Don't Touch That Dial?

Ask a machine tool supplier to comment on business right now, and you are likely to hear the word "deliveries. " Many of these companies have quickly gone from having too few customers to having more than they can satisfy in a timely way.

Columns From: 12/1/2004 Modern Machine Shop, ,

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Peter Zelinski

Ask a machine tool supplier to comment on business right now, and you are likely to hear the word "deliveries." Many of these companies have quickly gone from having too few customers to having more than they can satisfy in a timely way. Right now, you may face a longer wait than anticipated for your first choice in a new machine. If you do wait it out, the pressure falls to your existing equipment to carry the work a while longer. Is there an easy fix here?


That is, is there a short-term change you can make that will address some drain on productivity, to free up extra capacity?

For a whole lot of shops, I am convinced the answer is yes. In shops and plants of all sizes, an inexpensive component keeps many machine tools underused. Simply put, the "feedrate override" dial gets applied too often. Speed, technology and economics have all changed in ways that tend to make dialing down the feed rate by hand self-defeating.

Consider the following:

Speed. The use of higher feed rates today makes it more likely that an operator who responds in real time to a machining problem will be responding to that problem too late. Just as likely is the danger that the operator will not dial back up to the intended feed rate soon enough, thereby punishing a larger part of the program for a problem in a small area. Even more fundamental is the fact that feed rate might be the wrong parameter to touch. If the operator is responding to chatter, he would do better to change the spindle speed instead.

Technology. Software in your shop right now may offer solutions for the kinds of problems that manual override tries to address. For areas of heavy stock removal, some CAM systems can seamlessly change the tool path—switching from smooth lines to trochoidal milling, for example. Software can also tailor the feed to the stock size, adjusting it within the program to keep the cutting load constant.

Economics. An even better solution may be to use a more capable tool. The new tool may be expensive, but not as expensive as slowing down to protect a tool that costs less. For a typical machined part, tooling accounts for about 3 percent of cost, while machine time accounts for close to 30 percent. In other words, time is typically more precious than tools.

In summary, this established and well-intentioned approach to troubleshooting on the shop floor carries a risk of squandering resources instead of saving them. If your shop has long used feedrate override routinely, it may be time to reevaluate how that override is used.

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