1) The size of the tool used for hole machining is limited by the size of the hole being machined, resulting in poor rigidity and easy bending deformation and vibration;
2) When using fixed size cutting tools to process holes, the size of the hole often directly depends on the corresponding size of the tool, and the manufacturing error and wear of the tool will directly affect the machining accuracy of the hole;
3) When machining holes, the cutting area is inside the workpiece, and the conditions for chip removal and heat dissipation are poor, making it difficult to control the machining accuracy and surface quality.
(1) Drilling
Drilling is the first process of machining holes on solid materials, with a drilling diameter generally less than 80mm. There are two ways to drill: one is to rotate the drill bit; Another type is workpiece rotation. The errors generated by the above two drilling methods are not the same. In the drilling method of rotating the drill bit, when the cutting edge is asymmetric and the drill bit is not rigid enough, the centerline of the processed hole will be skewed or not straight, but the aperture will remain basically unchanged; In the drilling method of rotating the workpiece, on the contrary, the deviation of the drill bit will cause a change in the aperture, while the centerline of the hole remains straight.
The commonly used drilling tools include: Fried Dough Twists drill, center drill, deep hole drill, etc., of which the most commonly used is the Fried Dough Twists drill, whose diameter specification is Φ 0.1-80mm.
Due to structural limitations, the bending stiffness and torsional stiffness of the drill bit are both low, coupled with poor centering, resulting in low accuracy in drilling processing, generally only reaching IT13 to IT11; The surface roughness is also relatively high, with Ra generally ranging from 50 to 12.5 μ M; But the metal removal rate of drilling is high, and the cutting efficiency is high. Drilling is mainly used for processing holes with low quality requirements, such as bolt holes, threaded bottom holes, oil holes, etc. For holes that require high machining accuracy and surface quality, they should be achieved through reaming, reaming, boring, or grinding in subsequent machining.
(2) Expanding holes
Expanding a hole is the further processing of a hole that has already been drilled, cast, or forged using an expanding drill, in order to expand the aperture and improve the machining quality of the hole. Expanding a hole can be used as a pre-processing before precision machining, or as the final processing of a hole with low requirements. The reamer is similar to the Fried Dough Twists drill, but it has more teeth and no cross edge.
1) Expanding drilling has a large number of teeth (3-8 teeth), good guidance, and stable cutting;
2) The reaming drill does not have a horizontal edge and has good cutting conditions;
3) The machining allowance is small, the chip groove can be made shallower, the drill core can be made thicker, and the tool body has better strength and rigidity. The accuracy of hole expansion machining is generally IT11~IT10 levels, and the surface roughness Ra is 12.5~6.3 μ M. Expanding holes is commonly used to process holes with a diameter smaller than. When drilling holes with a larger diameter (D Greater than or equal to 30mm), it is common to pre drill with a small drill bit (with a diameter of 0.5-0.7 times the aperture), and then use a corresponding size reamer to enlarge the hole. This can improve the machining quality and production efficiency of the hole.
(1) Reamer
(2) Reaming process and its application
The allowance for reaming has a significant impact on the quality of reaming. If the allowance is too large, the load on the reamer will be high, and the cutting edge will be quickly dulled, making it difficult to obtain a smooth machining surface and ensuring dimensional tolerances; The margin is too small to remove the knife marks left by the previous process, which naturally has no effect on improving the quality of hole processing. The general rough hinge allowance is taken as {{0}}.35-0.15mm, and the fine hinge allowance is taken as 0.15-0.05mm.
To avoid the formation of chip deposits, reaming is usually carried out at a lower cutting speed (when using high-speed steel reamers to process steel and cast iron, v<8m/min). The value of feed rate is related to the aperture being processed. The larger the aperture, the larger the feed rate value. When high-speed steel reamers process steel and cast iron, the feed rate is usually set to 0.3-1mm/r.
The accuracy of hinge hole size is generally IT9 to IT7, and the surface roughness Ra is generally 3.2 to 0.8 μ M. For holes with medium size and high precision requirements (such as IT7 level precision holes), the drilling expanding hinge process is a typical machining scheme commonly used in production.
(1) Boring method
1) The rotation of the workpiece and the feed motion of the tool in boring holes on a lathe mostly belong to this type of boring method. The process characteristics are: the axis of the processed hole 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 geometric shape error of the hole mainly depends on the positional accuracy of the tool feed direction relative to the rotation axis of the workpiece. This boring method is suitable for machining holes with coaxiality requirements on the outer surface.
2) The tool rotates, and the workpiece moves in feed. The boring machine spindle drives the boring tool to rotate, and the worktable drives the workpiece to move in feed.
3) The tool rotates and performs feed motion using this boring method. The overhanging length of the boring bar changes, and the force deformation of the boring bar also changes. The aperture near the spindle box is larger, while the aperture far away from the spindle box is smaller, forming a tapered hole. In addition, as the overhanging length of the boring bar increases, the bending deformation of the spindle caused by its own weight also increases, and the axis of the machined hole will produce corresponding bending. This boring method is only suitable for processing shorter holes.
(2) Diamond boring
Compared with general boring, the characteristics of diamond boring are small back cutting amount, small feed rate, high cutting speed, and it can achieve high machining accuracy (IT{{0}}IT6) and very smooth surface (Ra is 0.4-0.05) μ m) . Diamond boring was originally processed with diamond boring cutters, but now it is generally processed with hard alloy, CBN, and artificial diamond cutting tools. Mainly used for processing non-ferrous metal workpieces, it can also be used for processing cast iron and steel parts.
The pre boring of the back cutting amount is 0.2-0.6mm,
Final boring is 0.1mm;
The feed rate is {{0}}.01~0.14mm/r;
The cutting speed for processing cast iron is 100-250m/min,
When processing steel, it is 150-300m/min,
When processing non-ferrous metals, the speed is 300-2000m/min.
In order to ensure that diamond boring can achieve high machining accuracy and surface quality, the machine tool (diamond boring machine) used must have high geometric accuracy and stiffness. The spindle support of the machine tool commonly uses precision angular contact ball bearings or hydrostatic sliding bearings, and high-speed rotating parts must be precisely balanced; In addition, the motion of the feed mechanism must be very smooth to ensure that the workbench can perform smooth low-speed feed motion.
Diamond boring has good machining quality and high production efficiency, and is widely used in the final machining of precision holes in large-scale production, such as engine cylinder holes, piston pin holes, and spindle holes on machine tool spindle boxes. However, it should be noted that when using diamond boring to process black metal products, only boring cutters made of hard alloy and CBN can be used, and boring cutters made of diamond cannot be used because the affinity between carbon atoms in diamond and iron group elements is high, and the tool life is low.
(3) Boring cutter
(4) The Process Characteristics and Application Range of Boring
Based on the above analysis, it can be concluded that boring has a wide processing range and can process holes of various sizes and accuracy levels. For holes and hole systems with larger apertures and higher requirements for size and positional accuracy, boring is almost the only processing method. The machining accuracy of boring holes is between IT9 and IT7 levels. Boring can be carried out on boring machines, lathes, milling machines and other machine tools, with the advantages of flexibility and flexibility, and is widely used in production. In mass production, boring dies are often used to improve boring efficiency.
(1) Honing principle and honing head
(2) The process characteristics and application scope of honing
1) Honing can achieve high dimensional and shape accuracy, with machining accuracy ranging from IT7 to IT6. The roundness and cylindricity errors of the hole can be controlled within a certain range, but honing cannot improve the positional accuracy of the processed hole.
2) Honing can achieve high surface quality, with a surface roughness Ra of 0.2~0.25 μ m. The depth of the metamorphic defect layer on the surface metal is extremely small, ranging from 2.5 to 25 μ M.
3) Compared with the grinding speed, although the circumferential speed of the honing head is not high (VC=16-60m/min), due to the large contact area between the sand strip and the workpiece, the reciprocating speed is relatively high (VA=8-20m/min), so honing still has a higher productivity.
Honing is widely used in large-scale production for the machining of precision holes in engine cylinder holes and various hydraulic devices. The aperture range is generally or larger, and deep holes with a length to diameter ratio greater than 10 can be machined. However, honing is not suitable for machining holes on non ferrous metal workpieces with high plasticity, nor for machining holes with keyways, spline holes, etc.
(1) Broaching and Broaching
During broaching, the cutting tool only performs low-speed linear motion (main motion). The number of teeth that the cutting tool should work on at the same time should generally not be less than 3, otherwise the cutting tool will not work smoothly and may produce circular ripples on the surface of the workpiece. In order to avoid excessive cutting force that may cause the broach to break, the number of teeth on the working tool should generally not exceed 6-8 during broaching.
1) The characteristic of layered broaching is that the broach sequentially cuts off the machining allowance of the workpiece layer by layer. In order to facilitate chip breaking, there are interlocking chip separation grooves on the cutter teeth. A broach designed according to a layered cutting method is called a regular broach.
2) The characteristic of block turning is that each layer of metal on the machined surface is cut off by a set of teeth (usually composed of 2-3 teeth in each group) that are basically the same size but interlocked with each other. Each blade only removes a portion of a layer of metal. A broach designed according to the block cutting method is called a wheel cutting broach.
3) The comprehensive broaching method combines the advantages of layered and block broaching. The rough cutting part adopts block broaching, while the fine cutting part adopts layered broaching. This can not only shorten the length of the broach, improve productivity, but also achieve better surface quality. A broach designed according to a comprehensive cutting method is called a comprehensive broach.
(2) The process characteristics and application scope of pulling holes
1) A broach is a multi edged tool that can sequentially complete the rough machining, precision machining, and finishing of holes in one cutting stroke, resulting in high production efficiency.
2) The accuracy of hole pulling mainly depends on the accuracy of the broach. Under normal conditions, the accuracy of hole pulling can reach IT9~IT7, and the surface roughness Ra can reach 6.3~1.6 μ M.
3) When pulling holes, the workpiece is positioned by the processed hole itself (the leading part of the cutting tool is the positioning component of the workpiece), and it is difficult to ensure the accuracy of the mutual position between the hole and other surfaces when pulling holes; For the machining of rotating parts with coaxiality requirements on the inner and outer surfaces, it is often necessary to first pull holes and then use the holes as the positioning reference to machine other surfaces.
4) Broaches can not only process circular holes, but also form holes and spline holes.
5) Broaches are fixed size cutting tools with complex shapes and expensive prices, which are not suitable for machining large holes.
Pulling holes are commonly used in mass production to process aperture sizes of Ф Through holes on small and medium-sized parts with a diameter of 10-80mm and a hole depth not exceeding 5 times the aperture.
