Why can't it be accomplished in one step from Rough Machining to Finishing?
In the manufacturing process of CNC machining, the two commonly used methods are milling and turning, collectively known as machining. In CNC lathe machining, these two methods are further categorized as “roughing” and “finishing”.
After “roughing” in CNC machining, the workpiece is close to the desired shape and size. However, there is still enough metal left on the machined surface to allow for finish machining, which ensures a smooth and dimensionally accurate surface.
Roughing
Roughing is the initial stage of machining, the main purpose of which is to quickly remove excess material from the workpiece and gradually approach the final shape. This process emphasizes rapid material removal and is often used to reduce the effort of subsequent machining steps.
Roughing methods include turning, milling, drilling, boring, etc. High-power machine tools and larger cutting volumes are used to increase productivity and reduce tool wear. Roughing does not require high surface quality, and the standard for tool dulling is generally a significant increase in cutting force, i.e., the width of wear on the back face VB.
Finishing
Finishing, on the other hand, is the final stage of machining and involves the removal of a small portion of material to achieve the surface quality, dimensional accuracy and precise geometry required by the design specifications.
Finishing methods include fine milling, grinding, plating, shot peening, polishing, anodizing, powder coating, sandblasting, painting, etc., using finer tools and smaller cutting parameters to ensure higher surface quality and accuracy. In finishing, the amount of wear on the back face of the tool is no longer the main criterion, but the surface quality of the workpiece is the primary concern.
In general, roughing and finishing are indispensable links in machining, each of them bears a different function, together constitute a complete machining process.
A brief overview of the difference between rough and finish machining
| Comparison Dimension | Rough Machining | Finishing Machining |
| Processing Objective | Quickly remove excess rough material and lay a good foundation | Achieve final dimensions, shape, and surface precision of parts |
| Precision Requirements | Lower, allowing a larger error range | Extremely high, must meet strict dimensional and surface roughness |
| Cutting Parameters | Large cutting depth, high feed rate, high cutting power | Small cutting depth, low feed rate, precise control |
| Equipment Type | High-power, high-rigidity general processing equipment | High-precision, high-stability special precision equipment |
| Tool Selection | High-strength, durable rough machining tools | High-precision, sharp-edged precision machining tools |
| Cutting Force & Heat Impact| | Large cutting force, prone to deformation and internal stress | Small cutting force, low heat impact, minimal deformation risk |
| Processing Efficiency | Focus on efficiency for rapid material removal | Focus on quality, slower speed but high precision |
| Subsequent Processes | Prepare for finishing, requiring further processing | Complete final machining, no major processing needed |
Why can't we go directly from roughing to finishing?
1. Thermal deformation and mechanical stress: Roughing directly after finishing, due to the thermal deformation and mechanical stress generated in the roughing process, may lead to unstable dimensions of the workpiece, affecting the accuracy of finishing.
2. Tool wear: Roughing process, tool wear is faster, if used directly for finishing, may lead to rough machined surfaces, and even produce machining defects.
3. Material hardening: Some materials may be hardened in the roughing process, direct finishing will increase tool wear and reduce machining efficiency.
This time it is necessary to add semi-finishing to the roughing to finishing process.
Semi-finish machining is based on rough machining, to further reduce the material allowance, improve the dimensional accuracy and surface finish of the workpiece, in preparation for finish machining.
Necessity of semi-finishing
Semi-finishing has the following important roles as a transitional stage:
1. Reduction of thermal deformation and mechanical stresses:
By semi-finishing, the stresses generated by roughing can be released and the effect of thermal deformation on finishing can be reduced.
2. Extend tool life
Semi-finishing allows the use of more durable tools than roughing, reducing tool wear during finishing.
3. Improve machining efficiency
Semi-finishing removes a large portion of the allowance, reducing the amount of material removed during finishing and improving machining efficiency.
To summarize, directly from roughing to finishing will result in machining quality not meeting the requirements for the reasons mentioned above, so it is necessary to carry out machining in stages to ensure machining accuracy and surface quality.