When decisions do not reflect your company’s identity, time, money and effort will be wasted.
Modern Machine Shop,
From the monthly column: CNC Tech Talk
Many factors contribute to “who” your company is. Just as individuals must make decisions based on who they are, people in a company must make decisions based on the company’s identity. Company type is the most important factor that contributes to a company’s identity. Another important factor is lot sizes. Most important decisions related to any job must begin with the question: “How many parts are we making?” The number of workpieces made during a single production run (which I am calling a lot size) as well as how many of these workpieces you run per year (the annual quantity) should dictate how several important decisions are made.
The more consistent these numbers are from job to job, the narrower your focus can be and the easier it is to make wise decisions. Unfortunately, most companies find quite a variance here. They might need to run anything from small lots with low annual quantities to large lots with high annual quantities. It is important to base your decisions on who your company is and focus on the majority of jobs you run. So, if 80 percent of your jobs are less than 10 parts and only run once or twice per year, then your decisions should be made according to this lot size.
Some obvious decisions relate to how workpieces are produced. Quantity dictates machine selection, cutting tools, workholding devices and cutting conditions. With small lots, just about any process that makes good parts is acceptable since jobs will not be in production very long. However, as quantities grow, it becomes more and more important to make good parts efficiently. Expenses related to more elaborate workholding devices, higher-quality cutting tools and edge materials, and more aggressive cutting conditions can be justified to machine workpieces more quickly.
Though selecting a workholding device is part of the machining process, I list the category separately because setup time is directly related to mounting and adjusting workholding devices. If you consistently run small lots, decisions should focus on achieving quicker turnaround time. For instance, you can easily justify subplates with quick-change-type component tooling because they shorten change-over time and eliminate program-zero assignment. As quantities grow, the emphasis should shift to machining multiple workpieces per cycle and/or shortening workpiece load time.
Distributed Numerical Control System
Smaller lot sizes mean that setups need to be made more often, and related tasks, such as loading and saving CNC programs, must be scrutinized. Setup people must be able to load and save programs quickly, and it should be easy to justify the cost of an automatic DNC system. These systems enable a setup person to load and save programs from the CNC machine’s control panel without having to walk to the DNC system computer. With larger lots, program loading and saving time will not be as critical. Even a laptop computer brought from an office may suffice for transferring programs.
Tasks Done Prior to Running a Job
The amount of preparation that can be done before a job is run is directly related to your average lot sizes and annual quantities. For example, there might not be much that can be done in preparation for upcoming jobs if your company is running small lots with short cycle times. A production run might be completed just minutes after it is started. However, even with small lots, if there is enough lead time (knowing when the lots will be run during the year in order to achieve annual quantities), then it might still be possible to do at least some preparation work.
Increased quantities increase production run times, and there is opportunity to do some, or all, of the preparation work. In this case, the goal should be to keep the setup person and operator at the machine for the entire time it takes to make the setup and complete the production run. That is, that person should not have to walk away from the machine to get a necessary component or perform a task that could have been done prior to the job coming to the machine.
Admittedly, we have barely scratched the surface of decisions that must reflect average lot sizes and annual quantities. At the very least, you should see how important it is to match these decisions to your own company’s situation.
Comments are reviewed by moderators before they appear to ensure they meet Modern Machine Shop’s submission guidelines.