selection principle of cutting parameters

In rough machining, productivity is generally improved, but economy and processing cost should also be considered; in semi finish machining and finish machining, cutting efficiency, economy and processing cost should be considered on the premise of ensuring processing quality. The specific value should be determined according to the machine tool manual, cutting data manual and experience.

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

determination of back knife amount

The back feed is determined by the rigidity of the machine tool, workpiece and cutter. If the rigidity is allowed, the back feed should be equal to the machining allowance of the workpiece as far as possible, so as to reduce the number of tool feed and improve the production efficiency.

The principle of determining the amount of back knife:

(1) When the surface roughness of the workpiece is required to be Ra12.5 μ m ~ 25 μ m, if the machining allowance of NC machining is less than 5 mm ~ 6 mm, one feed of rough machining can meet the requirements. However, when the allowance is large, the rigidity of the process system is poor or the power of the machine tool is insufficient, the feeding can be completed by several times.

(2) When the surface roughness of the workpiece is required to be between 3.2 μ m and 12.5 μ m, it can be divided into rough machining and semi finish machining. The amount of back cutting in rough machining is the same as before. The allowance of 0.5mm ~ 1.0mm shall be left after rough machining and cut off during semi finish machining.

(3) When the surface roughness of the workpiece is required to be Ra0.8 μ m ~ 3.2 μ m, it can be divided into three steps: rough machining, semi finish machining and finish machining. 5 mm ~ 2 mm for semi finishing. When finishing, the back knife is 0.3mm ~ 0.5mm.

determination of feed rate

The feed rate is mainly based on the machining accuracy and surface roughness requirements of parts, as well as the material selection of tools and workpieces. The maximum feed speed is limited by the stiffness of the machine tool and the performance of the feed system.

The principle of determining the feed speed is as follows:

1) When the quality requirements of the workpiece can be guaranteed, in order to improve the production efficiency, a higher feed speed can be selected. It is generally selected in the range of 100-200m / min.

2) When cutting, machining deep hole or machining with high speed steel tool, it is better to choose a lower feed speed, generally in the range of 20 ~ 50M / min.

3) When the requirements of machining accuracy and surface roughness are high, the feed speed should be smaller, generally in the range of 20 ~ 50M / min.

4) When the tool has no stroke, especially when it returns to zero in a long distance, the maximum feed speed set by the NC system of the machine tool can be selected.

determination of spindle speed

Spindle speed should be selected according to the allowable cutting speed and workpiece (or tool) diameter. The calculation formula is as follows:

n=1000*v/π*D

V — cutting speed, M / min, determined by tool life;

N — spindle speed, unit: R / min;

D — diameter of workpiece or cutter, unit: mm.

Finally, according to the instructions of the machine tool, the speed n of the spindle should be selected.

In a word, the specific value of cutting parameters should be determined by analogy according to the machine performance, relevant manuals and practical experience. At the same time, the spindle speed, cutting depth and feed speed can be adapted to each other to form the best cutting parameters.

reference formula

1) Depth of cut

The vertical distance between the machined surface and the surface to be machined is called back feed. Back draft is measured through the cutting edge base point and perpendicular to the direction of the work plane. It is the depth of the turning tool cutting into the workpiece during each feed, so it is also called cutting depth. According to this definition, if it is in the longitudinal outer circle, the back cutting amount can be calculated according to the following formula:

ap =（dw-dm）/2

Where, AP — the amount of back cutting (mm);

DW — diameter of workpiece surface to be machined (mm);

DM — diameter of machined surface of workpiece (mm).

Example 1: it is known that the diameter of the workpiece surface to be machined is Φ 95mm; now the diameter of the workpiece is Φ 90mm, and the back feed is calculated.

Solution: AP = (DW DM) / 2 = (95-90) / 2 = 2.5mm

2) Feed rate ﹥ f

The relative displacement between the tool and the workpiece in the direction of feed motion when the workpiece or cutter rotates one cycle. According to the different feed direction, it is divided into longitudinal feed and transverse feed. Longitudinal feed refers to the feed along the lathe bed guide direction, and transverse feed refers to the feed perpendicular to the lathe bed guide direction.

The feed speed VF refers to the instantaneous speed of the selected point on the cutting edge relative to the workpiece feed motion.

vf=f*n

Where VF — feed speed (mm / s);

N — spindle speed (R / s);

F — feed rate (mm / s).

3) Cutting speed ﹣ VC

The instantaneous velocity of the selected point on the cutting edge relative to the main motion of the workpiece.

vc=( π*dw*n)/1000

Where VC — cutting speed (M / min);

DW — diameter of workpiece surface to be machined (mm);

N — workpiece speed (R / min).

In the calculation, the maximum cutting speed should be taken as the criterion. For example, in turning, the value of the diameter of the surface to be machined should be taken as the basis, because the speed here is the highest and the tool wear is the fastest.

Example 2: when turning the outer circle of the workpiece with a diameter of Φ 60mm, the selected lathe spindle speed is 600r / min, and VC is calculated

Solution: V C = (π * D * w * n) / 1000 = 3.14x600x600/1000 = 113 M / min

In actual production, the diameter of the workpiece is often known. The cutting speed is selected according to the material of the workpiece, the material of the cutting tool and the processing requirements, and then the cutting speed is converted into the rotation speed of the lathe spindle, so as to adjust the lathe

n=( 1000*vc)/π*dw

Example 3: in CA6140 horizontal lathe, cut the outer circle of belt pulley with diameter of 260mm, select VC as 90m / min, and find n.

Solution: n = (1000 * VC) / π * DW = (1000×90) / (3.14×260) = 110R / min

After calculating the lathe spindle speed, the value close to the nameplate should be selected, that is, n = 100r / min should be selected as the actual speed of the lathe.

summary

The three elements of cutting parameters refer to cutting speed VC, feed rate f (or feed rate VF) and back cutting amount AP.

1. The amount of back knife AP (mm)

ap=（dw-dm) / 2

2. Feed rate f (mm / R)

vf=f*n

3. Cutting speed VC (M / min)

vc=( π*dw*n)/1000