Drilling Operations – Definition, Types and Calculations

There are various types of machining operations. One of these machining operations is a drilling operation. So, the drilling operation is one of the most classical operations that we use to create round holes in the work parts. Here we give a detailed explanation of them. Furthermore, this article include these topics;

  • What is drilling? What are the main motions in drilling? 
  • Calculations of the parameters of drilling operations,
  • Types of drilling operations,
  • Types of drilling machines,
  • FAQs about these operations and machines.

Explanation of the Drilling Operations

Drilling operation.
Image Source: Wikimedia.

Firstly, drilling operation is one of the most classical machining operations which they use to create a round hole on work parts. In drilling operations, a cylindrical rotating cutting tool which we also call a drill bit is directed toward the work part to drill holes on work parts. Drill bits have two cutting edges unlike turning machine tools. 

Drill bit make the two motions, primary motion, and the secondary or feed motion. Rotation of the drill bit is the primary motion that performs the material removal from the work part. And also, drill bit performs the secondary motion by feeding inside the work part. 

So, the machines that we perform drilling operations are drill presses. We use various types of drill presses for different purposes. 

Not only in the metal industry but drilling operations are also applied to different materials such as woods, polymers, etc. 

What are the Types of Drilling Operations?

Types of drilling operations.
General drilling operations; reaming, tapping, counterboring, countersiking, centering and spot facing respectively(Image Source: Mikell P. Groover; Fundamentals Of Modern Manufacturing, 4th edition, pg 521).

They apply various types of drilling operations in the machining industry. But they apply these drilling types after drilling the main hole with an ordinary drill bit. So, types of drilling operations are; Reaming, tapping, counterboring, countersinking, centering, and spot facing. 

Centering Operations

Centering is a primary and basic operation to create starting drill hole location for subsequent drilling operations. Also, centering is important because of the possible slippages of drill bits. So, Centering prevents these slippages that lead to wrong hole operations. 

The tool that we use in the centering operations is a center drill. So, as you understand that centering operations are not applied to a drilled hole. Because of this feature, the centering operation is not classified as a secondary operation. 

Spot Facing Operation

Spot-facing operation is similar to the milling operation. Also, around the drilled area on the workpiece, we obtain a machined smooth surface with a machine tool in a spot-facing operation. 

Reaming Operation

Reaming is a type of additional drilling operation that we apply to an existing hole. There are different purposes of reaming operations. Furthermore the first important purpose of the reaming operation is the improve the surface quality of the drilled hole. The second important purpose of the reaming operation is enlarging the existing hole. We use straight-edged drill bits in the reaming operations, unlike the classical drill bits.

Counterboring Operation

Above all in the counterboring operation, they create a larger diameter hole cap to obtain a stepped hole on work parts. We apply drill bits with a larger diameter to the existing drilled hole in the same center and drilled to a level. 

Counterbored holes are used in the attachments of screws that have standard heads. Enlarged sections of the counterbored holes are used for the attachment of screw heads. 

Countersinking Operation

In the countersinking operations, we create a tapered hole head with a cone-shaped drill bit. We use countersunk holes for the mounting of tapered head screws. 

Tapping Operation

They mainly use the tapping operations to create screw threads on work parts with a special drill bit called a tap. They apply the tapping operation also to an existing hole that they drilled by a conventional drill bit. 

And also these are the general types of operations that they apply in most machining facilities. 

What are the Types of Drill Presses?

According to the needs of a drill application, there are different constructions of drill machines or drill presses. So the general types of drill presses are; upright drill presses, radial drill presses, bench drill presses, gang drill presses, and CNC drill presses. 

Upright Drill Press Machine

Upright type.
Upright drilling machine(Image Source: Flickr).

The upright drill press machine is the most classical and the most used type of drilling machine. They clamp and hold the workpiece on the table of the upright drilling press, and the vertical spindle which has the rotating drill bit can make reciprocating motion over the work part with the help of the adjustable head. 

At the head section of the upright drill press, the electrical motor and the gear system are staying inside it. Column and base sections of the upright drill press machine are the main structural elements to carry and fit all of the systems to the ground. 

Bench Drill Press Machine

Bench drill press machines have nearly the same construction. The only difference between the bench drill machine is, that they constructed on a bench. They build the upright drill press directly to the ground. 

Smaller-sized work parts can be drilled on bench drill machines compared to upright drilling machines. 

Radial Drilling Machines

Radial machine.
Radial drilling machine press(Image Source: Wikimedia).

They use radial drilling machines to drill large work parts. In addition to the up and down movement of the drill bit, the drill bit can move right and left with the additional sliding arm on the radial drill machines. 

Also, we can attach additional spindles to radial drilling machines to drill multiple holes at the same time on the large work parts. 

For more large works, we use radial drill machines generally. 

Gang Drilling Machine

We use the gang drilling machines to drill work parts in a serial production manner. We attach multiple drilling bits which have separate power sources and the worktable is common for these different drill bits. At the same time, we can perform different drilling operations on gang drilling machines. 

Gang drilling machine.
Gang drilling machine(Image Source: Flickr).

In other words, the work table can move to gather the new part to be drilled. So a gang drill machine is very useful for high-count drilling works. 

Gang drill machines are also called multiple-spindle drilling machines in the sector. 

CNC Drill Presses

CNC drill presses are high-technology drill machines in which we use computer numerical control software to attain precise positioning and very high quality of machining. The main difference between CNC drill presses is that they have a special turret that has multiple spindles on it. According to the CNC code that we install on the CNC machine, CNC machine make the operation with different drill bits. We can change drill bits can instantly with the help of the turret. 

CNC operators create CNC codes by CNC operators and install on the machine. After starting the machine, the operation takes place automatically. 

Workholding on Drill Presses

Workholding is a very important parameter in machining operations. An ideal work holding device would not harm the workpieces and strictly hold the workpiece. If the work holding is not solid enough for the drilling operations, the drill bit would slip and unintended wrong holes can be drilled in the wrong places. 

There are 3 different workholding types in these operations. These work holding constructions are; vise, fixture, and jig. 

Vises are the workholding clamping unit that we primarily use in the upright drill press machines. On vises, there are two jaws to hold the work parts. 

Fixtures are specially designed tools to hold the work parts. For serial drilling purposes, fixtures are more useful than vises. 

Jigs are also specially designed to work part holding contraptions that also provide guiding the tool during the drilling operation, unlike the fixtures. 

Process Calculations in Drilling Operations

One of the most important parameters in drilling operations is the cutting speed. We calculate the cutting speed according to the outer perimeter of the drill bit. 

Cutting Speed of Drilling Operation

Cutting speed formula.

This is the general formula to calculate the revolutions per minute of the spindle of the drill bit. Here, ‘N’ is the revolution per minute, and ‘V’ is the cutting speed of the cutting tool mm/min or in/min in English units. ‘D’ is the diameter of the drill bit in mm or in. 

This formula says that, with the increasing diameter of the drill bit, the revolution speed decreases. But with the increasing speed, the revolution of the spindle increases.

Feed and Feed Rate in Drilling Operation

Feed is the secondary motion of the drill bit to the workpiece which has the unit of mm/rev or in/rev. Also feed rate is the value that shows how much distance that drill bit takes in a minute. The unit of the feed rate in mm/min or in/min and this equation can be used to convert the feed to feed rate; 

Feed rate formula for drilling operations.

So the feed is faster in the bigger diameters of drill bits. And for smaller diameters of drill bits, they apply slower feed rates in drilling operations. 

As you understand that there is a direct proportion between the RPM of the spindle and the feed and the feed rate. 

Calculation of the Approach Allowance of Drill Bit

In the calculation of the approach allowance, we are using basic trigonımetry. And in the user’s manual of the drill bits, probably you can find the approach allowance value. But in the calculation of the approach allowance of drill bits, we use this formulation; 

Approach allowance.

Furthermore the unit of the approach allowance ‘A’ is mm or in. We call ‘θ’ as the drill point angle. If the drill point angle increases, the total approach allowance of the drill bit decreases.

Calculation of the Total Machining Time

And then we calculate the total machining time according to the total length of the hole to be drilled. You can use the calculation below to calculate the total machining time of the drilling operations. 

Total machining time.

The ‘h’ in here is the total depth of the drilling operation by not adding the drill bit depth. The unit of the ‘h’ is mm or in and the unit of the time is min. 

With the increasing total height of the hole, the total machining time of the drilling operation increases. Also, with the increasing feed rate, the total machining time of the drilling operation decreases. 

Drilling Operation Calculator

Above all, as you see above, there are various calculations about the parameters of drilling operations. As an engineer or technician, you will be responsible for the total time that you take by the total drilling operation of multiple parts and multiple drilling operations. 

To calculate the required parameters to obtain solid numerical data about this operation, you can use the calculator below. 

-calc- 

The use of this operation parameter calculator is very simple. Firstly, you just need to enter the drilling speed, diameter of the drill bit, feed motion, height of the drilling operation, and drill point angle. After that, Click on the ‘Calculate!’ button to calculate the RPM of the spindle, feed rate of the operation, total drilling time, and approach allowance of the drill bit. If you want to make further calculations, click on the ‘Reset’ button and re-enter the values. 

Conclusion

Drilling operations may seem simple but on the manufacturing scale, there are different drilling operations. Also, there are different drilling operations in the industry for different kinds of workpieces and applications. 

So we tried to give general information about drilling operations in the industry and we made a calculator calculate the general parameters of the drilling operations. 

Above all, Mechanicalland does not accept any responsibility for calculations made by users in calculators. A good engineer must check calculations again and again.

Finally, you can find out much more calculators like this in Mechanicalland! Take a look at the other engineering calculators available in Mechanicalland!

Also do not forget to leave your comments and questions below about drilling operations. 

Your precious feedbacks are very important to us.

FAQs About Drilling Operations

What is the purpose of drilling?

They manufacture all types of parts according to the standards. Also they make holes on parts according to the standards. We use drilling operations in manufacturing sites to drill standard-sized holes on manufactured parts. 

What are the types of drilling machines?

In general, there are four types of drilling machines are used; upright drilling machines, radial drilling machines, gang drilling machines, and CNC drilling machines. 

What are the types of drilling operations? 

Different drilling operations are used. So, these operations are; tapping, counterboring, countersinking, centering, spot facing, and reaming.

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