如何提高车削表面光洁度？

In the finishing stage of turning operations, the last thing operators want to do is scrap parts due to poor surface finish quality. The factory also needs to consider many factors to improve surface finishing and meet customer requirements. Suitable cutting tools and parameters are indispensable, but taking a comprehensive approach to producing a better surface finish is also important.

A comprehensive approach should be taken to improve the surface finishment in turning operations

It is important to consider all the steps from roughing to finishing, as they are interrelated and interdependent.

To ensure a high-quality surface finish, the operator needs to remove an appropriate amount of material in roughing and semi-finishing so that finishing can be performed with minimal or no errors. Any issues can lead to poor surface quality.

Roughing removes most of the excess material so finishing tools are not under pressure, and reasonable roughing stock also prevents premature wear of finishing tools. Many shops prefer more aggressive feed rates in roughing which can cause larger burrs on the part walls that are hard to break off.

If these burrs are found on the part during finishing operations, they can be very hard (built-up edge) leading to extremely short tool life. Roughing tools should be programmed to remove notch burrs and should provide a good work surface for the finishing stage.

In a normally running lathe or steel turning operation, the operator needs to generate a good enough surface finish at the early stages to achieve the required finish. If you start looking at hardened steel components, the pre-heat treat rough rolled surface finish will greatly affect the final surface finish after heat treatment.

Choose the appropriate feed and speed

In finishing, you will use higher surface feed so the speed is faster and the feed rate is lower. Also, your depth of cut is generally smaller. But equally important is making sure the feed rate matches the surface finish you want. Too slow of a feed rate can lead to excessive rubbing and premature wear on the insert causing poor surface finish.

Faster cutting speeds help raise the temperature slightly for better surface finishes. It also prevents the material from sticking to the top or face of the tool. The operator should increase speed slightly over roughing applications but not too much or the opposite effect will happen. If the built-up edge is occurring on the insert sides, increase the feed rate.

Compared to roughing applications, many brands tend to lower speed which is a common mistake in finishing operations. Increasing speed is necessary for a high-quality surface finish.

Determining the right depth of cut will aid in the stability of the turning process. Too small of a depth of cut will cause the nose radius to radially apply all of the forces into the part which causes vibration and negatively impacts surface finish.

It is also important not to take too large of a depth of cut since most material should be removed in the roughing and semi-finishing stages. You typically want a light depth of cut and a lower feed rate.

Choose suitable brand inserts

Using brand name inserts helps produce better cutting performance. Another aspect is talking with insert manufacturers about new applications which will help determine which insert will produce high-quality surface finishes in turning operations to select the right insert. Machining conditions and workpiece material determine what type of insert is suitable, but some general characteristics can be recommended for finishing stages.

Whenever part geometry allows, larger radii are typically preferred for finishing. The larger radius helps streamline the material removal more effectively, almost like a burnishing tool. With a larger nose radius, slightly higher feeds can be run while still maintaining high surface quality. However, in thin wall applications, a smaller nose radius lowers radial cutting forces which can lead to deflection and vibration negatively impacting the surface finish. The shape of the insert has a big influence on initial chip formation and surface finish.

The role of burnishing tools in machining

Using brand name inserts helps produce better cutting performance. Another aspect is talking with insert manufacturers about new applications which will help determine which insert will produce high-quality surface finishes in turning operations to select the right insert. Machining conditions and workpiece material determine what type of insert is suitable, but some general characteristics can be recommended for finishing stages.

Whenever part geometry allows, larger radii are typically preferred for finishing. The larger radius helps streamline the material removal more effectively, almost like a burnishing tool. With a larger nose radius, slightly higher feeds can be run while still maintaining high surface quality. However, in thin wall applications, a smaller nose radius lowers radial cutting forces which can lead to deflection and vibration negatively impacting the surface finish. The shape of the insert has a big influence on initial chip formation and surface finish.

Choose suitable chip breakers

Choosing the right chip breaker is also key. When the insert is engaged in cutting, the top face of the insert has a direct relationship with the material being machined and the chip contact area. Therefore, the chip breaker will look different if you are taking a shallow depth of cut at a lower feed rate versus a higher feed rate and deeper depth of cut. You need to select the proper chip breaker for the material because chip control is critical for maintaining consistent good surface finishes especially across multiple parts.

How to control the chips

For most turning operations, it is recommended to direct high-pressure coolant directly at the cutting edge. This helps evacuate chips from the cutting zone. Chip control is critical for maintaining high-quality surface finishes. Removing the chips prevents the tool from encountering chips again which can damage the cutting edge. It also prevents chips from curling around the tool and moving across the workpiece surface which can potentially cause scratches or defects in the finish.

The coolant helps keep the part and tool cool so you can machine at faster speeds. If high-pressure coolant is not possible on the machine, flood coolant is the next best option.

Coolant is not recommended for all applications, however. For turning hardened materials (anything over HRC 50), ceramic tooling should avoid coolant since it has a tendency to thermally shock the tool which can lead to tool breakage. However, if the material is soft on one side, coolant can be used with ceramic inserts.

Chip control is essential because, in this process, we need to give the chips a way to dissipate heat. But you also need an adequate cutting area. If you decrease the cutting area, the mass that’s taking heat away from the cutting zone is reduced and you’ll start seeing effects of chemical wear, flank wear, and pitting on the insert. When you think about surface finish, chip control really becomes a challenge. This is why you have to choose the right geometries and maintain proper cutting parameters for a given application.

The importance of rigidity

Many agree that the tool holder and fixturing play a big role in achieving high-quality surface finishes. If the fixturing is not rigid it can lead to vibration which can affect the finish. Equally important is making sure the tool holder has the shortest overhang possible to help maintain its rigidity. The workpiece and tool should be well supported so there is no vibration during the finishing process.

One thing many may not necessarily think of is how the insert is placed in the tool holder. The design of the tool holder can play a big role. An overly open insert pocket decreases the contact area between the insert and pocket thus introducing movement into the pocket. This causes micro-vibration negatively affecting surface finish.

The movement in the tool also makes it difficult to hold part dimensional tolerances. The tool holder should match the insert size tolerances and be in good condition. There should be no wear or deformation since even the slightest movement will have detrimental effects.

Follow good machining practices

The best way to generate good surface finishes is to start with insert manufacturer recommendations and follow what the factory suggests which will be a good starting point. We can make adjustments in trial cuts but those recommendations are meant for experienced operators so it’s good to build up that expertise over time.

Choosing inserts with a positive rake angle is preferable for finishing. The positive rake helps produce a sharper cutting edge to sever the material. For roughing stages, the negative rake can be suggested since it places more force behind the cutting edge to remove more material providing a better starting point for the finishing stages.

Another consideration here is directional forces. In the final stages, you want as much force as possible axially into the part since that will give you the stability you need. Choosing inserts with a lead angle close to 0° gets you more force axially but you also need to increase the insert back rake to get high-quality surface finishes.

Tangential force is an important factor during machining. Tangential force, which is axial force plus radial force, can be thought of as constant during turning. If the operator increases axial force, they decrease the effect of radial forces allowing them to hold tighter tolerances and reduce micro-vibration by decreasing the natural instability. This is not necessarily a factor to consider during the roughing and semi-finishing stages of the process.

Finally, pay attention to the direction the tool is cutting. You want to make sure you are introducing cutting forces into a good supporting portion of the part, machining away from the fixture can lead to vibration which will also impact tool life, and certainly, your surface finish will be affected.

conclusion

With diligent effort at every stage of the turning process and utilizing science-based parameter selection, high-quality vibration-resistant tooling, ensured clamping rigidity, reinforced cooling and lubrication, and post-processing, mirror-like smooth and polished turned surfaces can be obtained.