Precautions for obtaining better surface finish for CNC machined parts
In order to obtain better CNC machining parts, we need to keep the following points in mind. Some of the main indicators appeared before we started manufacturing, such as correct size and tolerance, shape, quality of raw materials used, etc. But after the processing parts are produced, some work still needs to be done.
Surface finishing: The process that helps to define and refine the overall texture (laying, roughness and waviness) of the machined part. We cannot ignore the importance of impeccable surface finish, which is particularly important in aerospace and medical applications. Scrapped parts in the finishing stage are not the expected result of the workshop. But what variables need to be considered before entering the completion phase?
How can we ensure that the steps we are taking will result in a better surface finish? We have compiled a list of major surface treatment considerations to help you improve CNC machined parts.
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1. Understand the measurement surface finish
Surface finish measurement has a variety of technologies and characteristics, including profile analysis, area and microscopic examination, with a focus on the peak roughness (Ra) and its resolution (D). We need to know which technology is most suitable and can achieve the desired effect without spending a lot of effort and time.
2. Increase speed and reduce feed
When machining expensive parts, be sure to always follow the correct pre-defined feeds and speeds. The correct way to handle finishing is to increase the surface feet per minute (SFM) and reduce the surface inches per revolution (IPR). Increasing the surface feet per minute (SFM) will reduce the built-up edge (BUE). This will extend tool life and reduce the possibility of catastrophic tool failure damaging the finished product. Reducing inch per revolution (IPR) will reduce side wear and extend blade life.
In rough machining, it is better to use a tool that can advance to remove the material quickly. During finishing, it is better to cut deeply and keep the feed speed conservative.
3. Use chip breaker
Controlling chip is the key to obtain good surface finish. The chip generated greatly hinders the whole processing process. Before touching the workpiece, control it first.
We recommend that you use high-quality chip breaking groove, which can reduce the cutting pressure and make chip removal easier. In materials that produce long and thin chips, by breaking the chips into bits that are easy to fall into the cutting area, it helps the long chip string to leave the cutting area quickly and easily.
4. Increase the head radius
There is a direct relationship between the blade tip radius and the final surface finish. It is true that a smaller tip radius will reduce the pressure on the tool, but it will also limit the feedrate that can be used.
The blade can only be fed at half the tip radius. Once outside this range, the resulting surface resembles a thread. Therefore, use the largest radius possible to produce the best finish without chatter.
A larger tip radius can also perform heavier cuts, which are necessary when cutting difficult to cut materials. However, if the tool tip radius is large, more material must be left on the workpiece to be removed in the finishing feed.
5. Use balancing tools to reduce vibration
It is important to use balanced tool technology to reduce significant vibration during finishing. If your RPM is higher, this step becomes more important.
6. Use sharp blade, guide angle and positive angle
There is no doubt that we need sharper blades, larger lead angles and positive rake angles to achieve better surface finish.
7. Check the tool rest and workpiece rest
One factor that is often overlooked when trying to improve the surface treatment is the tool holder. If the tool 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 poor surface finish.
Improper tool fixing and chatter caused by jigs or non rigid machine tools can result in poor surface finish.
Strict and stable working environment is also key. Moreover, the higher the metal removal rate, the more important the stable workpiece clamping.
8. Do not use the same tool for rough and finish machining
Learn to reserve rough machining tools for rough machining and finish machining tools for finish machining.
Parts can be rough machined with blades with large tip radius, large rake angle and rapid feed speed. Then, the finishing tool with the required lead angle and radius can use the smoothing edge flatness to smooth the part, thus obtaining better surface finish.
The shallow depth of finish machining 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 hinder the correct completion of work. Remember that every time your tool stops moving when it comes in contact with the lathe or workpiece, it will leave traces.
If this happens frequently, I suggest you change the process completely! Make every effort to ensure that your tool does not stop or hesitate during the entire cutting process.
10. Avoid lowering the centerline
The best way to ensure the correct cutting process is to follow the 70:30 ratio, not the 50:50 method. The blade may be tapped along the edge of the material in the middle of the cut, which may cause burns. This may result in incorrect surface finish.
