CNC Trade Secrets. James Harvey
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imagine how anybody could machine parts simply by pressing buttons on a control panel.
When I signed up for a CNC programming course years ago, I asked the instructor how long he thought the average person would need to be able to set up and operate a CNC machine. His answer surprised me. He said “ a couple of days.” In my ignorance, I figured it would take at least a couple months of intense training. As it turns out, he was much closer to being correct than I was.
The other day while running a job on the CNC mill, I took some time to look over the list of G codes posted on the controller. To my surprise, I wasn’t familiar with a lot of the codes. I have literally programmed and run thousands of different parts through these machines. “How can that be?” I asked myself. The only logical conclusion I was able to come up was this: You don’t have to know “everything there is to know” to be able to make parts on these machines. It is sort of like using Microsoft Word. You don’t have to know everything there is to know about Microsoft Word to be able to write a letter.
This book is written from a machinist’s perspective. I work as a machinist in a small support shop. What I strive for and demand of myself is being able to accomplish whatever machining job comes through the door. If I don’t know some aspect of doing a job such as thread milling, I’ll learn it. I’m stubborn in that regard. I may forget what I learned a few weeks later, but that’s another story.
I have always preferred learning something new by doing a project. By doing a project you immediately to start to separate what is important from what is not.
The first chapter talks about planning jobs. The planning stage is where you may save countless hours of time and frustration later on. The second chapter is devoted to “work holding” which is often a challenging aspect of machining. The third chapter discusses ways to avoid crashes. Nobody likes to crash. The fourth chapter goes into some detail about applying edge dressings to parts. The fifth chapter discusses something dear to my heart, which is helping designers and engineers help us. The sixth, seventh, and eighth chapters are devoted to familiarizing yourself with CAD/CAM systems and G code. Once you get over the pain of learning a CAD/CAM system, you’ll never want to go back. In these chapters we’ll be modeling, programming, and machining a simple part. The methods applied to make this part are used over and over in shops and provide a solid base of knowledge from which to expand.
The ninth chapter provides an overview of the commonly used controls on CNC machines. The tenth chapter provides miscellaneous tips that may help you through your day in the shop.
The CNC machines and controllers discussed in this book are primarily Haas. The only reason for that is because Haas is what we use in our shop. The setup and programming procedures discussed are specifically for Hass machines, but will likely be useful in a generic sense for other brands of CNC machines. Nevertheless, if you don’t use a Haas right now, chances are you will in the future. At this time, Haas is outselling their competitors three to one.
Let’s get started.
When I began machining, CNC (computer numerical control) machines were just coming into mainstream use. The first few shops I worked in had only conventional machines. It wasn’t until the mid-1980s that I started to notice the incredible versatility of CNC machines. The contouring and shapes the machines could generate made many jobs much easier. The use of templates to file and sweep in surfaces was rapidly becoming a thing of the past.
Initially, I was intimidated by the technology. I couldn’t imagine how a person could machine parts simply by pushing buttons on a control panel. The mysterious code that ran the machines seemed to be the domain of computer people; it was seemingly beyond my comprehension. It wasn’t until later, when I took a class in programming, the mysteries started quickly dissolving.
As versatile as CNC machines are, they are only as good as the people programming and operating them. The cliché “garbage in garbage out” is well suited to CNC programming and machining. There is simply no substitute for proper planning and machining know-how. A programmer without much machining experience would likely struggle to produce good parts consistently.
One of the great virtues of CNC machines is that, while they are running, the machinists or operators can be working on other tasks. In essence, CNC machines take the labor out of machining.
Another virtue of CNC machines is that the cutting and measuring process so prevalent in conventional machining is virtually eliminated. If programmed and set up correctly, the cutter will go precisely where it should so that dimensional accuracy comes quickly. Usually only minor adjustments are needed to compensate for slight variations in tool size.
For anybody who wants to learn the machining trade, gaining experience on conventional machines has advantages, but it’s not entirely necessary. I was recently involved with teaching an inexperienced industrial engineer how to use our CAD/CAM system and CNC machines. Once he became familiar with the software and the different feeds and speeds that could be used with the various cutters and materials, he became an asset to our shop.
1.Initiate a new project with proper planning. (see Fig. 1-1 and Fig. 1-2)
It is one thing to create a beautiful model or drawing on a computer. It is quite another to figure out how you are going to hold the part for machining.
Planning involves a variety of decisions that have to be made before any material is cut. Table 1-1 lists many of the kinds of decisions that must be made.
Because of the infinite variety of machined parts — especially milled parts — it is somewhat difficult to categorize how to go about planning. There are enough similarities, however, that certain techniques can be used successfully time and again.
Figure 1-1 Parts are often easier to model than they are to machine.
| Table 1-1 Planning Decisions | |
| 1. | How is the part going to be held? Will a fixture be needed? |
| 2. | What sequence of setups will be used? |
| 3. | What size raw stock are you going to start with? |
| 4. | What size cutters will you choose and are they available? |
| 5. | Will squaring be done beforehand or in the program? |
| 6. | Is the part rigid or will flimsiness be an issue? |
| 7. | What feeds, speeds, and depth of cut will you choose? |
| 8. | Where are the G54 starting points going to be placed? |
| 9. | Will the part have to be roughed in first before finishing to avoid warping issues? |
Good planning is key to having jobs run smoothly. One paradox that machinists face is if a job runs smoothly, nobody pays much attention. Therefore, any time you planned and executed a job that runs smoothly, you should give yourself credit.
As a beginner, there will be no shortage of people willing to offer advice on what they think is the best way to run a job. It’s tough being a beginner. There are an infinite number of ways one could run a job; many have no advantage over the other. The bottom line? If a job runs smoothly, is in tolerance, and is completed in a reasonable amount of time, then you have accomplished your mission!
With that being said, it’s always humbling when you think you’ve planned a job well and it turns into a nightmare. I believe planning, programming, choosing speeds, feeds, and depth of cut before cutting anything are basically educated guesses. Yet, the more experience you gain, the better your guesses become.
