‘Greening’ the Factory Floor
At the higher levels in the manufacturing enterprise the facilities (office building and plant HVAC, administrative, employee services like cafeterias, etc., packaging and shipping) dominate.
As we move down through the production systems and their associated consumables (with their energy, water, materials, compressed air, part and material delivery and removal, maintenance, etc.) and further down to the machine and process level (with the tooling, both work holding and process tools, machine operation, resources and consumables, part and material handling, etc.) we want to be aware of every aspect of production for applying our wedges. But, remember, planting grass on your plant roof should not be the end of your green manufacturing initiative.
That accounts for energy, material and other resource use in the direct production of a product. If we are heavily dependent on our supply chain we should take that into consideration as well although at present it may not seem to impact directly our energy or other resource consumption.
Here we’d like to take a look closer to the factory floor for opportunities to employ some of these wedges – where the “tool hits the metal” one might say.
We can generally identify four distinct levels of influence with respect to greening production at this machine to part level – the system of machines making up a production line or cell, the individual machine tool or production machine itself, the overall operation of the machine and the detailed operation of the machine in production.
Let me elaborate starting with the lowest level (and we’ll rely on a machining analogy here):
1. Detailed machine operation (often called the process “microplan”) represents the particular speeds, feeds, depths of cut (for a machining process) and tooling required to accomplish the operation on the machine.
2. “Macroplan” or process sequence which represents the order in which the operations are carried out with requirements for “what comes first.”
3. Machine tool or production machine which represents the hardware (iron, actuators/motors and electronics) that provide the energy and coordinated motion for accomplishing the process sequence at the desired microplan.
4. The system, or assemblage of machines that comprise the production line or cell.
At the lowest level, we know that the shape transforming operation that occurs (forging, machining, grinding, rolling, etc.) will consume energy and resources to convert the incoming material to a new configuration. 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. The term “specific energy” is often used here – implying how much energy is needed to accomplish a shape transformation of a specific volume or, if we include the rate of transformation, the specific power.
Usually, very small volumes of material transformed require larger amounts of energy than would be proportionally determined from a larger volume due to inefficiencies (that’s why grinding heats up the metal more than a similarly scaled cutting process – the grinding removal is less efficient due to the small amount of material removed by each grain).
Moving up a notch, 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 matter.
The machine or production tool is the next level. (A word of caution – for some reason the words “tool and tooling” are used to represent a number of different elements in manufacturing. The machine is often referred to as a machine tool. The hardware that holds the part on the machine table during machining is a fixture or tooling. And the unique bit used to actually remove material — a drill or milling cutter, for example — is also called a tool. I don’t know why. It’s just that way.)
The machine tool has both embedded energy and resources resulting from its manufacture (as does the fixture and and other tooling). It also consumes energy and resources during operation. In addition to moving the axes and rotating the spindle there is energy consumed in the tool changer (providing the correct cutting tool from step to step), powering the computer controller, rotating the spindle, lubricating the moving parts and removing the heat generated during operation (to keep the machine from changing shape and losing precision during operation), mechanisms to remove chips, provide cutting fluid or mist and so on.
We can envision improvements at all these four levels beyond that considered as part of continuous improvement:
- The machine tool can be designed to be more thermally stable without the use of complex cooling systems, materials with higher stiffness and damping per unit of embedded energy of production chosen, hydraulics and spindles configured for reduced energy, component configured for re-use or remanufacture after their end of life, etc.
- Operation of the system of machines can be optimized to better balance the power load over the individual machine cycles to avoid peak demand, multi-purpose machines might be used in place of a series of discrete machines to eliminate material handling and consumables and duplicate operations, power might be harvested from one machine to offset the needs of another as spindles are stopped for tool change, for example.
- The process operation level could benefit from work holding and work orientation for minimum energy machining, process sequencing for minimum energy and consumables use finishing, etc.
- At the lowest level in the chain, the process planning level, machining feeds and speeds might be chosen for minimum energy machining or roughing and finishing designed for for minimum energy, consumables, and finishing. We might try optimized tool paths for high productivity and minimum energy.
We’ve tried many of these ideas – they work. And they are scalable and extensible to other processes.
David Dornfeld is the Will C. Hall Family Chair in Engineering in Mechanical Engineering at University of California Berkeley. He leads the Laboratory for Manufacturing and Sustainability (LMAS), and he writes the Green Manufacturing blog.
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