Increasing Shop Productivity with a Portable CNC Machine
By R. Leonhard
The challenge of organizing a small to medium size machine shop for optimal productivity is one that must be continuously overcome in order to succeed in today’s harshly competitive environment. Just how should a shop be arranged to remain profitable when skilled manufacturing labor is scarce and expensive and machines are relatively inexpensive on a cost-per-year basis?
In large companies and shops where parts, or families of parts, are made in large volumes, machines are often organized in cells. This allows one machinist to load, monitor, and check the output of several machines thereby spreading the expensive cost of one person over the output of several machines. That is, it maximizes the “spindle minutes’ (that make the parts) to the “labor minutes” (that cost the money) ratio. The higher this ratio, the higher the productivity and the lower the cost.
In smaller manufacturing shops and job shops that usually run a wide variety of parts in small to moderate lot sizes, this cell optimization is generally impractical. So most often a job is broken down into several independent operations where all parts go through one operation on one machine then another operation on another (or the same) machine, and so on until the job is complete. The job must be organized, scheduled, planned, set-up, tracked and followed up several times – lots of expensive overhead labor. And worse, during each operation the machinist stands idle or engages in low value-added work waiting for the machine to finish its cycle. In other words, expensive labor waiting for inexpensive machines. The result is a poor ratio of “spindle minutes” to “labor minutes”.
Before exploring solutions to the above problem, it is important to understand the nature of secondary operation machining. Few parts can be machined on a machining or turning center in one operation. Even in 5-axis milling, the part must be held on the sixth side and finished as a second operation. Often secondary operations are the first operation, for example machining a flat reference surface on a casting or facing off a part before it is chucked and turned. Most shops tackle their secondary work by adding an operation on a machining or turning center or by using manual machines. The former ties up an important primary machine and consumes labor and the latter is very labor intensive and risks operator error.
Optimally, a smaller shop would like to have all its machines organized to run a particular job just like the big, large lot size companies do in their cells. But with today’s machining and turning centers this is impossible since every day and every job is different and reorganizing substantial machines in the shop day by day is clearly unrealistic. A practical yet unique solution is to organize primary machine and turning centers in small clusters or cells of one or two machines and leave a little space to bring small portable secondary machines into the cell as needed for a particular job. In this way additional spindles are brought in to complete the job and maximize output. Theoretically, more and more secondary spindles can be added until the machinist’s total work minutes to tend the spindles reaches the longest cycle time of any of the spindles. An additional benefit of this strategy is that it combines operations and thereby lowers scheduling, organizing, inspection, expediting, etc. costs.
The portability of the secondary machines minimizes the shop’s capital costs as well. If they were not portable, more would be required to be used where ever they were needed. Portability allows them to be moved to where they can be used with the most efficacy. Some jobs may be able to be completely machined on the portable secondary machines. In that case bring the machines and work to any machinist who is running a machine with a cycle time that required him to wait. He may be able to do the additional job with no additional labor (and therefore labor cost).
Spreading a machinist’s time over more spindles reduces the labor content per part and therefore its cost. A second benefit of this strategy is that fewer machinists are required for any given level of work. This means minimizing the need to hire and train more machinists – a task that is long, hard and expensive.
To be effective, portable CNC second operation machines must:
- Be easy to move and use
- Have a small footprint
- Have a reasonable work envelope
- Be robust
- Be available as a CNC mill and CNC lathe
Smaller machine shops can become more competitive by embracing new ideas to increase their productivity. Portable CNC second operation machines, integrated into primary machining and turning center work, can provide that productivity. ME