1. The selection principle of cutting amount
Rough machining is generally based on improving productivity, but economy and processing cost should also be considered; semi-finishing and finishing should take into account cutting efficiency, economy and processing cost under the premise of ensuring processing quality. The specific value should be determined according to the machine tool manual, cutting parameter manual, and combined with experience.
2. Determining the amount of back knife
The amount of back-grabbing is determined by the rigidity of the machine tool, workpiece, and cutting tool. When the rigidity allows, the amount of back-grabbing should be equal to the machining allowance of the workpiece as much as possible. This can reduce the number of passes and improve production efficiency.
The principle of 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, one feed of rough machining can meet the requirement. 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.
(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-finish machining. The selection of the back-cutting amount during roughing is the same as before. Leave 0.5mm~1.0mm margin after rough machining, and cut off during semi-finish machining.
(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. For semi-finish machining, the back-cutting amount is 1.5mm~2mm. When finishing, the amount of the back knife is 0.3mm~0.5mm.
3. Determining the amount of feed
The feed rate is mainly selected according to the machining accuracy and surface roughness requirements of the parts and 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.
The principle of determining the feed rate:
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 selected in the range of 100～200m/min.
2) When cutting, processing deep holes or processing with high-speed steel tools, a lower feed rate should be selected, generally within the range of 20-50m/min.
3) When the machining accuracy and surface roughness are required to be high, the feed speed should be smaller, generally within the range of 20-50m/min.
4) When the tool is idle, especially when it is “zero return” at a long distance, the highest feed rate 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 allowable cutting speed and the diameter of the workpiece (or tool). The calculation formula is:
v—cutting speed, the unit is m/min, which is determined by the durability of the tool;
n—-spindle speed, the unit is 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 which has or is close to the machine speed.
In short, the specific value of the cutting parameters should be determined by analogy based on the performance of the machine tool, related manuals and actual 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.
5. Reference formula
1) The amount of back cutting (cutting depth) ap
The vertical distance between the machined surface of the workpiece and the surface to be machined is called the amount of back-cutting. The amount of back-grabbing is the amount of tool-grabbing measured in the direction perpendicular to the working plane through the base point of the cutting edge. It is the depth at which the turning tool cuts into the workpiece during each feed, so it is also called the depth of cut. According to this definition, for example, in the outer circle of the longitudinal car, the amount of the back knife can be calculated as follows:
ap = (dw-dm)/2
In the formula, ap — the amount of knife taken back (mm);
dw —-the diameter of the workpiece surface to be processed (mm);
dm —-The diameter of the machined surface of the workpiece (mm).
Example 1: It is known that the diameter of the workpiece surface to be machined is Φ95mm; now it feeds the car to a diameter of Φ90mm at one time, and finds the amount of the back tool.
2) Feed amount f
The relative displacement of the tool and the workpiece in the direction of the feed movement when the workpiece or tool makes one revolution. According to the feed direction, it is divided into longitudinal feed and transverse feed. The longitudinal feed refers to the feed along the direction of the lathe bed guide, and the transverse feed refers to the direction perpendicular to the lathe bed guide. Feed rate.
The feed speed v f refers to the instantaneous speed of the selected point on the cutting edge relative to the workpiece feed movement.
In the formula, vf — feed speed (mm/s);
n —- Spindle speed (r/s);
f —-Feeding amount (mm/s).
3) Cutting speed vc
The instantaneous speed of the main movement of the selected point on the cutting edge relative to the workpiece.
Where vc —cutting speed (m/min);
dw —-the diameter of the workpiece surface to be processed (mm);
n —- Workpiece rotation speed (r/min).
The calculation should be based on the maximum cutting speed. For example, when turning, use the value of the diameter of the surface to be machined for calculation, because here the speed is the highest and the tool wear is the fastest.
Example 2: Turning the outer circle of a workpiece with a diameter of Φ60mm, the selected lathe spindle speed is 600r/min, find vc
Solution: v c=( π*d*w*n)/1000 = 3.14x60x600/1000 = 113 m/min
In actual production, the diameter of the workpiece is often known. The cutting speed is selected according to the workpiece material, tool material and processing requirements, and then the cutting speed is converted into the lathe spindle speed to adjust the lathe, and the following formula is obtained:
Example 3: Turning the outer circle of a Φ260mm pulley on the CA6140 horizontal lathe, choose vc as 90m/min, and find n.
Solution: n=( 1000*vc)/ π*dw=(1000×90)/ (3.14×260) =110r/min
After the spindle speed of the lathe is calculated, the value close to the nameplate should be selected, that is, n=100r/min is selected as the actual speed of the lathe.
The three elements of cutting amount refer to the general term of the cutting speed vc, the feed amount f (or the feed speed vf), and the back-cutting amount ap.
ap=(dw-dm) / 2
2. Feed rate f(mm/r)
3. Cutting speed vc(m/min)