What can CNC machining do?
Essentially, this industrial machine has one function: to remove pieces of metal, plastic or other material from a larger block of that material to form a shape, which then becomes a component or a product. The method of removing that material varies by machine, either it can be by using a lathe, a milling machine, or by drilling, but with all these methods, the main function of removing material to shape an object is the same.
So how does the machine do this?
The clue is in fact in the name, CNC stands for computer numerical control. At the heart of all CNC machining is the central computer, which is fed data from a CAD program, or computer-aided design application. This data contains the information that the CNC machine needs to produce the object. The CNC computer is connected to the lathe itself, and so the entire process of shaping the metal through the three main ways is fully automated by the machine. One of the key benefits of this system is that each machine can be reprogrammed using a piece of software to accommodate different roles.
For example, if a factory had ten machines, on one day they could all be assigned to milling, the next day to drilling, and the following day to lathing. Compare this to a manual machine, where precious factory floor space needs to be filled with machines that cannot be reprogrammed. This means some machines will be left idle on certain days and the maximum productivity of the factory cannot be achieved.
Unlike a manual lathe or milling machine, the computer knows exactly what to do, so a skilled technician is not needed. Instead, the technician of a CNC machining system is relegated to a manager, who maintains the system, ensures it is fully operational and has enough materials to work with. The flip side is that one technician can manage more than one machine, maybe between 3 and 5 at a time, which means that one technician can increase their productivity five-fold. However, despite not actively engaging in any manual milling, lathing or drilling, the technician should be trained in these skills, as this knowledge is essential in operating these machines.
Because CNC machining equipment is fully computer controlled, accuracy is something that comes as standard. A computer controlled lathe has a steadier hand than a technician operating a manual one, so unprecedented levels of accuracy can be attained. CNC is actually now industry standard and required for the levels of accuracy needed for most components since manually made products cannot compete.
Tolerances of CNC machines
Tolerances of CNC machines completely dependent upon the type of CNC machine that is being used to complete the project. Each machine offers different tolerances. Of course, a machine that operates off of several different axes will allow for the closer, more specific tolerances. The four and five axes machines can produce specifically design parts that need precise measurements in order to work. Very tight tolerances are possible.
Exacting tolerances of 0.00004″ to 0.004″ is possible. (Otherwise, one to one hundred microns) Because these machines are computer numerical controlled (CNC) the possibility that production can be this precise is easy. No more do operators have to make sure that the placement of the material is precise and so on in order to achieve these standards. The computer technology can grind, cut, sand, etc. to the dimensions specified by the program. They will be extremely close if not all the way there.
One of the great benefits of CNC machining is the ability of high tolerances on a repetitive basis. Because the machines are computer operated, repetitive work is easy to perform. The machine keeps running to the program’s specifications until the work is complete. Another job that comes months down the road that has the same specifications can be performed easy. It just takes the downloading of the original program and the exact same specifications and tolerances will be repeated with this batch of work as well. Repetitive, high tolerance machining is one of the added benefits of CNC machining.
Tolerances are directly related to the quality of the CNC machines. This is why you will find that different companies who offer CNC machining services can offer different tolerances. Some may offer 0.001″ tolerances and others will offer a variety of different tolerances. The higher the quality of the CNC machine and the programs that operate it, the better tolerance that you will be able to get.
Top CNC Machine Brands
Yamazaki Mazak is a Japanese company that first opened its doors in 1919. In 1974, it established a base in North America.
Okuma was established in 1898. Since its inception, it has made a name for itself as a popular and quality brand for customers.
In 1983, Haas was established as a CNC manufacturer in California. Since then it has grown to become the biggest in America.
South Korea’s Doosan might be more up your alley if you are such a customer. For one thing, it’s relatively much cheaper.
Taiwanese in origin, Victor Taichung Machinery was born in 1954 and has since spread to many other countries.
What Are NC Machines?
Conventionally, an operator decides and adjusts various machines parameters and controls the slide movements by hand.
- A numerical control, or “NC”, system controls many machine functions and movements for high production rates, uniformity, and consistent part quality
- Programmed instructions are converted into output signals which control machine operations.
- Limited programming capability at the machine tool. Limited logic beyond direct input.
CNC Machine Definition
CNC stands for Computer Numeric Control
- In a CNC Machine, functions and slide movements are controlled by motors using computer programs.
- For a CNC, machine control unit (MCU) decides cutting speed, feed, depth of cut, tool selection, coolant on off and tool paths.
- The MCU issues command in form of numeric data to motors that position slides and tool accordingly.
- Comparison between CNC Machine and Conventional Machine
- Machine axis determination for horizontal and vertical spindle machines.
- Constructional details e.g. special configurations to increase accuracy.
- Recirculating ball lead Screws and anti-friction slideways.
- Use of servo and stepping motors in slide movementVisual displays unit-user interfaces.
- Multiple coolant nozzles around the Cutting Zone.
- Swarf removal systems.
Classification of CNC Machines
• Based on Feedback Control System
a. Open loop control system
b. Closed loop control system
• Based on Motion (Position Control) System
a. Point to Point Control System
b. Straight Path Control System
c. Continuous (contouring) Path Control System
• Based on the number of axes
a. 2 & 3 axes
b. 4 & 5 axes
Advantages and Limitations of CNC Machines
1. Increased productivity.
2. High accuracy and repeatability.
3. Reduced production costs.
4. Reduced indirect operating costs.
5. Capable of complex machining operations.
6. Greater flexibility.
7. Improved production planning and control.
8. Lower operator skill requirement.
9. Facilitation of flexible automation.
1. High initial investment.
2. High maintenance requirement.
3. Not cost-effective for low production cost.
Applications of CNC Machines
CNC machines are widely used in the metal cutting industry and are best used to produce the following types of product:
• Parts with complicated contours
• Parts requiring close tolerance and/or good repeatability
• Parts requiring expensive jigs and fixtures if produced on conventional machines
• Parts that may have several engineering changes, such as during the development stage of a prototype
• In cases where human errors could be extremely costly
• Parts that are needed in a hurry
• Small batch lots or short production runs
Specialized CNC Machines
Some common types of CNC machines and instruments used in industry are as following:
• Drilling Machine
• Lathe / Turning Centre
• Milling / Machining Centre
• Turret Press and Punching Machine
• Wirecut Electro Discharge Machine (EDM)
• Grinding Machine
• Laser Cutting Machine
• Water Jet Cutting Machine
• Electro Discharge Machine
• Coordinate Measuring Machine
• Industrial Robot
Steps for CNC Machining
- Study the part drawing carefully
- Unless the drawing dimensions are CNC adapted. Select a suitable programme zero points on the workpiece. The tool will be adjusted to this zero point during the machine set up.
- Determine the machining operations and their sequence.
- Determine the method of work clamping (vice, rotary table. Fixtures etc.).
- Select cutting tools and determine spindle speeds and feeds.
- Write programme (translate machining steps into programme blocks). If many solutions are possible, try the simplest solution first. It is usually longer, but better to proceed in this way.
- Prepare tool chart or diagram. Measure tool geometry (lengths, radii) and note.
- Clamp work piece and set up the machine.
- Enter compensation value if necessary.
• Check and test program It is a good practice to dry run the program
(i) Without the workpiece
(ii) Without the cutting tools or
(iii) By raising the tool to a safe height
- If necessary correct and edit the programme and check again.
- Start machining.