Comparing CNC Machines to NC Machines

In the machining manufacturing world there are two terms, CNC and NC that are sometimes used interchangeably. Comparing CNC machine to NC machines some definition and brief background history is required. Although they are related they are in fact distinct from each other.

Introduction to NC Machines

NC machines were the predecessors CNC machines. Their introduction and development paved the way for the present day  CNC machines. The introduction of NC machines happened in the mid-20^th century, primarily following WWII. The early NC machines took standard manual equipment (primarily lathes and mills), adapted electronic or hydraulic drives on each axes and the main spindle motor. A machine controller was installed that oversaw the machine feeds and speeds, as well as axis movements. This information was read into the controller via punch (memory) cards and/or punched tapes. Almost all calculations were done off the machines, prior to operation. This program information was then put pre-prepared punch cards or paper tape. There was a reader-head mechanism on the machine, that was able to read the information into the machine controller.

By today’s standards it is archaic, but in the mid-20th century it was a quantum leap in machining technology. It shifted the manufacturing paradigm. Running off these punch cards and tapes, allowed programs to be run through the machine and controller, making machine axes movements  and spindle speeds consistent and  repeatable. It removed much of the machining inconsistency due to operator variance. Machining efficiencies went way up. Part tolerance became more predictable and better  tolerance control greatly reduced scrap rates.

CNC Machine Introduction

By the late 1960’s with new developments in computer technology, NC (numerical control) machines began to transition to CNC (computer numerical control). The primary difference between CNC machines and NC machines is that CNC reads the machine program through an on-board computer. The axes movements and spindle speeds are read and monitored through the machine controller.

Flexibility

You no longer had to go in and modify or changeout the punch card or tape to change the program. Changes could be made through the computer and downloaded into the controller memory. This ability to easily change and/or update a program gave the machine and the manufacturing process a whole new level of flexibility.

Introduction of a machine language

All computers require a internal program language to carryout desired instructions. During this period of time a programming and/or machine language was developed, specifically for machine tools. It used a “G” and “M” code format to direct all machine movements. This basic format is still used today. The adaptation of this language through the controller gave the machines opportunity in real machine time to carryout machining instructions. The only limitation was the size of the work envelope and machine configuration. This allowed for more rapid changeover, shorter setup times and a higher degrees of repeatability and much greater flexibility.

Higher levels of precision

NC machines introduced a higher level of consistent precision by reducing the skill level required of the machine operator. Using NC machine operator error was reduced. Precision was primarily achieved by the rigidity of the machine and the accuracy of the programmed punch-card/tape. The introduction CNC machines took this precision to a higher level. The ability to generate programs offline, download them into the machine controller, run the machine through a test cycle using setup parts, and make necessary edits for running production, gave the production parts a greater precision, than previously experienced. Because with CNC, it was easier to make changes to the program, the efficiencies gained  enhanced both the setup and production. One of the net results was a higher level of precision.

Introduction of automation

The introduction of NC machines and greater control of the consistency of the part size, allowed for higher levels of automation, but in its infancy there was a limited access of interface between the machine and the automation. Most of the early automation was controlled by hard wiring or I/O outputs, making automating short and medium lot sizes difficult to justify.

CNC machines with their flexibility and ability for easy interface, to an ancillary piece of equipment (loader/robot, less operator intervention), broadened the opportunity for automation applications. The limitations of automation are still primarily a function of lot size and loader/robot changeover, but CNC makes automation possibilities more viable.

Improvements in maintenance

Maintaining NC machines were very similar to their manual predecessors. It was still a function of manual monitoring of the machine. There was little to no electronic diagnostics. CNC machines with every generational iteration have greatly expanded diagnostic capability used for maintenance. CNC machines can monitor not only machining operations but track maintenance schedules, remind the machine operator of any scheduled maintenance programs, and identify future potential machine failures. Later generations have online plug-in, allowing the end user to link up in real time for training and troubleshooting. These diagnostic capabilities help shorten service response and decrease down times.

Machine cost

Machine Cost is always a factor in capital equipment. The early NC machines were very expensive, when measured against their increases in productivity. Many times the Return on investment (ROI ) was difficult to justify. The efficiencies gained over manual equipment were certainly better, but capital justification was not a given . CNC machines with their greater flexibilities (noted above) and overall increases in gained efficiencies, made a much more compelling case for justification. When comparing cost of NC and early CNC  to the latest generations of CNC, factoring the current monetary values, the amount of CNC machines available on the market, the “bang-for-the-buck), today’s CNC are a much better buy. We now have a mature CNC machine tool market, capable of producing machine tools and a much higher production rate. The competitive forces within this market help drive efficiencies up and machine cost down.

Like all advances in technology, there is a progression, and machine tools are no exception. During the process of moving from manual to NC to CNC, each iteration was built on the back of its predecessor. The basic products ever evolving, became better. The manufacturing tolerances become tighter, the quality is higher, the overall products are better. Advancements made in servos, controllers, and automation continue to progress, making for better machine tools, resulting in superior end products, that the market demands.

We are pleased to be part of this evolution in manufacturing technology. If you would like more information about Intrex Aerospace, or would like to make comments about this article, please click here.