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How much is the CNC machining center cutting process, and it is necessary to do a good job in metal cutting.

Jun 05, 2019

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Among them, cutting is a relatively common tool for workpiece processing. Therefore, in order to ensure the accuracy of the machining of the workpiece has been cutting accuracy, the following points must be understood before the workpiece is machined:

 

1. Turning movement and formed surface

 

Turning movement: In the cutting process, in order to remove excess metal, the workpiece and the tool must be cut relative to each other. The movement of the excess metal on the workpiece by the turning tool on the lathe is called turning motion, which can be divided into main movement and advancement. Give exercise.

 

Feed motion: The new cutting layer is continuously put into the cutting motion. The feeding motion is the motion along the surface of the workpiece to be formed, which can be continuous motion or intermittent motion. For example, the horizontal lathe is continuously moving during the movement of the turning tool, and the feeding motion of the workpiece on the head planer is intermittent motion.

 

Surface formed on the workpiece: During the cutting process, the machined surface, the machined surface and the surface to be machined are formed on the workpiece. A machined surface is a new surface that has been formed by the removal of excess metal. The surface to be processed refers to the surface on which the metal layer is to be cut. The machined surface is the surface on which the turning edge of the turning tool is being turned.

 

Main motion: directly cut off the cutting layer on the workpiece and transform it into chips, thus forming the movement of the new surface of the workpiece, called the main motion. When cutting, the rotational motion of the workpiece is the main motion. Usually, the speed of the main motion is higher and the cutting power consumed is larger.

 

2. The three factors of machining center cutting amount refer to cutting depth, feed rate and cutting speed.

 

(1) Cutting depth: ap = (dw - dm) / 2 (mm) dw = diameter of the unmachined workpiece dm = diameter of the machined workpiece, the depth of cut is what we usually call the amount of knife.

 

Selection of cutting depth: The cutting depth αp should be determined according to the machining allowance. When roughing, except for the remaining allowance, the roughing allowance should be cut off as much as possible. This not only ensures a large product of cutting depth, feed rate ƒ, and cutting speed V under the premise of ensuring a certain degree of durability, but also reduces the number of passes. In the case of excessive machining allowance or insufficient rigidity of the process system or insufficient blade strength, it should be divided into two or more passes. At this time, the cutting depth of the first pass should be taken larger, which can account for 2/3 to 3/4 of the total allowance; and the cutting depth of the second pass is smaller to obtain the finishing process. Smaller surface roughness parameter values and higher machining accuracy.

 

When the surface of the cutting part has hard-hardened materials such as cast, forged or stainless steel, the cutting depth should exceed the hardness or chill layer to avoid cutting the cutting edge on the hard or chill layer.

 

(2) Selection of feed amount: the relative displacement of the workpiece and the tool in the direction of the feed motion, in units of mm, per revolution or reciprocation of the workpiece or tool. After the depth of cut is selected, a larger feed rate should be selected as much as possible. The selection of the reasonable value of the feed rate should ensure that the machine tool and the tool are not damaged by too much cutting force. The deflection of the workpiece caused by the cutting force does not exceed the allowable value of the workpiece precision, and the surface roughness parameter value is not too large. When roughing, the limit of the feed is mainly the cutting force. When semi-finishing and finishing, the limit of the feed is mainly the surface roughness.

 

(3) Selection of cutting speed: The instantaneous speed of a point on the cutting edge of the tool relative to the surface to be machined in the main moving direction during the cutting process, the unit is m/min. When the cutting depth αp and the feed amount ƒ are selected, the maximum cutting speed is selected on the basis of some, and the development direction of the cutting process is high-speed machining.

 

3. Roughness mechanics concept

 

In mechanics, roughness refers to the micro-geometric properties of the smaller pitches and peaks and valleys on the machined surface. It is one of the problems of interchangeability research. Surface roughness is generally formed by the processing methods employed and other factors, such as friction between the tool and the surface of the part during processing, plastic deformation of the surface layer metal during chip separation, and high frequency vibration in the process system. Due to the difference between the processing method and the material of the workpiece, the surface to be processed leaves a mark with a difference in depth, density, shape and texture. Surface roughness is closely related to the mechanical properties, wear resistance, fatigue strength, contact stiffness, vibration and noise of mechanical parts, and has an important impact on the service life and reliability of mechanical products.

 

4. Roughness representation

 

After the surface of the part has been machined, it looks very smooth and is uneven when viewed. Surface roughness refers to the microscopic geometric features of the smaller pitches and tiny peaks and valleys on the surface of the machined part, which are generally formed by the processing method and/or other factors taken. The function of the surface of the part is different, and the required surface roughness parameter values are also different. The surface roughness code is marked on the part drawing to illustrate the surface characteristics that must be achieved after the surface is finished. There are three types of surface roughness height parameters:

 

(1) Outline arithmetic mean deviation Ra

 

The arithmetic mean of the absolute distance between the point on the contour along the measurement direction (Y direction) and the reference line over the length of the sample.

 

(2) Micro unevenness 10 points height Rz

 

Refers to the sum of the average of the five largest contour peak heights and the average of the five largest contour valley depths within the sampling length.

 

(3) Maximum contour height Ry

 

The distance between the highest peak line and the bottom line of the profile over the length of the sample.

 

At present, Ra. is mainly used in the general machinery manufacturing industry.

 

5. The effect of roughness on the performance of the part

 

The surface quality after machining of the workpiece directly affects the physical, chemical and mechanical properties of the workpiece. The work performance, reliability and service life of the workpiece depend to a large extent on the surface quality of the main part. In general, the surface quality requirements of important or critical parts are higher than those of ordinary parts, because parts with good surface quality will greatly improve their wear resistance, corrosion resistance and fatigue resistance.


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