Is Your Machine Set Up For The Worst Possible Condition?

Machine tool builders are constantly striving to provide better machines capable of handling more applications efficiently. Yet they are also highly concerned with safety issues.

Columns From: 10/1/2001 Modern Machine Shop,

Machine tool builders are constantly striving to provide better machines capable of handling more applications efficiently. Yet they are also highly concerned with safety issues. Unfortunately, safety and efficiency don't always go hand in hand. What makes a machine safer to run usually compromises efficiency (and vise versa).

Most machine tool builders don't customize each machine they build to a specific end user's needs. Instead, they commonly set up the machine for the worst possible condition for every major efficiency or safety-related specification. And for liability reasons, if there is a compromise to be made, most put the emphasis on safety as opposed to efficiency. Most machine tool builders will make one universal adjustment for each time- and efficiency-related function. While this minimizes liability problems for the machine tool builders, you may not be coming close to approaching the worst-case condition for some of the functions that they universally adjust. And in turn, your machines may not be nearly as efficient as they could be. Here are a few examples.

Spindle acceleration/deceleration. Parameters control how quickly the spindle starts and stops. The response time of your spindle motor is, of course, directly tied to the weight the spindle must hold. Consider, for example, a 30-horsepower turning center. If the machine tool builder designs this machine to hold a 1,200-pound setup (including the chuck), it will set the spindle accel/decel parameters accordingly. Maybe this would be acceptable with a 15-inch chuck holding a large diameter steel shaft. But maybe you've placed an 8-inch chuck on the machine and exclusively run aluminum workpieces. Maybe your maximum setup is only 500 pounds. Since your spindle accel/decel parameters are set for a 1,200-pound setup, your spindle is not accelerating and decelerating as quickly as it could be.

If the machine tool builder claims a machining center can hold an 80-pound tool (maybe a large face mill), it likely has set the spindle accel/decel parameters for this worst case scenario. If your heaviest tool is 35 pounds, your spindle is not going to be responding as quickly as possible.

Axis acceleration/deceleration. In similar fashion, most machining center manufacturers set the axis acceleration/deceleration parameters for the worst case situation. Consider a bed-style machining center with which the table moves to form the X and Y axis. The machine tool builder may claim that the machine can hold a 2,000-pound setup. This means the X/Y axis accel/decel parameters must be set to allow smooth acceleration and deceleration of a very heavy setup. But you may never make setups heavier than 800 pounds. In this case, your X/Y axes are not responding as quickly as they could be.

Tool changing time. If your manufacturer claims a machining center can handle a 70-pound tool through the automatic tool changing system, it had to adjust the system to safely handle 70-pound tools. If your heaviest tool is 30 pounds, it's likely that your automatic toolchanging system can be adjusted to respond faster.

In-position check. Though this function goes by different names, parameters control how quickly a machine can transition from rapid to cutting movements. Like the axis accel/decel parameters, these parameters are related to the weight of the moving element attached to the axis. You may, for example, notice a pause every time the machine transitions from rapid to cutting motions. If not set appropriately for your specific needs, this can be a time waster.

How do you fine-tune the machine to your specific application? These are modifications that the typical CNC user would not want to tackle alone. First, contact your machine tool builder to see if it is willing to help. But, frankly speaking, many machine tool builders are reluctant to provide much help when it comes to changing the functions just mentioned. They're concerned for your safety and for the longevity of their machine tools. For these reasons, some may refuse to help (they may even threaten to void the warranty if you make changes to these functions). Many machine tool builders will ask you to sign a waiver relinquishing them of responsibilities should problems arise due to the changes you request before they'll provide help. Some may provide assistance to help you fine-tune a machine but charge for the service. If your machine tool builder is unwilling to help, note that there are a number of consultants in the industry capable of helping you fine-tune the machine to your application.

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