Making parts faster doesn’t always mean cheaper

By Mark Bos

In today’s difficult economy, we are all trying to make parts faster and cheaper. Unfortunately, making parts faster is sometimes at odds with making them cheaper.

I have learned from experience that sometimes a machine will consume less money if you slow it down a little and try to find the proverbial “sweet spot.” The fastest spindle speeds and the highest feed rates may not be the best way to run the machine. It may be difficult to convince your boss that this is true, but having real data to prove it can be helpful.

Tooling costs or machine repair costs are examples of costs that could go up significantly if the machine is pushed too hard when trying to reduce cycle time. When I get involved in a process, I like to track tool life. If I don’t know what my tool life was before I make a change, I can’t accurately measure the performance gains or losses caused by the change. I always share my tool log results with the machine operator, as part of his or her involvement in the process improvement.

The following is a good example of slowing a machine down to get better performance. The part was made on an Index ABC lathe. This machine was plunge-roughing the OD of a cylindrical part made of 8620 steel, using a .3” wide carbide insert-type OD roughing tool. The tool had problems with durability. The average tool life for this tool was little more than 200 hits per edge. When the edge went bad, it happened quickly and would cause problems for the finishing tool that followed it. This roughing tool is one of 11 tools used to make this part on this machine.

The first thing I checked was feeds & speeds. The actual surface speed for the roughing tool was 1100 ft/min, while the recommended surface speed range, per the insert manufacture’s catalog, was 250 to 600ft/min. After reducing the spindle speed by 50 percent to obtain a more suitable surface speed, I steadily increased the plunge feed rate to a value 60 percent higher than it was. After these adjustments the chips curled up tightly, with a nice sizzle when they came off.

The net effect of my changes on the cycle time was an increase in cycle time of less than 1 percent. I was lucky that I was nearly able to completely compensate for the decrease in surface speed by increasing the feed rate.

The tool life results went from little more than 200 hits per edge, to more than 1200 hits per edge. This resulted in a $330 reduction in tooling costs per month. Additionally, we reduced tool change time by 15 minutes per week.

This was a time where a slower cycle time actually enabled me to make similar quantities of parts per month and reduce tool usage costs. If someone forgot their machining 101 lessons, there may be room for improvement.