Judging Acceptable Spindle Run Time

What is an acceptable percentage of spindle run time for metalcutting CNC machines such as mills and lathes?

Columns From: 2/17/2009 Modern Machine Shop,

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Mike Lynch

Mike Lynch

A question that I often hear about CNC machine utilization typically boils down to: “What is an acceptable percentage of spindle run time for metalcutting CNC machines such as mills and lathes?” My general answer is: “You must achieve a spindle run-time percentage that is appropriate and feasible for your company’s needs.”

While this answer may be a little too generic, possibility and feasibility don’t always go together. Anything is possible, but not everything is feasible. Justifying a change that improves spindle run time may not always be possible given your company’s application for the CNC machine(s) in question.

Companies vary when it comes to the percentage of time that spindles should be running. In some cases, especially companies with large product lots, it may be feasible to justify doing almost anything to achieve more than 95 percent spindle run time. On the other end of the spectrum, some companies may be satisfied with machines that have less than 20 percent spindle run time.

I must admit, I’ve always linked the level of CNC machine utilization to the percentage of time that spindles are actually running. It seems obvious to me that you’re only making money when your machines are in cycle. I’ve always felt that a better-utilized CNC machine has a higher percentage of spindle run time.
While I still hold firm beliefs in this regard, there are many good applications for CNC machine tools that do not require the utmost in spindle run time. Many companies don’t need or desire to run their CNC machines at maximum output capacity. Consider the following examples:

Manufacturing Cells
With a manufacturing cell, machining a completed workpiece or assembly in a timely fashion is a priority. The CNC machine in the cell may perform one of several processes done on the workpiece or assembly. Other processes in the cell may not even be related to CNC or machining. Unless the CNC process is the constraint for the cell (taking the longest time to complete) and unless the cell is a constraint for the company (more parts or assemblies are required than the cell is currently outputting), there is no need to improve the CNC machine’s productivity. A CNC machine in the cell may sit idle for long periods of time and still be appropriately utilized.

Broader Company Picture
This cell concept can be expanded to a company’s overall performance. While the company may not use manufacturing cells to complete workpieces, a given CNC machine may not be a constraint for the company’s throughput. If the CNC machine is achieving the required production volume, there will be no need to improve it.

Lean Manufacturing
More and more companies are putting the emphasis on getting as much from their machine tools as possible with a limited number of people and resources. While I sometimes question the specific techniques used to accomplish lean manufacturing, this manufacturing concept—when properly applied—can minimize costs while allowing
the company to achieve required production
volumes.

In a lean company, it is not uncommon for CNC machines to sit idle. Decreased machine utilization and increased personnel utilization is an easily accepted compromise in the overall process of reducing costs. Though I’m over-simplifying, my point is that as long as the
lean company is meeting production schedules, this compromise in machine utilization is
acceptable.

Tool Rooms, Laboratories And Research And Development Departments
CNC machines are not always placed in production departments. They are often used as support machinery, not part of the company’s product-producing environment. It is not uncommon for machines in this situation to sit idle for long periods of time. 

Staffing Problems
Some companies can’t find, hire and keep enough people to fully attend their CNC machines. There just aren’t enough qualified (or interested) people in the area who are willing to work for the wages the company can afford to pay. When there aren’t enough people to run CNC machines, the machines are going to sit idle—even when there is a need to run them.

When faced with staffing problems, many companies turn to lean manufacturing to improve. If lean manufacturing principles are appropriately applied, the company may be able to find an acceptable balance between a reduced workforce and required production volume.

Unfortunately, some companies don’t embrace or understand the principles of lean manufacturing fully enough to achieve its benefits. When faced with staffing problems, they simply expect their employees to do more with less. They call that lean manufacturing. This becomes a vicious circle. Overworked people will leave the company for greener pastures, causing even more staffing problems.

So what are your specific needs?

My general suggestion is to consider constraints in your company. If you are meeting or exceeding output goals, you should be pretty comfortable with spindle run-time percentage. If not, look for constraints. Improve those machines that are causing the bottlenecks.  

Also, keep an eye on your future needs. A great deal has changed in the manufacturing environment in the last 10 years. As you see a drop in lot sizes, reduced lead times, higher quality requirements and the demand for better quality reporting, your current methods and processes will have to improve. It wouldn’t hurt to determine those areas in your company that are marginal. By knowing those areas, you can think about how to improve before the need arises.
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