Precision Hole Manufacturing: Understanding Process Capabilities and Limitations
Drilling and Reaming
1. Drilling
Drilling is the first step in creating a hole in solid materials, typically with a diameter of less than 80 mm. There are two main drilling methods: one uses a rotating drill bit, while the other involves rotating the workpiece. The errors produced by these two methods differ.
In the rotating drill bit method, the drill bit can deviate from the centerline of the hole due to factors like asymmetric cutting edges and insufficient drill rigidity. However, the hole’s diameter generally remains unchanged. Conversely, in the rotating workpiece method, any deviation of the drill bit can alter the hole’s diameter, but the centerline tends to stay straight.
Common drilling tools include twist drills, center drills, and deep-hole drills. Twist drills are the most widely used, with diameters ranging from 0.1 mm to 80 mm. Due to their structural limitations, drill bits have low bending and torsional stiffness and poor centering, which results in low drilling accuracy—typically only reaching an IT grade of 13 to 11. Additionally, the surface roughness is relatively high, with Ra values typically ranging from 50 to 12.5 μm.
Despite these limitations, drilling offers a high metal removal rate and cutting efficiency. It is primarily used for creating holes with lower quality requirements, such as those for bolts, threaded bottoms, and oil passages. For holes requiring higher precision and better surface quality, additional processing methods such as reaming, boring, or grinding should be employed.
2. Hole reaming
Hole reaming is a process used to further refine a hole that has been drilled, cast, or forged. It involves using a reaming drill to enlarge the hole’s diameter and enhance its quality. Hole reaming can serve as a preliminary step before finishing a hole or as a final operation for holes with less stringent requirements.
A reaming drill is similar to a twist drill but features more cutting teeth—usually between three and eight—and lacks a chisel edge. This leads to several advantages when compared to drilling:
1. The reaming drill has more teeth, providing better guidance and more stable cutting.
2. Without a chisel edge, the cutting conditions are improved.
3. The machining allowance is small, enabling shallower chip grooves and a thicker drill core, which enhances the strength and rigidity of the tool body.
The accuracy of hole reaming typically falls within the IT11 to IT10 range, with a surface roughness of Ra 12.5 to 6.3. This technique is commonly used for processing holes with diameters less than Ф100mm. When drilling larger diameter holes (D ≥ 30mm), it is common practice to pre-drill with a smaller drill bit (with a diameter 0.5 to 0.7 times that of the intended hole) before reaming with a suitably sized countersink drill. This approach improves hole quality and production efficiency.
In addition to producing cylindrical holes, countersink drills, also known as countersinks, can take on various special shapes to create countersunk holes and countersink end faces. These countersink drills often include a guide pin at the front end to help ensure accurate drilling.
Reaming
Reaming is a method for finishing holes that is widely used in production. For smaller holes, reaming is more economical and practical compared to internal grinding and fine boring.
1. Reamers
Reamers are typically classified into two categories: hand reamers and machine reamers. Hand reamers feature straight shanks and longer working sections, which offer improved guidance during use. They come in both integral and adjustable outer diameter (OD) configurations. On the other hand, machine reamers are available in both shank-mounted and sleeve-mounted designs. Reamers are versatile tools that can be used for creating not only round holes but also tapered holes, which are specifically addressed by tapered reamers.
2. Reaming Process and Its Application
Reaming allowance has a significant impact on the quality of the reamed holes. If the allowance is excessive, it places a heavy load on the reamer, which can quickly dull the cutting edge. This makes it difficult to achieve a smooth surface finish and maintain dimensional tolerances. Conversely, if the allowance is insufficient, it may not effectively remove tool marks left by previous processes, resulting in a failure to improve hole quality. Typically, rough reaming allowances range from 0.35 mm to 0.15 mm, while fine reaming allowances are between 0.1 mm and 0.05 mm.
To prevent built-up edge (BUE), reaming should be performed at low cutting speeds—less than 8 m/min for high-speed steel reamers machining steel and cast iron. The feed rate also varies depending on the diameter of the hole; larger diameters require a higher feed rate. For high-speed steel reamers working on steel and cast iron, the typical feed rate is between 0.3 mm and 1 mm per minute.
While reaming, it is essential to use appropriate cutting fluids for cooling, lubrication, and cleaning to prevent BUE and ensure effective chip removal. Compared to grinding and boring, reaming provides higher productivity and greater accuracy in hole dimensions. However, reaming cannot correct positional errors in the hole axis, so accuracy must be guaranteed in the preceding processes. Additionally, reaming is not suitable for producing stepped or blind holes.
The dimensional accuracy of reamed holes generally falls within grades IT9 to IT7, with a surface roughness (Ra) ranging from 3.2 to 0.8. For medium-sized holes with high precision requirements, such as those classified as IT7, the drill-reamer-reamer process is commonly used in production.
Boring
Boring is a machining process that uses a cutting tool to enlarge a prefabricated hole. Boring can be performed on either a boring machine or a lathe.
1. Boring Methods
Boring methods are divided into three different types:
1) Workpiece Rotation, Tool Feed.
Most boring operations performed on lathes fall into this method. This process is characterized by aligning the centerline of the hole with the rotational axis of the workpiece. The roundness of the hole primarily depends on the rotational accuracy of the machine spindle, while the axial geometric error is mainly influenced by the positional accuracy of the tool’s feed direction relative to the workpiece’s rotational axis. This boring method is suitable for creating holes that require coaxial alignment with the outer cylindrical surface.
2) Tool Rotation, Workpiece Feed.
The boring machine’s spindle rotates the boring cutter, while the worktable feeds the workpiece. In this process, the tool both rotates and feeds. This method allows for variation in the length of the boring bar overhang, which leads to deformation of the bar in different ways. As a result, the hole diameter is larger near the spindle housing and smaller farther away, creating a tapered hole. Additionally, as the overhang of the boring bar increases, the spindle experiences bending deformation due to its weight, which in turn causes a corresponding bend in the axis of the hole being machined. Therefore, this boring method is only suitable for shorter holes.
2. Diamond Boring
Diamond boring differs from conventional boring in several key aspects, including reduced back boring depth, lower feed rates, and higher cutting speeds. This method can achieve exceptional machining accuracy (IT7-IT6) and produces very smooth surfaces (Ra 0.4-0.05). Initially, diamond boring was carried out using diamond tools; however, carbide, CBN (cubic boron nitride), and synthetic diamond tools are now more commonly employed.
Diamond boring is primarily used for machining non-ferrous metal workpieces, though it can also effectively machine cast iron and steel.
Typical cutting parameters for diamond boring include a back boring depth of 0.2-0.6 mm for pre-boring and 0.1 mm for final boring, a feed rate of 0.01-0.14 mm/rev, and cutting speeds ranging from 100-250 m/min for cast iron, 150-300 m/min for steel, and 300-2000 m/min for non-ferrous metals. To achieve high machining accuracy and surface quality, the diamond boring machine used must exhibit high geometric accuracy and rigidity. Precision angular contact ball bearings or hydrostatic plain bearings are often utilized for spindle support, and high-speed rotating components must be precisely balanced. Additionally, the feed mechanism should function smoothly to ensure a consistent low-speed feed motion of the worktable.
Diamond boring offers excellent machining quality and high production efficiency, making it widely adopted in large-scale mass production for the final machining of precision holes, such as engine cylinder bores, piston pin holes, and spindle holes in machine tool spindle boxes. It is crucial to note that when boring ferrous metals, only carbide and CBN boring tools should be used, as diamond tools are not suitable. This is due to the strong affinity of carbon atoms in diamond for iron group elements, which results in a reduced tool life.
3. Boring Tools
Boring tools can be divided into single-edge and double-edge boring tools.
4. Boring Process Characteristics and Applications
Boring, in comparison to the drilling-reaming-reaming process, is not restricted by tool size when it comes to hole diameter. It also provides strong error correction capabilities, allowing for multiple passes to rectify deviations in the original hole axis. Furthermore, boring maintains a high level of positional accuracy between the bored hole and the positioning surface.
However, when compared to external turning, boring has some drawbacks. It generally results in lower machining quality and production efficiency due to the lower rigidity and greater deformation of the toolholder system. Additionally, there are issues with poor heat dissipation and significant thermal deformation of both the workpiece and tool.
From this analysis, it is evident that boring has a wide range of machining applications, being capable of producing holes of various sizes and precision levels. For larger holes and systems that require high dimensional and positional accuracy, boring is often the preferred machining method. It achieves accuracy levels of IT9 to IT7, along with a low surface roughness, Ra. Boring can be performed on various machine tools, including boring machines, lathes, and milling machines. Its flexibility makes it widely used in production, and in large-scale mass production, boring dies are often employed to enhance efficiency.
Honing a Hole
1. Honing Principle and Honing Head
Honing is a finishing method used to create precise holes using a honing head equipped with a grinding wheel (often referred to as an oilstone). In this lathe process, the workpiece remains stationary while the honing head is rotated and reciprocated by the machine tool spindle. The grinding wheel exerts pressure on the surface of the workpiece, removing a very thin layer of material. This cutting action creates a cross-shaped pattern on the surface.
To prevent the abrasive grains in the grinding wheel from following the same paths, both the rotational speed of the honing head and the number of reciprocating strokes per minute must be prime numbers.
The intersection angle (θ) of the honing paths is influenced by the reciprocating speed (va) and the peripheral speed (vc) of the honing head. The size of this angle significantly affects the quality and efficiency of the honing process. Generally, an angle of θ = 40-60° is used for rough honing, while θ = 0.05° is preferred for fine honing. To aid in the removal of broken abrasive particles and chips, reduce cutting temperatures, and improve machining quality, it is essential to use sufficient cutting fluid during honing.
To ensure uniform machining of the hole wall, the abrasive strip must extend beyond the travel distance at both ends of the hole. Additionally, to maintain a consistent honing allowance and to minimize the effects of spindle rotation errors on machining accuracy, a floating connection is typically employed between the honing head and the machine spindle.
The radial extension and retraction of the honing head strips can be adjusted manually, pneumatically, or hydraulically.
2. Process Characteristics and Applications of Honing
1) Honing is capable of achieving high dimensional and shape accuracy, with machining precision levels reaching IT7-IT6. While it can control hole roundness and cylindricity errors within a specific range, honing does not improve the positional accuracy of machined holes.
2) This technique can also attain excellent surface quality, with a surface roughness (Ra) ranging from 0.2 to 0.025 μm, and it introduces minimal surface metal deterioration defects, typically between 2.5 and 25 μm.
3) Although the circumferential speed of the honing head is relatively low compared to grinding speed (vc = 16-60 m/min), honing maintains high productivity due to the large contact area between the abrasive strip and the workpiece, combined with a relatively high reciprocating speed (va = 8-20 m/min).
Honing is widely utilized in mass production for machining precision holes in engine cylinder bores and various hydraulic systems. It can accommodate a broad range of hole diameters and is capable of machining deep holes with aspect ratios greater than 10. However, honing is not suitable for machining holes in non-ferrous metal workpieces with high plasticity, nor can it be used for machining holes with keyways, splines, and similar features.
Broaching
1. Broaching and Broaching Tools
Broaching is a high-productivity finishing method carried out on a broaching machine using a specially designed tool called a broach. Broaching machines are categorized into horizontal and vertical types, with horizontal machines being the most common.
During the broaching process, the broach moves in a low-speed linear motion, which is the main motion involved. To ensure smooth operation, the number of teeth on the broach that are engaged at the same time should generally be at least three. If there are fewer than three teeth in contact, the broach may not function properly and can create annular ripples on the surface of the workpiece. Conversely, to prevent excessive broaching force that could lead to the breakage of the broach, it is advisable that the number of working teeth engaged simultaneously does not exceed six to eight.
There are three different broaching methods, described below:
(1) Layered Broaching:
This broaching method is characterized by the sequential removal of machining allowances from the workpiece, layer by layer. To aid in chip breaking, the cutting teeth are designed with interlocking grooves. Broaches specifically intended for this layered broaching technique are referred to as ordinary broaches.
(2) Segmented Broaching:
This broaching method involves removing each layer of metal from the machined surface using a set of interlocking cutting teeth, typically consisting of 2 to 3 teeth per set. Each tooth is responsible for removing only a portion of a single layer of metal. Broaches that utilize this segmented broaching technique are known as rotary broaches.
(3) Composite Broaching:
This method combines the benefits of layered and segmented broaching. The roughing teeth are created using segmented broaching, while the finishing teeth are produced through layered broaching. This approach reduces the length of the broach, enhances productivity, and results in improved surface quality. Broaches designed using this technique are referred to as composite broaches.
2. Hole Broaching Process Characteristics and Applications
1) Broaches are multi-edged tools designed to complete the roughing, finishing, and polishing of a hole in a single broaching stroke, which significantly enhances production efficiency.
2) The accuracy of hole broaching largely depends on the precision of the broach. Under normal conditions, broaching accuracy can achieve levels between IT9 and IT7, with surface roughness values ranging from Ra 6.3 to 1.6μm.
3) During the hole broaching process, the aluminum machined parts is positioned concerning the hole being machined, using a broaching guide as the positioning element. This can make it challenging to maintain the relative positional accuracy of the hole and other surfaces. For parts that require coaxial alignment of internal and external circular surfaces, the hole is typically broached first, after which other surfaces can be machined using the hole as a reference.
4) Broaches are capable of machining not only round holes but also form holes and spline holes.
5) Broaches are fixed-size tools with complex shapes and can be quite expensive, making them less suitable for machining large holes. In mass production, hole drawing is often employed to process through holes in small to medium-sized parts, typically with a diameter ranging from 10 to 80 mm and a hole depth not exceeding five times the hole diameter.
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