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fixtured on a machine without reposting. Part coordinates also are the foundation of making programs that can run on any machine. An example of an offset that can be used to define this part coordinate system is the FANUC G54.4 WSEC offset.


Programming Four or More Axes For four, five, or more axis machines, the rotary axes that change the tool and/or part orientation can be tricky to program. In the past, four or more axis machines required functions like “inverse time” and complex postprocessors to do the trigonometry so the tool ends up in the correct lo- cation. Modern CNC’s have functions typically referred to as Tool Center Point control or simply “TCP.” For reference, on a FANUC control, TCP is G43.4 or G43.5 depending on how you want to define the tool orientation. TCP algorithms put all the required math in the CNC instead of the postprocessor so the part program points are the actual contact point of the tool with the part. The TCP algorithm requires that the machine dimensions relat- ed to the rotary axes be accurately entered into the CNC. The dimensions entered in the CNC should be the exact measured dimensions rather than the dimensions on the mechanical prints. In CNC terminology, this is known as “defining the kinematic.” The tool lengths and diameters also need to be accurately entered into the CNC. Since CNCs only do what the program and CNC parameters describe, lying to a CNC by entering zero tool lengths or zero gage lengths never works out well in the end and severely limits flexibility and efficiency.


Inverse Time is Bad


With inverse time (G93), the program tells the CNC that all machine axes should reach their programmed positions at the same time as defined by the F-word in the part program for every programmed point. This is not true simultaneous motion and this artificially limits the capabilities of machines because the expected cycle time is effectively hard-coded into the program. With part coor- dinates and TCP algorithms defining the part and the tool relationship to the part, Feed Per Minute (G94) should be used instead of inverse time. With Feed Per Minute, the part program will only have a few F-words programmed and the value of the F-word describes the expected tool tip speed through the material. The parameterization of the servo system and CNC will adjust the actual speed as necessary to maintain accuracy.


With this programming process, the geometry section of a particular part program is the same regardless of what machine is used. For the machine specific functions like tool changes and M-codes to enable/disable specific machine devices, macros with common names can be used on every machine to do the machine specific func- tions. The major CAD/CAM systems have ways to create logical functions for the part programmer to select that are translated by a simple postprocessor function into machine specific calls.


Taking Control—and Getting Assistance After defining, implementing, and enforcing a good


part programming process, the process needs to be clearly defined and communicated to any potential equip- ment supplier. The best way to clearly define the expected process is to have example part programs that show the expected process. The example programs should be able to run and produce real parts for process testing. Every facility does a circle-diamond-square and might include a tilted cone as defined in the NAS specifications developed several decades ago. These parts are designed to check mechanical capabilities and alignments. Neither part does a good job of defining the programming expectations. At a minimum, every new equipment specification should include one example test program that is designed to show the expected programming method and verify that all the features and functions required are enabled and properly configured. If additional functions like prob- ing are planned, there should be an example test program that shows the expected probe functions and the ex- pected offsets. The best resources to help define and set up machine independent processes that will help drive application consistency are the CNC experts. Since the CNC is the brain of the machine and the device that interprets and executes the commands necessary to produce a part, make sure a CNC expert is included as a member of any equipment engineering or specification team. The only way to improve consistency at a facility is to


adopt processes that are machine-independent and make those processes requirements for any new equipment. A quality partner and CNC vendor will invest the time and money to help develop the necessary specification and related tests to help start down the path of consistency and the associated cost savings and flexibility.


89 — Aerospace & Defense Manufacturing 2015


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