pipes, plants, & process
points are the authentic contact points of the tool with the part. The TCP algorithm requires exact machine dimensions in relation to the rotary axes be entered into the CNC system. These dimensions are measured using probing or laser tools instead of the dimensions on the mechanical prints. This is known as “defi ning the kinematic.” The tool lengths and diameters also need to be accurately entered into the CNC. Any shortcuts, like entering zero tool lengths or zero gage lengths into the CNC for dimensions, rarely work out well and severely limit fl exibility and effi ciency.
Stop Using Inverse Time! With inverse time (G93), the CNC guarantees all machine
axes will reach their programmed positions at the same time as defi ned by the F-word in the part program for every programmed point. This is not true simultaneous motion, and it limits the capabilities of machines because the expected cycle time is effectively hard coded into the program. By using part coordinates and TCP algorithms to defi ne the part
Using this programming process, the geometry section of a particular part program will be identical, regardless of ma- chine. For machine-specifi c functions, like tool changes and M-codes to enable/disable specifi c machine devices, macros with common names are used on every machine to do these machine-specifi c functions. The foremost CAD/CAM systems have ways to create logical functions for the part programmer to select which ones are translated by a simple postproces- sor function into machine specifi c calls.
Establishing Control After defi ning, implementing, and enforcing the part
programming process, it needs to be clearly defi ned and communicated to any potential equipment supplier. The best approach is to have sample part programs showing the expected process. The example programs need to run and produce real parts for process testing. Every facility follows the standard circle-diamond-square test and might include a tilted cone as defi ned in the NAS specifi cations developed decades ago. These tests are designed to check mechanical capabilities and alignments; however, neither test does well enough in defi ning the programming expectations. Every new equipment purchase must include an example
test program designed to show the expected program- ming method and verify that all of the required features and functions have been enabled and properly confi gured by the supplier. For additional functions like probing there should be an example test program showing the expected probing functionality and offsets.
Support
Generating programs in part coordinates leads to programs that are machine independent.
and the tool relationship to the part Feed Per Minute (G94) can be used instead of inverse time. With Feed Per Minute, the part program will have fewer F-words programmed and the value of the F-word describes the expected tool tip speed through the material. The optimized parameters of the servo system and CNC will adjust the actual speed as required in order to sustain accuracy.
The CNC experts are the key resource for defi ning and setting machine-independent processes to drive application consistency. The CNC system is the “brain,” interpreting and executing the commands necessary to create a part. It is imperative a CNC expert is an active participant in any equip- ment engineering or specifi cation team. Improving consistency at a facility means having to
adopt processes independent of machines and to make those processes the requirements for any new equip- ment. A quality CNC partner will invest the time and money necessary to create the specifi cation and related tests to ensure you are on a path toward consistency, cost savings and fl exibility.
44 — Energy Manufacturing 2016
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