February 2004, updated February 2013
In older days, most CNC programs were developed manually, using simple common and everyday aids — pocket calculator, pencil, paper, and the proverbial five-pound eraser. Combine this human effort with punched tapes and some extra hardware, and you have what seems today a history long past. The tape is gone today, and even though many programmers still sharpen their pencils and purchase new erasers, the times have clearly changed in favor of computer-based programming, or CAM programming. True, CNC programming using personal computers started in the mid-1980s, but initial costs, the state of computer technology, long learning curves, and a lack of features prevented their wide usage for some time.
Using CNC software to make part programs is not a novelty. Feature rich software from a number of vendors offer the whole gamut of options — from entry level 2D to sophisticated 3D, solids, multi-axis machining, and multi-tasking — all available for desktop and laptop computers. Public and private institutions offer many training programs in this area, often in addition to standard CNC programming courses.
This brings up the ever more important question that many students ask: “Why should I know manual programming, when I am learning Mastercam?” Of course, there is software available other than Mastercam, but the question raises a good point. I wish I had a dollar for every time this question pops up in conversation or during a training session. So what gives?
Every CNC programmer has to possess certain skills. One skill I consider the most important is the ability to machine a part. Deciding on the best setup, selecting the most suitable tools, and creating safe and efficient machining operations is the core skill of any successful CNC programmer. The actual method of programming has nothing to do with it. Computers cannot make intelligent decisions.
Although the part program, in the form of a file or printed copy, is designed for a particular CNC system, we — the programmers and machine operators — have to be able to understand it.
Let me go through five main reasons for this statement:
Reason 1 — Myths about Post-Processors
Most CAM software comes with pre-configured postprocessors in the form of standard text files. These files are intended to be customized by the end user. There is no way for any software vendor to cover all possible combinations that exist between machine tools and control systems. In addition, vendors cannot predict the individual preferences of each user. As a postprocessor is used to format the final output of the part program, you still have to know what that output should be.
Reason 2 — At the Control with No Control
When the program runs at the CNC machine, you can watch it scroll by on the control display screen. Do you understand what all the letters and numbers mean? What do you do when there is an error in the program? CNC operators who can make minor (and sometimes even major) changes to the program are much valued by their employers. Some operators do not mind just pushing the buttons, but if you want a certain level of control and responsibility, you have to be able to understand the program itself.
Reason 3 — Programmer / Operator as an Opportunity
Small shops and job shops often do not have the luxury of employing a full time CNC programmer. As important as such a person would be to the company, many owners and managers opt for a qualified CNC operator who can also prepare the part programs, then run them. This programmer/operator concept has many benefits for both the employer and the employee. Combine Reasons 1 and 2, and you have a Reason 3.
Reason 4 — Do You MDI?
Any operator who sets up a CNC machine has to perform many pre-machining activities, for example, make tool changes, set tools, activate the spindle, orient it, move the table by a certain distance, check a position, and so on. Even with many switches on the panel, one of the best methods is to use the Manual Data Input feature of the control — the MDI. This feature allows you to input and execute data at the control on a temporary basis. The data is always provided in the program format of that control. You cannot use the MDI feature if you do not understand at least the basics of manual programming.
Reason 5 — Learning from others
A part program printed on paper does not teach programming or machining methods, yet there is so much to learn from programs that have already been verified. How many of us started our careers by adapting existing programs to our own? A part program that works well offers tremendous opportunity for study and learning. Of course, one has to be able to read such programs — to interpret them — and that means being able to understand the G-codes and M-functions, the program format and structure, offsets, modes, and the many addresses and their subtle meanings.
Of course, one has to be willing to learn. When you just look at a part program, without understanding it, without being able to interpret it, do you care? Does it bother you? Do you feel you want to know more? If the answer is “yes”, you are on the way of becoming a better programmer or operator — a true professional. But that is reason number six, and I only promised five.
 | Running the First Part — Economically, That Is March 2004, updated February 2013 |
Machining a batch of the same parts in a single production run is probably the most common reason for purchasing a CNC machine. The CNC technology offers a great amount of predictability. There is consistency between parts, dimensional accuracy is maintained over many parts, and the program and setup — once verified — can be used over and over in the future.
Yet, even with proven part programs, the production always hits one weak spot when running a new batch. Yes, it is the first part. Every supervisor knows that the first part of the batch is also the most expensive part of the whole batch; its cost of manufacturing proportionally affects all remaining parts. To minimize the cost of making the first part, it is worth looking at several influencing factors and possible solutions.
Comparing Programs
There is a significant difference between running the first part using a new program and a program that had already been verified.
New part programs
CNC programs that have never been used must be carefully inspected. Even programs generated by a CAD/CAM system require certain scrutiny, although on a much smaller scale. Programming errors and oversights can find their way into any program in various forms, some quite hard to detect. For example, the depth and width of a cut may be out of a reasonable range, clearances may be too large or too small, spindle speeds and feed rates may be overrated or underrated, a tool may not be the most suitable one for the job, and so on. These oversights are virtually impossible to see in print or even on the screen — they show only when the first part is run on the CNC machine. In addition to the program itself, the machine setup is always new, regardless of whether the program is new or previously verified.
Verified part programs
These are part programs accepted for production and verified at some earlier date. Keep in mind that only the programs have been verified, not the setups or tools. Setups and tools are subjects to frequent changes; they must be verified every time. Another important consideration is the CNC machine tool being used. Even verified programs have to be checked if they are used on a different machine than the previous time.
Decreasing Costs
Decreasing the cost of the first part — and, therefore,
