Wednesday, December 9, 2009
Diving Deeper - Green at the Process Level (Last of a 2 Part Series)
The more information you have the easier it is to understand what is going on and what you should do next. This simple statement lays out the basic strategy to green manufacturing - at any level. If you recall the major improvements (or leaps forward) in manufacturing we spoke about in an earlier posting (on July 27th to be precise - see http://green-manufacturing.blogspot.com/2009/07/why-green-manufacturing-part-of-next.html) you will remember that each of these "leaps" was based on observation of the process from a new perspective, data to document what was being observed and then a plan of improvement built on that observation and data. That's diving deeper in to the process at many different levels.
Last time we identified two distinct modes of performance of a manufacturing process (and, of course, there will be exceptions but in general this is a reasonable classification) that distinguished between processes (or machine or tools). One mode was where the process energy dominates or, at least, is a significant component of consumption relative to tare energy, and another where the tare power dominates. And, depending on which "mode" you are in will determine what potential approaches you'll take to reduce energy, or consumption, during the process. The table below summarizes this (and recall that where the tare energy dominates, Et >> Ep and, when, Ep >> Et, process energy is much greater than tare energy.)
Depending on the units of measure (here meaning power or energy per unit of time or per unit of production) our strategies may be somewhat different, but, in general, our strategy for the tare dominant operation vs the process dominant operation are as shown in the table. I'll define some of the other terms as we go along.
For the operation of the machine under the tare dominant mode we should try to make the cycle time on the machine as short as possible (cycle time is tc) which will give the part produced the lowest energy footprint from the process. The machine itself (operation with out process) we need to look at ways to reduce the tare consumption. For a numerically controlled machine tool (following our milling example from part 1 of this posting) with reasonable precision we know that the main sources of energy consumption are related to the power for the controller itself, the control panel of the machine, rotating the spindle, table motion, and coolant pump. In fact, the coolant pump is a big one since machines of this quality need to be kept thermally stable to avoid thermal distortion. So idling the machine controller between process steps and finding a way to keep the spindle thermally stable (material? design? air cooling?) without the use of the coolant pump would be first on the list.
For operation under the process dominant mode we focus on optimizing the process itself. You may recall a posting on the 29th September on "Greening the factory floor: (see http://green-manufacturing.blogspot.com/2009_09_01_archive.html). In that I distinguished between different levels of machine operation from the "microplan" (the particular speeds, feeds, depths of cut (for a machining process) and tooling required to accomplish the operation on the machine), the "macroplan" (process sequence which represents the order in which the operations are carried out with requirements for "what comes first") and the machine tool or system of machines itself. Our interest here is on the first two, micro and macroplan.
For the microplan we know from research and experience that the choice of process settings will impact energy and resource use. It follows then that the correct choice will yield reduced energy consumption. This would be, for milling, the cutting speed (rate of rotation of the tool in the spindle translated to peripheral velocity), the feed rate (rate of advancement of the cutter through the work) and type of tools used (for example, material type and any coating to reduce friction, resist temperature, etc.)
For the microplan (and still speaking about process energy) the process sequence level determines the path that a cutting tool takes across the workpiece and the sequence of operations. Machines use more or less energy depending on how their axes move, accelerate and decelerate, how many times the spindle starts and stops, tools are changed, etc. So sequence and paths can have a big effect.
As an example, consider a workpiece that requires motion of the machine table in two directions (two orthogonal axes) to produce. Usually, the machine is built with one of these axes stacked at a right angle on top of another - like a sandwich. The workpiece is fixed to a table on the top axis. But, if I need to move that workpiece in the direction of the bottom axis (that is at a right angle to the direction of the top axis) I need to move the bottom axis in the correct direction which also carries the top axis. Not surprisingly, it takes a lot more energy to move one carrying the other than to move the top axis by itself. So, a workpiece which has a lot of features that need to be machined using the bottom axis motion will consume more process energy than one that doesn't.
Make sense? So by either position the workpiece the table so that the maximum "top axis" features can be machined or, at least, adjusting the tool motion to incorporate as many top axis moves as possible as the tool sweeps over the part we can substantially reduce energy consumption. And that is both on a per part and a per unit time basis.
Machine "warmup" is also a big issue that requires the machine to operate much longer than needed for the specific process at the start of the production run. Strategies for minimizing that vary from thermal insensitive materials to special process plans that use the relative inaccuracy of the machine when it is "cold" to work on less accurate sections of the workpiece or for roughing cuts.
Embedded energy was a factor in either mode of operation since it accounts for the energy and materials used to build the machine in the first place. Here, we'd need to look at selection of materials for the machine (specially trade-off between "low embedded energy" materials and those that meet the structural or thermal requirements of the machine design) as well as ease of recycling or reuse of components, etc.
Whatever level of scrutiny we apply to the manufacturing process we can find potential for improving the energy or resource performance of the process or machine or system.
Next time we'll discuss some ideas about "line balancing" and the potential advantages of machines that do more than one process.
The webinar on Thursday, December 10, 2009 on "Built to Last: Sustainable Manufacturing" is history. Go to the Future State Solutions website to see the archived webinar (http://futurestatesolutions.com/.) There will be a follow-on webinar on "Sustainable Manufacturing: The Details You Need to Know" that I will participate in on Tuesday the 14th December. Go to http://bit.ly/91eAfO to register.