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Analysis of CNC Machining Tools commonly used and Machining Allowances of CNC Machined Parts

Common tools for CNC machining

Ball Nose End Mill

The ball nose end mill is commonly used for rough machining. During the entire process of rough machining of raw materials, the ball nose end mill with a large angle R radius is usually used. The ball nose end mill with a small angle R radius can also be used for milling around the sloping contour or milling around the side wall.

End Mill (Flat End)

The end mill, also known as the flat end, is commonly used for milling around the inclined side wall, and can also be used for vertical grooving. In special cases, it can also be used for milling product workpiece slopes.

Bull Nose End Mill

The bull nose end mill is commonly used for precision machining of sloping contours, and it is generally used in the final stage of shaping product workpieces. It can also be used for milling around side wall contours and grooves. The spherical end mill is also commonly used for machining font styles.

Boring Tool

The boring tool is commonly used for boring holes. When processing standard diameters, a spiral drill with a smaller diameter is used to drill first, followed by a boring tool for boring. It operates at a lower speed than normal.

Slotting Cutter

The slotting cutter is used for milling male and female slots. There are also common dovetail guide cutters with viewing angles.

Pointed Tool

The pointed tool is often used for carving fonts or hand-carving product workpiece numbers, with different angles. It can also be used for grooving and chamfering.

Why leave machining allowances for CNC machining parts?

The thickness of the metal layer cut off by each process in CNC machining is called the inter-process machining allowance. For inner holes and holes with twisted outer edges, the machining allowance is considered in diameter, so it is called the symmetrical allowance (that is, the polygonal allowance), that is, the actual thickness of the cut metal layer is half of the machining allowance in diameter. The machining allowance for flat surfaces is a one-sided allowance, which is the actual thickness of the cut metal layer.

The overall goal of leaving machining allowances on the workpiece is to better cut the machining errors and external defects left by the previous process, such as the cold and hard layer, vent holes, and wind-cured layer on the outside of castings, the oxide layer, carburized layer, and external cracks on the outside of cast steel parts, and the thermal stress layer and surface roughness after drilling and machining. This further develops the precision and surface roughness of the workpiece. The size of the machining allowance has a relatively large impact on the quality of the machining and production efficiency.

If the machining allowance is too large, it not only increases the workload of mechanical processing, reduces production efficiency, but also increases material, labor, and power consumption, and increases the cost of CNC precision machining. If the machining allowance is too small, it cannot eliminate various defects and errors left by the previous process or compensate for the clamping errors during this process, resulting in waste. The clamping rules are to ensure the quality of CNC machining, and to make the allowance as small as possible. Generally, the smaller the machining allowance, the more precise the machining.

When determining the CNC machining sequence, it is also necessary to first determine whether the part needs pre-processing before machining. Pre-processing is usually performed by a general CNC lathe. If the accuracy of the blank is relatively high, the accurate positioning is also relatively advanced, or the machining allowance is sufficient and symmetrical, then it can be processed directly on the CNC lathe without pre-processing. At this time, the division of the CNC lathe process should be based on the accuracy of the blank standard. It can be a single process or divided into several processes。

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