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How to get better surface finish on CNC machined parts

In order to obtain a better surface finish, we need to keep the following points in mind. Some of these main indicators come before we start manufacturing, such as correct dimensions and tolerances, shape, quality of raw materials used, etc. But after the machined part is produced, some work needs to be done.

Surface Finishing: A process that helps define and refine the overall texture (lay, roughness, and waviness) of a machined part. We cannot ignore the importance of an impeccable surface finish, especially in aerospace and medical applications. Scrapped parts in the finishing stage are not what the shop expects. But what variables need to be considered before entering the completion stage?

How can we ensure that the steps we are taking will result in a better surface finish? We've compiled a list of dos and don'ts for major surface finishes to help you improve your CNC machined parts.

1. Learn about measuring surface finish

There are a variety of techniques and properties for surface finish measurement, including profiling, area, and microscopic inspection, with a focus on peak roughness (Ra) and its degree of separation (D). We need to understand which techniques are best suited and can achieve the desired results without spending a lot of effort and time.

2. Increase the speed and reduce the feed

When machining expensive parts, make sure to always follow the correct pre-defined feeds and speeds. The correct way to handle finishing is to increase surface feet per minute (SFM) and decrease inches per revolution (IPR). Increasing Surface Feet per Minute (SFM) will reduce Bulk Edge (BUE). This will extend tool life and reduce the likelihood of catastrophic tool failure damaging the finished product. Reducing inches per revolution (IPR) will reduce side wear and extend insert life.

When roughing, it is best to use a tool capable of high feeds to remove material quickly. When finishing, it is best to have a shallow depth of cut and keep the feed rate conservative.

3. Use a chipbreaker

Chip control is the key to a good surface finish. The resulting chips largely hinder the entire machining process. Before touching the workpiece, it should be controlled first.

We recommend that you use a high-quality chipbreaker, which reduces cutting pressure and makes chip evacuation easier. In materials that produce slender chips, it helps longer chip strings leave the cutting zone quickly and easily by breaking the chips into bits that tend to fall into the cutting zone.

4. Increase the nose radius

There is a direct relationship between the blade tip radius and the final surface finish. It is true that a smaller nose radius reduces stress on the tool, but also limits the feed rates that can be used.

The insert can only be fed at half the nose radius. Beyond this range, the resulting surface resembles a thread. So use the largest possible radius to produce the best finish without chattering.

The larger nose radius also enables heavier cuts, which are necessary when cutting difficult-to-cut materials. However, if the nose radius is larger, more material must be left on the workpiece to be removed in the finishing pass.

5. Use balancing tools to reduce vibration

It is important to use a balanced tool technique to reduce noticeable vibration during finishing. This step becomes even more important if your RPM is higher.

6. Use sharp blades, leading and positive angles

There is no doubt that we need sharper blades, larger lead angles and positive rake angles for better surface finishes.

7. Check the tool holder and workpiece holder

One factor that is often overlooked when trying to improve the finish is the tool holder. If the blade holder is old and the groove used to hold the blade is worn, the blade may move. Any movement of the blade will cause chatter and result in a poor surface finish.

Chatter from incorrect tool holding and fixtures or non-rigid machine tools can produce poor surface finish.

A strict and stable work environment is also key. Also, the higher the metal removal rate, the more important stable workpiece holding is.

8. Do not use the same tool for roughing and finishing

Learn to reserve roughing tools for roughing and finishing tools for finishing.

Parts can be roughed with inserts with large nose radii, large rake angles and rapid feed rates. A finishing tool with the desired lead angle and radius can then use wiper flatness to flatten the part for a better surface finish.

The shallow depth of finishing is good, but it must be equal to or greater than the radius. Otherwise, the blade will push the material instead of cutting, resulting in poor surface quality, edge burrs, and reduced blade life.

9. Avoid Pauses

Unnecessary pauses and pauses can also get in the way of getting work done properly. Remember, every time your tool stops moving while in contact with the lathe or workpiece, it leaves a mark.

If this happens often, I suggest you completely change the process! Do your best to ensure that your tool does not stop or hesitate throughout the cutting process.

10. Avoid lowering the centerline

The best way to ensure a proper cutting process is to follow the 70:30 ratio, not the 50:50 method. Mid-cuts can tap the blade along the edge of the material, which can cause burns. This may result in an incorrect surface finish.