece. Machining away from the support will result in vibration, which can also impact tool life, and your surface finish will certainly be affected.