What are the main advantages of numerical control?
It is important to establish which areas of machining will benefit from it and which are better done the conventional way. It is absurd to think that a two horse power CNC mill
will win over jobs that are currently done on a twenty times more powerful manual mill. Equally unreasonable are expectations of super improvements in cutting speeds and
feedrates over a conventional machine. If the machining and tooling conditions are the same, the total cutting time will always be very close in both cases.
A list of some major areas where CNC users can and should expect improvement includes:
- Setup time reduction
- Lead time reduction
- Accuracy and repeatability
- Contouring of complex shapes
- Simplified tooling and work holding
- Consistent cutting time
- General productivity increase
Each area offers only a potential improvement. Individual CNC users will experience different levels of actual improvement, depending on the product manufactured, CNC machine used, setup methods applied, complexity of fixturing, quality of cutting tools, management philosophy and engineering design, experience level of the workforce, individual attitudes, and many others.
Setup Time Reduction
In many cases, actual setup times for CNC machines can be reduced, sometimes quite dramatically. It is important to realize that setup is a manual operation, greatly dependent on the performance of CNC operators, the type of fixturing and general machine shop practices. Setup time is unproductive, but necessary – it is part of the overall costs of doing
business. To keep setup time to minimum should be the primary consideration of any machine shop supervisor, programmer and operator.
Because of the design of CNC machines, real setup time should not be a major problem. Modular fixturing, standardized tooling, fixed locators, automatic tool changing, pallets, and other advanced features, make the setup time more efficient than a comparable setup of conventional machines. With good knowledge of modern manufacturing, productivity can be increased quite significantly.
The number of parts machined in a single setup is also important, in order to assess the actual cost of setup time. If a great number of parts is machined in one setup, the setup
cost per part can be rather insignificant. A very similar reduction can be achieved by grouping several different operations into a single setup. Even if the setup time is longer, it may be justified when compared to the time required to setup several conventional machines and operations.
Lead Time Reduction
Once a part program is written and proven correct, it is ready to be used again in the future, even at a short notice. Although the first run lead time is usually longer, it is virtually nil for all subsequent runs. Even if an engineering change of the part design requires program modification, it can be done usually quickly, reducing the lead time.
Long lead time, required to design and manufacture several special fixtures for conventional machines, can often be reduced by using simplified fixturing.
Accuracy and Repeatability
The high degree of accuracy and repeatability of modern CNC machines has been the single major benefit to many users. Whether part program is stored on a disk or in the computer memory, or even on a tape (the original method, now obsolete), it always remains the same. Any program can be changed at will, but once proven, no changes are usually required any more. A given program can be reused as many times as needed, without losing a single bit of data it contains. True, program has to allow for such changeable factors as tool wear and operating temperatures, it has to be stored safely, but generally very little interference from the CNC programmer or operator will be required. The accuracy of modern CNC machines and their repeatability allows high quality parts to be produced consistently, time after time.
Contouring of Complex Shapes
CNC lathes and machining centers are capable of contouring a large variety of different shapes. Many CNC users acquired their machines only to be able to handle complex parts. A good examples are CNC applications in the aircraft and automotive industries. Any use of some kind of computerized programming is virtually mandatory for any three dimensional tool path generation.
Complex shapes, such as molds,manifolds, dies, etc., can be manufactured without the additional expense of making a model for tracing. Mirrored parts can be achieved literally at the switch of a button. Storage of part programs is a lot simpler than storage of paper patterns, templates, wooden models, and other pattern making tools.
Simplified Tooling and Work Holding
Non-standard and ‘homemade’ tooling that clutters the benches and drawers around a conventional machine can be eliminated by using standard tooling, specially designed for numerical control applications.Multi-step tools such as pilot drills, step drills, combination tools, counter borers and others, are replaced with several individual standard tools. These tools are often cheaper and easier to replace than special and non-standard tools. Cost-cutting measures have forced many tool suppliers to keep a low or even a nonexistent inventory, while increasing delivery time to the customer. Standard, off-the-shelf tooling can usually be obtained faster then non-standard tooling.
Fixturing and work holding for CNC machines have only one major purpose – to hold the part rigidly and in the same position for all parts within a batch. Fixtures designed for CNC work do not normally require special jigs, pilot holes and other hole locating aids.
Cutting Time and Productivity Increase
Cutting time on a CNC machine is commonly known as the cycle time – and is always consistent. Unlike a conventional machining, where the operator’s skill, experience and personal fatigue are subject to changes, CNC machining is under the control of a computer. Only a small amount of manual work is restricted to the setup and part loading and unloading. For large batch runs, the high cost of unproductive time is spread among many parts, making it less significant. The main benefit of a consistent cutting time is for repetitive jobs, where production scheduling and work allocation to individual machine tools can be done very efficiently and accurately.
One of the main reasons companies often purchase CNC machines is strictly economic – it is a serious investment with great potential. Also, having a competitive edge is always on the mind of every plant manager. Numerical control technology offers excellent means to achieve significant improvements in manufacturing productivity and increasing the overall quality of manufactured parts. Like any means to an end, it has to be used wisely and knowledgeably. When more and more companies use CNC technology, just having a CNC machine does not offer the extra edge anymore. Companies that grow and get forward are those where the use of technology is managed efficiently, with the goal to be competitive in the global economy.
To reach the goal of major increase in productivity, it is essential that users understand the fundamental principles on which CNC technology is based. These principles take many forms, for example, understanding the electronic circuitry, complex ladder diagrams, computer logic, metrology, machine design, machining principles and practices, and many others. Each discipline has to be studied and mastered by all persons in charge.