Drilling is easily the most common machining process. One estimate is that 75% of all metal-cutting material removed comes from drilling operations.
Drilling involves the creation of holes that are right circular cylinders. This is accomplished most typically by using a twist drill, something most readers will have seen before. The figure below illustrates a cross section of a hole being cut by a common twist drill:
The chips must exit through the flutes to the outside of the tool. As can be seen in the figure, the cutting front is embedded within the workpiece, making cooling difficult. The cutting area can be flooded, coolant spray mist can be applied, or coolant can be delivered through the drill bit shaft.
Drilling Characteristics
The characteristics of drilling that set it apart from other powered metal cutting operations are:
A view of the metal-cutting area of a drill press is shown below. The workpiece is held in place by a C-clamp since cutting forces can be quite large. It is dangerous to hold a workpiece by hand during drilling since cutting forces can unpredictably get quite large and wrench the part away. Wood is often used underneath the part so that the drill bit can overshoot without damaging the table. The table also has holes for drill overshoot as well as weight reduction. A three-jaw chuck is used since three points determine a circle in two dimensions. Four-jaw chucks are rarely seen since offset of the bit is not necessary.
Twist Drill Bit
The figure below labels the important angles for a typical twist drill bit.
Drill Bit Variety
The figures below illustrate various drill bits and their cut hole configurations.
Turning: Chucks
The chuck is integral to a lathe's functioning because it fixtures the part to the spindle axis of the machine. Below is shown a three-jaw chuck with jaws that are all driven by the same chuck key. This arrangement provides convenience in that parts can be mounted and dismounted quickly.
Three-Jaw Chuck
Four-Jaw Chuck
If the part needs to be off center or is not a solid of revolution (axially symmetric), a four-jaw chuck with independently-actuated jaws needs to be used. Such a chuck is depicted below.
Drill Press Detail
A typical manual drill press is shown in the figure below. Compared to other powered metal cutting tools, a drill press is fairly simple, but it has evolved into a versatile necessity for every machine shop.
The most adjustable part of the drill press is the vertical movement of the drill bit, since this is the motion that is used in production.
A detail of the work area of the drill press is shown below.
Drilling can be accomplished more accurately on alath or mill , although drill presses are much cheaper and more accessible machines.
Jig Boring
Jig boring is used to accurately enlarge existing holes and make their diameters highly accurate. Jig boring is used for holes that need to have diameter and total runout controlled to a high degree. Typically, a part has holes machined on regular equipment and then the part is transferred to a dedicated jig boring machine for final operations on the especially accurate holes. Jig boring can also maintain high accuracy between multiple holes or holes and surfaces. Tolerances can be held readily within ±.005 mm (±0.0002 inches). Dedicated jig boring machines are designed to machine holes with the tightest tolerances possible with a machine tool.
Standard boring can be carried out on a mill fitted with a boring head or on a lathe. Boring is most accurate on a lathe since a lathe is dedicated to solids of revolution (axially symmetric parts).
Gun Drilling
For long holes such as those found in gun bores, gun drills are used. The length of the hole requires that coolant be delivered through the shaft of the gun drill to the cutting front. The coolant also serves to eject chips from the cutting area and to move them back and out of the hole entrance. The figures below illustrate a gun drill and the cutting/cooling configuration.
Computer Numerical Control (CNC) Drilling
Computer Numerical Control (CNC) Drilling is commonly implemented for mass production. The drilling machine, however, is often a multi-function machining center that also mills and sometimes turns. The largest time sink for CNC drilling is with tool changes, so for speed, variation of hole diameters should be minimized. The fastest machines for drilling varying hole sizes have multiple spindles in turrets with drills of varying diameters already mounted for drilling. The appropriate drill is brought into position through movement of the turret, so that bits do not need to be removed and replaced. A turret-type CNC drilling machine is shown below.
A variety of semi-automated drilling machines are also used. An example is a simple drill press which, on command, drills a hole of a set depth into a part set up beneath it.
In order to be cost-effective, the appropriate type of CNC drilling machine needs to be applied to a particular part geometry. For low-volume jobs, manual or semi-automated drilling may suffice. For hole patterns with large differences in sizes and high volume, a geared head is most appropriate. If holes are close to each other and high throughput is desired, a gearless head can locate spindles close together so that the hole pattern can be completed in one pass. For further reference for CNC processes, please refer to the
Detailed Nomenclature for a Reamer
A more complete listing of reamer nomenclature is provided below.
Reamed holes should not intersect with drilled holes, so the configuration below should NOT be implemented:
As with a drilled hole, clearance for chips is needed at the bottom of a reamed hole. This is illustrated below:
Reaming should not be relied upon to correct the location or alignment of a hole. Its primary purpose is to fine-tune the diameter of the hole.
Honing
Honing is a final finishing operation conducted on a surface, typically of an inside cylinder, such as of an automotive engine block. Abrasive stones are used to remove minute amounts of material in order to tighten the tolerance on cylindricity. Honing is a surface finish operation, not a gross geometry-modifying operation. Hones can be of the multiple pedal type (pictured below) or the brush type. Either type applies a slight, uniform pressure to a light abrasive that wipes over the entire surface.
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Drilling involves the creation of holes that are right circular cylinders. This is accomplished most typically by using a twist drill, something most readers will have seen before. The figure below illustrates a cross section of a hole being cut by a common twist drill:
Drilling Characteristics
The characteristics of drilling that set it apart from other powered metal cutting operations are:
- The chips must exit out of the hole created by the cutting.
- Chip exit can cause problems when chips are large and/or continuous.
- The drill can wander upon entrance and for deep holes.
- For deep holes in large workpieces, coolant may need to be delivered through the drill shaft to the cutting front.
- Of the powered metal cutting processes, drilling on a drill press is the most likely to be performed by someone who is not a machinist.
A view of the metal-cutting area of a drill press is shown below. The workpiece is held in place by a C-clamp since cutting forces can be quite large. It is dangerous to hold a workpiece by hand during drilling since cutting forces can unpredictably get quite large and wrench the part away. Wood is often used underneath the part so that the drill bit can overshoot without damaging the table. The table also has holes for drill overshoot as well as weight reduction. A three-jaw chuck is used since three points determine a circle in two dimensions. Four-jaw chucks are rarely seen since offset of the bit is not necessary.
Twist Drill Bit
The figure below labels the important angles for a typical twist drill bit.
Drill Bit Variety
The figures below illustrate various drill bits and their cut hole configurations.
Drill Chucks
Drill chucks can be of several types, but are typically three-jaw since three points on the circumference define a circle in two dimensions. A standard three-jaw and a multi-jaw chuck are shown in the figures below.
Drill chucks can be of several types, but are typically three-jaw since three points on the circumference define a circle in two dimensions. A standard three-jaw and a multi-jaw chuck are shown in the figures below.
Turning: Chucks
The chuck is integral to a lathe's functioning because it fixtures the part to the spindle axis of the machine. Below is shown a three-jaw chuck with jaws that are all driven by the same chuck key. This arrangement provides convenience in that parts can be mounted and dismounted quickly.
Three-Jaw Chuck
The inner construction of the three-jaw chuck is shown below. A spiral gear meshes with cog teeth on the jaws to move all three jaws in or out simultaneously. Parts can be fixtured on outer or inner surfaces since there are gripping surfaces on the inner and outer surfaces of the chuck jaws.
Four-Jaw Chuck
If the part needs to be off center or is not a solid of revolution (axially symmetric), a four-jaw chuck with independently-actuated jaws needs to be used. Such a chuck is depicted below.
Drill Press Detail
A typical manual drill press is shown in the figure below. Compared to other powered metal cutting tools, a drill press is fairly simple, but it has evolved into a versatile necessity for every machine shop.
The most adjustable part of the drill press is the vertical movement of the drill bit, since this is the motion that is used in production.
The capstan wheel (A) moves the drill head up and down.
- This movement can be locked (D)
- and there are point-to-point stops (B) for maintaining a specific length of travel.
- Gradations marked on the stationary part of the drill press (C) let the operator know where he is vertically.
- Both the drilling head and table can move vertically and rotate about the vertical guide post (b).
- The base of the drill press incorporates a work surface similar to the table's for oversize workpieces. The base can be bolted down, but often is not since forces on a drill press do not typically cause it to tip over.
The drill is powered by an electric motor (I)
A detail of the work area of the drill press is shown below.
Drilling can be accomplished more accurately on alath or mill , although drill presses are much cheaper and more accessible machines.
Jig Boring
Jig boring is used to accurately enlarge existing holes and make their diameters highly accurate. Jig boring is used for holes that need to have diameter and total runout controlled to a high degree. Typically, a part has holes machined on regular equipment and then the part is transferred to a dedicated jig boring machine for final operations on the especially accurate holes. Jig boring can also maintain high accuracy between multiple holes or holes and surfaces. Tolerances can be held readily within ±.005 mm (±0.0002 inches). Dedicated jig boring machines are designed to machine holes with the tightest tolerances possible with a machine tool.
When designing a part with holes, it is important to determine what holes must be jig bored. The reason for this is that jig boring requires extra time and attention, and the jig boring machine at the machine shop may have a back log of jobs. Jig boring can therefore have a big impact on the lead time of a part. A cross section of a hole being jig bored is shown below.
Standard boring can be carried out on a mill fitted with a boring head or on a lathe. Boring is most accurate on a lathe since a lathe is dedicated to solids of revolution (axially symmetric parts).
Gun Drilling
For long holes such as those found in gun bores, gun drills are used. The length of the hole requires that coolant be delivered through the shaft of the gun drill to the cutting front. The coolant also serves to eject chips from the cutting area and to move them back and out of the hole entrance. The figures below illustrate a gun drill and the cutting/cooling configuration.
Computer Numerical Control (CNC) Drilling
Computer Numerical Control (CNC) Drilling is commonly implemented for mass production. The drilling machine, however, is often a multi-function machining center that also mills and sometimes turns. The largest time sink for CNC drilling is with tool changes, so for speed, variation of hole diameters should be minimized. The fastest machines for drilling varying hole sizes have multiple spindles in turrets with drills of varying diameters already mounted for drilling. The appropriate drill is brought into position through movement of the turret, so that bits do not need to be removed and replaced. A turret-type CNC drilling machine is shown below.
A variety of semi-automated drilling machines are also used. An example is a simple drill press which, on command, drills a hole of a set depth into a part set up beneath it.
In order to be cost-effective, the appropriate type of CNC drilling machine needs to be applied to a particular part geometry. For low-volume jobs, manual or semi-automated drilling may suffice. For hole patterns with large differences in sizes and high volume, a geared head is most appropriate. If holes are close to each other and high throughput is desired, a gearless head can locate spindles close together so that the hole pattern can be completed in one pass. For further reference for CNC processes, please refer to the
The Computer Numerical Control (CNC) fabrication process offers flexible manufacturing runs without high capital expenditure dies and stamping presses. High volumes are not required to justify the use of this equipment.
Tooling is mounted on a turret which can be as little as 10 sets to as much as 100 sets. This turret is mounted on the upper part of the press, which can range in capacity from 10 tons to 100 tons in capacity.
The turret travels on lead screws, which travel in the X and Y direction and are computer controlled. Alternatively, the workpiece can travel on the lead screws, and move relative to the fixed turret. The tooling is located over the sheet metal, the punch is activated, and performs the operation, and the turret is indexed to the next location of the workpiece. After the first stage of tooling is deployed over the entire workpiece, the second stage is rotated into place and the whole process is repeated. This entire process is repeated until all the tooling positions of the turret are deployed.
- Advantages of drilled holes include accuracy and sharpness of edges. Since machining is expensive compared to other manufacturing processes, drilling to create a hole should be justified by looking at alternatives. Before adding drilled holes to a design, ask yourself whether the hole is needed and/or whether it can be cast, molded, or pierced with sufficient accuracy instead of drilled.
- Specify standard drill bit sizes. Unusual hole sizes bring up the cost of manufacturing through purchasing and inventory costs.
- Through holes are preferred over blind holes. This has to do with the fact that a blind hole does not provide as much leeway for chip exit and cooling. Operations such as reaming and threading after drilling are more easily conducted on a through hole.
- Do not specify flat-bottomed holes. Twist drills create cone-bottomed holes and flat-bottom holes cause problems with reaming, etc.
- If possible, do not specify holes that are smaller than one-eighth inches in diameter. Drills for smaller holes tend to break and for convenient mass production, are not recommended.
- For large holes, try to cast in a preliminary hole that must only be bored out to specification. This saves material, transportation cost, and drilling cost.
- When dimensioning holes, it is better to use rectangular rather than angular (or polar) coordinates. Angular coordinates will require the machinist to set up a dividing head or to re-dimension the part, both of which take time.
- Minimize the number of drilled hole sizes so that tool changes are minimized.
- Minimize the number of directions on the part that holes must be drilled from.
- The entrance and exit surfaces of a drilled hole should be perpendicular to the hole axis. The reasons for this are as follows:
- Upon entrance of the drill, the drill tip will wander if the surface that the tip contacts is not perpendicar to the drill axis.
- Exit burrs will be uneven around the circumference of the exit hole. This can make burr removal difficult.
Bad and good examples of entrance and exit lands are shown in the figure below. - Intersections of drilled holes with other cavities should be avoided if at all possible. If interesection with a cavity is unavoidable, the drill axis should at least be outside of the cavity, as shown below.
- On drawings, multiple holes in a flat surface should be located from the same horizontal and vertical datums.
- If there are protrusions surrounding a drilled hole, it may be difficult to bring the drill press head close to the entrance surface, resulting in a drill bit that is prone to wandering, chatter, and other instabilities. This problem can be solved by providing a fixture with a drill bushing close to the drill bit. However, part design must allow for this fixture, as shown below.
- Drilled Hole Depth:
Deep, narrow holes with length to diameter ratios of larger than three should be avoided. Deeper holes are possible but the drill will tend to wander and possibly break. One way to avoid a deep, narrow hole is to use a stepped entrance. Blind holes should be drilled to a depth 25% deeper than the actual hole in order to provide space for chips.
Detailed Nomenclature for a Reamer
A more complete listing of reamer nomenclature is provided below.
Reamed holes should not intersect with drilled holes, so the configuration below should NOT be implemented:
As with a drilled hole, clearance for chips is needed at the bottom of a reamed hole. This is illustrated below:
Reaming should not be relied upon to correct the location or alignment of a hole. Its primary purpose is to fine-tune the diameter of the hole.
Honing
Honing is a final finishing operation conducted on a surface, typically of an inside cylinder, such as of an automotive engine block. Abrasive stones are used to remove minute amounts of material in order to tighten the tolerance on cylindricity. Honing is a surface finish operation, not a gross geometry-modifying operation. Hones can be of the multiple pedal type (pictured below) or the brush type. Either type applies a slight, uniform pressure to a light abrasive that wipes over the entire surface.
The figure below illustrates the configuration of the abrasive stones of an external hone. 
Honed Part Design
Below are illustrated bad and good honed part geometries. Both pairs of figures show that a through hole design is always best for coolant flow, etc. The upper pair shows that a certain portion at the end of a blind hole is not completely honed. The lower pair shows that a right circular cylinder is the easiest to hone.
Below are illustrated bad and good honed part geometries. Both pairs of figures show that a through hole design is always best for coolant flow, etc. The upper pair shows that a certain portion at the end of a blind hole is not completely honed. The lower pair shows that a right circular cylinder is the easiest to hone.
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