What is Honing
What is Honing?
Table of Contents
Honing, also known as boring, is the finishing of a finishing surface with an oil stone (also known as a honing bar) set in a honing head.
The principle of honing
Considering the honing oil stone and the workpiece as two mutually researched surfaces, in order to achieve high machining quality, they should be in relative reciprocating motion, and the trajectory of each abrasive grain on the oil stone on the hole wall should not be repeated. During processing, the oil stone and the workpiece are in contact with each other in a face-to-face condition, with lower cutting speed and pressure to reliably grind away a smaller machining margin of the workpiece (generally 0.01-0.08mm, depending on different processing materials, processing lot and processing requirements). Honing can significantly improve the size accuracy of the workpiece (small hole up to 1-2μm, medium hole up to 10μm, both or even smaller) and shape accuracy (small hole roundness up to 0.5μm, cylindricity up to 1μm, medium hole roundness up to 3μm or even smaller; hole length 300-500mm, cylindricity up to 5μm or less, processing error dispersion range is small, only 1-3μm, processing surface quality is high (The surface roughness value Ra is only about 0.4-0.04μm or even smaller), generally because the average pressure P of the oil stone on the workpiece is small (about 0.4-0.8MPa), so the heat generation is small, and the processing surface deterioration layer is also less, because the honing head and the workpiece are in contact with the surface, and at the same time the abrasive grains participating in cutting are more, so it is also an efficient processing method.
Features of Honing
The characteristics of honing processing mainly include the following points: high processing accuracy, after honing, the size accuracy and shape accuracy of the workpiece can be improved, and the surface roughness can reach Ra0.2-Ra0.8μm, some even lower than Ra0.025μm; good surface quality, the honing processing surface has crossed reticulation, which is conducive to the storage of lubricant and the maintenance of oil film, and has a high surface support rate (the actual contact area between the hole and the shaft). The actual contact area and the ratio between the two fit area), so it can withstand larger load, wear resistance, and can extend the service life.
In addition to the machining accuracy, shape accuracy of roundness, cylindricity and high surface quality, the surface of the workpiece has a cross pattern, which is conducive to the formation and maintenance of oil film. Therefore, the service life of the machined workpiece is often more than double that of the workpiece machined by other processing methods, especially suitable for the processing of precision coupling parts with high relative motion requirements. Honing processing range can be wider, in addition to through-hole, interrupted hole, blind hole, multi-step hole, residual cycloid hole, plane, spherical surface and tooth surface can be processed, can adapt to processing many varieties of metal materials.
The machine spindle drives the honing tool (honing head) to rotate on one side and make a linear up and down reciprocating motion on the other side, and the oil stone (grinding bar) on the honing head removes the abrasive chips on the surface of the workpiece under the effect of certain outward expansion pressure, and grinds out the spiral cross mesh abrasion marks, which is mainly used for the processing of precision holes, such as engine cylinder hole, compressor cylinder hole, connecting rod, pump body and control block, etc.
Figure 1 shows the trajectory of the oil stone in honing process, where lw is the length of the workpiece, πdw is the circumference of the workpiece hole, and θ is the cross angle of the grinding marks. Ⅰ, Ⅱ, Ⅲ are the positions of the oil stone in a reciprocal stroke when it bends back in the sequence. In order not to let the grinding marks repeat, the return position Ⅲ should deviate from the starting position Ⅰ by a distance of S. In the early days, the honing head that pushed the conical bevel to expand the oil stone by spring force was used, as shown in Figure 2. At present, the new honing head mainly relies on hydraulic expansion out, 1 in Fig. 2 is oil stone (grinding strip), oil stone is a strip-shaped abrasive made of abrasive plus bonding agent, according to the different materials to be processed, you can choose the abrasive in the corresponding oil stone, shape, type, particle size, bonding agent, hardness, organization and performance. The honing head is usually composed of multiple oil stones evenly distributed, which can process multiple parts of the hole at the same time.
Fig.1 Trajectory of oil stones during machining
Fig.2 Early honing head structure
Honing process characteristics
- 1) The contact area between the abrasive strip and the hole wall of the workpiece is large during honing, and the vertical load of the abrasive grain is only 1/50-1/100 of that of grinding. in addition, the cutting speed of honing is low, generally below 100m/min, which is only 1/30-1/100 of that of ordinary grinding. during honing, a large amount of cutting fluid is injected, which can make the dislodged abrasive grain flushed away in time, and can also make the processed surface fully cooled, so The workpiece is less heated, not easy to be burned, and the deformation layer is very thin, so that a higher surface quality can be obtained.
- 2) Honing can achieve high dimensional accuracy, shape accuracy and low roughness, honing can obtain the hole accuracy of IT6-IT7 grade, surface roughness Ra is 0.2-0.025. Because in honing, the protruding part of the surface is always first contacted with the sand bar and ground away first, until the sand bar is completely in contact with the surface of the workpiece, thus honing can make a certain degree of correction to the geometric shape error left behind by the previous process. The shape error of the hole is generally less than 0.005mm.
- 3) Honing head is connected with machine tool spindle by floating, when honing head is working, it is guided by the workpiece hole wall and moves reciprocally along the center line of the pre-finished hole, so honing process cannot correct the relative position error of the hole, therefore, pre-finishing process must be arranged in the hole before honing to ensure its position accuracy. The honing allowance after boring is 0.05-0.08mm, after reaming is 0.02-0.04mm and after grinding is 0.01-0.02mm. The allowance can be divided into coarse and fine honing twice when the allowance is larger.
- 4) Honing hole productivity is high, short maneuvering time, honing 1 hole only need 2-3min, high processing quality, processing range, can process cast iron parts, quenched and non-quenched steel parts and bronze parts, etc., but not suitable for processing tough non-ferrous metals, processing hole diameter of 15-500mm, hole depth-diameter ratio of up to 10 or more.
The use of honing
Finishing processing of the finishing surface with the oil stone (also called honing strip) set on the honing head (see cutting processing). Honing is mainly used for machining various cylindrical holes with bore diameters of 5-500 mm or larger, such as cylinder barrels, valve holes, connecting rod holes and box holes, etc. The ratio of hole depth to bore diameter can reach 10 or even larger. Under certain conditions, honing can also process outer circle, flat surface, spherical surface and tooth surface, etc. The surface roughness of cylindrical honing is generally up to Ra0.32-0.08 micron, and the surface roughness of fine honing can be up to Ra0.04 micron or less, and can improve the geometric accuracy by a small amount, and the processing accuracy can be up to IT7-4. The surface quality of flat honing is slightly worse.
Honing generally uses honing machine, the machine tool spindle and honing head are generally floating coupling; but in order to improve the ability to correct the geometry of the workpiece, it can also use rigid coupling. When honing the hole, the honing head is generally set with 2-10 oil stones around the periphery, driven by the machine tool spindle to rotate in the hole and make linear reciprocating motion at the same time, which is the main motion; meanwhile, the oil stones are controlled by the spring or hydraulic pressure in the honing head to rise evenly and make radial feed to the wall of the hole to be processed. Figure 1 shows the diagram of internal honing. The ratio of the number of reciprocations per minute and the number of revolutions of the honing head should be taken as a non-integer number, so that the processing traces formed by the abrasive on the surface of the workpiece become a crossed net pattern without repeating each other. Figure 2 shows the trajectory of a single oil stone when honing in a hole. The oil stone reciprocates up and down once, and the workpiece rotates more than one circle. The abrasive grit of rough honing oil stone is 120-180, and the fine grit oil stone below W28 is used for fine honing. The width of oil stone is 3-20mm, and the length is about 1/3-3/4 of the hole length. when the oil stone moves reciprocally in the hole, the length beyond the hole at both ends should not be greater than 1/3 of the full length of the oil stone, otherwise it is easy to produce a flare; but when the overtravel is less than 1/4 of the length of the oil stone, it will make the hole drum-shaped. The honing principle and operation requirements for external circle and plane are the same as those for internal circle honing.
Honing allowance is generally not more than 0.2mm. The circumferential speed of honing is about 15-30m/min for steel processing, and can be increased to more than 50m/min for cast iron or non-ferrous metal processing; the reciprocating speed of honing should not exceed 15-20m/min. The pressure of the oil stone on the hole wall is generally 0.3-0.5 MPa, up to about 1 MPa for rough honing, and less than 0.1 MPa for fine honing. Since the oil stone and the workpiece are in contact with each other, the vertical pressure of each abrasive grain on the surface of the workpiece is only 1/50-1/100 of that of grinding, plus the low honing speed, so the temperature of the cutting area can be kept within the range of 50-150℃, which is conducive to reducing the residual stress on the processed surface and improving the surface quality. In order to flush the chips and avoid blocking the oil stone, and at the same time reduce the temperature of the cutting area and reduce the surface roughness, the cutting fluid used in honing should have a certain working pressure and be filtered. Most of the cutting fluids are made of kerosene, or kerosene with spindle oil, and some of them are made of extreme pressure emulsion. In the absence of a special honing machine, the honing tool holder can also be mounted on a vertical drilling machine to achieve the task of honing the inner hole.
Honing technology improvement
In the 1970s, the diamond oil stone and cubic boron nitride oil stone with metal bond were adopted to process hardened steel or cast iron, and the abrasive consumption is only 1/150-1/250 of the ordinary oil stone, while the pressure of the oil stone on the workpiece can be increased by 2-3 times, so that the efficiency and surface quality of honing can be further improved.
Source: Network Arrangement – China Flange Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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