Q: We are thinking of setting up a work cell to produce the parts enclosed. I have also enclosed some routings for these parts as they are run today. Do you have some recommendations on equipment and how to lay out this cell?
A: The writer sent some prints and routings to go with these parts, and a couple of phone calls back and forth allowed me to give her what help I could. However, the process reminded me of some of the key elements in designing work cells, so I thought I would share some of those key concepts in this month's article.
Let's start with some work cell basics. A work cell is a group of machines located closely together for the purpose of producing a certain item or family of items. This item can be a single part, a subassembly, or a complete assembly. The machines in a work cell are normally arranged in a "U" configuration to reduce the distance that has to be traveled from one operation to the next.
To be clear, let's look at a simple example. Say that we want to set up a work cell to produce a corner block.
The corner block starts as a random length of material, already ripped to width. This stock is brought into the work cell where the following operations are performed on it:
- The ends are mitered to 45 degrees.
- Two holes are drilled perpendicular to the angle cuts.
- A third hole is drilled from top to bottom.
In a normal production plant, this part would be mitered on a double end miter saw, then bored on one or two multi-spindle boring machines. However, we have decided to make these parts in a work cell. This is the layout I chose for my work cell.
The operations are:
1. The random length material is brought to a miter saw and the trailing edge is mitered.
2. The part moves to a second miter saw where it is mitered to length.
3. The top hole is bored.
4. One horizontal hole is bored.
5. Second horizontal hole is bored.
6. Completed part moves out of the cell.
What would be the advantages of doing production in this manner?
- There is no setup involved in this cell. Each machine is permanently set up to perform its task.
- The cost to setup such a cell would be low. In this example we would need two miter saws, three single spindle boring machines, and the workbenches for them. The total cost for such a cell would be less than $2,000.
- The production level of the cell is very flexible. By adding people to the cell, you can quickly increase production if demand changes.
- Each step of the operation is fairly simple, so the training required is minimal.
There are also cases where a work cell is advisable for new product introductions. Say that you had to produce a new part, product, etc. that involved a number of complex shaping operations. You felt you were at maximum on your present shaping capacity and were going to have to look at buying additional capability. In the traditional mindset, we would probably hit the trade shows looking at CNC equipment. However, suppose that the new product is something of a market risk, and we didn't want to sink a lot of money into new equipment to support until we knew the product sales would support it.
We could set up a work cell with unused equipment, smaller "shop" style machines, and used items. We would set it up with multiple shapers, for example, that would each do a small portion of the work. Then, we have only one person in the cell, moving from station to station. As sales increase, we continue to add people. Once the product line has proven itself, we can then decide whether or not to make the investment in new equipment or in an additional work cell.
What if the product line dies? Then you use the equipment in other work cells in the plant, and your lost investment is kept to a minimum.
Work cells even provide the opportunity to improve quality. To see how, let's go back to our block example earlier. Say that we were having a problem with the width of the block being correct. We find that the problem is caused by slight movement of the piece as we make the second cut. We might change our operations so that, after the first miter cut is made, we then bore the top hole. Then, we could put a pin on the fixture for the second miter cut that would fit in the hole. This would help hold the piece in place and ensure that the part was the correct width. (See drawing on page 23.)
The work cell concept also works very well with subassemblies that have to feed into a main line. I have worked at companies where we made pedestals for desks in a work cell and fed them directly to the line. This work cell not only included assembling of the pedestals, but went all the way back through boring and attaching drawer slides, notching for toe kicks, and even edgebanding of the panels. It had some drawbacks, certainly, but we proved it could be done in a production setting with only one piece of inventory between each station.
A key factor to keep in mind is to design your work cell so that the flow of parts is equal to demand, not the capacity. Because of the inevitable variations that occur in a production environment, trying to match the capacity with demand is doomed to failure. Instead, make sure that the parts leave the work cell at a rate that will meet customer demand. As the demand fluctuates over time, you can adjust the manning of the work cell to compensate for it.
Work cells are one way to improve production flexibility and another is in the building of specialty design equipment.
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