A brief discussion on the difficulties and solutions in CNC machining operations

1. Tool setting

(I) Common problems

Before executing the processing program, the first thing to do is to set the tool to determine the starting point position. Tool setting is often a headache for the operator. After the tool is sharpened, it needs to be set again. The tool tip is worn. Especially when multiple tools are used for processing, the tool compensation value needs to be measured.

(II) Solution

1. Inching tool setting

Press the inching key on the control panel, touch the tool tip lightly to the surface of the workpiece (inching twice in the X and Z directions), reset the counter, and then return to the initial position to be set (X, Z initial design value), and then reset to get the initial position of the tool. Determine the initial position of each tool in turn, and adjust it to the accurate design position (starting point) after trial processing. This method does not require any auxiliary tools and can be operated at hand, but it takes a long time, especially every time the tool is sharpened, it must be readjusted once.

2. Use tool setting instrument

Now many lathes are equipped with tool setting instrument. Using tool setting instrument to set tool can avoid the error caused by measurement, which greatly improves the accuracy of tool setting. Since the tool setting instrument can automatically calculate the difference between the length and width of each tool and store it in the system, only standard tools are needed when processing other parts, which greatly saves time. It should be noted that there are generally no standard tools when using tool setting instrument to set tools. When setting tools, standard tools should be set first. The disadvantage is that the use of tool setting instrument requires the purchase of tool setting instrument auxiliary equipment, which is costly and laborious to load and unload.

3. Use CNC tools

After the tool is installed and positioned for the first time, it will be worn after a period of cutting and needs to be sharpened. The position of the tip of the ordinary tool after sharpening has changed and needs to be re-set. The characteristics of CNC tools are high tool manufacturing precision and the repeated positioning accuracy of the blade after indexing is about 0.02mm, which greatly reduces the time of tool setting. At the same time, the blade is coated with a metal layer (Sic, Tic, etc.) to greatly improve its durability, but the cost is also a bit high.

4. Use tool setting blocks to set the tool

Use self-made tool setting method. Simple tool setting blocks made of plastic, plexiglass, etc. can easily achieve repeated positioning of the tool after sharpening, but the positioning accuracy is poor, usually 0.2~0.5mm. It is still a fast positioning method, and it can be quickly and conveniently adjusted.

2. Processing

(I) Common difficulties

CNC machine tools are processed according to the program, and the processing process is automatic. All processes, steps, cutting amount, feed route, processing allowance of each process, and the size, type, and tool setting points of all tools must be programmed into the program. Therefore, the process programming problem directly affects the processing quality and efficiency of parts. However, operators often fail to pay attention to details during programming, resulting in program errors and dimensional deviations. In serious cases, there will be crashes.

(II) Solve routines

1. When dividing the CNC machining process of parts, you should pay attention to the following points:

① The CNC machine tool process tends to be centralized, the number of machine tools required and the number of parts clamping times are reduced, and the processing efficiency is high, but the process is too centralized, the program is lengthened, the error rate is high, and it brings difficulties to inspection, etc. Therefore, the process concentration and dispersion procedures should be flexibly determined according to the amount of part processing content and the specific situation of the current CNC machining equipment.

② Follow the principle of roughing first and then finishing. For parts with less machining rigidity, deformation may occur after roughing, and it is necessary to calibrate or eliminate residual internal stress and correct errors during finishing.

2. The following points should be paid attention to when arranging the sequence of processes:

① It is best to connect the processes with the same positioning, clamping method or the same tool to reduce the number of repeated positioning and tool changes.

② First arrange the process that causes less damage to the rigidity of the workpiece.

3. Pay attention to checking and trial running the program to ensure the accuracy of the program.

3. Processing of special-shaped surface parts

(I) Processing difficulties

Some parts on the machine have surfaces that are not straight lines but curves, such as rocker handles, balls, and cams. These surfaces with curves are called special-shaped surfaces or forming surfaces. For the processing of these sharp swords, different turning methods should be used according to the characteristics of the product, the precision requirements, and the batch size. However, the processors often cause shape errors due to improper method selection, uneven system clearance machining allowances, and uneven tool selection.

(II) Solutions

1. Correctly select the turning method.

① Two-hand control method

Special-shaped surface parts with a small number or a single piece can be turned using two-hand control, that is, hold the middle slide handle with the right hand and the large slide handle with the left hand. Through the combined movement of both hands, the turning tool moves out of the curve required by the special-shaped surface to turn out the special-shaped surface. The two-hand control method can turn out special-shaped surfaces of general precision without other special tools, but it requires the operator to have skilled skills and has low production efficiency.

②Use forming turning tool to turn special-shaped surfaces

A large number of special-shaped surface parts can be turned with forming tools. The turning tool with the blade ground to the same shape as the workpiece surface is called a forming tool, also called a template tool. It is very important to correctly select the prop material and the reasonable collective shape. If the processed material is steel, the surface variation range is small and relatively simple, and the tool material can be selected from YT15 carbide turning tools. For complex surfaces, high-speed steel is generally used. The front angle is selected from 15 degrees to 20 degrees, and the back angle is generally selected from 6 degrees to 10 degrees.

③Use the template to turn special-shaped surfaces

The principle of turning the forming surface is the same as the template turning the conical surface. During processing, just replace the slide plate with a roller and replace the tapered template with a template with the required curve. This method is not limited in the size of the workpiece, and can use motorized feeding. It has high production efficiency and high processing accuracy, and is widely used in mass production.

④ Use special tools to turn special-shaped surfaces

When machining internal and external arcs, we can design a special tool so that the tool tip moves along the trajectory of the external arc or the internal arc, and various arcs can be turned. If the distance from the tool tip to the center of rotation is adjustable, internal and external arc workpieces of various radii can be turned.

2. System clearance

In the transmission pair of the equipment, there is a certain gap between the lead screw and the nut. As the equipment is put into operation for a longer time, the gap gradually increases due to wear. Therefore, the corresponding gap compensation during reverse motion is the main factor to overcome the convex shoulder on the machined surface. The gap measurement usually adopts the dial indicator measurement method, and the error is controlled within 0.01-0.02mm. It should be pointed out here that the meter seat and the meter rod should not be extended too high or too long, because the meter seat is easily moved by force due to the long cantilever during measurement, resulting in inaccurate counting and unreal compensation value.