Compared with external cylindrical surface machining, the conditions for hole machining are much worse, making hole operations more challenging than external cylindrical machining. This is due to:
1) The size of the hole machining tool is restricted by the dimensions of the hole, leading to low rigidity, which can easily result in bending, deformation and vibration.
2) When machining holes with a fixed size tool, the size of the machined hole generally depends directly on the size of the tool. Any manufacturing errors or tool wear will directly affect the accuracy of the machined hole.
3) During hole machining, the cutting area is inside the workpiece, resulting in poor chip removal and heat dissipation conditions, making it challenging to control machining accuracy and surface quality.
1. Drilling and Reaming
(1) Drilling
Drilling is the main operation for making holes in solid materials, with a typical drilling diameter of less than 80 mm. There are two drilling methods: drill rotation and workpiece rotation.
The errors produced by these methods are different.
In the drill rotation method, the central axis of the hole may deviate or become misaligned due to asymmetrical cutting edges and insufficient rigidity of the drill, although the diameter remains essentially unchanged.
In contrast, with the part rotation method, any misalignment of the drill will result in diameter changes, but the center axis of the hole remains straight.
Common drilling tools include twist drills, center drills, and deep drills. The most commonly used is the twist drill, with diameter specifications ranging from Φ0.1-80mm.
Due to design limitations, the drills have low bending and torsional rigidity. Coupled with poor centering, drilling accuracy is generally only between IT13 ~ IT11.
The surface roughness is also relatively high, typically between Ra 50~12.5μm. Drilling is mainly used for holes with lower quality requirements, such as screw holes, threaded bottom holes and oil holes.
For holes that require greater precision and surface quality, subsequent operations such as reaming, boring or grinding must be applied.
(2) Enlargement
Reaming is used to process already drilled, cast or forged holes to increase their diameter and improve their machining quality.
It can serve as a pre-machining step for precision hole machining or as a final process for holes with lower requirements. Reamers resemble twist drills, but have more teeth and no cross edges.
Compared to drilling, reaming has the following characteristics:
1) The reamers have multiple teeth (3-8) ensuring better guidance and more stable cutting.
2) Without transverse edges, reamers provide better cutting conditions.
3) Due to smaller machining tolerances, the chip grooves can be shallower and the reamer core can be thicker, ensuring greater strength and rigidity.
The reaming accuracy is generally between IT11~IT10, with surface roughness ranging from Ra 12.5~6.3μm. Reaming is often used for holes with diameters less than 30 mm.
For larger diameter holes (D ≥30mm), a smaller drill bit (0.5-0.7 times the hole diameter) is initially used, followed by the corresponding reamer to improve machining quality and efficiency.
In addition to cylindrical holes, special reamers can be used to machine countersunk holes and flat faces. They typically feature a guide column using a pre-machined hole for guidance.
2. Boring
Boring is one of the precision machining methods for holes and is widely applied in manufacturing.
For smaller holes, compared with internal cylindrical grinding and precision boring, reaming is a more economical and practical method.
(1) Boring tools
Boring tools typically come in two types: manual and machine-operated. Hand tools have a straight handle with a longer working part, providing better guidance. Machine-operated tools come with a handle or socket design. Boring tools can machine cylindrical and conical holes.
(2) Boring process and applications
The amount of material remaining for boring significantly influences the quality of the hole. Too much material increases tool load and wear, resulting in poor surface finish and dimensional tolerance.
Too little material will not remove tool marks from the previous operation and will not improve hole quality.
Generally, rough boring requires a tolerance of 0.35 ~ 0.15mm, while fine boring requires 0.15 ~ 0.05mm.
To avoid chip build-up, boring typically uses lower cutting speeds (for high-speed steel tools working on steel and cast iron, v<8m/min).
The feed rate depends on the hole diameter, with larger holes requiring higher feed rates, typically between 0.3~1 mm/r for high speed steel and cast iron tools.
Adequate cutting fluids are essential during boring for cooling, lubrication and chip removal to prevent chip build-up and ensure timely chip removal.
Compared to grinding and precision boring, reaming offers higher production rates and easier precision maintenance.
However, enlargement cannot correct positioning errors in the hole axis, which must be guaranteed by the previous operation. Reaming is not suitable for stepped holes and blind holes.
The accuracy of milled holes is generally between IT9 ~ IT7, with surface roughness ranging from Ra 3.2 ~ 0.8μm.
For medium-sized holes that require greater precision (such as IT7), a drill-ream-hole sequence is a typical manufacturing approach.
3. Trepanation
Trepanning is a machining method that enlarges prefabricated holes using a cutting tool. This operation can be performed both on a trepanning machine and on a lathe.
1. Trepanation Methods
There are three distinct methods of trepanation.
1) The part rotates while the tool advances linearly. This method is mainly used on lathes.
A characteristic of this method is that the centerline of the machined hole aligns with the axis of rotation of the part.
The circularity of the hole mainly depends on the rotation accuracy of the main spindle of the lathe, while the axial geometric error is influenced by the accuracy of the tool feed direction relative to the workpiece rotation axis.
This method is ideal for machining holes that require concentricity with external surfaces.
2) The tool rotates while the part advances linearly. The spindle of the trepanning machine drives the rotation of the tool and the work table moves the part forward.
3) The tool rotates and advances simultaneously. In this method, the overhang length of the trepanning bar changes, causing varying forces and deformations in the bar. The diameter of the hole near the spindle housing is larger than that further away, resulting in a tapered hole.
Furthermore, as the overhang length increases, bending deformities due to the weight of the spindle also increase, causing bending in the axis of the machined hole. This method is only suitable for shorter holes.
2. Diamond trepanning
Compared to general trepanning, diamond trepanning features fewer back cuts, lower feed rates, and higher cutting speeds.
It can achieve high machining precision (IT7 to IT6) and a very smooth surface finish (Ra between 0.4 and 0.05 μm). Initially, diamond trepanning was performed with diamond tools, but now tungsten carbide, CBN and synthetic diamond tools are commonly used.
It is mainly used for non-ferrous metals, but can also be used for cast iron and steel.
Standard cutting parameters for diamond trepanning are:
- Posterior cut for pre-trepanation: 0.2 to 0.6mm,
- Final trepanation: 0.1mm;
- Feed rate: 0.01 to 0.14 mm/rev;
- Cutting speeds: 100 to 250 m/min for cast iron, 150 to 300 m/min for steel and 300 to 2,000 m/min for non-ferrous metals.
To ensure high precision and surface quality in diamond trepanning, the machine (diamond trepanning machine) must have high geometric precision and rigidity.
The main spindle bearing typically uses precise angular contact ball bearings or hydrostatic sliding bearings, and the high-speed rotating components must be accurately balanced.
Furthermore, the feeding mechanism must work very smoothly to ensure a constant and low-speed feeding movement of the work table.
Diamond trepanning offers excellent machining quality and productivity. It is widely used for final machining of precision holes in mass production, such as engine cylinder holes, piston holes and main spindle holes in machine tool heads.
However, it is important to note that when machining ferrous metal products with diamond trepanning, only tungsten carbide or CBN tools should be used.
Diamond tools are unsuitable due to the high affinity between carbon atoms in diamond and ferrous elements, leading to reduced tool life.
3. Trepanning Tools
Trepanning tools can be categorized into single-edged and double-edged tools.
4. Characteristics and Applications of Trephination
Compared to the drilling-expanding-reaming process, trepanning is not limited by tool size. It has a strong ability to correct errors, allowing multiple tool passes to adjust initial hole misalignments.
Furthermore, it maintains high positional accuracy in relation to the reference surface.
When contrasted with external turning, trepanning faces challenges such as reduced rigidity of the tooling system, increased deformations, inadequate cooling and chip removal conditions, and significant thermal deformations of the workpiece and tool. This results in lower machining quality and productivity in trepanning than in external turning.
From the above analysis, it is clear that trepanning offers a wide processing range, capable of machining various hole sizes and accuracy classes.
For large diameter holes that require high dimensional and positional accuracy, trepanning is often the only machining option.
Its machining precision ranges from IT9 to IT7. Trepanning can be carried out on trepanning machines, lathes, milling machines and other machine tools, offering versatility and wide application in production.
In high-volume production, trephination models are often used to increase efficiency.
4. Sharpening
(1) Principles of Sharpening and Head Sharpening
Honing is a finishing process that uses a honing head equipped with sharpening rods (whetstones) to smooth holes.
During honing, the part remains stationary while the honing head, driven by the machine's main spindle, rotates and alternates in a linear manner.
The grinding sticks exert pressure on the surface of the part, removing an extremely thin layer of material, resulting in an intersecting cross-hatched pattern.
To avoid repetitive tracking of abrasive grains, the revolutions per minute of the honing head rotation and its reciprocating movements must be equal.
The angle of the hatch pattern is associated with the reciprocating speed and the circumferential speed of the honing head. The size of this angle affects the quality and efficiency of the honing.
Typically, a coarser angle is used for rough honing and a finer angle for finish honing. To facilitate the removal of broken abrasives and chips, reduce the cutting temperature and improve machining quality, plenty of cutting fluid should be used during honing.
To ensure uniform grinding across the hole wall, the grinding stick must extend beyond both ends of the hole to a certain extent.
To ensure uniform honing and minimize the impact of spindle rotation errors on machining accuracy, most honing heads are floatingly connected to the main spindle.
Various structures such as manual, pneumatic and hydraulic are adopted for the radial expansion adjustments of the grinding rods in the honing head.
(2) Technical characteristics and scope of application of honing
1) Honing achieves high dimensional and geometric precision. Machining accuracy ranges from IT7 to IT6. Hole roundness and cylindricity errors can be controlled within a narrow range. However, honing does not improve the positional accuracy of the processed hole.
2) Honing produces a superior surface finish with a surface roughness Ra of 0.2 ~ 0.25 μm and a minimum altered metal layer depth of 2.5 ~ 25 μm.
3) Compared to the grinding speeds, the circumferential speed of the honing head may not be high (vc=16~60m/min).
However, due to the large contact area between the grinding stick and the workpiece, and a relatively high reciprocating speed (va=8~20m/min), honing still maintains a high production rate.
Honing is widely used in mass production to machine engine cylinders and precise bores in various hydraulic devices.
It typically handles holes with diameters of (specific size) or larger and can machine deep holes with a length-to-diameter ratio greater than 10.
However, honing is not suitable for holes in non-ferrous metals with significant plasticity, nor can it process holes with keyways or fluted grooves.
5. Broaching
(1) Broaching and broaching tool
Hole broaching is a high-production precision machining method performed with a specially designed broaching tool on a broaching machine.
Broaching machines are categorized into horizontal and vertical types, with horizontal being the most prevalent.
During broaching, the broaching tool performs a low-speed linear motion (primary motion).
Generally, the broaching tool must have at least 3 functional teeth engaged; otherwise, it may operate unstable and probably produce circular ripples on the surface of the workpiece.
To avoid excessive broaching force that could break the tool, the number of teeth working simultaneously should normally not exceed 6 to 8.
There are three distinct broaching techniques:
1) Layer-by-layer broaching:
This technique sequentially removes the machining margin from the part, layer by layer. To facilitate chip breaking, the tool teeth are designed with interlocking chip breaking grooves. Broaching tools designed for this technique are called standard broaches.
2) Segmental Broaching:
The characteristic of this technique is that each metal layer of the machining surface is removed by a set of staggered teeth of almost the same size (generally consisting of 2 to 3 teeth). Each tooth only removes part of the metal layer. Brooches designed for this method are called wheel-cut brooches.
3) Combined Broaching:
This approach combines the advantages of layer-by-layer and segmented broaching. Coarse-cutting sections use segmented broaching, while fine-cutting sections adopt layer-by-layer technique. This not only reduces the length of the broaching tool, increasing productivity, but also provides a better surface finish. Brooches designed for this method are known as combination brooches.
(2) Technical characteristics and scope of application of hole broaching
1) The broaching tool has multiple edges; In a single broaching stroke, it sequentially completes the roughing, finishing and polishing machining of the hole, making the process highly efficient.
2) The accuracy of hole broaching largely depends on the accuracy of the broaching tool. Under standard conditions, the hole broaching accuracy can reach IT9 to IT7, and the surface roughness Ra can be between 6.3 to 1.6 μm.
3) During hole broaching, the workpiece is positioned by the hole being machined (the front part of the broaching tool serves as the positioning component). This makes it challenging to ensure accurate positioning between the hole and other surfaces. For rotational parts where the inner and outer circular surfaces require concentricity, broaching is usually done first and then other surfaces are machined using the hole as a reference.
4) Broaching tools can not only machine round holes, but also shape holes and spline holes.
5) Broaching tools are fixed size tools; they have complex shapes and are expensive, making them unsuitable for machining larger holes.
Hole broaching is often used in mass production to machine through holes in small and medium-sized parts with diameters ranging from Ф10 to 80 mm and hole depths no more than five times the diameter.
