401 Definition of Work Cells and Systems
The short answer is a group of machines organized to minimize or eliminate labor content while maximizing production and quality of one or several parts.
A work cell is an arrangement of machines to process a part from raw stock through several operations, ideally to the point at which it can be shipped or assembled. The way parts flow through the particular arrangement of machines in a work cell is what sets it apart from other methods of manufacturing. The machine for each subsequent operation is positioned so that parts go through without stopping or waiting in tubs and bins.
The biggest reason for using a work cell to make parts is that you get more through-put or production, which means you get more profits.
The term through-put will reoccur several places, it refers not to what the salesman said you can do (parts per hour) but rather what you did in actual production.
Flexibility in a work cell, particularly in a job shop, is very desirable. As the range in OD and length increases, so does the number of different part numbers that can be run through the work cell, which helps in justification.
With a manual cell this flexibility is relatively inexpensive. However, as systems get more complex flexibility gets very dear.
How does a work cell differ from conventional manufacturing methods? Many shops are divided by function into departments, like cut to length, end finishing, bending, etc... This kind of shop floor organization is the most flexible so far as work flow is concerned. Work flows from department to department as the process dictates.
Machine positions seldom need to be changed to accommodate any given part. This has worked out very well for many manufacturers but, between departments the work in process sits in tubs and bins while waiting. As it sits, it costs money in storage and inventory.
A work cell has very clearly defined paths that work flows through for either specific parts or for families of parts. The parts go directly to the next operation from the last with no waiting in baskets or the use of fork lift trucks to move the parts. A work cell is less flexible as a result.
Most people in the industry are familiar with the different kinds and makes of benders, end formers, robots, and automated material transfer devices now available. These machines and their CNC versions are the building blocks from which a work cell or system can be created. Many commonly used tube fabricating machines other than benders are now available in CNC versions, including:
Ram end formers with programmable pressure, length, and rate of advance, as well as programmable punch and clamp selection.
Spinning end formers with programmable reduction rates and shapes.
Cut to length machines which can alternate between different length parts individually or in groups.
For end forming, punch and clamp changes can be made during the run (3 hits with 3 different punches per part) or between runs (OD changes). It saves time to control these features through a computer rather than with a wrench.
CNC machines are preferred for use in work cells because set up is much faster and they should be more accurate than manual machines.
CNC Tube fabricating systems and work cells take many forms, starting with a group of individual machines that cycle automatically, and winding up with groups of automatic machines that work in synchronization with full communication through a master controller. These various forms break down into groups as follows:
Group 1- Machines that cycle automatically but require manual loading and unloading.
Group 2- Single dedicated (not flexible) machines that cycle automatically with separate automatic material handling.
Group 3- Flexible groups of automatic cycling machines with separate material handling arranged in a work cell.
Group 4- A group of machines dedicated to one part (not flexible), controlled from a central point with communication.
Group 5- A group of machines capable of making any part within a range of materials (flexible), controlled from a central point with communication.
There is a progression here towards increasing complexity.
The difference between a work cell and a system involves whether or not the machines in the work cell can communicate with each other. According to this, Groups 1, 2, and 3 are work cells, and groups 4, and 5 are systems.
The kind of information transferred in a system would include; part program, part in place, part not in place, working, ready for loading, ready for unloading, start, off line, and emergency stop among others. The controller of a machine (a machine in a system) should be capable of tracking the machine's operating condition relative to the performance of its’ function. If the machine can't do its' job because it has broken down, the controller should be able to determine that and send an off line signal.
A major reason for grouping machines together in a work cell is that the amount of goods in process can be reduced. If all the machines required to produce finished goods can be arranged in a work cell there will be no goods in process sitting around in bins in front of separate machines. Of course, the more machines involved, the better the chances are that one can get out of adjustment and cause the whole group to halt.
This is why it pays to have machines communicate. As work cells get more automatic, they require less human attention and are more productive. If something goes wrong in an unattended work cell that doesn't communicate, it can make piles of automatic junk very quickly. Conversely, lights will flash and bells will ring when something goes wrong in a system that does communicate.
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