Attention to dimensional tolerance of CNC machined parts
In the process of machining and manufacturing industry and precision mechanical parts processing, the most contact is the mutual cooperation of mechanical equipment parts. So what is the cooperation of precision machining industry? What aspects should be paid attention to in dimensional tolerance fit in the machining process?
Overview of interchangeability of precision part fitting: the interchangeability of machining fitting usually includes the interchangeability of geometric parameters (such as size) and mechanical properties (such as hardness and strength). Only the interchangeability of geometric parameters is discussed here. The allowable variation of dimensions and geometric parameters of mechanical parts is called “tolerance”. The interchangeability of mechanical equipment spare parts in terms of geometric parameters is reflected in the tolerance standard. The tolerance standard is the basic standard in machining and manufacturing. It provides a theoretical basis for the standardization, serialization and generalization of machines, so as to shorten the time of mechanical design and promote the rapid development of new products.
Definition of “size” of mechanical parts: size is a number representing the standard value in specific units. Basic size of mechanical parts: the size given by the design. Actual size: the size obtained by measurement. Limit dimension: refers to the two limit values of allowable dimension change. Maximum solid state (MMC) and maximum solid size: the maximum solid state refers to the state when the hole or shaft has the largest amount of material within the dimensional tolerance. The size in this state is called the maximum entity size, which is a general designation of the minimum limit size of the hole and the maximum limit size of the shaft. Minimum solid state (LMC) and minimum solid size: the minimum solid state refers to the state when the hole or shaft has the least material within the dimensional tolerance. The size in this state is called the minimum entity size, which is a general designation of the maximum limit size of the hole and the minimum limit size of the shaft. Action size: the maximum ideal shaft size connected with the actual hole on the full length of the mating surface is called the action size of the hole. The size of the smallest ideal hole external to the actual shaft is called the action size of the shaft.
Definition of “tolerance and deviation” of mechanical parts. Dimension deviation: refers to the algebraic difference obtained by subtracting a certain dimension from its basic dimension; Dimensional tolerance: refers to the variation of allowable dimensions; Zero line: in the tolerance and fit diagram (tolerance zone diagram for short), a datum line for determining the deviation, i.e. zero deviation line; Tolerance zone: an area defined by two straight lines representing the upper and lower deviations in the tolerance zone diagram; Basic deviation: it is used to determine the upper deviation or lower deviation of the tolerance zone relative to the zero line. Generally, it refers to the deviation close to the zero line. When the tolerance zone is above the zero line, its basic deviation is the lower deviation; When it is below the zero line, its basic deviation is the upper deviation; Standard tolerance: any tolerance specified in the national standard to determine the size of the tolerance zone.
Definition of “fit” of mechanical parts: it refers to the relationship between combined holes and shaft tolerance zones with the same basic dimensions. Basic hole system: it is a system in which the tolerance zone of a hole with a certain basic deviation forms various fits with the tolerance zone of an axis with different basic deviations; Base axis system: it is a system in which the tolerance zone of an axis with a certain basic deviation forms various fits with the tolerance zone of holes with different basic deviations; Fit tolerance (or clearance tolerance): it is the variation of the allowable clearance, which is equal to the absolute value of the algebraic difference between the maximum clearance and the minimum clearance, and also equal to the sum of the matched hole tolerance zone and the shaft tolerance zone; Clearance fit: the tolerance zone of the hole is completely above the tolerance zone of the shaft, that is, the fit with clearance (including the fit with minimum clearance equal to zero); interference fit: the tolerance zone of the hole is completely below the tolerance zone of the shaft, that is, the fit with interference (including the fit with minimum interference equal to zero) ; transition fit: in the fit between hole and shaft, the tolerance zones of hole and shaft overlap each other. Any pair of holes and shaft match, which may have clearance or interference fit.
The machining tolerance fit of mechanical equipment parts can also be matched according to the actual machining process effect on the customer’s drawings to meet the best requirements in the process of equipment assembly. The above are the basic concepts of machining tolerance fit of precision mechanical parts. Thank you for your attention.
ken. tang@chengcg.com
CNC computer gong processing method (Part 1)
General principles of tool path:
Roughing: under the maximum load of the machine tool, in most cases, the tool should be as large as possible, the feed should be as large as possible, and the feed should be as fast as possible. In the case of the same tool, the feed is inversely proportional to the feed. Generally, the load of the machine tool is not a problem. The principle of tool selection mainly depends on whether the two-dimensional angle and three-dimensional arc of the product are too small. After selecting the tool, the length of the tool will be determined. The principle is that the length of the tool is greater than the processing depth, and the interference of the chuck should be considered for large workpieces.
Light knife: the purpose of light knife is to meet the processing requirements of workpiece surface finish and appropriate allowance. Similarly, the light knife should be as large as possible and as fast as possible, because it takes a long time to finish the knife, and the most appropriate feed and feed should be used. Under the same feed, the larger the transverse feed, the faster it will be. The surface feed is related to the finish after machining, and the feed size is related to the surface shape of the surface. In the case of not damaging the surface, the minimum allowance, the maximum tool, the fastest speed and proper feed should be left.
Clamping method:
- All clamps are long horizontally and short vertically.
- Vise clamping: the clamping height should not be less than 10 mm, and the clamping height and processing height must be specified when processing the workpiece. The machining height should be about 5mm higher than the plane of the vise, so as to ensure the firmness and not damage the vise. The clamping height is also related to the size of the workpiece. The larger the workpiece is, the higher the clamping height will be.
- Splint clamping: the splint is fixed on the worktable with a code, and the workpiece is locked on the splint with a screw. This kind of clamping is suitable for the workpiece with insufficient clamping height and large processing force. Generally, the effect is better for medium and large workpieces.
- Clamping with iron Code: when the workpiece is large, the clamping height is not enough, and it is not allowed to lock the wire at the bottom, the clamping with iron code is used. This kind of clamping needs second clamping, first code the four corners and process other parts, then code the four sides and process the four corners. During the second clamping, do not let the workpiece loose, first code and then loosen. You can also code two sides first and process the other two sides.
- Tool clamping: the clamping length shall not be less than 30mm when the diameter is more than 10mm; the clamping length shall not be less than 20mm when the diameter is less than 10mm. The clamping of the cutting tool should be firm, and the tool collision and direct insertion into the workpiece should be strictly prevented.
Classification and application scope of cutting tools:
(1) By material:
White steel knife: easy to wear, used for roughening copper and small steel.
Tungsten steel knife: used for angle cleaning (especially steel material) and light knife.
Alloy knife: similar to tungsten steel knife.
Purple knife; used for high speed cutting, not easy to wear.
(2) According to the cutter head:
Flat bottom knife: used for plane and straight side to clear plane angle.
Ball knife: used for all kinds of surface medium light and light knife.
Ox nose knife (one side, two sides and five sides): used for roughening steel (r0.8, r0.3, r0.5, r0.4).
Rough leather knife: used to cut rough, pay attention to the remaining method (0.3).
(3) By arbor:
Straight bar knife: straight bar knife is suitable for various occasions.
Inclined bar knife: but it is not suitable for straight face and face whose slope is less than the inclination of the bar.
(4) Blade points:
The more the number of blades, the better the effect, but the more the work, the more the speed and feed are adjusted, the more the number of blades and the longer the service life.
(5) The difference between ball knife and flying knife is as follows:
Ball knife: when concave ruler is smaller than ball ruler and plane ruler is smaller than ball R, the light can’t reach (clear the bottom angle).
Flying knife: the advantage is that it can clear the bottom corner. Comparison of the same parameters: v = R * ω, the speed is much faster (flying knife), the light is bright when the force is big, flying knife is mostly used for contour shape, sometimes flying knife doesn’t need middle light. The disadvantage is that the concave size and the plane ruler are less than the diameter of the flying knife.
ken.tang@chengcg.com
Tool path used in five axis CNC machining center
Although there is no clear requirement for the tool magazine selection of five axis CNC machining center, we always choose a better tool magazine when we choose the tool magazine of five axis CNC machining center. This is the choice based on the characteristics of five axis CNC machining center. The workpieces processed by five axis CNC machining center are generally very complex, and the requirements for accuracy are very high. The selection of tool magazine and tool directly affects the machining efficiency and the quality of workpieces processed by five axis CNC machining center, so the selection of tool magazine of five axis CNC machining center can not be ignored.
When selecting tool magazine for 5-axis CNC machining center, we need to choose better tool magazine and more precise tool. A good magazine is based on its type and brand. For example, for a five axis vmc850 CNC machining center, we can choose the hat type tool magazine and arm type tool magazine, so we’d better choose arm type tool magazine.
Five axis CNC machining center can make large vertical CNC machining center, can also be small vertical CNC machining center, but these two kinds of CNC machining center’s tool magazine only has the hat type and the knife arm type choice, generally five axis CNC machining center we can’t see the hat type tool magazine, few people will use the hat type tool magazine on the five axis CNC machining center. The type of tool magazine of five axis high-speed CNC machining center is different from that of ordinary CNC machining center. The tool magazine used in high-speed CNC machining center is arm type and synchronous type. Generally, synchronous type tool magazine is better and tool changing speed is faster. The same is true for drilling and tapping centers. There is no big difference between the two types of tool magazine. The tool changing speed of the flying saucer type tool magazine is faster than that of the clamping arm type tool magazine. Generally, the capacity of the tool magazine is larger than that of the clamping arm type tool magazine, which can meet the needs of most customers.
ken.tang @ chengcg.com
Loss of the electrode cutting nozzle in NC plasma cutting machine
Operators of CNC plasma cutting machine are clear, if CNC plasma cutting machine nozzle appears damaged, it is easy to lead to no arc in the plasma processing process, to prevent this situation, in the use of CNC plasma cutting machine electrode cutting nozzle, how to use the electrode cutting nozzle more flexible.
Causes and solutions of electrode cutting nozzle loss in CNC plasma cutting machine
In the process of NC plasma cutting machine, under normal conditions, the service life of a set of electrode cutting nozzle is about 3-4 hours. In order to reduce the cost of consumables, many users buy electrode cutting nozzle with low price and poor quality. This method will only aggravate the loss of cutting nozzle, and it is easier to burn the cutting gun, which will bring some uncertain safety hazards, so we do not recommend this How to do it. In order to reduce the loss of electrode cutting nozzle, we summarized the following solutions in the long-term after-sales work with customers
Method 1: remove the oxide from the air or oxygen nozzle
When air or oxygen plasma is used in the nozzle, oxide will accumulate in the nozzle, which will affect the air flow and increase the service life of consuming parts. Wipe the inside of nozzle with clean flannelette to eliminate oxide.
Second, reflect on the air flow and cooling methods every day
One of the most rare reasons for cutting torch damage is the short cooling flow. The air flow, air pressure or cooling liquid required for cutting torch are often reflected. If the air flow is insufficient or leakage is found, the machine should be shut down immediately to clean up the problem.
The third method is to inject hardened water into the cutting torch
Hard water will precipitate metal impurities on the nozzle ring, which will affect the air flow, reduce the quality of the cutting torch and prolong the service life of the parts.
Method 4: ensure the correct pressure and activity of plasma
It is very important to correct the pressure and movement of plasma to counteract the service life of consumable parts. If the air pressure is too high, the life of the electrode will be greatly increased; if the air pressure is too low, the life of the nozzle will be affected.
The fifth method is to adopt reasonable cutting distance
According to the requirements of the application specification, the reasonable cutting distance should be adopted. The cutting distance is the interval between the cutting nozzle and the surface of the workpiece. When piercing, the interval of twice the normal cutting distance or the maximum height that can be transferred by plasma arc should be adopted as far as possible.
Method 6: do not overload the nozzle
Overloading the nozzle (i.e. exceeding the task current of the nozzle) will quickly damage the nozzle. The current intensity should be 95% of the task current of the nozzle. For example, the current intensity of 100A nozzle should be set to 95A. Method 7: the perforation thickness should be within the allowable range of the machine parts. The cutting machine can not pierce the steel plate beyond the task thickness. The usual perforation thickness is 1 / 2 of the normal cutting thickness.
Method eight, adhere to the dry and clean plasma gas
Plasma system needs dry and clean plasma gas to be able to work normally. Dirty gas is usually the result of gas compression, it will prolong the service life of consuming parts, forming abnormal damage.
Method 9, prevent the extension of the plasma arc. If the plasma arc can only contact the surface of the workpiece by extending, this kind of extension and extension will occur at the beginning and end of the cutting, which will cause abnormal damage to the nozzle. If we adopt the right edge terminal technique and choose the appropriate “arc breaking” signal time, this result can be prevented.
Method 10, cutting should start from the edge, as far as possible from the edge of the beginning of cutting, rather than perforation cutting. Using the edge as the starting point will extend the service life of the workpiece. The correct way is to align the nozzle with the edge of the workpiece indirectly and then start the plasma arc.
Therefore, in the purchase of CNC plasma cutting machine, we should pay attention to whether the cutting gun and electrode cutting nozzle match. In addition, the appropriate cutting thickness of nozzles with different apertures is also different. Therefore, before the cutting task of CNC plasma cutting machine, the electrode cutting nozzle with cutting ability and plate to be cut should be selected.
ken.tang @ chengcg.com
Computer gong processing with multi axis
- Fully enclosed protection. All CNC computer gongs have protective doors. When processing, closing the protective doors can effectively prevent personal injury accidents.
- The CNC computer gong generally has multiple feed axes (more than three axes) and even multiple spindles, and the number of linkage axes is also very large, such as three-axis linkage, five axis linkage, seven axis linkage and so on. Therefore, it can actively complete the processing of multiple planes and multiple viewpoints, and finish the high-precision processing of messy parts. The CNC computer gong can finish milling, boring, drilling, expanding, reaming, tapping and other processing with high concentration.
- The CNC computer gong is equipped with a tool magazine and an active tool changing device. Before processing, the required tools are loaded into the tool magazine, and the tools can be replaced actively through program control.
- The use of multiple worktables, worktable active exchange CNC computer gong processing, if with active exchange worktable, can end a worktable in the processing together, another worktable finish workpiece clamping, and then greatly shorten the auxiliary time, improve the processing power.
- CNC computer gongs can be used for turning and grinding. For example, the circular worktable can drive the workpiece to rotate at high speed, and the cutting tool only makes the main motion without feeding. This makes CNC computer gongs have a wider processing scale
- CNC computer gongs with high initiative, high precision and high power have high spindle speed, feed speed and rapid positioning accuracy. Through reasonable selection of cutting parameters, they can fully show the cutting performance of cutting tools and reduce cutting time. All kinds of auxiliary actions are fast and active, which reduces auxiliary action time and downtime. Therefore, CNC computer gongs are very popular The production power is very high.
- High investment due to the high degree of intelligence, disordered structure and powerful function of CNC computer gong, the one-time investment and maintenance cost of CNC computer gong is much higher than that of ordinary machine tools.
- In the appropriate conditions, the ability to perform the best benefits, that is, in the process of using the performance of the advantages of CNC machine tools, ability to fully reflect the benefits, which is very important for the rational use of CNC computer gongs.
Precision CNC machining of mass parts
It is very simple to use the tool magazine for the cost of the CNC system, but the spindle and tool magazine, air compressor and various tool handles will increase the cost, and the programming and adjustment of the tool magazine will also take the corresponding time. Therefore, for the production of less than one or two hundred workpieces of the same variety, the machining center should not be used as far as possible, with high cost and low efficiency.
Precision CNC machining can use the automatic tool setting system, a tool up, a button, machine tool automatic tool setting, direct processing, error within 0.001 ~. 0003mm, compared with the automatic change time, not much slower. If it is a machining center, but there is no automatic tool setting device, the efficiency of the latter is much higher than that of the former.
For the domestic manufacturing industry, especially the mold production enterprises, generally single piece production, and sufficient labor resources, therefore, for the processing of small batch of industrial parts, we should fully consider the use value of funds, do not easily use the processing center equipment.
- Precision CNC has high machining precision and high machining quality;
- CNC machining can carry out multi coordinate linkage, and can process complex parts;
- When the CNC machine tool changes the parts, it only needs to change the NC program, which can save the production preparation time;
- CNC lathe processing itself has high precision and high rigidity. It can choose favorable processing amount and high productivity (generally 3 ~ 5 times of ordinary machine tools);
- CNC lathe has a high degree of automation, which can reduce the labor intensity;
- CNC lathe processing mass production, product quality is easy to control;
CNC lathe processing disadvantages:
- CNC lathe processing requires lower quality of operators and higher technical requirements of maintenance personnel.
- CNC machine tool processing route is not easy to control, not as intuitive as ordinary machine tools.
- The maintenance of lathe is inconvenient and the technical requirements are high;
- CNC lathe processing technology is not easy to control.
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