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However, in the case of parts produced on well-maintained computer numerical controlled (CNC) equipment, nearly all problems that result in deviation from specification can be eliminated at the front end of the manufacturing process using powerful analytical tools available in the computer aided manufacturing (CAM) software. This is not only the smart thing to do; it is also the essential thing in many industries where short runs mean that a single defective part could prevent a project from being profitable.
Analyze the ModelAn ounce of prevention is worth a pound of cure. That is why good CAM systems provide users with a number of tools for analyzing models. Sometimes the model itself is inaccurate. It is good to know this up front to avoid hours of fruitless programming. With access to a print you can verify the model in a matter of minutes.
If you are modifying an existing part, it is also important to know that your current CAM program is based on the latest version of the model. Some CAM software has tools that will check the program you are about to modify against the most current CAD model to make sure that all of the model changes have been incorporated into the CAM program.
Once the model has been verified, there are other CAM tools that allow the user to bounce around the model quickly to detect such things as flats, radii and draft angles. This makes it possible to decide on the best machining strategies and the most appropriate tools in advance. Five minutes spent using these tools can later save hours of backtracking through your program to correct problems that were based on false assumptions.
Define Cutting ToolsA lot of shops might be roughing or semi-finishing with the tools they have been using for a while. Of course, using an undersized tool will leave a little extra material at this stage and that could introduce problems later particularly if the material is very hard.
If your half-inch mill actually measures 0.492 your results will automatically be off by .008 inches before you cut anything. To avoid this problem you need to get in the habit of actually measuring the tools you want to use with a micrometer or shadow graph and plug in real numbers instead of nominals.
This is really important for high-precision work and micromanufacturing, where discrepancies between nominal tool definitions and actual values can dramatically magnify dimensional errors. If you peek into the tool library of CAM programs for micromanufacturing you will see all kinds of numbers that rarely end in zeros. These shops know the difference this little step can make.
Manage CAM Tolerance StackupsNearly everyone is familiar with tolerance stackups that relate to CNC machines. Fewer people understand that CAM programs themselves have tolerances that can also lead to stacking errors. The error budget for a toolpath is the sum of the individual tolerances for the lines, arc filters and smoothing interventions used in it.
You should familiarize yourself with how your CAM system deals with tolerance stackups. Some will actually allow the user to adjust and balance stack-ups so that tolerances are never exceeded and you can have total confidence in your programs.
Inspect Virtual PartsOnce you have created a CAM program to manufacture a part on a particular CNC machine, you can actually create a simulation that uses a color-coding on the 3-D model to show where too little or too much stock will be removed from the part.
This gross check will identify most stock removal errors, but some CAM systems allow users to inspect their virtual parts in much finer detail. Users can employ search tools to instantly locate and measure microscopic flaws-those needles in a haystack that can cause close tolerance parts to fail inspection. These flaws can now be corrected in the CAM software prior to manufacturing.
Virtual inspection is also a great way to reconfirm the integrity of features that may not be accessible to measurement unless they are cross-sectioned. This may make it possible to reduce the required frequency of time-consuming and costly cross-sectioning procedures.
Simulate MachiningComputer simulation is also used to show the programmer that his toolpaths will run safely. In other words, there will be no collisions between the tool or tool holder and the part, which could break the tool, damage the machine or compromise the part. This simple form of collision checking is used routinely by most CAM system users throughout the programming process.
In addition, some CAM programs allow for full machine simulation that will not only detect all the aforementioned collisions but also ones involving fixturing, the machine table and/or other machined components in a set-up. These simulation capabilities are particularly comforting for those who are programming multi-axis CNC equipment and need to confirm that the setup is indeed safe.
Refine the QuoteThe engineers who prepare quotes can use a combination of past experience and analytical tools in CAM to prepare competitive quotes based more on information than on optimism. The tools used for analyzing the model prior to programming can also be very instructive to the engineer preparing the quote.
Machine simulation can provide a close approximation of part cycle times. For short runs, meticulous virtual collision checking can give machinists courage to not stand over the machine running the program slowly to make sure there will be no crashes. As a result, cost factors relating to being overly cautious may be eliminated.
Abandon HopeMost machinists are familiar with the sinking feeling in the pit of their stomachs when they run a part program for the first time. They hope it is not going to crash. They hope a tool doesn’t break. They hope that the part will not be scrap. Hope is not a viable strategy for invariably producing quality parts the first time out. It should be abandoned in favor of analytical knowledge.
The analytical tools available on good CAM systems make it possible to detect and eliminate essentially all of the part programming risk factors. As long as you trust your post-processor, there is no reason not to have total confidence in the code you are sending to your CNC machine. The machinist can verify his setup, press the green “go” button and get on with his next task.