Generally, CNC machining fundamentals include standard material reduction manufacturing operations, such as turning, milling, end face machining, drilling, grooving, boring, etc. These processes involve removing excess material layer by layer from the solid workpiece to achieve the required part dimensions and features.
However, it is impossible to obtain these features in a single machining operation. Roughing is usually the first process, followed by finishing. After all stages, the required CNC products can be obtained from the blank. In this paper, we will focus on what is rough machining, finish machining and the difference between rough machining and finish machining.
What Is Rough Machining?
The rough machining process in mechanical processing is mainly used to quickly remove large pieces of material and rough process the workpiece into the desired shape, so as to make the subsequent processing more convenient and efficient. The purpose of rough machining is to quickly remove the blank allowance. Generally, large feed and cutting depth are selected to remove as much chips as possible in a short time. Therefore, rough machined products often have low precision, rough surface and high productivity. Rough machining is often the preparation for semi finishing and finishing, which cannot provide good surface finish and tight tolerance.
Advantages Of Rough Machining
–Rough machining can realize rapid feed, and the error can be corrected by subsequent finishing to ensure quality.
–Divide the processing stages and give full play to the advantages of rough and finish processing equipment. Rough machining equipment has high power, high efficiency, strong rigidity, and finish machining equipment has high precision and small error.
–Rough machining can find various defects of the blank, such as sand hole, air hole, insufficient machining allowance, etc., which is convenient for timely repair or scrapping, so as to avoid wasting processing time and cost.
–After hot working, the residual stress of the workpiece is large, so the rough and finish machining can be separated. The aging can be arranged to eliminate the residual stress, and the finishing can be arranged to eliminate the deformation after cooling.
What Is Finishing In Machining?
Finishing in machining refers to a manufacturing process that involves changing the surface of a manufactured part or component for a specific resolution. This mainly includes eliminating aesthetic defects to improve the appearance of parts or obtain some mechanical properties that can improve performance.
Generally, finishing includes precision machining, grinding, electroplating, sandblasting, polishing, anodizing, powder coating, sandblasting, painting and other processes. Therefore, the manufacturer shall use a specific finishing process or a combination of appropriate finishing operations according to the required part characteristics to increase or improve the performance of the manufactured parts, such as hardness, adhesion, corrosion resistance, etc.
In most CNC manufacturing projects, finish machining is usually the last process after the engineers rough machining the workpiece. In addition, the purpose of the finishing process is to remove the excess materials required and complete the manufactured parts to achieve the final dimensions in terms of flatness, roughness, thickness, tolerance and surface finish.
Difference Between Rough Machining And Finish Machining
In order to meet the basic requirements of CNC machining, it is necessary to perform many operations in the machining workshop, including turning, milling, end face machining, etc. Conventional machining process is applicable to high cutting amount and good surface quality, involving two stages or two processes. Rough machining operation is used to produce part geometry close to finished product shape in a short time, and finish machining operation is carried out after rough machining to obtain final geometry and other details. What is the specific difference between rough machining and finish machining?
Rough cuts are made to quickly give the basic shape according to the required features. Here, surface roughness is not an important factor; On the contrary, the ultimate goal is to remove as much unnecessary material as possible. In contrast, finish machining is performed to improve the surface finish, dimensional accuracy, and tolerance of the required features. In the case of qualified finishing, the cutting rate is not important.
2. Process parameters and MRR:
Cutting speed (Vc), feed rate (s or f) and cutting depth (t or a) are three process parameters of each traditional machining process. These parameters greatly affect the overall machining action and capability. Higher speeds, feeds, and cutting depths can improve material removal rate (MRR) at the expense of surface finish.
MRR is proportional to speed, feed and cutting depth, so it can be expressed mathematically by multiplying speed, feed and cutting depth by the normal number of unit conversion. During processing, the speed is usually kept constant, because the speed is selected according to the workpiece and tool materials, machine capacity, vibration level and other important factors.
In order to achieve the basic goal, higher feed and cutting depth are used in rough machining, so MRR is increased. On the other hand, the finishing tool path adopts low feed and cutting depth, so MRR is reduced.
3. Surface finish and dimensional accuracy:
Due to the feed speed, every traditional machining process will have fan-shaped marks or feed marks on the finished product surface. This serrated scallop mark will cause primary surface roughness.
In addition to the geometry of the cutting tool, the surface roughness also depends directly on the feed rate. Higher feed rates will result in poor surface finish. Higher cutting depths also tend to reduce surface finish and machining accuracy.
In rough cutting, higher feed and cutting depth are used, so poor surface finish is obtained. It also fails to provide high dimensional accuracy and tight tolerances.
On the other hand, due to the very low feed and cutting depth, the finishing tool path can improve the finish, accuracy and tolerance.
Different levels of roughness have different requirements for the cutting tool and cutting angle. Negative rake inserts are best suited for rough machining because they absorb cutting forces and thus achieve higher cutting speeds. For finishing, the front angle blade is usually selected to obtain better surface finish.
Precautions During Machining
Precautions for rough machining
Rough machining operation provides an efficient and fast method for manufacturers to manufacture workpiece datum shapes for subsequent processing. However, some considerations may come into play when rough machining is performed in machining. Check them below:
1. Processing parameters
The CNC roughing tool software includes preselected options for feed rate, cutting speed and depth. However, these default machining parameters cannot predict the precautions for each specific roughing operation. In addition, applying these default parameters may cause processing errors. Therefore, you must select and optimize all rough machining parameters to adapt to each workpiece and tool to achieve machining efficiency.
2. Machine tool type and control software
Rough machining process requires equipment with high power, high efficiency and rigidity. As a result, the manual equipment cannot handle the tool movement required to perform rough machining. Similarly, software programmed for complex 3D milling programs may not be able to maintain constant cutting on workpieces with narrow corners. Therefore, you must select the machining tools and software suitable for rough machining operations.
3. Heat and cutting fluid
The roughing uses a higher feed rate, resulting in a greater return. This, in turn, creates greater cutting resistance, which generates a lot of heat. In addition, heat is transferred to the cutting tool and workpiece. At the same time, the heat aggravates the tool wear and the thermal deformation of the workpiece.
Therefore, heat treatment measures should be formulated in the rough machining process to avoid processing complications. Machinists often use water-based cutting fluid in rough machining, which has considerable lubricating and cooling effects. If necessary, you can use oil bath or air cooling to reduce the impact of the heat generated.
Precautions for finishing In Machining
Finishing in machining is as important as any other operation in the manufacturing cycle. In addition, finishing will make your entire manufacturing effort go to waste. The following are some important factors to consider before implementing the collation process:
1. Dimensional accuracy
It is important to note that the application of surface treatment to manufactured components may change their GD&T and other dimensional features. For example, coating metal parts with powder paint may increase their surface thickness. Therefore, it will be helpful if you always check these factors to ensure machining accuracy and precision before applying surface treatment.
2. Some applications
When selecting finishing operations, careful consideration of the application of parts and the potential conditions to which such components will be subjected will help to make the right choice. For example, the finishing process of automobile hidden parts will pay less attention to beauty and more attention to improving the durability of parts.
After considering the above factors, you must also consider the overall cost of completing the project. The best finishes usually require high-quality materials, tools and complex processes.