When NC programming, the programmer must determine the cutting amount of each process and write it into the program in the form of instructions. Cutting amount includes cutting speed, back cutting amount and feed speed, etc. For different processing methods, different cutting parameters need to be selected

1. The selection principle of cutting amount

In rough machining, productivity is generally the main focus, but economy and processing costs should also be considered; in semi-finishing and finishing machining, cutting efficiency, economy, and processing costs should be considered on the premise of ensuring processing quality. The specific value should be determined according to the machine tool manual, cutting quantity manual, and experience.

Starting from the durability of the tool, the selection order of the cutting amount is: first determine the back cutting amount, then determine the feed rate, and finally determine the cutting speed.

2. Determination of the amount of back knife

The amount of back cut is determined by the rigidity of the machine tool, workpiece and tool. Under the condition of rigidity, the back cut should be equal to the machining allowance of the workpiece as much as possible, so as to reduce the number of tool passes and improve production efficiency.

Principles for determining the amount of back cut:

(1) When the surface roughness value of the workpiece is required to be Ra12.5μm~25μm, if the machining allowance of CNC machining is less than 5mm~6mm, the rough machining can meet the requirement with one feed. However, when the margin is large, the rigidity of the process system is poor or the power of the machine tool is insufficient, it can be completed in multiple feeds.

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(2) When the surface roughness value of the workpiece is required to be Ra3.2μm~12.5μm, it can be divided into two steps of rough machining and semi-finishing. The amount of back cut during rough machining is the same as before. Leave a margin of 0.5mm~1.0mm after rough machining, and cut it off during semi-finishing.

(3) When the surface roughness value of the workpiece is required to be Ra0.8μm~3.2μm, it can be divided into three steps: rough machining, semi-finishing machining, and finishing machining. During semi-finishing, the back-feeding knife measures 1.5mm~2mm. During finishing machining, the back-feeding knife measures 0.3mm~0.5mm.

3. Determination of feed rate

The feed rate is mainly selected according to the machining accuracy and surface roughness requirements of the parts, as well as the materials of the tools and workpieces. The maximum feed rate is limited by the rigidity of the machine tool and the performance of the feed system.