What is hole machining

What is hole machining?

The operation of machining holes so that the holes meet the requirements is called hole machining. No machine can be made without holes. To connect parts, various sizes of screw holes, pin holes or rivet holes are required; In order to fix the transmission parts, various mounting holes are needed; Machine parts also have many kinds of holes (such as oil holes, process holes, weight reduction holes, etc.). Inner hole surface is one of the important surfaces of mechanical parts. In mechanical parts, perforated parts generally account for 50% – 80% of the total number of parts. The types of holes are also varied, including cylindrical holes, conical holes, threaded holes and formed holes.

Classification of hole machining

Drilling processing

All kinds of holes of mold parts, such as screw holes, screw holes, pin holes, ejector pin holes, circular core fixing holes, etc., need to be drilled and reamed to meet the requirements of aperture, hole spacing accuracy and roughness. The common processing methods are shown in the table.

Type	Content
Drilling of individual parts	Single parts are drilled directly according to the marking position
Drilling pilot	First drill the hole of one part, and then guide to drill the hole of other parts. One part can be used for direct drilling in opposite direction during pilot drilling; The drill hole can also be drawn out, which can be used as the opposite direction drilling.
Combined drilling	In order to ensure the hole distance of the parts, the two parts can be clamped with parallel collets or combined with screws, and drilling shall be conducted at the same time according to the marking.

Reaming processing

There is often a distribution pin hole, ejector pin hole, core fixing hole, etc. in the mold that needs to be processed after scribing or during assembly. Its processing accuracy is generally IT6 – IT8, and the roughness is not lower than RA3.2 μm。
General principles of reaming

Type		Content
Workpiece diameter	<10	It is drilled and reamed by fitter.
	10-20	Adopt drilling, countersinking, reaming and other processes.
	>20	It is pre controlled by fitter and then processed by milling and boring machine.
Hole to be quenched	0.02 ~ 0.03 grinding amount should be reserved during reaming. The hole should be protected during heat treatment and then grinded after assembly.
Combined reaming of different materials	When reaming parts of different materials, they should be reamed from harder materials
Quenching hardware Reaming	In order to ream a hole through a hardened hardware, first check whether the hole is deformed, ream it with a standard carbide reamer or ream with an old reamer, and then grind it to the required size with a cast iron grinding bar.
Reamed hole	When reaming is not possible, the reaming depth should be deepened, and the length of the reamer cutting part should be reserved to ensure the hole depth of effective diameter; The standard reamer can also be used for reaming, and then the old reamer for grinding off the cutting part is used to ream the UN reamed bottom of the hole.
Mechanical hinge	After the workpiece is clamped once, drill, countersink and ream continuously to ensure the perpendicularity and parallelism of the hole.

Deep hole machining

Deep hole processing is required for cooling water channel holes, heater holes and some ejector rod holes in the plastic mold. Generally, the accuracy requirements of cooling water holes are not high, but deflection should be prevented; In order to ensure the heat conduction efficiency, there are certain requirements for the aperture and roughness of the heater hole. The aperture is 0.1 ~ 0.3mm larger than the heating rod, and the roughness is Ra12.5 ~ 6.3 μ m; The ejector pin hole requires higher accuracy, generally it8, and has requirements for verticality and roughness.

Hole system processing

Many holes on the mold are required to ensure the hole distance, hole edge distance, the parallelism of each hole axis, the perpendicularity with the end face, and the coaxiality of the holes after the assembly of the two parts. When processing this kind of hole system, the benchmark is usually processed first, and then each hole is processed by scribing.

Method of hole machining

Spiral interpolation milling

Use the milling cutter to mill the workpiece blank or the machined pre hole obliquely. Then spiral milling down along the Z axis while moving in the x/y direction to realize reaming.

Circular interpolation milling

The milling cutter carries out cutting and milling with full tooth depth around the outer diameter or inner diameter of the machined pre hole to realize reaming.

Insert milling or z-axis milling

By inserting and cutting along the shoulder wall of the workpiece step by step, a new hole is machined (drilled) while rough milling the cavity.

Commonly used hole machining tools

The tools for machining holes from solid materials or expanding existing holes are called hole processing tools. For example, twist drill, center drill, flat drill, deep hole drill, etc. can be machined on solid materials, while reamers, reaming drills, boring tools, etc. can be used to expand holes in existing materials.

Drilling tools

(1) Twist drill.

Twist drill
Twist drill is the most common hole processing tool, which can drill holes in solid materials and can also be used to ream holes, mainly for processing holes below φ30mm.
(2) Deep hole drilling.
The hole length-diameter ratio (L/D) is greater than 5 for deep holes, because processing deep holes is cutting in the depth, cutting fluid is not easy to inject, poor heat dissipation, chip removal difficulties, poor rigidity of the drill rod, easy to damage the tool and cause the axis of the hole deflection, affecting the processing accuracy and productivity, so the deep hole tool should be selected for processing.
(3) Hole reaming drill.
The existing hole on the workpiece (cast, forged or drilled holes) to expand the processing method is called reaming. Machining center to ream more holes using reaming drill, you can also use the keyway milling cutter or end mill to ream the hole, than the ordinary reaming drill processing accuracy.
(4) Center drilling and centering drilling

Center drilling and centering drilling
The center drill is mainly used for drilling the center hole, but also can be used for pre-drilling the centering hole before twist drilling; the centering drill is mainly used for pre-drilling the centering hole before twist drilling, but also can be used for chamfering the hole, α is mainly 90 ° and 120 ° two kinds.

Boring tool

In the machine tool with a boring tool for large and medium-sized holes for semi-finishing and finishing is called boring. There are many types of boring tools, which can be divided into single-edge boring tools and double-edge boring tools according to the number of cutting edges.

(1) Single Flute Boring Tools.
Single-edged boring tools can be used for boring through holes, stepped holes and non-through holes. Single-edged boring tools have only one insert and are screw-mounted to the boring bar. Vertically mounted inserts bore through holes, while angled inserts bore through holes or stepped holes.
Single-edged boring tools are not rigid and are prone to vibration when cutting. To reduce radial forces, it is advisable to choose a larger main offset angle. Boring cast iron holes or fine boring, often take κr = 90 °; rough boring steel holes, in order to improve tool life, generally take κr = 60 ° -75 °. The single-edge boring tool has a simple structure and wide adaptability. By adjusting the overhanging length of the boring insert, the hole of different diameters can be bored.
(2) Double-edged boring tool.
The double-edged boring tool can be used for boring large diameter holes. Double-edged boring tools have two symmetrical cutting edges working simultaneously, also known as boring blocks (fixed-size tools). The head of the double-edged boring tool can be adjusted within a wide range, and it is easy to adjust the maximum boring diameter up to 1000 mm. two symmetrical cutting edges participate in cutting at the same time, which not only can eliminate the impact of cutting force on the boring bar, but also has high cutting efficiency. The double-edged boring tool has good rigidity, large chip space, two radial forces offset, not easy to cause vibration, high machining accuracy, can obtain good surface quality, suitable for mass production.

Reamer

Reaming is a method of finishing a hole with a reamer. Reaming is often used as a finishing process after drilling and reaming of small and medium-sized holes, and can also be used for pre-machining before grinding or researching holes. Reaming can only improve the dimensional accuracy and shape accuracy of the hole and reduce its surface roughness value, but cannot improve the position accuracy of the hole or correct the axis skew of the hole. The general reamed hole size accuracy up to IT7-IT9, surface roughness JPRa value up to 1.6-0.8μm.

In addition to the correct selection of reaming dosage and cooling lubricant, the selection of reaming tool is also crucial to the quality of reaming. When reaming holes on machining centers, in addition to the use of ordinary standard reamers, single-edged reamers and floating reamers with machined carbide inserts are often used.
(1) Standard reamer.
There are three kinds of standard reamers: straight shank, tapered shank and sleeve type. The diameter of taper shank reamer is φ10-φ32mm, the diameter of straight shank reamer is φ6-φ20mm, the diameter of small hole straight shank reamer is φ1-φ6mm, and the diameter of set reamer is φ25-φ80mm.
(2) Single-edged reamer with carbide insert.
Single-edged reamer blade 3 is fixed to the tool body by screw 1 through wedge sleeve 4, and the reamer size can be adjusted by screw 7 and pin 6. The guide block 2 can be fixed by bonding and brass welding. The single-edge reamer with a machined carbide insert not only has a long life, but also processes holes with high accuracy and a surface roughness Ra value up to 0.7μm. For the single-edge reamer with an internal cooling channel, a cutting speed of 80m/min is allowed.
(3) Floating reamer.
Floating reamers not only ensure the stability of the tool during tool change and tool feed, the insert will not slip out of the rectangular hole of the toolholder, but also can be accurately “centered” by free floating. Since the floating reamer has two symmetrical edges, it can automatically balance the cutting force and compensate the machining error caused by the tool installation error or the radial circular runout of the toolholder during the reaming process, so the machining accuracy is stable. The life of floating reamer is 8-10 times higher than that of HSS reamer and has the continuity of diameter adjustment, so it is an ideal reamer for machining center.

Countersinks

Countersinks are mainly used for countersinking step holes, countersinking planes, chamfering holes and other processes in various materials.

Machine taps

Machine taps are mainly used for machining threaded holes from M6 to M20. In principle, a tap is a tool for making external threads.

Thread milling cutter

There are cylindrical thread milling cutters, machine thread milling cutters and combined multi-station special thread boring and milling cutters.

Characteristics of hole machining tools

• ① Most of the hole processing tools are fixed size tools, the size accuracy and shape accuracy of the tool itself inevitably has an important impact on the hole processing accuracy.
• ② Hole machining tool size is limited by the diameter of the processed hole, the tool cross-sectional size is small, characterized by the tool used for processing small diameter holes and deep diameter ratio (the value of the ratio of the depth of the hole to the diameter) of the larger hole, its cross-sectional size is even smaller, so the tool rigidity is poor, cutting instability, easy to produce vibration.
• ③ Hole machining tool is in the workpiece has been processed in the surface surrounded by cutting processing, cutting is closed or semi-closed state, so chip removal is difficult, cutting fluid is not easy to enter the cutting area, it is difficult to observe the actual situation in cutting, the quality of the workpiece, tool life will have a negative impact.
• ④ There are many types and specifications of hole machining tools.

Hole processing tools

Technical requirements of hole machining

In the hole processing process, the hole diameter should be avoided to expand the hole straight line is too large, the workpiece surface roughness is poor and the drill bit wear too quickly to prevent the impact of drilling quality and increase processing costs. The following technical requirements should be ensured as far as possible.

• (1) Dimensional accuracy: the accuracy of the diameter and depth dimensions of the hole.
• (2) Shape accuracy: roundness, cylindricity and straightness of the axis of the hole.
• (3) Position accuracy: coaxiality of hole and hole axis or hole and outer circle axis; parallelism perpendicularity between hole and hole or hole and other surfaces, etc.

At the same time, the following five elements should be considered.

• (1) Hole diameter hole depth, tolerance surface roughness hole structure.
• (2) The structural characteristics of the workpiece, including the stability of clamping overhang and slew.
• (3) The power speed, coolant system and stability of the machine tool.
• (4) Processing lot size.
• (5) Processing costs.

Complete collection of hole machining methods

There are more methods of internal hole surface processing, commonly used are drilling, reaming, reaming, boring, grinding, pulling, grinding, honing, rolling, etc.

Compared with the external surface processing, hole processing conditions are much worse, processing holes to be more difficult than processing the external circle. This is because.

• (1) The size of the tool used for hole machining is limited by the size of the hole to be machined, poor rigidity, easy to produce bending deformation and vibration.
• (2) Processing holes with fixed-size tools, hole processing size often depends directly on the corresponding size of the tool, the manufacturing error and wear of the tool will directly affect the hole processing accuracy.
• (3) When machining holes, the cutting area is inside the workpiece, the chip removal and heat dissipation conditions are poor, and the machining accuracy and surface quality are not easy to control.

Drilling

Drilling is the first process of processing holes in solid materials, and the diameter of the hole is generally less than 80 mm. there are two ways of drilling: one is drill rotation; the other is workpiece rotation. In the drilling method of drill rotation, the centerline of the hole will be skewed or not straight due to the asymmetry of the cutting edge and the lack of rigidity of the drill bit, but the hole diameter will remain the same.
Commonly used drilling tools are: twist drill, center drill, deep hole drill, etc. The most common one is twist drill, whose diameter specification is Φ0.1-80mm.
Due to the limitation of construction, the bending stiffness and torsional stiffness of the drill are low, and the centering is not good, so the accuracy of drilling processing is low, generally only IT13~IT11; the surface roughness is also large, Ra is generally 50-12.5μm; but the metal removal rate of drilling is large, 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 the holes with high machining accuracy and surface quality requirements, they should be reached by reaming, reaming, boring or grinding in the subsequent processing.

Reaming

Reaming is to use reaming drill to further process the hole that has been drilled, cast or forged to enlarge the hole diameter and improve the quality of the hole processing, reaming can be used both as pre-processing before finishing the hole and as the final processing of the hole with low requirements. Reaming drills are similar to twist drills, but have a higher number of teeth and no cross edge.

Compared to drilling, reaming has the following characteristics.

• (1) A high number of teeth (3-8 teeth) and good guidance for reaming drills, which makes cutting more stable.
• (2) Reaming drills do not have a cross-edge and have good cutting conditions.
• (3) The machining allowance is small, the chip tank can be made shallower, the core can be made coarser, and the strength and rigidity of the tool body is better. The accuracy of reaming is generally IT11-IT10 grade, and the surface roughness Ra is 12.5-6.3μm. reaming is often used to process holes with a diameter smaller than that. In the drilling of larger diameter holes (D ≥ 30mm), often first use a small drill (diameter of 0.5-0.7 times the diameter of the hole) pre-drilling, and then use the corresponding size of the reaming drill reaming, which can improve the quality of hole processing and production efficiency.

In addition to reaming cylindrical holes, a variety of special-shaped reaming drills (also known as countersinks) can be used to process a variety of countersunk seat holes and countersinks with flat faces. The front end of the countersink is often equipped with a guide pillar to guide the machined hole.

Reaming

Reaming is one of the finishing methods 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 fine boring.
1. Reamer
Reamers are generally divided into two types: hand reamers and machine reamers. The hand reamer has a straight shank, a long working part and a good guiding effect, and the hand reamer has two structures: integral and adjustable outside diameter. There are two types of reamers for machine use: those with shanks and those with sleeves. Reamers can not only process round holes, but also tapered holes with tapered reamers.
2. Reaming process and its application
The reaming allowance has a great influence on the quality of reaming. If the allowance is too large, the load on the reamer is large, the cutting edge is quickly dulled, and it is not easy to obtain a bright and clean surface, and the dimensional tolerance is not easily guaranteed. General rough reaming allowance is taken as 0.35-0.15mm, fine reaming is taken as 01.5-0.05mm.
To avoid chip tumors, reaming is usually done at a lower cutting speed (v < 8m/min for HSS reamers processing steel and cast iron). The larger the diameter of the hole, the larger the value of the feed, HSS reamer processing steel and cast iron feed is often taken as 0.3-1mm/r.
Reaming must be done with appropriate cutting fluid for cooling, lubrication and cleaning to prevent chip tumors and timely chip removal. Compared with grinding and boring holes, reaming is highly productive and easy to ensure the accuracy of the hole; however, reaming cannot correct the position error of the hole axis, and the position accuracy of the hole should be ensured by the previous process. Reaming is not suitable for processing stepped holes and blind holes.
Reamed hole size accuracy is generally IT9 to IT7, surface roughness Ra is generally 3.2-0.8μm. for medium size, high accuracy requirements of the hole (such as IT7 accuracy hole), drilling – expansion – reaming process is a typical processing program commonly used in production.

Boring

Boring is a processing method to enlarge a prefabricated hole with a cutting tool. Boring work can be done either on a boring machine or on a lathe.
1. Boring method
There are three different ways of boring.
(1) The workpiece rotates and the tool makes a feed motion in the lathe to bore most of the holes belonging to this boring method. The process is characterized by: the axis of the hole is the same as the axis of rotation of the workpiece, the roundness of the hole mainly depends on the rotary accuracy of the machine tool spindle, and the axial geometry error of the hole mainly depends on the position 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 makes a feed motion. The boring machine spindle drives the boring tool to rotate and the table drives the workpiece to make a feed motion.
(3) The tool rotates and feeds the workpiece. With this boring method, the overhang length of the bar varies and the force deformation of the bar also varies. In addition, as the overhang of the boring bar increases, the bending deformation of the spindle caused by its own weight also increases, and the axis of the hole to be machined will be bent accordingly. This boring method is only suitable for short holes.
2. Diamond Boring
Compared with general boring, diamond boring is characterized by small back draft, small feed, high cutting speed, it can obtain very high machining accuracy (IT7 ~ IT6) and very smooth surface (Ra for 0.4-0.05μm). Diamond boring initially with diamond boring tool processing, now commonly used carbide, CBN and artificial diamond tool processing. Mainly used for processing non-ferrous metal workpieces, can also be used for processing cast iron parts and steel parts.
Diamond boring commonly used cutting amount: back eat tool amount pre-boring for 0.2-0.6mm, final boring for 0.1mm; feed 0.01-0.14mm/r; cutting speed processing cast iron for 100-250m/min, processing steel for 150-300m/min, processing non-ferrous metal for 300-2000m/min.
In order to ensure that diamond boring can achieve high machining accuracy and surface quality, the machine tool (diamond boring machine) must have high geometric accuracy and rigidity, the machine tool spindle support commonly used precision angular contact ball bearings or hydrostatic sliding bearings, high-speed rotating parts must be accurately balanced; in addition, the movement of the feed mechanism must be very smooth, to ensure that the table can do a smooth low-speed feed movement.
The machining quality of diamond boring is good, high production efficiency, in a large number of mass production is widely used in the final processing of precision holes, such as engine cylinder holes, piston pin holes, machine tool spindle box on the spindle hole, etc.. But must draw attention to: with diamond boring processing ferrous metal products, can only use carbide and CBN made boring tool, can not use diamond made boring tool, because the diamond in the carbon atom and the affinity of the iron group elements, tool life is low.
3. Boring tool
Boring tools can be divided into single-edged boring tools and double-edged boring tools.
4. Boring process characteristics and application range
Compared with the drilling-expanding-reaming process, the bore size is not limited by the size of the tool, and the bore has a strong error correction capability, which can correct the original hole axis skew error by multiple tooling, and can make the bore and the positioning surface maintain high positional accuracy.
Compared with turning, the quality and productivity of boring are not as high as turning due to the poor rigidity and deformation of the tool bar system, poor heat dissipation and chip removal conditions, and large thermal deformation of the workpiece and tool.
From the above analysis, it can be seen that boring has a wide range of machining, and can process a variety of different sizes and different accuracy levels of holes. For holes and hole systems with large hole diameters and high requirements for size and position accuracy, boring is almost the only machining method. The machining accuracy of boring is IT9 to IT7 grade. Boring can be carried out on boring machines, lathes, milling machines and other machine tools, with the advantages of mobility and flexibility, production is widely used. In mass production, in order to improve the efficiency of boring, often use boring die.

Honing holes

1. Honing principle and honing head
Honing is the use of honing head with a grinding bar (oil stone) on the hole for finishing processing method. When honing, the workpiece is fixed, and the honing head is driven by the machine tool spindle to rotate and make reciprocating linear motion. In the honing process, the abrasive strip acts on the surface of the workpiece with certain pressure to remove an extremely thin layer of material from the surface of the workpiece, and its cutting trajectory is a crossed mesh pattern. In order to make the trajectory of the abrasive strip not repeat, the number of revolutions per minute of the rotary motion of the honing head and the number of reciprocating strokes per minute of the honing head should be a prime number of each other.
The cross angle of honing trajectory is related to the reciprocating speed and circumferential speed of honing head, the size of the angle affects the processing quality and efficiency of honing, generally take ° when rough honing, and take when fine honing. In order to facilitate the discharge of broken abrasive particles and chips, reduce the cutting temperature and improve the processing quality, sufficient cutting fluid should be used when honing.
In order to make the machined hole wall can be evenly processed, the stroke of the sand bar should exceed a section of overtravel amount at both ends of the hole. In order to ensure uniform honing allowance and reduce the influence of machine tool spindle rotation error on machining accuracy, the floating connection is mostly used between honing head and machine tool spindle.
The radial expansion and contraction adjustment of the honing head has various structural forms such as manual, pneumatic and hydraulic.

2. Honing process characteristics and application range

• (1) Honing can obtain high dimensional accuracy and shape accuracy, the machining accuracy is IT7-IT6 grade, the roundness and cylindricity error of the hole can be controlled within the range, but honing cannot improve the position accuracy of the machined hole.
• (2) Honing can obtain high surface quality, the surface roughness Ra is 0.2-0.25μm, and the depth of the metamorphic defect layer of the surface metal is extremely slight 2.5-25μm.
• (3) Compared with the grinding speed, the circumferential speed of honing head is not high (vc=16-60m/min), but the reciprocating speed is relatively high (va=8-20m/min) due to the large contact area between the abrasive strip and the workpiece, so honing still has a high productivity.

Honing is widely used in mass production for engine cylinder holes and various hydraulic devices in the processing of precision holes, aperture range is generally for or larger, and can process deep holes with length-diameter ratio greater than 10. But honing is not suitable for processing plasticity of the non-ferrous metal workpiece on the hole, but also can not be processed with keyway holes, spline holes, etc.

Broaching

1. Broaching and broach
Broaching is a high-productivity finishing method, which is carried out on a broaching machine with a special broaching tool. Broaching machine is divided into two kinds of horizontal broaching and vertical broaching machine, horizontal broaching is the most common.
When broaching, the broach only makes low-speed linear motion (main motion). The number of teeth of the broach working at the same time should generally be not less than 3, otherwise the broach will not work smoothly and will easily produce ring ripples on the surface of the workpiece. In order to avoid excessive broaching force and broach breakage, broach work, the number of simultaneous working knife teeth should generally not exceed 6-8.
Broaching has three different ways of broaching, which are described as follows.

• (1) Layered broaching This broaching method is characterized by the broach will workpiece machining allowance layer by layer sequential removal. In order to facilitate chip breaking, the teeth of the tool are ground with interlocking chip slots. The broaches designed in accordance with the layered broaching method are called ordinary broaches.
• (2) Block broaching This broaching method is characterized by the fact that each layer of metal on the processing surface is removed by a group of teeth (usually each group consists of 2-3 teeth) of basically the same size but with interlocking teeth. Each tooth removes only a portion of a layer of metal. The broach designed by block broaching is called wheel-cut broach.
• (3) Integrated broaching combines the advantages of layered and block broaching, with block broaching for the coarse cutting part and layered broaching for the fine cutting part. This can shorten the broach length, increase productivity and obtain better surface quality. The broach designed in accordance with the integrated broaching method is called integrated broach.

2. Process characteristics of broaching and application range

• (1) Broaches are multi-edged tools, which can sequentially complete the roughing, finishing and polishing of holes in one broaching stroke with high productivity.
• (2) The accuracy of broaching mainly depends on the accuracy of broach, under normal conditions, the accuracy of broaching can reach IT9-IT7 and the surface roughness Ra can reach 6.3-1.6μm.
• (3) Broaching, the workpiece to be machined hole itself positioning (broach front guide is the workpiece positioning elements), broaching is not easy to ensure the mutual position accuracy of the hole and other surfaces; for those internal and external circular surface coaxiality requirements of the processing of the rotary body parts, often are first broaching, and then the hole as a positioning reference for processing other surfaces.
• (4) Broaches can not only process round holes, but also can be processed forming holes, spline holes.
• (5) Broach is a fixed-size tool, complex shape, expensive, not suitable for processing large holes.

Pulling hole commonly used in mass production in the processing of a large number of hole diameter Ф10-80mm, hole depth of not more than 5 times the diameter of the small and medium-sized parts on the through-hole.
Source: China machining solutions provider – Yaang Pipe Industry (www.machinedsgn.com)