CNC Tips and Techniques. Peter Smid
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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, the whole batch — has to be considered at four levels, all interrelated:
Planning
Process planners or supervisors have to evaluate many aspects of the production. One of their key considerations is the number of parts produced in a single batch. The more batches per period of time, the more expensive each batch will be, mainly because of the repeated setups. Assuming that more parts produced in a batch will have no negative side effects (for example, on inventory), fewer batches per period of time might provide part of the solution.
Programming
CNC programmers can literally do wonders to decrease the cost of the first part through the program. Here are only some ideas. Using consistent tool numbers for commonly used tools saves time during setup, as tool registration and many related offsets will not change. Another very effective programming method is to include one or more trial cuts with the block skip function in the program. These cuts will only be used for the first part testing, but turned off for a full run (except the occasional inspection). Don’t forget to provide sufficient comments in the program, so the operator knows the purpose of these special cuts. Another consideration, especially for programs developed manually, is to invest in reliable CNC simulation software. Although no software can simulate every detail of CNC machining, it can be an excellent tool to find errors and areas of improvement.
Setup
At the machine, the CNC operator is normally responsible for setting up tools and fixtures, registering tool numbers and various offsets, troubleshooting the program, and monitoring the operation. Speeding up the non-productive time required for setup is the most significant single saving available. Modular fixturing, common tooling, general setup consistency, and the operator’s skill all add up to make the transition between setup and operation faster and smoother.
Machining
During machining, the CNC operator relies heavily on the program itself. During the first part run, it should not be enough for the CNC operator to look just for errors or monitor the program flow. Operators can and should do much more at the same time. For example, they can think about optimizing the program for better performance. Speeds and feeds are the most common areas to focus on, but many more “little things” also influence the cost of manufacturing (including the first part).
Lowering the cost of the first part should be a team effort. CNC programmers may be the key people in such efforts. However, mutual cooperation of everybody involved will offer many benefits and have a positive effect on the proverbial bottom line.
 | Are You a CAM Machinist? April 2004, updated February 2013 |
The main topic at a discussion I had several years ago was quite simple: CAD/CAM Learning Curve for CNC part programming. How long does it take to learn CAD/CAM software to produce a quality part program for a CNC machine tool? This type of part programming is often associated with the term CAM programming. Having looked at many courses offered by many more providers, the picture is still a bit cloudy. From an introductory three-day course (eighteen hours in reality, breaks included) to a three-level course running more than one hundred hours, there is something for everyone. All that in addition to self-starters, books, and multi-media resources.
Learning a CAM system can be quite intuitive for someone experienced with computers. For such a person, creating geometry (mainly points, lines, and arcs) can be child’s play after a few short hours. On the other hand, the same CAM software can be somewhat intimidating for somebody whose experience is with machining parts but not with using computers. That brings up an important question: What is required from a CAM programmer? What type of knowledge and skill should a CAM programmer have in order to develop often complex tool paths that make parts on expensive CNC machines?
During the discussion I mentioned above, the word skills was indeed used many times, mostly related to computer skills. Even the term creating geometry (also used quite frequently) is simplistic. The correct phrase should be creating toolpath geometry — and here lies the big difference. It is fairly easy to develop a geometry that looks right on the screen. It is not easy to create a toolpath geometry that guides the cutting tool and can often make or break a small shop.
In CAM programming, the purpose of geometry is not to recreate a paper drawing as many beginners try to do. Its purpose is to create a working toolpath. True, most CAM systems have hundreds of features that make the entire process simpler and faster. Features such as lead-in and lead-out, multiple contouring passes, segmentation of depth cuts, and stock allowances all contribute to a software that becomes easier and more efficient to use. Their inclusion requires another important skill — understanding the actual purpose of all these features. You must understand when to use them and how to use them correctly. You must also have a solid knowledge of machining techniques.
There are many times when even the best features of CAM software may not provide the desired result. Often it is necessary to add a bit of geometry here and there, a little extra tweak to get the exact result required. The part programmer should have an exact vision of the tool path in mind — before even starting to use the computer.
With all this in mind, what is a reasonable learning curve for using CAM software? Is the computer guru in the shop, who has minimal if any knowledge of machining, better suited for the task than the experienced machinist with limited computer knowledge? Most people will agree that learning how to operate and use a computer and software may take a few weeks at the most, whereas learning the art of machining takes years, even a lifetime.
The learning curve for any individual will also depend on other personal and professional skills. Even in CAM programming, there is always some math involved. There is also the entire area of understanding productivity,