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General steps of plastic parts designPlastic parts are designed on the basis of industrial modeling. First, see whether there are similar products for reference, and then carry out detailed functional decomposition of products and parts to determine the main process problems such as parts folding, wall thickness, demoulding slope, transition treatment between parts, connection treatment and strength treatment of parts.1. Similar reference
Before design, first look for similar products of the company and peers, what problems and deficiencies have occurred in the original products, and refer to the existing mature structure to avoid problematic structural forms.2. Determine the part discount, transition, connection and clearance treatment between partsUnderstand the modeling style from the modeling drawing and effect drawing, cooperate with the functional decomposition of the product, determine the number of parts (different surface states are either divided into different parts, or there must be over treatment between different surfaces), determine the over treatment between parts' surfaces, and determine the connection mode and fit clearance between parts.
3. Determination of part strength and connection strengthDetermine the wall thickness of the part body according to the product size. The strength of the part itself is determined by the wall thickness of the plastic part, the structural form (the plastic part in the shape of a flat plate has the worst strength), the stiffener and the stiffener. While determining the single strength of parts, the connection strength between parts must be determined. The methods to change the connection strength include: adding screw column, adding stop, adding buckle position and adding reinforcing bone against top and bottom.4. Determination of demoulding slope
The demoulding slope shall be comprehensively determined according to the material (PP, PE silica gel and rubber can be demoulded forcibly), surface state (the slope of decorative grain shall be greater than that of smooth surface, and the slope of etched surface shall be 0.5 degrees greater than that required by the template as far as possible, so as to ensure that the etched surface will not be damaged and improve the yield of products), transparency or not determines the demoulding slope of parts (the transparent slope shall be greater).Material types recommended by different product series of the companySurface treatment of plastic parts
Wall thickness selection of plastic partsFor plastic parts, the uniformity of wall thickness is required, and the workpiece with uneven wall thickness will have shrinkage traces. It is required that the ratio of stiffener to main wall thickness should be less than 0.4, and the maximum ratio should not exceed 0.6.Demoulding slope of plastic parts
In the construction of stereoscopic drawing, where the appearance and assembly are affected, the slope needs to be drawn, and the slope is generally not drawn for stiffeners.The demoulding slope of plastic parts is determined by the material, surface decoration status and whether the parts are transparent or not. The demoulding slope of hard plastic is greater than that of soft plastic. The higher the part, the deeper the hole, and the smaller the slope.Recommended demoulding slope for different materials
Numerical values of different accuracy in different size rangesDimensional accuracy of plastic partsGenerally, the accuracy of plastic parts is not high. In practical use, we mainly check the assembly dimensions, and mainly mark the overall dimensions, assembly dimensions and other dimensions that need to be controlled on the plan.
In practice, we mainly consider the consistency of dimensions. The edges of the upper and lower covers need to be aligned.Economic accuracy of different materialsNumerical values of different accuracy in different size ranges
Surface roughness of plastics1) The roughness of the etched surface cannot be marked. Where the plastic surface finish is particularly high, circle this range and mark the surface state as mirror.2) The surface of plastic parts is generally smooth and bright, and the surface roughness is generally ra2.5 0.2um.
3) The surface roughness of plastic mainly depends on the surface roughness of mold cavity. The surface roughness of mold is required to be one to two levels higher than that of plastic parts. The mold surface can reach ra0.05 by ultrasonic and electrolytic polishing.FilletThe fillet value of injection molding is determined by the adjacent wall thickness, generally 0.5 1.5 times of the wall thickness, but not less than 0.5mm.
The position of the parting surface shall be carefully selected. There is a fillet on the parting surface, and the fillet part shall be on the other side of the die. It is difficult to make, and there are fine trace lines at the fillet. However, fillet is required when anti cutting hand is required.Stiffener problemThe injection molding process is similar to the casting process. The non-uniformity of wall thickness will produce shrinkage defects. Generally, the wall thickness of reinforcement is 0.4 times of the main body thickness, and the maximum is no more than 0.6 times. The spacing between bars is greater than 4T, and the height of bars is less than 3T. In the method of improving the strength of parts, it is generally reinforced without increasing the wall thickness.
The reinforcement of the screw column shall be at least 1.0mm lower than the end face of the column, and the reinforcement shall be at least 1.0mm lower than the part surface or the parting surface.When multiple bars intersect, pay attention to the non-uniformity of wall thickness caused by the intersection.Design of stiffeners for plastic parts
Bearing surfacePlastic is easy to deform. In terms of positioning, it should be classified as the positioning of wool embryo. In terms of positioning area, it should be small. For example, the support of plane should be changed into small convex points and convex rings.Oblique roof and row position
The inclined top and row position move in the parting direction and perpendicular to the parting direction. The inclined top and row position shall be perpendicular to the parting direction, and there shall be sufficient movement space, as shown in the following figure:Treatment of plastic limit process problems1) Special treatment of wall thickness
For particularly large workpieces, such as the shell of toy cars, the wall thickness can be relatively thin by using the method of multi-point glue feeding. The local glue position of the column is thick, which is treated as shown in the following figure.Special treatment of wall thickness2) Treatment of small slope and vertical surface
The die surface has high dimensional accuracy, high surface finish, small demoulding resistance and small demoulding slope. In order to achieve this purpose, the parts with small inclination of the workpiece are inserted separately, and the inserts are processed by wire cutting and grinding, as shown in the figure below.To ensure that the side wall is vertical, the running position or inclined top is required. There is an interface line at the running position. In order to avoid obvious interface, the wiring is generally placed at the junction of fillet and large surface.Treatment of small slope and vertical surface
To ensure that the side wall is vertical, the running position or inclined top is required. There is an interface line at the running position. In order to avoid obvious interface, the wiring is generally placed at the junction of fillet and large surface.Problems often to be solved for plastic parts1) Transition processing problem
The accuracy of plastic parts is generally not high. There must be transition treatment between adjacent parts and different surfaces of the same part.Small grooves are generally used for transition between different surfaces of the same part, and small grooves and high-low staggered surfaces can be used between different parts, as shown in the figure.Surface over treatment
2) Clearance value of plastic partsParts are directly assembled without movement, generally 0.1mm;The seam is generally 0.15mm;
The minimum clearance between parts without contact is 0.3mm, generally 0.5mm.3) The common forms and clearance of plastic parts are shown in the figureCommon forms and clearance taking method of stop of plastic parts
The processing methods of holes include drilling, reaming, reaming, boring, drawing, grinding and finishing of holes. The following small series for you to introduce several hole processing technology in detail, crack the hole processing problems.
The hole is an important surface on the box, bracket, sleeve, ring, and disk parts, and it is also a surface often encountered in machining. In the case of the same processing accuracy and surface roughness requirements, it is difficult to process the hole than the outer round surface, low productivity and high cost.
This is because: 1) the size of the tool used in hole processing is limited by the size of the hole being processed, and the rigidity is poor, which is easy to produce bending deformation and vibration; 2) When machining the hole with a fixed-size tool, the size of the hole processing often directly depends on the corresponding size of the tool, and the manufacturing error and wear of the tool will directly affect the processing accuracy of the hole; 3) When machining holes, the cutting area is inside the workpiece, the chip removal and heat dissipation conditions are poor, and the processing accuracy and surface quality are not easy to control.
Drilling
Drilling is the first process of machining holes on solid materials, and the diameter of the drilling hole is generally less than 80mm. There are two ways of drilling: one is the bit rotation; The other is workpiece rotation. The error generated by the above two drilling methods is not the same, in the drilling method of the bit rotation, due to the asymmetry of the cutting edge and the insufficient rigidity of the bit and the bit deflection, the center line of the hole will be skewed or not straight, but the aperture is basically unchanged; On the contrary, in the drilling method of workpiece rotation, the bit deflection will cause the aperture to change, but the center line of the hole is still straight.
Commonly used drilling knives have: twist drill, center drill, deep hole drill, etc., of which the most commonly used is twist drill, its diameter specification is Φ0.1-80mm.
Due to structural limitations, the bending stiffness and torsional rigidity of the drill bit are low, coupled with poor centering, the drilling accuracy is low, generally only IT13 ~ IT11; The surface roughness is also large, Ra is generally 50~12.5μm; However, the metal removal rate of drilling is large and the cutting efficiency is high. Drilling is mainly used for processing holes with low quality requirements, such as bolt holes, thread bottom holes, oil holes, etc. For holes with high machining accuracy and surface quality requirements, they should be achieved by reaming, reaming, boring or grinding in subsequent processing.
Reaming
Reaming is to further process the hole that has been drilled, cast or forged with a reaming drill to enlarge the aperture and improve the processing quality of the hole. Reaming can be used either as a pre-processing before finishing the hole or as the final processing of the hole with low requirements. Reaming drill is similar to twist drill, but has more teeth and no cross edge.
Compared with drilling, reaming has the following characteristics:
(1) the number of reaming drill teeth (3~8 teeth), good guidance, cutting is relatively stable; (2) reaming drill without cross edge, cutting conditions are good;
(3) The processing allowance is small, the chip sink can be made shallower, the drill core can be made thicker, and the tool body strength and rigidity are better. The precision of reaming is generally IT11~IT10, and the surface roughness Ra is 12.5~6.3μm. Reaming is often used to process holes with smaller diameters. When drilling a large diameter hole (D ≥30mm), often use a small drill bit (diameter of 0.5 to 0.7 times of the aperture) to pre-drill, and then use the corresponding size of the hole reaming drill, which can improve the processing quality and production efficiency of the hole.
In addition to processing cylindrical holes, reaming drills of various special shapes (also known as countersinks) can be used to process various countersunk seat holes and countersinks. The front face of the countersink is often equipped with a guide post, guided by a machined hole.
Reaming is one of the finishing methods of holes, which is widely used in production. For smaller holes, reaming is a more economical and practical machining method than internal grinding and fine boring.
1. Reamer
Reamer is generally divided into two kinds of hand reamer and machine reamer. The handle part of the hand reamer is straight handle, the working part is longer, and the guiding function is better. The hand reamer has two kinds of structures: integral and adjustable outside diameter. The machine reamer has two kinds of structure with handle and sleeve. The reamer can not only process round holes, but also taper reamer can process taper holes.
2. Reaming process and its application
Reaming allowance has a great influence on the quality of reaming, the allowance is too large, the load of the reamer is large, the cutting edge is soon blunted, it is not easy to obtain a smooth machining surface, and the dimensional tolerance is not easy to guarantee; The margin is too small to remove the knife marks left by the previous process, and naturally there is no role in improving the quality of hole processing. Generally, the margin of coarse hinge is 0.35~0.15mm, and the fine hinge is 01.5~0.05mm.
To avoid chip nodules, reaming is usually processed at a lower cutting speed (v <8m/min for steel and cast iron with HSS reamers). The value of feed is related to the aperture to be machined, the larger the aperture, the larger the feed value, the feed rate of high-speed steel reamer processing steel and cast iron is usually 0.3~1mm/r.
Reaming must be cooled, lubricated and cleaned with appropriate cutting fluid to prevent chip buildup and remove chips in time. Compared with grinding and boring, the reaming productivity is higher and the accuracy of the hole is easily guaranteed. However, reaming can not correct the position error of the hole axis, and the position accuracy of the hole should be guaranteed by the previous process. Reaming is not suitable for processing step holes and blind holes.
The dimensional accuracy of reaming is generally IT9 ~ IT7, and the surface roughness Ra is generally 3.2~ 0.8μm. For medium-size holes with high precision requirements (such as IT7 precision holes), the driller - reamer - reamer process is a typical processing scheme commonly used in production.
Boring is a machining method in which the prefabricated hole is enlarged with a cutting tool. The boring work can be carried out either on the boring machine or on the lathe.
1. Boring method
There are three different machining methods for boring.
(1) The workpiece rotates and the tool makes feed movement
Boring on the lathe mostly belongs to this boring method. The characteristics of the process are: the axis line of the hole after processing is consistent with the rotation axis of the workpiece, the roundness of the hole mainly depends on the rotation accuracy of the machine tool spindle, and the axial geometry error of the hole mainly depends on the position accuracy of the tool feed direction relative to the rotation axis of the workpiece. This boring method is suitable for machining holes with coaxial requirements on the surface of the outer circle.
(2) The tool rotates and the workpiece is fed
The boring machine spindle drives the boring tool to rotate, and the table drives the workpiece to feed.
(3) The tool rotates and makes feed motion
Using this kind of boring boring method, the overhanging length of the boring bar is changed, the force deformation of the boring bar is also changed, the aperture near the headstock is large, and the aperture away from the headstock is small, forming a cone hole. In addition, with the increase of the overhang length of the boring bar, the bending deformation of the main shaft caused by its own weight also increases, and the axis of the machined hole will have a corresponding bending. This boring method is only suitable for machining short holes.
2. Diamond boring
Compared with general boring, diamond boring is characterized by a small amount of back cutting, small feed, high cutting speed, it can obtain a high processing accuracy (IT7 ~ IT6) and a very smooth surface (Ra is 0.4~ 0.05μm). Diamond boring was originally processed with diamond boring tools, and is now commonly processed with cemented carbide, CBN and artificial diamond tools. Mainly used for processing non-ferrous metal workpieces, can also be used for processing cast iron and steel parts.
The commonly used cutting parameters of diamond boring are: pre-boring of 0.2~0.6mm and final boring of 0.1mm; The feed rate is 0.01~0.14mm/r; The cutting speed is 100~250m/min when processing cast iron, 150~300m/min when processing steel, and 300~2000m/min when processing non-ferrous metals.
In order to ensure that the diamond boring machine can achieve high machining accuracy and surface quality, the machine tool (diamond boring machine) must have high geometric accuracy and stiffness, the main shaft of the machine tool supports the commonly used precision angular contact ball bearing or static pressure plain bearing, and the high-speed rotating parts must be accurately balanced; In addition, the movement of the feed mechanism must be very smooth to ensure that the table can do smooth low-speed feed movement.
The machining quality of diamond boring is good, the production efficiency is high, and it is widely used in the final processing of precision holes in a large number of mass production, such as the engine cylinder hole, the piston pin hole, the main shaft hole on the spindle box of the machine tool. However, it should be noted that when machining ferrous metal products with diamond boring, only the boring tool made of cemented carbide and CBN can be used, and the boring tool made of diamond can not be used, because the carbon atoms in diamond have a large affinity with the iron group elements, and the tool life is low.
3. Boring tool
Boring tool can be divided into single-edge boring tool and double-edge boring tool.
4. Boring process characteristics and application range
Compared with the drilling, expanding and reaming process, the bore size is not limited by the tool size, and the boring has a strong error correction ability, and the deviation error of the original hole axis can be corrected by multiple cutting, and the boring can maintain a higher position accuracy with the positioning surface.
Compared with the outer circle of the boring, due to the poor rigidity of the tool bar system, large deformation, poor heat dissipation and chip removal conditions, the hot deformation of the workpiece and the tool is relatively large, and the processing quality and production efficiency of the boring are not as high as the outer circle of the car.
In summary, it can be seen that the processing range of boring is wide, and holes of different sizes and different precision levels can be processed. For holes and hole systems with large aperture, high size and position accuracy requirements, boring is almost the only processing method. The machining accuracy of boring is IT9 ~ IT7. Boring can be carried out on the boring machine, lathe, milling machine and other machine tools, which has the advantages of flexibility and flexibility, and is widely used in production. In mass production, boring die is often used to improve boring efficiency.
1. Honing principle and honing head
Honing is the method of finishing the hole by using a honing head with a grinding rod (whetstone). When honing, the workpiece is fixed, and the honing head is rotated by the spindle of the machine tool and moves in a reciprocating straight line. In honing processing, the grinding strip acts on the workpiece surface with a certain pressure, and cuts an extremely thin layer of material from the workpiece surface. In order to make the movement of the abrasive particle not repeat, the number of revolutions per minute of the turning movement of the honing head and the number of reciprocating strokes per minute of the honing head should be prime.
The cross Angle of the honing track is related to the reciprocating speed and circular speed of the honing head, and the size of the Angle affects the processing quality and efficiency of the honing. In order to facilitate the discharge of broken abrasive particles and chips, reduce the cutting temperature and improve the processing quality, sufficient cutting fluid should be used when honing.
In order to make the machined hole wall can be uniformly machined, the stroke of the sand bar at both ends of the hole must exceed a section of overpass. In order to ensure the uniform honing allowance and reduce the influence of the spindle rotation error on the machining accuracy, the floating connection between the honing head and the spindle of the machine tool is mostly adopted.
The radial expansion adjustment of honing head grinding rod has various structural forms such as manual, pneumatic and hydraulic.
2. Honing process characteristics and application range
(1) honing can obtain higher dimensional accuracy and shape accuracy, the processing accuracy is IT7~IT6, the roundness and cylindricity error of the hole can be controlled within the range, but honing can not improve the position accuracy of the hole to be machined.
(2) Honing can obtain a higher surface quality, the surface roughness Ra is 0.2~0.25μm, the surface metal metamorphic defect layer depth is very small 2.5~25μm.
(3) Compared with the grinding speed, the circular speed of the honing head is not high (vc=16~60m/min), but due to the large contact area between the sand bar and the workpiece, the reciprocating speed is relatively high (va=8~20m/min), so the honing still has a high productivity.
Honing is widely used in the machining of engine cylinder holes and precision holes in various hydraulic devices in a large number of mass production, and can process deep holes with a length-diameter ratio greater than 10. However, honing is not suitable for processing holes on non-ferrous metal workpieces with large plasticity, nor can it process holes with keyways, spline holes, etc.
1. Broach and broach
Drawing is a high productivity finishing method, which is carried out on a broaching machine with a special broach. Broaching machine divided into horizontal broaching machine and vertical broaching machine two kinds, the horizontal broaching machine is the most common.
Broaching only uses low-speed linear motion (main motion). The number of teeth of the broach working at the same time should generally be not less than 3, otherwise the broach is not stable, and it is easy to produce ring ripples on the surface of the workpiece. In order to avoid generating too much broaching force and causing the broach to break, the number of teeth of the broach working at the same time should not exceed 6 to 8.
There are three different broaching methods, which are described as follows:
(1) Layered broaching
This broaching method is characterized by the broach cutting the workpiece machining allowance layer by layer in sequence. In order to facilitate chip breaking, the cutter teeth are ground with interleaved chip grooves. The broach designed according to the layered broaching method is called the ordinary broach.
(2) block broaching
The characteristic of this broaching method is that each layer of metal on the machined surface is cut by a set of tool teeth that are basically the same size but interlaced with each other (usually each set consists of 2-3 tool teeth). Each tooth cuts only part of a layer of metal. Broach designed according to the block broach method is called rotary broach.
(3) Comprehensive broaching
In this way, the advantages of layering and block broaching are concentrated. Block broaching is used in the rough cutting part and layer broaching is used in the fine cutting part. In this way, the broach length can be shortened, productivity can be increased, and better surface quality can be obtained. The broach designed according to the comprehensive broach method is called the comprehensive broach.
2. Process characteristics and application range of drawing holes
(1) The broach is a multi-edge tool, which can finish the roughing, finishing and finishing of the hole in a sequence in one broaching stroke, and has high production efficiency.
(2) The drawing accuracy mainly depends on the accuracy of the broach, under normal conditions, the drawing accuracy can reach IT9~IT7, and the surface roughness Ra can reach 6.3~ 1.6μm.
(3) When drawing a hole, the workpiece is positioned by the machined hole itself (the leading part of the broach is the positioning element of the workpiece), and the drawing hole is not easy to ensure the mutual position accuracy of the hole and other surfaces; For the processing of rotary parts whose inner and outer circular surfaces have coaxial requirements, it is often necessary to first pull holes, and then process other surfaces with holes as the positioning reference.
(4) broach can not only process round holes, but also process forming holes and spline holes.
(5) broach is a fixed size tool, complex shape, expensive, not suitable for processing large holes.
Drawing holes are commonly used in a large number of mass production to process holes on small and medium-sized parts with a diameter of 10~80mm and a hole depth of not more than 5 times the aperture.
Honscn Precision Technology Co., LTD., offering a wide range of machining processes, including hardware parts foundry, precision hardware parts, turret turning and milling complex machining, and core walking complex machining.Our products are widely used in automobiles, motorcycles, communications, refrigeration, optics, home appliances, micro electronics, measuring tools, fishing gear, instruments, electronics and other professional fields to meet their parts needs.Contact us
Contact: Ada Li
Tel: +86 17722440307
WhatsApp: +86 17722440307
E-mail: Ada@honscn.com
Add: 4F, No. 41 Huangdang Road, Luowuwei Industrial, Dalang Street, Longhua, Shenzhen, 518109, China