Calculation formulas for cutting speed, cutting amount and feed speed in CNC machining centers

In CNC machining center machine tool programming, programmers must determine the cutting parameters for each process and write them into the program in the form of instructions. Cutting parameters include cutting speed, back-cutting amount and feed speed. Different cutting parameters need to be selected for different machining methods.

1. Principles for selecting cutting parameters

The productivity of machining centers is generally improved in rough machining, but economy and machining cost must also be considered. In semi-finishing and finishing, cutting efficiency, economy and machining cost must be taken into account under the premise of ensuring machining quality. The specific values are determined according to the machine tool manual, cutting parameter manual and experience.

From the perspective of machining center tool durability, the order of selecting cutting parameters is: first determine the back-cutting amount, then determine the feed rate, and finally determine the cutting speed.

2. Determination of back-cutting amount

The back-cutting amount is determined by the rigidity of the machining center machine tool, workpiece and tool. When the rigidity allows, the back-cutting amount should be as equal to the machining allowance of the workpiece as possible, which can reduce the number of tool passes and improve production efficiency.

Principles for determining the back-cutting amount:

(1) When the machining center requires the workpiece surface roughness value to be Ra 12.5 microns ~ 25 microns, if the machining allowance of CNC machining is less than 5mm ~ 6mm, rough machining can meet the requirements in one pass. However, when the allowance 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 machining center requires the workpiece surface roughness value to be Ra 3.2 microns ~ 12.5 microns, it can be divided into two steps: rough machining and semi-finishing. During rough machining, the back-cutting amount is the same as before. After rough machining, leave 0.5 mm ~ 1.0 mm allowance, which is removed during semi-finishing.

(3) When the machining center requires the workpiece surface roughness value to be Ra 0.8 microns ~ 3.2 microns, it can be divided into three steps: rough machining, semi-finishing and finishing. The back-cutting amount during semi-finishing is 1.5mm ~ 2mm ~ 2 mm, and 0.3 mm ~ 0.5 mm is taken for back-cutting during finishing.

3. Determination of feed amount

The feed speed of the machining center is mainly based on the requirements of part processing accuracy and surface roughness, as well as the material selection of the machining center tool and workpiece. The maximum feed speed is limited by the rigidity of the machine tool and the performance of the feed system.

Principles for determining the feed speed:

1) Under the condition of ensuring the quality requirements of the workpiece, a higher feed speed can be selected to improve production efficiency. Generally, it is selected in the range of 100 ~ 200 m/min.

2) When cutting or processing deep holes or using high-speed steel tools, it is advisable to select a lower feed speed, generally in the range of 20 ~ 50 m/min.

3) When processing accuracy and surface roughness are required, the feed speed should be smaller, generally in the range of 20 ~ 50m/min.

4) When the tool is idle, especially when "returning to zero" for a long distance, the highest feed speed set by the machine tool CNC system can be selected.

4. Determination of the spindle speed of the machining center

The spindle speed should be selected according to the allowable cutting speed and the workpiece (or tool) diameter. The calculation formula is: n=1000v/D

V - cutting speed, in meters per minute, determined by tool durability;

N - spindle speed, in revolutions per minute;

D - workpiece diameter or tool diameter, in millimeters.

Finally, the calculated spindle speed n should be selected according to the machine tool manual.

In short, the specific value of the cutting parameter should be determined by analogy based on machine tool 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) Back cutting amount (cutting depth) ap

The vertical distance between the workpiece machining surface and the surface to be machined is called the back cutting amount. The back cutting amount is the cutting amount measured by the cutting edge base point in the direction perpendicular to the working plane. It is the depth of the turning tool cutting into the workpiece each time it feeds, so it is also called the cutting depth. According to this definition, when turning the outer circle longitudinally, the back-cut amount can be calculated as follows:

a p = (d w-d m) / 2

a p - back-cut amount (mm);

D w - diameter of the workpiece to be machined (mm);

Dm - diameter of the workpiece machining surface (mm).

Example 1: The diameter of the machined surface is known to be 95mm; now the diameter is 90mm after one turn, and the back-cut amount is required.

Solution: A P = (D W-D M) / 2 = (95-90) / 2 = 2.5 mm

(2) Feeding

When the workpiece or tool rotates once, the relative displacement of the tool and the workpiece in the feed motion direction.

According to the different feeding directions, it can be divided into longitudinal feeding and transverse feeding. Longitudinal feeding refers to feeding along the bed rail direction, and transverse feeding refers to feeding perpendicular to the bed rail direction.

(Note) Feed speed vf refers to the instantaneous speed of the selected point on the cutting edge relative to the workpiece feed motion.

vf=fn

vf——Type feed speed (mm/s);

N——Spindle speed (rev/s);

F——Feed speed (mm/s).

(3) Cutting speed vc

The instantaneous speed of the selected point on the cutting edge of the machining center relative to the main motion of the workpiece. The formula is as follows:

vc=(dwn)/1000

vc——Type cutting speed (m/min);

Dw——Diameter of the workpiece surface to be machined (mm);

N——Workpiece speed (r/min).

The maximum cutting speed shall be used as the basis for calculation. For example, when turning, the value of the machining surface diameter shall be used for calculation, because the speed here is the highest and the tool wears the fastest.

Example 2: When turning the outer circle of a workpiece with a diameter of 60mm, select a lathe spindle speed of 600r/min and calculate vc. Solution: VC=(d w n)/1000=3.14 x60x 600/1000=113m/min

In actual production, the diameter of the workpiece is often known, and the cutting speed is selected based on factors such as the workpiece material, tool material, and processing requirements. If you want to learn UG programming, you can add QQ: 770573829 to receive learning materials and courses, and then convert the cutting speed to the lathe spindle speed, so as to adjust the lathe and get the following formula:

n=(1000v c)/ d w

Example 3: Turning the outer circle of a 260mm pulley on a CA6140 horizontal lathe, select vc as 90m/min, and calculate n.

Solution: n=(1000v c)/d w=(1000 x 90)/(3.14 x 260)=110 r/min

After calculating the lathe spindle speed, you should choose a value close to the nameplate, that is, you should choose n=100r/min as the actual lathe speed.

Summary:

Cutting parameters

1. Reverse cutting amount ap (mm) ap = (dw-dm) / 2 (mm)

2. Feed flow (mm / s)

3. Cutting speed vc (m / min) Vc = dn / 1000 (m / min)

n = 1000vc / d (r / min)