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Impact of rapid motion on program execution time

CNC Programmers have the tendency to ignore the effects of rapid motions on cycle time because of the very fast rate of motion. While today's CNC machines do rapid at amazing rates (some over 1,500 inches per minute), no rapid motion can be made instantaneously. In order to minimize cycle time, you must minimize rapid motions.

To stress this point, we offer a simple rule of thumb called the one second rule. One second of saved cycle time will total 16.6 minutes of saved production time in one thousand cycles. While this may not sound like a lot, it can add up fast. If but four seconds can be saved per cycle in a one thousand piece order, over one hour of production time can be saved. And if this four seconds can be saved without spending money (by simply formatting your programs efficiently), all the better.

It helps to know just how far your machine must travel (at rapid) to accumulate one second of cycle time. Of course, the faster the machine's rapid rate, the further it must move to accumulate one second. Here is a chart that shows the relationship of rapid rate to motion distance at common rapid rates. Notice that rapid motion is never instantaneous! In fact, you may be somewhat unpleasantly surprised to learn how little your machine must move to accumulate one second. These values, of course, do not reflect any acceleration/deceleration, meaning these numbers represent the best condition. In reality, the effects of rapid motion are even worse!

• 100 IPM - 1.666 inches

• 200 - 3.332 inches

• 300 - 4.998 inches

• 400 - 6.664 inches

• 500 - 8.330 inches

• 600 - 9.996 inches

• 700 - 11.662 inches

• 800 - 13.328 inches

• 900 - 14.994 inches

• 1000 - 16.660 inches

• 1100 - 18.300 inches

• 1200 - 20.000 inches

• 1300 - 21.700 inches

• 1400 - 23.300 inches

• 1500 - 25.000 inches

Knowing these values, let's look at a simple example. Say you have a ten-tool machining center program. Say the motion distance from the workpiece to the machine's tool changing position is about 10 inches (a relatively small machine). At each tool change, about 20 inches of motion distance will be required (ten inches to and from the workpiece). For ten tools, this totals 200 inches of motion (ten tools times twenty inches of motion per tool). Cycle time in minutes is calculates by dividing the motion distance by the inches per minute rapid rate. One second is equal to 0.0166 minutes.

With a rapid rate of 500 inches per minute, rapid motion for tool changing will require 24 seconds of cycle time! And that's assuming no acceleration/deceleration. In one thousand cycles, that's over six hours of production time, just for rapid motions. Anything you can do to minimize rapid motions will have a direct impact on cycle time!

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