# Yijin Hardware Tells You Cutting Speed, Knife Amount, Feed Speed and Three calculation formulas for CNC Machining

Keywords: CNC, CNC machining, CNC machining cutting speed, knife amount, feed speed three calculation formula

In CNC programming, the programmer must determine the cutting amount of each process, in the form of instructions written into the program. Cutting parameters include cutting speed, knife back and feed speed, etc. For different processing methods, different cutting parameters need to be selected.

### 1. The selection principle of cutting parameters

Rough machining, generally to improve productivity, but also should consider the economy and processing costs; In semi-finishing and finishing, cutting efficiency, economy and processing cost should be taken into account on the premise of ensuring the processing quality. The specific value should be based on the machine tool manual, cutting manual, and combined with experience.

Starting from the tool’s durability, the selection sequence of cutting parameters is: first to determine the amount of back, then determine the amount of feed, and finally determine the cutting speed.

### 2. The determination of back eating knife

Back to eat by the CNC machine tool, the workpiece, and the tool stiffness to determine, in the stiffness allowed conditions, should be as far as possible to make back to eat the knife is equal to the workpiece machining allowance, so that can reduce the number of cutting, improve production efficiency.

### The principle of determining the amount of back cutting:

When the workpiece surface roughness value is Ra12.5μm ~ 25μm, if the machining allowance of CNC machining is less than 5mm~6mm, the rough machining can meet the requirements of a feed. However, when the allowance is large, the rigidity of the processing system is poor or the power of the machine is insufficient, it can be divided into multiple feeds.

When the workpiece surface roughness is Ra3.2μm ~ 12.5μm, it can be divided into rough machining and semi-finishing. Rough machining of the back knife selection with the front. 0.5mm ~ 1.0mm allowance is left after rough machining and removed during semi-finishing.

When the workpiece surface roughness is Ra0.8μm ~ 3.2μm, it can be divided into rough machining, semi-finish machining, and finish machining. Half finish when the back of the knife takes 1.5mm ~ 2mm. When finishing back to take 0.3mm ~ 0.5mm.

### 3. Determination of feed quantity

The feed is mainly based on the machining accuracy and surface roughness requirements of parts and the selection of tools and workpiece materials. The maximum feed speed is limited by the stiffness of the machine and the performance of the feed system.

### The principle for determining the rate of feed:

• When the quality of the workpiece can be guaranteed, in order to improve production efficiency, a higher feed speed can be selected. Generally, it is selected in the range of 100 ~ 200m/min.
• When cutting, machining deep holes, or machining with high-speed steel tools, it is advisable to choose a lower feed speed, generally in the range of 20 ~ 50m/min.
• When the machining accuracy and surface roughness are high, the feed speed should be smaller, generally within the range of 20 ~ 50m/min.
• The maximum feed speed set by the CNC system of the machine tool can be selected when the tool is traveling in the air, especially when the distance is “back to zero”.

### 4. Determination of spindle speed

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

N = 1000 v/PI D

V —- cutting speed, in m/min, determined by the tool durability;

N — – Spindle speed, unit: r/min;

D—- Workpiece diameter or tool diameter, in mm.

The calculation of the spindle speed n finally according to the machine tool manual to select the machine tool some or close to the speed.

In short, the specific values of cutting parameters should be determined by analogy according to machine tool performance, relevant manuals, and practical experience. At the same time, the spindle speed, cutting depth, and feed speed can adapt to each other, to form the best cutting amount.

(a) back knife (cutting depth) AP

The vertical distance between the machined surface of the workpiece and the surface to be machined is called the back bite. Backbite is measured through the cutting edge base point and perpendicular to the direction of the working plane. It is the depth of the turning tool into the workpiece when each feed, so it is also called the cutting depth. According to this definition, if the longitudinal car is in the outer circle, its back cutting amount can be calculated as follows:

A p = (d w – d m) /2

In the formula, A p — back bite (mm);

D W — workpiece surface diameter (mm);

Dm — machined surface diameter of the workpiece (mm).

Example 1: known workpiece surface diameter is 95mm; Now a feed car to the diameter of φ 90mm, the back to eat knife.

A p = (d w — 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 revolution of the workpiece or tool.

According to the different feed directions, it is divided into vertical feed and horizontal feed. The vertical feed refers to the feed along the guide rail direction of the lathe bed, and the horizontal feed refers to the feed perpendicular to the guide rail direction of the lathe bed.

(Note) The feed speed V F refers to the instantaneous speed of feed motion relative to the workpiece at the selected point on the cutting edge.

v f=fn

Where V f — the feed speed (mm/s);

N — spindle speed (r/s);

F — feed (mm /s)

### (3) Cutting speed v C

The instantaneous velocity of the selected point on the cutting edge is relative to the main motion of the workpiece. The calculation formula is as follows

V c= PI d w n /1000

Where v C — cutting speed (m/min);

Dw — workpiece surface diameter to be machined (mm);

N — workpiece speed (r/min).

The maximum cutting speed should be used in the calculation, such as the number of surface diameters to be machined during turning, where the speed is the highest and the tool wears the most.

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

V c=(π d w n)/1000 = 3.14x60x600/1000 = 113 m/min

In the actual production, is often known as workpiece diameter, according to the workpiece material, tool material and processing requirements and other factors selected cutting speed, want to learn UG programming can please contact us to get learning materials and courses, and then the cutting speed into the lathe spindle speed, in order to adjust the lathe, the following formula has been obtained:

N =(1000V c)/ π d w

Example 3: In CA6140 horizontal lathe turning φ 260mm outer circle with wheel, select VC 90m/min, find n.

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

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

### Cutting parameter

1. Back cutting ap(mm) AP = (DW-DM) / 2 (mm)
2. Feed f(mm/r)

Vc (m/min) vc =∏dn/1000(m/min)

N = 1000 vc / ∏ d (r/min)

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