Machining operations are employed to cut or form materials into a planned part. It is an integral part of the manufacturing process.
It is performed using manual tools or machines that remove workpiece material through milling, drilling, cutting, turning, shearing, grinding, punching, etc.
Machining is the process of controlled removal of material from a workpiece to shape it into the desired part. It is a subtractive manufacturing process where a cutting tool glides through the surface of the workpiece and delivers the cutting force to remove the material in the form of chips.
This article is an introduction to machining. It overviews the basics of the machining process by discussing its evolution, fundamentals, operations, etc.
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What is Machining?
Machining involves the removal of material from the workpiece to get the desired shape.
While machining of some metals like tungsten can be extremely difficult, other metals like aluminum, free machining steel, etc., are relatively easy to machine.
Traditionally, machining operations were carried out by hand tools like chisels, reamers, wrenches, pliers, hacksaws, etc., and other manually operated machines.
But with the advancements in technology, these machines were integrated with computer systems to automatically cut and shape these metals with a minimal physical effort from the machinist.
Manual Machining: How it Works
Manual machining is where it all started. This process is close to using hand tools for machining, but with the addition of machines that provide the ability to deliver strong cutting forces.
The machines used in manual machining have hand crankings that the machine operator rotates to move the workbed or the cutting head along the X, Y, or Z axes.
Sometimes different cranks are turned simultaneously to make smooth cuts on the workpieces.
Lathes, grinders, hand routers, bandsaws, and drill presses are some of the most used manual machines.
Workflow of Manual Machining
The workflow of manual machines is quite basic.
First, the part drawings are made with proper dimensions. It can also have drawings of sub-parts required to complete the main part.
Next, the stock workpieces are properly measured and marked for making each part.
While machining, the parts are measured between cycles with tools like micrometers, vernier calipers, dial indicators, rulers, gauges, optical comparators, etc.
These measurements help minimize the dimensional flaws the part can incur during the manual machining process.
Usually, the machinist will measure and draw lines on the metal workpieces using dye coatings or scribe markings to machine parts with good accuracy.
CNC Machining Explained
Operating machines manually is quite hard, especially when the machine is required to move along different axes simultaneously.
A CNC (Computer Numerical Control) machine overcomes this by controlling the motion of these machines using servo/stepper motors and CNC controllers.
The microcontrollers onboard the CNC controller enables the machine to link together different working parts and allows the operator to control all the processes from a computer.
Workflow in CNC Machining
The workflow of CNC machining includes the following
- Preparing the design
- Converting the design to code
- Setting up machining parameters
- Simulating the machining process
- Running the job
For CNC machining a part, the design of that part is first created on a computer with the help of CAD (Computer Aided Design) software.
Then the design is converted to code using CAM (Computer Aided Manufacturing) software. It does so by analyzing the geometrical data of the design.
Next, this code (G-code) is fed to the CNC controller, which can decode the program code and send appropriate electrical signals to each part of the machine.
Before running, you can test the code using simulation software. It allows you to call up different tools and materials to check for any errors in the code and verify that you can get the desired output.
You have to use control software to set the machining parameters like speed, feed rate, depth of cut, coolant on/off, etc., and run the job.
Note that some CNC software are capable of performing all these operations, eliminating the need for multiple software packages.
Manual Machining vs CNC Machining: Differences
| Parameter | Manual Machining | CNC Machining |
|---|---|---|
| Mode of operation | Manipulation of the tool by hand | Manipulation of the tool by computer |
| Accuracy | Low | High |
| Productivity | Low | High |
| Repeatability | Low | High |
| Cost per unit | High | Low |
| Cost of the machine | Low | High |
When compared to manual machining, CNC machining has a greater advantage. It helps reduce the work cycle and increases productivity.
Also, they can machine highly accurate and repeatable parts because of the automated process involved in CNC manufacturing.
This helps reduce the number of scraped units and the cost per unit.
Unfortunately, the cost of purchasing a CNC machine and maintaining it is much higher than manual machines.
Also, to operate a CNC machine, you need to have some working knowledge of computers and related software programs.
10 Basic Machining Operations
Milling
In the milling operation, the machine removes material using a rotating cutter. Face milling and peripheral milling are the two types of basic milling operations.
Machining centers, CNC mills, CNC routers, etc., perform these operations.
Drilling
In the drilling operation, a rotating cutter pushes downward and removes material in a circular cross-section.
Depending on the required depth of cut, machinists use lengthy drill bits. Drilling operations are performed by drill presses, CNC drills, mills, machining centers, routers, and lathes having live tools.
Turning
Turning operations are performed on a lathe and turning centers. Here a cylindrical workpiece rotates while a sharp tool is fed to the workpiece surface to make symmetrical cuts.
Boring
Unlike turning, in boring, the material is removed from the inside of a rotating workpiece using ID (Internal Diameter) tools.
This operation can be performed on a lathe by using a special cutting tool, known as a boring bar.
Reaming
Reaming is a finishing operation done to size a previously drilled hole accurately.
Threading
Threading operations are performed to make helical screw threads on cylindrical workpiece surfaces. These operations are performed on CNC mills, machining centers, and turning machines.
Generally, CNC lathes are used to perform CNC threading operations by using special G-code cycles such as G32 and G76 cycles.
This process can be used to cut different types of threads such as acme threads, circular threads, etc.
Tapping
In the tapping operation, holes with internal threads are made in the workpieces. Sometimes, a hole is drilled using a drill bit. Then a tapping tool is used as a finishing cycle.
Tapping is generally considered a metal threading process as it is primarily applicable to metals.
Rigid tapping is a CNC tapping method that eliminates the need for costly tool holder to cut threads with high precision.
Grinding
In the grinding operation, the surface of a material is removed using a rotating belt or an abrasive wheel.
This is usually a finishing operation performed to provide a better surface quality to the machined material.
Generally, grinding wheels with silicon carbide abrasive are used for grinding aluminum, whereas grinding wheels with the term “Inox” printed on them, are suitable for grinding stainless steel.
Etching, Engraving, and Marking
Etching, engraving, and marking operations are performed using laser cutters and engravers.
The difference between these operations is that they are performed at different depths. Marking is done on the surface, while engraving can reach a depth of up to 0.125”.
Cutting
In the cutting operation, a workpiece is completely cut across its cross-section. Most machines, like mills, routers, laser cutters, plasma cutters, water jets, etc., cut through metal workpieces.
Comparatively, Wire EDMs have a higher cutting accuracy and are also very effective in cutting thick material blocks.
Important Machining Parameters
The machining parameters of each CNC machine vary as they operate in different ways. The following parameters are common for machines like lathes, mills, machining centers, VMCs, drills, etc.
Cutting Speeds, Feeds, and Depth of Cut
Cutting speeds and feed rates are an important part of machining. The speeds and feeds will depend on the material toughness.
Generally, ferrous metals are hard to machine. Hence they are processed at lower speeds than soft metals like aluminum, brass, etc.
Machining Power
The power of the machine tool determines the mechanical force exerted on the workpiece. A machine must have a high-powered motor to cut harder materials.
For example, wood lathes generally have a motor with power anywhere between 0.5 hp – 1 hp. In contrast, metal lathes have high-powered motors (above 1 hp).
Cutting Tools and holding devices
The cutting tools and holding devices play a major part in providing a steady machining environment. These devices must be rigid for the machine to cut off chips at a stable rate.
If they are loosened up, the machine won’t be able to transfer the machining power to the workpiece properly.
Cutting Compound
The cutting compound is a coating applied to the tool. Each coating has specific advantages and hence is used to machine different metals.
Following is a list of popular cutting compounds regularly used in end mills and router bits for machining metals.
| Cutting Compound | Benefit | Used to Cut |
|---|---|---|
| Titanium Nitride(TiN) | High lubricity, increases chip flow, heat resistance | Stainless steel, cast iron, aluminum alloys |
| Titanium Carbonitride (TiCN) | Higher spindle speed applications | Stainless steel, cast iron, aluminum alloys |
| Titanium Aluminum Nitride (TiAlN) | High hardness | Stainless steel, carbon steel, nickel, and titanium alloys |
| Aluminum Titanium Nitride (AlTiN) | Hardest coating, Abrasive-resistant | Nickel, stainless steel, titanium, cast iron, carbon steel |
| Zirconium Nitride (ZrN) | Higher oxidation temperature, Resists sticking | Aluminum, brass, rubber, titanium, plastic |
| Diamond-Like Carbon | Wear, corrosion, and abstraction resistance, low friction | Stainless steel, carbon steel, nickel, and titanium alloys |
Cutting Fluid
Cutting Fluids provide lubrication and regulate the temperature at the contact point between the cutting tool and workpiece. This helps extend the tool’s life.
Also, cutting fluid delivery mechanisms like flood coolant helps wash off chips from the material surface.
Final Thoughts
To better understand the machining processes, in addition to theoretical knowledge, you also need to gain a practical understanding of working with machines and tools involved in the process.
With the developments in technology, machine tool manufacturers are releasing hybrid machines. These machines are capable of performing many different operations.
Some machines even combine additive and subtractive manufacturing in a single machine, providing the good of both worlds. Snapmaker is a small-scale example of one such machine.
Frequently Asked Questions (FAQ)
Do you need to be good at math to be a machinist?
Yes, a machinist must have a good grasp of the basics of maths to program and work with manufacturing machines. Your understanding of maths will help you set proper dimensions, speeds, feed, power, etc., and it also comes in handy while selecting cutting tools.
What are the basic techniques in machining?
Some of the basic machining processes include turning, milling, and drilling. Turning operations are performed by lathes, while drilling and milling operations are done on mills and drill presses.
What metals can be machined?
You can machine metals like aluminum, brass, titanium, bronze, copper, stainless steel, carbon steel, mild steel, nickel, cast iron, etc.
