What is the tolerance range of precision screws?
What is the tolerance range of precision screws?
The produced screws, due to unreasonable production aspects, are likely to cause quality problems in the screws. During production, many screw quality problems may be found upon delivery. Let's talk about the quality problems often encountered by screws and the reasons for the quality problems, and finally put forward some solutions. 1. The head of the screw is deformed and the head is crooked. The possible reasons are the poor installation of the first punch of the screw die and the improper adjustment of the machine. 2. The head of the screw is not round. The reason is that the selection of the first punch of the screw mold is improper or the first punch is not full enough. 3. The screw has burrs or burrs. The reason is the poor forming of one punch, mainly caused by too large gap between punch and die hole or too short punch. 4. The screw head is cracked and the screw head is cracked. The reason may be that there is a problem with the quality of the screw wire itself, so before the screw wire is headed, the quality department must check it and use the potion. Especially the stainless steel screw wire, it is necessary to check that it is stainless steel 201 and that is stainless steel 304. It is also possible that a die is used incorrectly (such as a die with a hexagonal washer head for a pan head), and the viscosity of the lubricating oil fails. For the problem of the screw head, you can read the article written by Manager Zhu above - high-strength screw fracture or head crack detection. In this article, some problems encountered by the screw head and how to detect the screw head problem are clearly introduced [4]. The common quality reasons for screws are of course more than the above, this is only part of it. There are other quality issues, then Manager Zhu will talk about it. The above screw quality reasons are for reference. If you encounter these problems, you can try the solutions proposed.
Hydraulic wrench or pulling method Use hydraulic torque wrench or puller to fasten large screws, which saves effort and has high precision. However, hydraulic tools are expensive, screws of different sizes need to be equipped with different wrenches, and now the hydraulic pullers in the world are generally only M160×6. If the screws are too large, special specifications or very few in number, it is extremely uneconomical to equip them with hydraulic tightening tools.
Usually, the forming of the bolt head adopts cold heading plastic processing. Compared with the cutting process, the metal fiber (metal wire) is continuous along the shape of the product, and there is no cut in the middle, thus improving the strength of the product, especially the mechanical properties. The cold heading forming process includes cutting and forming, single-station single-click, double-click cold heading and multi-station automatic cold heading. An automatic cold heading machine performs multi-station processes such as stamping, heading forging, extrusion and diameter reduction in several forming dies. The processing characteristics of the original blank used by the single-station or multi-station automatic cold heading machine are determined by the size of the bar with a length of 5-6 meters or the wire rod with a weight of 1900-2000KG, that is, the characteristics of the processing technology. The point is that cold heading does not use pre-cut single blanks, but uses the automatic cold heading machine itself to cut and upset (if necessary) blanks from bars and wire rods. Before extruding the cavity, the blank must be shaped. A blank that meets the technological requirements can be obtained by shaping. Before upsetting, reducing and positive extrusion, the blank does not need to be shaped. After the blank is cut, it is sent to the upsetting and shaping station. This station can improve the quality of the blank, reduce the forming force of the next station by 15-17%, and prolong the life of the die, and the bolts can be made with multiple diameter reductions. The precision that can be achieved by cold heading is also related to the selection of the forming method and the procedure used. In addition, it also depends on the structural characteristics of the equipment used, the process characteristics and its state, the precision of the tool and die, the life and the degree of wear. For high-alloy steel used in cold heading forming and extrusion, the roughness of the working surface of the cemented carbide mold should not be greater than Ra=0.2um. When the roughness of the working surface of this type of mold reaches Ra=0.025-0.050um, it has the highest life.
With the rapid development of electronic technology, the integration of circuit boards is getting higher and higher, and the number of layers of circuit boards is increasing. Press-fit and fix to ensure accurate alignment during riveting and pressing. The rivets used on PCB boards are basically brass rivets. When riveting and pressing multi-layer PCB boards, metal chips are prone to appear, inner layer short circuits or foreign objects between layers, and because of the brass rivets With higher hardness and larger wall thickness, the riveted area of the PCB is thicker than the non-riveted area, which is easy to cause damage to the middle steel plate of the auxiliary tool. At the same time, the protruding rivets hinder the free expansion of the copper foil, which is easy to produce copper foil. wrinkling phenomenon. Now there are improved plastic rivets used to manufacture multi-layer PCB boards. The advantage is that no metal debris is generated, and at the same time, the tool steel plate is not damaged to the greatest extent. However, the existing plastic rivets are insufficient due to the inherent strength of plastics. When the PCB board is riveted and pressed, it is easy to deform, and the positioning is inaccurate, resulting in dislocation between layers, resulting in poor improvement of the product defect rate.
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