Disclaimer: CNC Concepts, Inc. accepts no responsibility for the use
or misuse of techniques shown in this web page. We simply publish information
we feel will be of interest to CNC users. In all cases, the reader is totally
responsible for considering the implications, good and bad, of implementing one
or more of the techniques we show.
How fast can you rapid?
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!
