3. How to improve surface finishing in CNC machining?
Adjusting the mentioned above parameters is already one of the best tool recommendations. Here are some of the complementary advice.
3.1. Increase the Speed
As a rule, higher values of surface feet per minute (SFM) directly result in the reduction of built-up edge (BUE). It also prolongs tool life and reduces the risks of tool failure and damage to a component.
Just ensure that the coolant pressure is sufficient, raw material is not prone, and the tooling is appropriate.
3.2. Reduce the Feed
Another rule is that a slower inch per revolution (IPR) or simply feed rate reduces flank wear and prolongs wiper insert life. It also helps to avoid poor finish outcomes and achieve a more accurate tool path.
When rouging, it’s especially vital to use a tool capable of high feeds, while when finishing, light depth of cut at high speeds works better. Although, you should be cautious about the phenomenon when the tool rub and not cut. Use the depth of cut of 0.0020 inches. It suits most applications.
3.3. Increase the Top Rake Angle
Take angle is definite as a variable in the cutting tool insert’s design. Let’s take a 45-degree lead face mill as an example. If the cutting force acts downward with this tool, it; may cause a part to flex after the cutter passes over it.
On the contrary, a 90-degree cutter creates the parallel force and does not make it flex, resulting in better dimensional tolerances and an excellent finish. Play with both lead angle and the tool pressure to achieve the best outcomes.
3.4. Use a Chip Breaker
Metal chips are the waste procedure in the process of machining. Tools are constructed in a way that helps chips evacuate. Yet, manufacturers experience problems with this waste on and off since it scratched the surface in the process.
Use a chip breaker to reduce cutting pressures and produce smaller or shorter chips that can be evacuated easily. Significantly, you deal with materials that produce long, stringy chips. A chip break will not let them interfere with high-end finishing.
See what chip breakers look like in the image below.
Every material is different, so choosing an appropriate chip breaker is not easy. Some options may work better for you, so give several of them a try.
Besides, pay attention to the setup stability, roughing application, and coolant usage. These factors combined together are what matters when choosing a chip breaker.
3.5. Use a Large Nose Radius
Besides, there is a direct dependency between the size of the insert’s nose radium and the type of surface finish produced. The interesting thing about this factor is that smaller nose radiuses decrease the pressure of tooling. Yet, they also limit the feed rate that can be set.
Any insert is capable of feeding only one-half of the nose radius. Basically, to avoid producing thread, you should use the largest radiuses possible. This would also eliminate chatter.
Another benefit of using a larger radius is a heavier cut that you can make. It’s useful when you have to process hard-to-cut materials. Such tooling is also less prone to wear.
Yet, larger nose radiuses take more materials from the workpiece. For example, using a 0.032-inch nose radius, you cannot finish there is only 0.01 inches to be removed.
3.6. Use a Wiper
Basically, using an insert with a wiper is the first piece of advice you may get from an experienced CNC mechanic. An insert with a wiper is capable of creating a much smoother surface in the milling pass.
It also makes it possible to increase feed rates without compromising the quality of the finished, as detailed in the above section. It qualifies when the speed of machining is crucial.
Besides, pay close attention to the tool diameter that you use. The cutter should be engaged somewhere between two-thirds and three-fourths of it for the best outcomes.
3.7. Use the Correct Technique
Unsurprisingly, the technique is crucial for achieving fine surface finishes. Thick-to-thin should be your very first rule to apply.
Arc-in and arc-out cutting ensures minimal impact on the insert edge due to the creation of a smooth transition. The same is for going into a corner.
Another piece of advice is to choose a cutter that is smaller than the radius. This way, you can program the cutter for the most smooth transition possible and eliminate sharp moves. This ensures the optimal tool path, deprived of deviations.
3.8. Use Different Tools to Rough and Finish
Well, some inserts may be used for routing, regular cutting, and finishing. You just change some parameters such as feed rate, speed, etc. Some manufacturers just utilize slightly used inserts for roughing.
Yet, it’s far not the most optimal way to get a smooth finish. Just because roughing should be done by using a large nose radius nose and a rapid feed rate. At the same time, the finishing tool with the desired angle should have a smaller nose, as described above.
Another vital consideration is the very material that inserts are made of. Cermet insert often produces better finishes, but it’s simply not applicable for heavy-duty roughing.
Remember, a light depth of cut should be equal to or more than the radius. Overview the tool will be just using the material forward.
3.9. Clear the Chips
Chip evacuation is a big deal, as you can see. It’s accompanied by a choice of whether or not to use coolant. Considerations here are the type of operation to perform, the material type, and the insert type.
You want to avoid applying coolant in milling operations and in turning applications involved with interrupted cuts.
On the contrary, with sticky materials such as aluminum or low-carbon steel, coolant will help you prevent material from sticking to the tool and, respectively, chips from not evacuating properly and damaging the surface.
3.10. Check the Toolholding and Workholding
When it comes to surface finishing, another overlooked factor is a tool holder itself. As an example, if the tool holder is old and it has a worn pocket, the insert may move. Fluctuations are minor, yet, they impact the end quality. Every uncontrolled movement results in a poor surface finish.
These fluctuations, called chatter, are caused by improper fixturing and tool holding. Another case is when a matching tool is not rigid, which is an essential problematic aspect.
Rigid workholding is also important. If both tooling and workpiece move uncontrollably, a good surface finish is absolutely cannot be achieved.
