How to ensure that the CNC machining center milling PA nylon workpiece is not deformed?
The English abbreviation of nylon is PA, and the Chinese full name is polyamide. There are many types of nylon, including PA6, PA66, PA610, PA11, PA12, PA1010, PA612, PA46, etc. Nylon is a kind of engineering plastics, and CNC machining centers can process engineering plastics, including PA nylon. PA nylon has the advantages of high mechanical strength, good toughness, smooth surface, small friction coefficient, outstanding wear resistance, fatigue resistance, excellent electrical properties, easy dyeing, and easy molding.
PA nylon is used in transportation, machinery, cables and wires, automobile industry, electronic and electrical industry, etc.
PA nylon is specifically used for various bearings, gears, pulley pump impellers, blades, fans, air filter housings, radiator water chambers, brake pipes, engine covers, etc.
The real-time and long-term deformation of the PA nylon workpiece is milled by the CNC machining center, so the accuracy is difficult to guarantee. So how can we avoid this from happening?
Pay attention to these 4 points to ensure that the CNC machining center milling PA nylon workpiece does not deform!
The CNC machining center mills PA nylon workpieces without deformation, mainly from the four aspects of clamping, cutting tools, cutting heat and original internal stress of materials.
1. The first is clamping: no matter what material the workpiece is, in the process of clamping, there will always be a clamping force, especially for very thin workpieces, which are very prone to deformation. After unloading the clamping force, the elasticity of the workpiece The deformation is automatically restored. The size of the workpiece under the free condition of no force is not the same as the processing size. Once the clamping force is too large, it will exceed the yield limit of the workpiece, especially when clamping for a long time, it is easy to cause plastic deformation of the workpiece, then the clamping part of the processed part does not match the processing size; conversely, It will cause the clamping is not tight, the vibration during processing is large, and the final processing size and weight will be affected.
Different from metal materials, PA nylon material has the characteristics of easy deformation, low density, and easy processing. In the table clamping of CNC machining center, it is very easy to be deformed by clamping; after processing, the elasticity recovers, making PA nylon size and shape. All have undergone certain changes, and due to the greater the clamping force, the greater the deformation after processing is completed. Therefore, when processing PA nylon workpieces, it is recommended to adopt the sequence of strong clamping for preliminary machining and slight clamping for finishing, so that the clamping force will not affect the machining accuracy of the workpiece size.
Alright, that's the end of the clip.
2. Let's talk about the tool: we need to avoid the excessive extrusion force brought by the tool itself when cutting PA nylon. Since the tool continuously moves to the inside of the PA nylon during cutting, the lateral cutting of the PA nylon by the tool will be removed, and there will be a direct push pressure. If the propulsion pressure is too high, it will not only affect the clamping stability of the PA nylon workpiece, but also cause the PA nylon workpiece to deform, so that the dimensional deviation of the PA nylon workpiece after elastic deformation recovery is too large.
Compared with the tool with stronger stiffness and the tool with weaker stiffness, the former has poor elasticity, which is more likely to cause a propulsion force on the PA nylon workpiece, which causes the workpiece to deform. Therefore, we recommend using a relatively weak alloy tool for better machining accuracy. Suitable for.
The sharpness of the blade also affects the machining accuracy. The sharper the cutting edge of the tool, the smaller the cutting resistance, the smaller the propulsion force on the PA nylon workpiece, the smaller the deformation of the PA nylon workpiece, and the smaller the rebound phenomenon, the better the dimensional accuracy can be guaranteed. Therefore, we use alloy knives to process PA nylon workpieces. Among them, triangular knives are better than quadrangular knives, and the edges can ensure the surface roughness when the workpiece is finished. The use of new blades can ensure dimensional accuracy better than the old ones, and can also sharpen the blade. Sharpen to make the sharp angle of the blade smaller.
3. It is the turn of cutting heat: no matter what part is processed, it will generate a lot of heat, such as elastic deformation and plastic deformation during milling, chip separation and energy consumed by the friction between the tool and the workpiece, most of these can be converted into heat energy. A small portion of this thermal energy is carried away by the chip or radiated by the air, but a large portion is still absorbed by the workpiece. The remaining heat energy will cause thermal stress in the profile of the workpiece, and then with the continuous advancement of processing, heat energy will be generated continuously, and the thermal stress will continue to change. Finally, the workpiece will deform and crack seriously.
However, for PA nylon workpieces, the thermal stability of this material itself is very weak, and it is easy to deform with a little heat absorption.
If the heat generated during cutting is generated at the cutting point, it is assumed that:
1) The temperature of the workpiece is uniform before cutting;
2) The generated heat energy is not radiated outward;
3) The cutting process is stable and uniform, then any point M (x0, y0, z0) of the workpiece is affected by the temperature of the moving point heat source:
In the formula, Q(τ) is the instantaneous heating value of the point heat source; ρ is the density of the medium; c is the specific heat capacity of the heat-conducting medium; α is the thermal conductivity of the heat-conducting medium; τ is any moment after the heat source heats up instantaneously; x0, y0, z0) is the position of the fixed point, which is a known value; the coordinates (x, y, z) are the position of the point heat source, which is the change value; t is the temperature rise at the fixed point after the influence of the point heat source. It can be seen from the formula that the closer to the point heat source is affected by its temperature, the cutting surface is directly the heat source surface, which is heated the most, and the deformation caused by heat is also greater; therefore, workpieces with high machining accuracy requirements should It cools down. Cooling can be done by kerosene flushing or coolant flushing.
4. Finally, the original internal stress of the material: we need to remove the original internal stress in the process of processing, then this will change the overall structural correlation of the workpiece, which will cause the internal stress balance of the material to be broken, and it is necessary to find new internal stress. balance, which causes the material to deform during cutting. Therefore, when we process metal materials, we should use methods such as quenching and tempering and vibration aging to eliminate internal stress, so as to ensure that the internal stress and structure of the material are as stable as possible and reduce machining deformation.
PA nylon is made by casting, resulting in large and small holes and pores; when the mold temperature is too high, nylon shrinks; on the contrary, because the instantaneously separated polymer is not completely dissolved in the monomer, resulting in micropores; in addition, PA nylon is easily mixed into Volatile or easily decomposed products, casting produces volatile products, which eventually form bubbles and holes. These large and small holes cause the instability of PA nylon. If the structure is changed, the internal stress will change the balance again, and the material will easily deform.
If it is assumed that there are air holes inside, then the holes inside the PA nylon board are not processed, and the structures are balanced by mutual traction and support; after a part of the cutting, the holes lose their original balance and shrink inward to the center of the holes under the action of edge stress, which leads to the milling finished. The workpiece is bent and deformed toward the machining side.
Four aspects of clamping, tool, cutting heat and material internal stress will affect the processing effect of PA nylon workpiece.
CNC machining center milling of PA nylon workpieces and stable precision are mainly affected by four factors: clamping, tool, cutting heat and material internal stress, and these four factors affect each other. For example, if the tool wear is serious, the propulsion force of the milling cutter on the part needs to be increased, and the professional can increase the heat generated by cutting, and the cutting heat can change the internal stress balance of the material. It can be seen that when the CNC machining center mills PA nylon workpieces, the influence of these four factors needs to be comprehensively considered, and the influence of each factor needs to be minimized. Is it a headache? Now, don't think that the CNC machining center is so easy to operate, there is a lot of knowledge that needs to be understood.