Modern Machine Shop

SEP 2018

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MMS SEPTEMBER 2018 64 CAD/CAM SOFTWARE individual machine or job. However, SmartCAM is distinct in that it was developed to be highly customizable, he notes. Its postprocessor (or "code generator" in this company's terminology) follows logic that is accessible and revisable by the user, and he reports seeing users increasingly applying this access to refine the cycle time esti- mates according to user-defined logic to get more accurate cycle-time predictions. "Cycle times are important because of the direct effect on cost," he notes. Why are CAM cycle time estimates inaccurate? Machine acceleration is a significant reason. The published rapid traverse rate for a machine is an ideal that many machining cycles fail to achieve. In a drilling cycle involving hundreds of holes, one SmartCAM user seeking to learn the pro- gram's actual rapid rate timed the machining, cal- culating an actual average rapid traverse rate of 2 m/min. The machine's nominal rate was 10 m/ min. The acceleration/deceleration rates deter- mining the difference between these two values are not always known, particularly in the case of older machines, and they may vary for different axes or different workpiece weights. Similarly, spindle ramp-up/ramp-down time is a factor affecting real cycle time. So are varia- tions in tool change time. CAM software often provides the option for including acc/dec values in cycle time estimates and might also include the option to apply a fudge factor to these estimates, Mr. Antrobus says. SmartCAM carries this further by allowing the user to write conditional statements into the postprocessor logic that apply different acc/dec rates and/or different fudge factors to different types of cuts. For example, if an arc machining pass is consistently slower than expected on a given machine, conditional statements can achieve different, more accurate cycle time esti- mates for those portions of every cut. Of course, learning where these refinements are needed requires an investment in itself. "But if cycle time is critical, it is worth spending some time to get it as accurate as possible," he says. One shop he knows that lacked acceleration data on its machines ran cuts of known length with a stopwatch to measure these values. A similarly valuable approach is to apply the stopwatch to every machining cycle, searching for jobs show- ing an actual cycle time departing from the pre- dicted time by a significant margin. These jobs can then be diagnosed to determine which parts of the cutting played out in significantly more or less time (likely more) than was predicted. An investment such as this is increasingly warranted, he says. Competitive pressures or mandated cost reductions from OEMs push profit margins tighter, making it all the more important for the shop to know its machining requirements precisely. Mr. Antrobus points to these advantages to be won from more accurate CAM predictions: • Competitive quoting. Accurate cycle time esti- mates can reduce the shop's "safety factor" in its quoting, resulting in more competitive bids. • Capacity utilization. A similar safety factor might apply to scheduling. More accurate cycle time predictions mean more precise scheduling, which might in turn mean more parts per day scheduled on existing equipment. • Managing expensive tools. High-end cutting tools capable of machining hard metals might be short-lived. Knowing the cycle time might be the difference between trusting one tool to per- form the cut versus the added cost of changing to a second tool in mid-cycle for safety. • Process analysis. In cases when either of two different processing strategies might make sense for machining the part, cycle time esti- mates for the options might not help the evalu- ation if there is high uncertainty around those estimates. Precise time predictions offer a way to identify the best method. • Unattended production. Running lights-out in many shops means completing a given machin- ing cycle within the six to eight hours when no one is in the shop. If cycle time prediction is accurate, then the machining cycle can be pre- cisely fitted to the available time. A very long program could be broken into multiple programs so that just the right amount of the job can be done within the available lights-out window. The predictability of a process is arguably even more valuable than its productivity. | SmartCAMcnc | 541-344-4563 |

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