Conventional and CNC Machining

What makes CNC machining methods superior to conventional methods? Are they superior at all? Where are the main benefits? While comparing CNC and conventional machining processes, common general approach to machining a typical part will emerge:

  1. Obtain and study the engineering drawing
  2. Select the most suitable machining method
  3. Decide on the setup method (work holding)
  4. Select cutting tools and holders
  5. Establish spindle speeds and cutting feedrates
  6. Machine the part

Conventional Lathe Machine

This general approach is the same for both types of machining. One major difference is how various data are input. A feedrate of 10 inches per minute (10 in/min) is the
same in manual or CNC applications, but the method of applying it is not. The same can be said about a coolant – it can be activated by physically turning a knob, pushing a switch
or programming a special code. All these actions will result in coolant rushing out of a nozzle. In both kinds of machining, a certain amount of knowledge by the user is required.
After all, metal working, and metal cutting specifically, is mainly a skill, but it is also, to a great degree, an art and a profession of large number of people. So is the application
of Computerized Numerical Control. Like any skill, or art, or profession, mastering it to the last detail is necessary to be successful. It takes a lot more than just technical knowledge
to be a CNC machinist, operator or CNC programmer. Work experience, intuition, and what is sometimes called a ‘gut-feel’, are much needed supplements to any skill.

In conventional machining, the operator sets up the machine and moves each cutting tool, using one or both hands, to produce the required part. Design of a manual machine tool offers many features that help the process of machining a part – levers, handles, gears and dials, to name just a few. The same body motions are repeated by the operator for every part machined. However, the word ‘same’ in this context really means ‘similar’rather than ‘identical’. Humans are not capable to repeat every process exactly the same at all times – that is the job of machines. People cannot work at the same performance level all the time, without a rest. All of us have some good and some bad moments. Such moments, when applied to machining a part, are difficult to predict. There will always be some differences
and inconsistencies within each batch of parts. Parts will not always be exactly the same. Maintaining dimensional tolerances and surface finish quality are the most typical problems encountered in conventional machining. Individual machinists may have their own ‘time proven’ methods, different from those of their fellow colleagues. Combination of these and other factors create a large field of inconsistency.

Machining under numerical control does away with the majority of inconsistencies. It does not require the same physical involvement as manual machining. Numerically controlled machining does not need any levers or dials or handles, at least not in the same sense as conventional machining. Once the part program has been proven, it can be used any number of times over, always returning consistent results. That does not mean there are no limiting factors. Cutting tools do wear out,material blank in one batch is not identical to the material blank in another batch, setups may vary, etc. These factors should be considered and compensated for, whenever necessary.
Emergence of numerical control technology does not mean an instant – or even a long term – demise of all manual machines. There are times when a traditional machining method is preferable to a computerized method. For example, a simple one time job may be done more efficiently on a manual machine than on a CNC machine. Certain types of machining jobs will benefit from manual, semiautomatic or automatic machining, rather than machining under numerical control. CNC machine tools are not meant to replace every manual machine, only to supplement them.
In many instances, the decision whether certain machining will be done on a CNC machine or not is based on the number of required parts and nothing else. Although the volume of parts machined as a batch is always an important criteria, it should never be the only factor. Consideration should also be given to the part complexity, its tolerances, the required quality of surface finish, etc. Often, a single complex part will benefit from CNC machining, while fifty relatively simple parts will not.
Keep in mind that numerical control has never machined a single part by itself. Numerical control is only a process or a method that enables a machine tool to be used in a productive, accurate and consistent way.