Generally, CNC machining basics comprise standard subtractive manufacturing operations such as turning, milling, facing, drilling, grooving, boring, etc. These processes involves the layer-by-layer removal of excess materials from solid workpieces and adding quality finishes to achieve the required dimensional features for a part.
However, attaining these features in a single machining operation is impossible. Thus, manufacturers usually carry out production in two steps with variable parameters – roughing and finishing processes. This article spells out the difference between roughing and finishing in machining.
What is Roughing in Machining?
Roughing in machining refers to a process that involves removing a substantial amount of excess material from workpieces. Roughing is usually the first step during the processing stages in machining. Therefore, CNC roughing produces the material shape or figure close to the needed part geometry, making subsequent machining processes easier and more efficient.
In addition, the purpose of roughing cuts is to remove blank allowances quickly. Manufacturers usually conduct roughing processes at high depths of cut and large feed rates to eliminate large quantities of chips within a short period. However, these machining parameters impact product surface and dimensional features.
Even though roughing processes offer high production efficiency, rough products are often poorly finished, inaccurate, and imprecise. Therefore, roughing operations produce components that do not meet high dimensional accuracy and tight tolerances.
What is Finishing in Machining?
Finishing in machining refers to a manufacturing process that involves altering the surface of already fabricated parts or components for specific resolutions. This majorly includes removing aesthetic defects to improve a part’s appearance or to attain certain mechanical properties that enhance performance.
In common, finishing comprises several processes, such as precision machining, grinding, electroplating, bead blasting, polishing, anodizing, powder coating, sandblasting, painting, etc. Therefore, depending on required parts features, manufacturers use a particular finish process or a suitable combination of finishing operations to add or improve properties of fabricated parts like the following:
- Electrical conductivity
- Corrosion resistance
- Wear resistance, etc.
In most CNC manufacturing projects, finishing is usually done as the final procedure after the engineer performs roughing operations on workpieces. Furthermore, the purpose of the finishing process is to remove the required excess material and complete the manufactured component to achieve final dimensions in terms of flatness, roughness, thickness, tolerances, and surface finish.
Difference Between Roughing and Finishing in Machining
Amidst the machining operations carried out in a typical CNC machine shop, finishing and roughing processes remain vital manufacturing processes to meet basic machining requirements.
While some assume these machining steps are the same or mistake one for the other, keep in mind that they have clear distinctions from an engineering point of view. Check the differences between roughing and finishing in machining below:
Definition of Roughing
Roughing is a machining operation to remove excess material from workpieces quickly. It imparts the primary shape and dimensions needed to make other subsequent machining operations faster and more efficient.
Definition of Finishing
Finishing is a machining process to modify the surface of manufactured parts to improve appearance or achieve specific properties that enhance the part’s capabilities. In addition, finishing is usually carried out to meet standard machining requirements in terms of flatness, roughness, thickness, and tolerances and to improve the surface finishes for various components.
2. Rate of Material Removal
The material removal rate for roughing operations is high, while the stock removal rate for finishing processes is comparatively low.
3. Feed Rate and Depth of Cut
Roughing processes utilize higher feed rates and depth of cut. In contrast, manufacturers perform finishing operations at lower feed rates and cutting depths.
4. Surface Roughness/Finish
The higher feed rate and depth of cut employed in roughing produce more visible scallop marks or feed marks. Therefore, rough products usually have poor surface roughness. On the flip side, the shallow cuts and small feed rates utilized in finishing operations leave fewer scallop marks, thus easing the fabrication of products with good surface finishes.
5. Dimensional Accuracy and Tolerance
Proper finish machining completes manufactured components to achieve final dimensions that meet high dimensional accuracy and the requirements for close tolerances while roughing cannot meet the requirements for machining accuracy and close tolerances.
6. Cutter Type
The edge and nose sharpness of cutters influence the level of attainable surface finish and accuracy in machining. Finishing machining requires sharp cutters that take a low chip load to get a better surface finish and dimensional accuracy. On the contrary, rough machining can utilize cutters with less sharp edges that take a high chip load since the quality surface finish is not a vital requirement.
Typical CNC roughing procedures precede finishing while finishing processes can be carried out only after roughing operations.
For much better contrast and clarity, check the differences between roughing cuts and finishing cuts in the chart form below:
|High feed rate
|Low feed rate
|High cutting depth
|Low cutting depth
|Higher material removal rate
|Lesser stock removal rate
|Poor surface quality
|Good surface finish
|Low dimensional accuracy
|High dimensional accuracy
|Poor adherence to tight tolerance levels
|Good adherence to close tolerance levels
|Larger chip load on the cutter
|Lesser chip load on the cutter
|Higher production efficiency
|Lower production efficiency
Considerations for Roughing in Machining
Roughing operations provide an efficient and rapid means for manufacturers to fabricate the benchmark shape of workpieces for subsequent machining. Nevertheless, certain considerations come into play when carrying out roughing in machining. Check them below:
1. Machining Parameters
CNC roughing tool software comprises pre-selected options for feed rate, cutting velocity, and depth. But these default machining parameters cannot predict the considerations for each specific roughing operation. Besides, applying these default parameters may result in processing errors. Therefore, you must select and optimize all roughing parameters to suit each workpiece and cutting tool to achieve processing efficiency.
2. Type of Machine Tool and Control Software
Roughing processes need equipment with high power, efficiency, and rigidity. That way, manual equipment cannot handle the tool movements needed to perform roughing. Likewise, software programmed for complex 3D milling procedures may not maintain constant cutting on workpieces with narrow corners. Thus, you must select machining tools and software that adapt to roughing operations.
3. Heat and Cutting Fluid
Roughing utilizes larger feed rates, leading to greater back-feeding. This, in turn, produces large cutting resistance that generates a substantial amount of heat. Further, the heat gets transferred to the cutting tool and the workpiece. At the same time, the heat intensifies tool wear and thermal distortion of the workpiece.
Therefore, you should establish heat treatment measures during roughing operations to avoid machining complications. Machinists often use water-based cutting fluid with considerable lubricating and cooling effects during roughing. If necessary, you can use oil baths or air cooling to mitigate the effect of the generated heat.
Considerations for Finishing in Machining
Finishing in machining is just as vital as any other operation in the manufacturing cycle. Besides, getting the finishing process makes your whole manufacturing efforts futile. Here are some important factors to consider before performing the finishing process:
1. Dimensional Accuracy
It is essential to note that applying finishes to manufactured components may alter their GD&T and other dimensional features. For instance, applying powder coating to metal parts may increase their surface thickness. So, it would help if you always examined the factors before applying finishes to ensure machining accuracy and precision.
2. The Part’s Application
Careful consideration of a part’s application and the potential conditions to which such component will be subjected will help make the right choice when choosing a finishing operation. For example, the finishing process for hidden parts used in an automobile will place less emphasis on aesthetics and more on improving the part’s durability.
After considering the above factors, you must also consider the overall cost of your finishing project. The best finishes often require high-quality materials, tools, and complex processes. Therefore, you must compare all these cost drivers with your manufacturing project before selecting a finishing procedure.
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Performing the right machining step at each manufacturing stage is crucial to the project’s success. Therefore, understanding the working principle of roughing and finishing remains pivotal to attaining accurate parts with quality finishes.