Whether the selection of cutting parameters (ap, f, v) is reasonable plays an important role in whether the potential of the machine tool and the cutting performance of the tool can be fully utilized to achieve high quality, high output, low cost and safe operation. In 2.3.3, the general principle of cutting amount selection is introduced. Here, the selection principle of turning amount is mainly discussed: during rough turning, first consider choosing a back engagement amount ap as large as possible, and secondly choose a relatively large cutting amount ap. Large feed f, finally determine a suitable cutting speed v. Increasing the back cutting amount ap can reduce the number of tool passes, and increasing the feed rate f is beneficial to chip breaking. Therefore, selecting the roughing cutting amount according to the above principles is beneficial to improve production efficiency, reduce tool consumption, and reduce processing costs.
In finish turning, the requirements for machining accuracy and surface roughness are relatively high, and the machining allowance is not large and relatively uniform. Therefore, when selecting the cutting amount for finish turning, you should focus on how to ensure the machining quality, and on this basis, try to increase productivity. Therefore, a small (but not too small) back cutting amount ap and feed rate f should be selected for finishing turning, and a cutting tool material with high cutting performance and reasonable geometric parameters should be selected to increase the cutting speed v as much as possible.
1. Determination of the back cut ap
Under the condition that the rigidity of the process system and the power of the machine tool allow, choose a larger amount of back cutting as much as possible to reduce the number of feeds. When the precision of the part is required to be high, it should be considered to leave a finishing allowance, which is generally smaller than that left during ordinary turning, usually 0.1-0.5mm.
2. Feed amount f (some CNC machine tools use feed speed Vf)
The selection of the feed rate f should be compatible with the amount of back cutting and the spindle speed. On the premise of ensuring the processing quality of the workpiece, a higher feed rate (below 2000mm/min) can be selected. When cutting off, turning deep holes or finishing turning, a lower feed rate should be selected. When the tool travels empty, especially when it is "returning to zero" at a long distance, the feed rate can be set as high as possible.
For rough turning, f=0.3～0.8㎜/r is generally used, for fine turning, f=0.1～0.3㎜/r is usually taken, and f=0.05～0.2㎜/r for cutting.
3. Determination of spindle speed
(1) Spindle speed for bare outer circle
Spindle speed should be determined according to the diameter of the part to be machined on the part and the cutting speed allowed by the parts, tool materials and processing properties during light turning of the outer circle.
In addition to calculation and table lookup, the cutting speed can also be determined according to practical experience. It should be noted that the low-speed output torque of the CNC lathe with AC frequency conversion speed regulation is small, so the cutting speed cannot be too low.
After the cutting speed is determined, use the formula n = 1000 vc/πd to calculate the spindle speed n (r/min). Table 5-9 is the reference value of cutting speed of cemented carbide cylindrical turning tool.
How to determine the cutting speed during machining, in addition to referring to the values listed in Table 5-6, is mainly determined based on practical experience.
Note: The tool durability is about 60 minutes when cutting steel and gray cast iron.
(2) Spindle speed during threading
When turning threads, the spindle speed of the lathe will be affected by various factors such as the pitch P (or lead) of the thread, the frequency up and down characteristics of the drive motor, and the speed of thread interpolation calculations. Therefore, different CNC systems are recommended. Spindle speed selection range. The spindle speed n (r/min) recommended by most economical CNC lathes for threading is:
n ≤ (1200/P)－k (5-1)
In the formula, P--the thread pitch to be processed, mm;
k--insurance factor, generally taken as 80.
In addition, when arranging the amount of rough and fine turning, attention should be paid to the allowable range of cutting amount given in the machine tool manual. For CNC lathes with AC frequency conversion speed regulation as the spindle, due to the low torque of the spindle at low speeds, special attention should be paid to cutting at this time. Dosage selection.
