The Appeal of CNC
While the specific applications of CNC vary greatly from one machine to the next, all these sophisticated machines have become widely used in a variety of industries. A few of the major benefits offered by CNC technology are described below.
The first benefit offered by all forms of CNC machine tools is improved automation. The skill level of the operator in producing the work can be reduced or eliminated. Many CNC machines can run unattended during their entire machining cycle, freeing the operator to do other tasks. This gives the CNC user several side benefits including reduced operator fatigue, fewer mistakes caused by human error, and consistent and predictable machining time for each work piece.
The second major benefit of CNC technology is consistent and accurate work pieces. Todays CNC machines feature a typical accuracy rate in the range of 2 to 4 thousandths of an inch or 0.05 to 0.10 mm and repeatability near or better than 8 ten-thousandths of an inch or 0.0001 mm. This means that once a program is verified, two, ten, or one thousand identical work pieces can be easily produced with the same precision and consistency.
A third benefit offered by most forms of CNC machine tools is flexibility. Since these machines are run from computer programs, running a different work piece is as easy as loading a different program.
This leads to yet another benefit, quick changeover. Since these machines are very easy to set up and run, and given the ease with which programs can be loaded, they allow for a very short set up time. This is critical with today’s Just-In-Time production environments.
The resulting reduction in the number of machines needed in a wood manufacturing shop is yet another benefit worthy of noting. In the past, a great number of dedicated machines were needed to produce furniture or cabinets. With the advent of CNC technology, this reality has changed drastically.
Less time spent between work centers means faster production time. Less Work-in-Progress (WIP) also translates into lower inventory and less investment in non value-added resources.
As a result, machinery requirements decrease, employee workloads are simplified, and waste is minimized while production is maximized.
The CNC machine takes no breaks and although the human operator does, he can prepare work for the machine to perform on its own while he is undertaking other work.
For example, a company could run a carving program whenever the machine is not scheduled to be used. This carving program runs for many hours while the operator performs other work, providing extra revenue for the business.
Machines are made to optimally perform a set of functions and they don’t inherently have the same mobility and versatility as humans. Newer machines have evolved to become multitasking and more versatile and although there are still some limitations with CNC software technology, manufacturers are constantly improving their machines and creative users are finding new ways of using them beyond their limitations.
Since the machine will be running under program control, the skill level required of the CNC operator is also reduced as compared to a worker producing work pieces with conventional machine tools. Of course this is offset by the skill needed in the office to draw and program the parts for the machine.
CNC machines allow for precise control of all the variables that affect how the tool performs its cut into the material being machined. This translates into consistent high quality, smooth, even cuts.
It is generally believed that better cut quality is achieved by using a more rigid and accurate machine. While rigidity of the frame and accuracy have major roles to play, other factors are very important in affecting cut quality, such as control features, acceleration and deceleration, tool holding, and part holding.
Mass alone will tend to compound machining errors since a heavy moving part carries more momentum and is harder to start and stop. Conversely, too light a frame will allow for more vibrations and limit the loads that can be applied.
Bear in mind that not one single feature of the machine will determine its quality but a combination of all of these features.
Accuracy is a simple machine characteristic to measure although simply comparing cut length with programmed length alone is not a good evaluation of accuracy.
Absolute positioning accuracy means the ability to reach a point in space within a certain tolerance. This measure can change greatly whether it is measured on a single or on multiple axes, or on whether or not there is a load applied to the cutter head. This measurement is also dependent on the position of the piece on the work table. Different numbers can be obtained in different areas as ball screw compensation tables can be off or missing altogether.
Spatial accuracy is mostly dependent on encoder resolution. A high quality, properly adjusted servo system can normally position within plus or minus ten times the encoder resolution. Therefore, a system with 0.0005″ resolution can only be expected to achieve plus or minus 0.005″ or 0.1 mm positioning accuracy.
Repeatability is the ability to return to a point in space each time a program is executed. Just like absolute accuracy, repeatability can either be measured on a single axis or on multiple axes. On most systems, repeatability always outshines absolute positioning accuracy.
This is important in the business of cutting parts using computer controlled equipment. You want the control portion of the machine to work the same way every time no matter what program is running. A good controller will calculate the tool path many steps ahead and alert you or decide on a different course of action when it finds problems.
In the early days of Point-to-Point machines, the computer told the head to move to a certain position in X-Y. The drives executed this operation without consideration for the path that the tool would take.
On newer CNC machines, the movement of the axes are coordinated with each other to obey a set of given rules. This gives them the ability to circle around fixtures and to ease the cutter in and out of a part for example. This is known as an interpolated path.
In today’s competitive manufacturing environment, it is imperative for companies to maximize their production performance.
As can be seen in the table below, the average North American furniture plant still lags behind its international counterpart regarding performance.
As Lean Manufacturing principles are becoming more popular in the wood manufacturing sector, manufacturing practices such as “one piece flow” and “mass customization” are being implemented. These practices can be accomplished through using the full potential of CNC automation.
As seen in the next table, a batch size of one can be made for a much lower unit cost using CNC technology compared to traditional methods.
Note : All accuracy data’s and statistical data’s get from CNC Router machines. The article shows mostly common benefits of all kind CNC machines and only examples from CNC Router Business.