Drilling, boring, reaming and threading: differences explained

1. Drilling

Drilling is the initial process of creating holes in solid materials, typically with diameters less than 80 mm. There are two drilling methods: one involves rotating the drill bit, while the other rotates the workpiece. The errors produced by these two methods are different.

In the drill rotation method, due to the asymmetrical cutting edges and insufficient rigidity of the drill, the drill may deviate, causing the center line of the hole to be distorted or not straight, but the diameter of the hole remains unchanged.

In contrast, when the part rotates, drill runout may cause a change in hole diameter, but the centerline remains straight.

Common drilling tools include twist drills, center drills and deep drills, with twist drills being the most commonly used, ranging in diameter from 0.1 to 80 mm.

Due to structural limitations, drills have low bending and torsional rigidity and poor centering capacity, resulting in low drilling accuracy, typically between IT13 and IT11; the surface roughness is also relatively high, generally between Ra 50 and 12.5 μm.

However, drilling features high metal removal rate and cutting efficiency. It is mainly used for holes that do not require high precision, such as screw holes, threaded bottom holes and oil holes.

Holes that require greater precision and surface quality must be finished with subsequent processes, such as reaming, reaming, boring or grinding.

2. Enlargement

Reaming is the process of further machining pre-drilled, cast or forged holes to increase diameter and improve hole quality. Reaming can serve as a pre-finishing operation before precision machining or as a final process for holes with less stringent requirements. Reamers are similar to 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 to 8), providing good guidance and stable cutting;

(2) The reamers do not have transverse edges, which improves cutting conditions;

(3) The machining tolerance is small, allowing for shallower chip grooves and a thicker core, resulting in stronger and more rigid tool bodies. Flaring accuracy is generally between IT11 and IT10, with surface roughness values ​​between Ra 12.5 and 6.3. Reaming is commonly used for holes with diameters less than 100 mm. When drilling larger holes (D ≥ 30 mm), it is common practice to pre-drill with a smaller drill bit (0.5 to 0.7 times the hole diameter) and then enlarge to the desired size, thus improving quality and efficiency. of hole machining.

In addition to cylindrical holes, various special-shaped reamers, also known as countersinks, can be used to machine countersunk holes and flatten end faces. The front end of a counterbore usually features a guide column, which is guided through the already machined hole.

3. Countersinking

Countersinking is one of the precision machining methods for holes and is widely used in production. For smaller holes, compared to internal grinding or precision boring, countersinking is a more economical and practical method.

(1) Countersinks

Reamers are generally divided into hand-operated and machine-operated types. Hand reamers have a straight shank with a longer workpiece, providing better guidance, and come in full and adjustable outer diameter types.

Machine-operated reamers come in shank and sleeve types. Countersinks can process not only round holes but also conical holes with conical countersinks.

(2) Countersinking process and application

The tolerance for countersinking has a major impact on the quality of the finish. Excessive clearance increases the load on the reamer, quickly dulling cutting edges and making it difficult to obtain a smooth surface and maintain dimensional tolerances. Insufficient tolerance cannot remove marks left by previous processes, thus not improving the machining quality of the hole.

The general tolerance for rough countersinking is between 0.35 and 0.15 mm, while for fine countersinking it is between 0.15 and 0.05 mm.

To avoid the formation of built-up edges, countersinking is normally performed at low cutting speeds (for high-speed steel countersinks machining steel and cast iron, v < 8 m/min). The feed depends on the diameter of the hole being machined; Larger diameters require higher feed rates, with common feed rates for high-speed steel reamers machining steel and cast iron being between 0.3 and 1 mm/r.

Reaming requires the use of appropriate cutting fluids for cooling, lubrication and cleaning to prevent built-up edges and timely chip removal.

Compared to grinding and boring, countersinking offers higher productivity and easily maintains hole accuracy; however, it cannot correct hole axis positioning errors, which should be guaranteed by the previous process. Countersinking is not suitable for machining stepped holes and blind holes.

The dimensional accuracy of countersinking is generally between IT9 and IT7, with surface roughness typically between Ra 3.2 and 0.8. For medium-sized holes with higher accuracy requirements (e.g. IT7 class holes), the typical machining sequence in production is drilling – reaming – countersinking.

4. Boring

Boring is a machining process that enlarges a pre-drilled hole with a cutting tool. This operation can be performed on both boring machines and lathes.

1. Boring methods

There are three different boring methods:

a) Rotation of the part with forward movement of the tool: This method is commonly used on lathes. The process ensures that the axis of the hole is aligned with the axis of rotation of the workpiece. The circularity of the hole mainly depends on the rotation accuracy of the machine spindle, while the axial geometric shape error is mainly determined by the accuracy of the tool feed direction relative to the workpiece rotation axis. This method is suitable for boring holes that require concentricity with the outer cylindrical surface.

b) Rotation of the tool with forward movement of the part: The boring machine spindle drives the rotation of the boring tool, while the work table moves the part forward.

c) Rotation of the tool with feed movement: When using this method, the projection length of the boring bar changes, as does the deformation under load, resulting in a conical hole with a larger diameter close to the spindle housing and a smaller diameter further away . Furthermore, as the projection length of the boring bar increases, the bending deformation caused by the spindle's own weight also increases, which causes a corresponding curvature in the axis of the hole being machined. This method is only suitable for drilling short holes.

2. Diamond Drilling

Compared to general boring, diamond boring is characterized by a smaller amount of backlash, lower feed rate and higher cutting speed. It can achieve high machining precision (IT7 to IT6) and a very smooth surface finish (Ra 0.4 to 0.05). Initially, diamond boring was carried out with diamond boring tools, but now hard alloy, CBN and synthetic diamond tools are commonly used. It is mainly used for machining non-ferrous metal parts and can also be applied to cast iron and steel parts.

Typical cutting parameters for diamond boring are: backlash amount of 0.2 to 0.6 mm for rough boring and 0.1 mm for finish boring; feed rate from 0.01 to 0.14mm/r; cutting speed of 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 machining precision and surface quality in diamond boring, the machine (diamond boring machine) must have high geometric accuracy and rigidity. Main spindle bearings generally use precision angular contact ball bearings or hydrostatic sliding bearings, and high-speed rotating parts must be accurately balanced. Furthermore, the feeding mechanism must move smoothly to ensure that the worktable can perform slow and stable feeding movements.

Diamond boring is widely used in mass production for machining final precision holes such as engine cylinder holes, piston pin holes and main spindle holes in machine tool spindle housings. However, it is important to note that when machining ferrous metals with diamond boring, boring tools made of hard alloy or CBN should be used instead of diamond, as the carbon atoms in diamond bond strongly with iron group elements, reducing tool life.

3. Boring Tools

Boring tools can be categorized into single-edge and double-edge boring tools.

4. Technological Characteristics and Scope of Application of Boring

Compared to the process of drilling, expanding and reaming, boring is not limited by the size of the tool and has a strong ability to correct errors. It can correct the initial hole axis deviation through multiple passes and maintain high positional accuracy with the locating surface.

Compared to external turning, boring has less rigidity in the toolbar system, greater deformation, poor heat dissipation and chip removal conditions, and both the workpiece and tool experience significant thermal deformation. Consequently, the machining quality and production efficiency of boring are not as high as those of external turning.

In summary, boring has a wide range of applications, capable of machining holes of different sizes and levels of precision. It is almost the exclusive method for holes with large diameters and high requirements for dimensional and positional accuracy. The machining accuracy of boring ranges from IT9 to IT7, and the surface roughness is Ra. Boring can be performed on boring machines, lathes, milling machines and other types of machine tools, offering the advantage of flexibility. In mass production, to improve boring efficiency, boring jigs are often used.

5. Sharpening

1. Honing principle and honing tool

Honing is a machining method of finishing holes with a honing tool equipped with abrasive sticks (oil stones). During honing, the part remains stationary while the honing tool, driven by the machine tool spindle, rotates and alternates linearly.

In the honing process, abrasive sticks apply a certain pressure to the surface of the part, removing an extremely thin layer of material, resulting in a hatched pattern on the surface. To ensure that the abrasive particles do not follow the same path, the number of revolutions per minute of the honing tool and the number of reciprocating strokes per minute must be relatively close together.

The crossing angle θ of the honing pattern refers to the reciprocating speed (va) and circumferential speed (vc) of the honing tool. The size of the angle θ affects the quality and efficiency of honing; Typically, θ is set to 40-60° for rough honing and finer for precision honing. To facilitate the expulsion of broken abrasive particles and chips, reduce the cutting temperature and improve the processing quality, plenty of cutting fluid should be used during honing.

To ensure uniform machining of the hole wall, the travel of the abrasive sticks must extend beyond both ends of the hole. To ensure a uniform honing margin and minimize the impact of spindle rotation errors on machining accuracy, a floating connection is commonly used between the honing tool and the machine spindle.

The radial expansion and contraction adjustments of the honing tool's abrasive rods can be manual, pneumatic, hydraulic and other structures.

2. Technological characteristics and application range of honing

1) Honing achieves high dimensional and shape accuracy, with processing accuracy at IT7-IT6 level. Hole roundness and cylindricity errors can be controlled within a very restricted range. However, honing does not improve the position accuracy of the machined hole.

2) Honing achieves a high surface quality, with surface roughness (Ra) ranging from 0.2 to 0.025μm and an extremely shallow depth of the altered defect layer on the metal surface (2.5-25μm).

3) Although the circumferential speed of the honing tool is not high (vc=16-60m/min) compared with grinding speeds, the larger contact area between the abrasive sticks and the workpiece and the relatively high reciprocating speed (va=8-20m /min) still allow honing to maintain high productivity.

Honing is widely used in mass production to machine precision holes in engine cylinders and various hydraulic devices. The range of hole diameters typically starts at 5 mm or larger, and honing can process deep holes with length-to-diameter ratios greater than 10. However, honing is not suitable for machining holes in non-ferrous metal parts with high plasticity , nor can it process holes with keyways or splines.

6. Broaching

1. Broaching and Brooches

Broaching is a highly productive precision machining process performed on a broaching machine using specially designed broaches. There are two main types of broaching machines: horizontal and vertical, with horizontal being the most common.

During broaching, the broach performs a slow linear motion (the primary motion). The number of broach teeth engaged simultaneously should generally not be less than three to ensure stability; otherwise, uneven cutting may create ring-shaped ripples on the surface of the part. To avoid excessive broaching forces that could break the broach, the number of cutting teeth working at the same time should generally not exceed six to eight.

There are three distinct broaching methods, described below:

1) Layer-by-layer broaching involves sequentially cutting excess material from the part, layer by layer. To facilitate chip breaking, the broach teeth are ground with interlocking chip-breaking grooves. Brooches designed for this method are called simple brooches.

2) Segmental broaching is characterized by each layer of the machined surface being cut by a group of similarly sized staggered teeth (normally 2 to 3 teeth per group). Each tooth removes only part of a single metal layer. Brooches designed for this method are called rotary style brooches.

3) Combined broaching combines the advantages of layer-by-layer and segmented broaching. The roughing portion uses segmented broaching, while the finishing portion uses layer-by-layer broaching. This not only reduces broach length and improves productivity, but also provides better surface quality. Brooches designed for this method are called combination brooches.

2. Technological Characteristics and Applications of Broaching

1) Broaches are multi-edge tools that can sequentially perform grinding, finishing and polishing of a hole in a single broaching stroke, resulting in high production efficiency.

2) Broaching accuracy mainly depends on broaching accuracy. Under normal conditions, broaching can reach tolerances of IT9 to IT7, with surface roughness (Ra) reaching 6.3 to 1.6 μm.

3) During broaching, the part is self-located by the hole being machined (the front part of the broach serves as a positioning element), making it difficult to guarantee the accuracy of the hole positioning in relation to other surfaces; For rotational parts that require concentricity between the inner and outer surfaces, broaching is usually performed first, followed by machining other surfaces based on the hole as a reference.

4) Broaches can machine not only round holes, but also shaped holes and spline holes.

5) Broaches are tools with fixed sizes, complex shapes and high costs, making them unsuitable for machining large holes.

Broaching is commonly used in mass production to machine through holes in small and medium-sized parts with diameters ranging from 10 to 80 mm and hole depths no greater than five times the diameter.