10 Classifications of Linear Transmission Mechanisms (Compiled from Essential Information)
HIWIN KK Specifications and Parameters Introduction
Compared to machining external cylindrical surfaces, machining holes presents much more challenging conditions, making it much more difficult than machining external cylindrical surfaces. This is because:
1) The size of the cutting tools 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 machining holes with a fixed-size tool, the size of the hole often depends directly on the corresponding size of the tool. The manufacturing error and wear of the tool will directly affect the machining accuracy of the hole.
3) When machining holes, the cutting zone is inside the workpiece, resulting in poor chip removal and heat dissipation, making it difficult to control machining accuracy and surface quality.
Drilling and reaming
1. Drilling
Drilling is the first step in machining holes in solid materials, and the hole diameter is generally less than 80mm. There are two methods of drilling: one is drilling with the drill bit rotating; the other is drilling with the workpiece rotating. The errors produced by these two drilling methods are different. In drilling with the drill bit rotating, due to the asymmetry of the cutting edge and insufficient rigidity of the drill bit, when the drill bit deviates, the center line of the machined hole will be skewed or not straight, but the hole diameter remains basically unchanged. In drilling with the workpiece rotating, the opposite is true; when the drill bit deviates, it will cause a change in the hole diameter, but the hole center line will still be straight.
Commonly used drilling tools include twist drills, center drills, and deep hole drills. Among them, twist drills are the most commonly used, and their diameter specifications are used to solve machining problems—the classification and comparison of hole machining.
Due to structural limitations, drill bits have low bending and torsional stiffness, and poor centering, resulting in low drilling accuracy, typically only reaching IT13 to IT11. Surface roughness is also relatively high, with Ra generally ranging from 50 to 12.5 μm. However, drilling offers a high metal removal rate and cutting efficiency. Drilling is mainly used for machining holes with low quality requirements, such as bolt holes, threaded pilot holes, and oil holes. For holes requiring higher machining accuracy and surface quality, these should be achieved through reaming, boring, or grinding in subsequent machining processes.
2. Enlarging the hole
Reaming is a process using a reamer to further process holes that have already been drilled, cast, or forged, in order to enlarge the hole diameter and improve the quality of the hole. Reaming can be used as a pre-processing step before finishing holes, or as a final processing step for holes with less demanding requirements. A reamer is similar to a twist drill, but it has more cutting teeth and no chisel edge.
Compared with drilling, reaming has the following characteristics:
(1) The number of teeth in the reaming drill is large (3 to 8 teeth), the guidance is good, and the cutting is relatively stable;
(2) The reamer has no chisel edge, resulting in good cutting conditions;
(3) With a smaller machining allowance, the chip flute can be made shallower, the drill core can be made coarser, and the tool body has better strength and rigidity. The accuracy of reaming is generally IT11 to IT10, and the surface roughness Ra is 12.5 to 6.3. Reaming is often used to machine holes with a diameter smaller than 30mm. When drilling a larger diameter hole (D≥30mm), it is common to first pre-drill with a small drill bit (0.5 to 0.7 times the diameter of the hole), and then use a reamer of the corresponding size to enlarge the hole. This can improve the machining quality and production efficiency of the hole.
Besides machining cylindrical holes, reaming can also be used to machine various countersunk holes and countersunk end faces using reamers of various special shapes (also known as countersunk drills). The front end of a countersunk drill often has a guide post to guide it using the pre-machined hole.
Reamed hole
Reaming is a finishing method for holes and is widely used in production. For smaller holes, reaming is a more economical and practical machining method compared to internal grinding and precision boring.
1. Reamer
Reamers are generally divided into two types: hand reamers and machine reamers. Hand reamers have a straight shank, a longer working part, and better guiding effect. Hand reamers are available in two structures: integral and adjustable outer diameter. Machine reamers are available in two structures: shanked and sleeved. Reamers can not only machine round holes, but taper reamers can also machine tapered holes.
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 is too heavy, the cutting edge will wear down quickly, it is difficult to obtain a smooth machined surface, and dimensional tolerances are difficult to guarantee. If the allowance is too small, it cannot remove the tool marks left by the previous process, and naturally it will not improve the quality of the hole. Generally, the rough reaming allowance is 0.35~0.15mm, and the finish reaming allowance is 0.15~0.05mm.
To avoid built-up edge, reaming is usually performed at a lower cutting speed (v < 8 m/min when machining steel and cast iron with high-speed steel reamers). The feed rate is related to the diameter of the hole being machined; the larger the diameter, the larger the feed rate. When machining steel and cast iron with high-speed steel reamers, the feed rate is usually 0.3–1 mm/r.
When reaming, appropriate cutting fluid must be used for cooling, lubrication, and cleaning to prevent built-up edge formation and to remove chips promptly. Compared to grinding and boring, reaming offers higher productivity and makes it easier to ensure hole accuracy; however, reaming cannot correct positional errors in the hole axis, and the hole's positional accuracy must be ensured by the preceding process. Reaming is not suitable for machining stepped holes and blind holes.
The dimensional accuracy of reamed holes is generally IT9 to IT7, and the surface roughness Ra is generally 3.2 to 0.8. For medium-sized holes with high precision requirements (such as IT7 precision holes), the drilling-reaming-reaming process is a typical machining scheme commonly used in production.
Boring hole
Boring is a machining method that uses a cutting tool to enlarge a pre-made hole. Boring can be performed on a boring machine or a lathe.
1. Boring method
There are three different machining methods for boring.
(1) The workpiece rotates and the tool makes a feed motion. Most boring on a lathe belongs to this type of boring. The process characteristics are: the axis of the hole after machining is consistent with the axis of rotation of the workpiece, the roundness of the hole mainly depends on the rotational 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 axis of rotation of the workpiece. This boring method is suitable for machining holes that have coaxiality requirements with the outer cylindrical surface.
(2) The tool rotates and the workpiece makes a feed motion. The boring machine spindle drives the boring tool to rotate, and the worktable drives the workpiece to make a feed motion.
(3) The tool rotates and makes a feed motion. When boring in this way, the overhang length of the boring bar is variable, and the stress deformation of the boring bar is also variable. The hole diameter is larger near the spindle box and smaller further away from the spindle box, forming a tapered hole. In addition, as the overhang length of the boring bar increases, the bending deformation of the spindle due to its own weight also increases, and the axis of the machined hole will bend accordingly. This boring method is only suitable for machining relatively short holes.
2. Diamond halberd
Compared to conventional boring, diamond boring is characterized by a smaller depth of cut, smaller feed rate, and higher cutting speed, resulting in higher machining accuracy (IT7–IT6) and a very smooth surface (Ra 0.4–0.05). Originally using diamond boring tools, diamond boring now commonly employs carbide, CBN, and synthetic diamond tools. It is primarily used for machining non-ferrous metal workpieces, but can also be used for machining cast iron and steel parts.
The commonly used cutting parameters for diamond boring are: depth of cut: 0.2-0.6 mm for pre-boring and 0.1 mm for final boring; feed rate: 0.01-0.14 mm/r; cutting speed: 100-250 m/min for machining cast iron, 150-300 m/min for machining steel, and 300-2000 m/min for machining non-ferrous metals.
To ensure that diamond boring achieves high machining accuracy and surface quality, the machine tool used (diamond boring machine) must have high geometric accuracy and rigidity. The machine tool spindle support commonly uses precision angular contact ball bearings or hydrostatic sliding bearings, and high-speed rotating parts must be precisely balanced. In addition, the movement of the feed mechanism must be very smooth to ensure that the worktable can make smooth low-speed feed movements.
Diamond boring tools offer high-quality machining and production efficiency, and are widely used in mass production for the final machining of precision holes, such as engine cylinder bores, piston pin bores, and spindle bores on machine tool spindle boxes. However, it is important to note that when machining ferrous metals with diamond boring tools, only boring tools made of cemented carbide or CBN can be used; diamond boring tools cannot be used because the carbon atoms in diamond have a high affinity for iron group elements, resulting in a shorter tool life.
3. Boring tool
Boring tools can be divided into single-edged boring tools and double-edged boring tools.
4. Technological characteristics and application scope of boring
Compared with drilling-reaming-reaming, boring is not limited by the size of the tool. It also has a strong error correction capability, which can correct the original hole axis deviation error through multiple tool passes. Moreover, it can maintain a high positional accuracy between the bored hole and the positioning surface.
Compared to turning, boring has lower machining quality and lower production efficiency due to the poor rigidity and large deformation of the tool holder system, poor heat dissipation and chip removal conditions, and greater thermal deformation of the workpiece and tool.
In summary, boring has a wide processing range, capable of machining holes of various sizes and precision levels. For holes and hole systems with large diameters and high requirements for dimensional and positional accuracy, boring is almost the only processing method. The machining accuracy of boring is IT9 to IT7, with a surface roughness Ra of [value missing]. Boring can be performed on machine tools such as boring machines, lathes, and milling machines, offering advantages in flexibility and making it widely used in production. In mass production, boring dies are often used to improve boring efficiency.
Honing holes
1. Honing principle and honing head
Honing is a method of finishing holes using a honing head equipped with abrasive stones (whetstones). During honing, the workpiece remains stationary, while the honing head rotates and reciprocates linearly, driven by the machine tool spindle. In honing, the abrasive stones apply pressure to the workpiece surface, removing a very thin layer of material; the cutting path is a crisscrossing pattern. To ensure that the abrasive grains do not repeat their paths, the rotational speed of the honing head (revolutions per minute) and the number of reciprocating strokes per minute should be coprime.
The intersection angle of the honing trajectory is related to the reciprocating speed and circumferential speed of the honing head. The size of the angle affects the machining quality and efficiency of honing. Generally, it is 0° for rough honing and 0° for fine honing. In order to facilitate the removal of broken abrasive grains and chips, reduce the cutting temperature, and improve the machining quality, sufficient cutting fluid should be used during honing.
To ensure uniform machining of the hole walls, the abrasive strip's stroke must exceed a certain amount at both ends of the hole. To guarantee uniform honing allowance and reduce the impact of machine tool spindle rotation error on machining accuracy, a floating connection is usually used between the honing head and the machine tool spindle.
The radial extension and retraction adjustment of honing heads and grinding bars can be achieved through various structural forms, including manual, pneumatic, and hydraulic methods.
2. Process characteristics and application scope of honing
(1) Honing can achieve higher dimensional and shape accuracy, with a machining accuracy of IT7 to IT6. The roundness and cylindricity errors of the hole can be controlled within the range, but honing cannot improve the positional accuracy of the machined hole.
(2) Honing can achieve high surface quality, with a surface roughness Ra of 0.2 to 0.025 μm and a very small depth of the alteration defect layer of the surface metal (2.5 to 25 μm).
(3) Compared with the grinding speed, the circumferential speed of the honing head is not high (vc = 16~60m/min), but because the contact area between the abrasive strip and the workpiece is large, the reciprocating speed is relatively high (va = 8~20m/min), so honing still has a high productivity.
Honing is widely used in mass production for machining precision holes in engine cylinder bores and various hydraulic devices. The hole diameter range is generally 10 or larger, and it can machine deep holes with a length-to-diameter ratio greater than 10. However, honing is not suitable for machining holes on non-ferrous metal workpieces with high plasticity, nor can it machine holes with keyways, splines, etc.
Pull hole
1. Broaching and broaching tools
Broaching is a highly productive finishing process performed on a broaching machine using a specially designed broaching tool. There are two types of broaching machines: horizontal and vertical, with the horizontal type being the most common.
During broaching, the broach performs only low-speed linear motion (primary motion). The number of teeth working simultaneously on the broach should generally be no less than three; otherwise, the broach will operate unevenly and is prone to producing annular ripples on the workpiece surface. To avoid excessive broaching force that could cause the broach to break, the number of teeth working simultaneously on the broach should generally not exceed six to eight.
There are three different broaching methods, which are described below:
(1) Layered broaching
The characteristic of this broaching method is that the broach removes the machining allowance from the workpiece layer by layer sequentially. To facilitate chip breaking, the cutting teeth are ground with interlocking chip-breaking grooves. Broaches designed according to the layered broaching method are called ordinary broaches.
(2) Segmented broaching
The characteristic of this broaching method is that each layer of metal on the machined surface is removed by a set of roughly the same size but interlaced cutting teeth (usually each set consists of 2-3 teeth). Each tooth removes only a portion of a layer of metal. Broaches designed using this segmented broaching method are called rotary broaches.
(3) Comprehensive broaching
This method combines the advantages of layered and segmented broaching. The roughing teeth are broached in segments, while the finishing teeth are broached in layers. This shortens the broach length, increases productivity, and achieves better surface quality. Broaches designed using this combined broaching method are called combined broaches.
2. Technological characteristics and application scope of hole drawing
(1) The broach is a multi-bladed tool that can complete the roughing, finishing and smoothing of the hole in sequence in one broaching stroke, resulting in high production efficiency.
(2) The accuracy of hole drawing mainly depends on the accuracy of the broach. Under normal conditions, the accuracy of hole drawing can reach IT9 to IT7, and the surface roughness Ra can reach 6.3 to 1.6 μm.
(3) When drawing holes, the workpiece is positioned by the hole itself (the broach guide is the positioning element of the workpiece). Drawing holes does not easily guarantee the relative positional accuracy between the hole and other surfaces. For the machining of rotating parts with coaxiality requirements for inner and outer cylindrical surfaces, the holes are often drawn first, and then other surfaces are machined with the holes as the positioning reference.
(4) The broach can not only process round holes, but also form holes and spline holes.
(5) Broaches are fixed-size cutting tools with complex shapes and high prices, and are not suitable for machining large holes.
Pulling is commonly used in mass production to process through holes in small and medium-sized parts with a diameter of Ф10～80mm and a hole depth not exceeding 5 times the hole diameter.
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