When CNC programming, the programmer must determine the cutting amount of each operation and write it into the program in the form of instructions. The cutting amount includes cutting speed, back feed amount and feed rate. For different machining methods, different cutting amounts need to be selected.
1. The principle of selection of cutting amount
In rough machining, it is generally necessary to improve productivity, but economy and processing cost should also be considered; In the case of semi-finishing and finishing, cutting efficiency, economy and processing cost should be taken into account under the premise of ensuring the processing quality. The specific value should be determined according to the machine tool manual, the cutting quantity manual, and combined with experience.
Starting from the durability of the tool, the order of selection of cutting amount is: first determine the amount of back eating tool, then determine the feed, and finally determine the cutting speed.
2. Determine the amount of knife eaten on the back
The amount of back eating tool is determined by the stiffness of the machine tool, the workpiece and the tool, and under the condition that the stiffness allows, the amount of back eating tool should be equal to the machining allowance of the workpiece as much as possible, so that the number of passes can be reduced and the production efficiency can be improved.
Principles for determining the amount of back knife:
(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 roughing can be fed once to meet the requirements. 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 by multiple feeds.
(2) When the surface roughness value of the workpiece is required to be Ra3.2μm~12.5μm, it can be divided into two steps: rough machining and semi-finishing. The amount of back eating knives during rough machining is selected as before. After rough machining, leave a margin of 0.5mm~1.0mm, 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 and finishing. The amount of back eating knife during semi-finishing is 1.5mm~2mm. When finishing, the back knife is 0.3mm~0.5mm.
3. Determination of feed
The feed is mainly based on the machining accuracy and surface roughness requirements of the parts, as well as the material selection of tools and workpieces. The maximum feed rate is limited by the stiffness of the machine and the performance of the feed system.
Principles for determining feed rates:
1) When the quality requirements of the workpiece can be guaranteed, in order to improve the production efficiency, a higher feed rate can be selected. Generally, it is selected in the range of 100~200m/min.
2) When cutting, processing deep holes or processing with high-speed steel tools, it is advisable to choose a lower feed speed, generally in the range of 20~50m/min.
3) When the 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 is empty, especially when the tool is "back to zero" at a long distance, the maximum feed speed set by the CNC system of the machine tool can be selected.
4. Determination of spindle speed
The spindle speed should be selected according to the permissible cutting speed and the diameter of the workpiece (or tool). It is calculated as follows:
n=1000*v/π*D
v---- cutting speed, in m/min, is determined by the durability of the tool;
n----主轴转速，单位为 r/min；
D---- workpiece diameter or tool diameter in mm.
The calculated spindle speed n should finally be selected according to the machine tool manual.
In short, the specific value of the cutting amount should be determined by analogy according to the performance of the machine tool, the relevant manuals and practical experience. At the same time, the spindle speed, cutting depth and feed rate can be adapted to each other to form the optimal cutting amount.
5. Reference formula
1) Back eating tool amount (cutting depth) AP
The vertical distance between the machined surface of the workpiece and the surface to be machined is called the back-eating amount. The back feed is measured by the cutting edge base point and perpendicular to the direction of the working plane, which is the depth of the turning tool cutting into the workpiece at each feed, so it is also called the cutting depth. According to this definition, if in the longitudinal outer circle of the car, the amount of back knife can be calculated as follows:
ap =（dw-dm）/2
式中 ap----背吃刀量（mm）；
dw---- surface diameter of the workpiece to be machined (mm);
dm----工件已加工表面直径（mm）。
Example 1: The surface diameter of the workpiece to be machined is known to be Φ95mm; Now a feed car to a diameter of Φ90mm, to find the back of the knife.
解：ap=（dw-dm）/2=（95-90）/2=2.5mm
2) Feed f
The relative displacement of the tool and the workpiece in the direction of feed motion for each rotation of the workpiece or tool. According to the different feed directions, it is divided into longitudinal feed and transverse feed, longitudinal feed refers to the feed along the direction of the lathe bed guide rail, and the transverse feed refers to the feed perpendicular to the direction of the lathe bed guide rail.
The feed rate v f refers to the instantaneous velocity of the feed movement of the workpiece at the selected point on the cutting edge.
vf=f*n
式中 vf----进给速度（mm/s）；
n----主轴转速（r/s）；
f----进给量（mm /s）。
3) Cutting speed vc
The instantaneous velocity of the selected point on the cutting edge with respect to the main motion of the workpiece.
vc=( π*dw*n)/1000
式中 vc----切削速度 (m/min) ；
dw---- surface diameter of the workpiece to be machined (mm);
n----工件转速（ r/min）。
The maximum cutting speed should be used in the calculation, for example, in the case of turning, the value of the diameter of the surface to be machined, because here the speed is the highest and the tool wears the fastest.
Example 2: Turning the outer circle of the workpiece with a diameter of Φ60mm, the selected lathe spindle speed is 600r/min, and vc is found
解：v c=( π*d*w*n)/1000 = 3.14x60x600/1000 = 113 m/min
In actual production, the diameter of the workpiece is often known, and the cutting speed is selected according to factors such as workpiece material, tool material and processing requirements, and then the cutting speed is converted to the spindle speed of the lathe, so as to adjust the lathe, and the following formula is obtained:
n=( 1000*vc)/π*dw
Example 3: Turn the outer circle of the belt wheel with Φ260mm on the CA6140 horizontal lathe, select vc as 90m/min, and find n.
解：n=( 1000*vc)/ π*dw=(1000x90)/ (3.14x260) =110r/min
After calculating the spindle speed of the lathe, the value close to the nameplate should be selected, that is, n=100r/min should be selected as the actual speed of the lathe.
6. Summary
The three elements of cutting amount refer to the general term of cutting speed vc, feed f (or feed speed vf), and back eating tool ap.
1. Back eating knife amount ap (mm)
ap=(dw-dm) / 2
2.进给量 f(mm/r)
vf=f*n
3.切削速度vc(m/min)
vc=( π*dw*n)/1000