Another issue that must be considered is what other machine characteristics will affect your cycle times, like rapid, acceleration, and deceleration. You can spend a significant amount of time in non-cut, or non-productive times, that are very dependent on rapids, acceleration, and deceleration. You have a lot of interplay between these things. On most machines there’s a certain length of a move before you can get to rapid rate. Any of these can dramatically impact your cycle time, influencing every tool change and every positioning move. For example, as you move from one feature on a part to another, or even the same tool with multiple holes, you’ll have an impact for how fast your acceleration, deceleration, and rapid are. This means that even a very accurate machine, if slow on the acceleration, deceleration, or rapid can dramatically change your potential cycle times.

If we look back at history, in the 1960s most rapid traverse rates were about 200 inches per minute. Today, it’s not unusual to see rapids of 3,000 inches per minute or more. Acceleration and deceleration rates have gone from 0.2G in the 1960s to about 1.5G. So improvements have been made.

As a result of this, many people have assumed that you can compare these numbers directly. For example, if you have a machine with 3700 IPM rapid traverse and 1G acceleration/deceleration, and another with a 2,000 IPM traverse and 0.6G, that means machine 2 is about half as fast as machine 1, right? Not exactly, because we run into the same measuring issues as we did in accuracy and repeatability.

For instance, there’s a certain distance to obtain a full, rapid traverse rate in many machines. There are also two different types of acceleration, linear versus bell-shaped. There’s also a relationship between acceleration/deceleration and velocity. Finally, not all machines have the same acceleration/deceleration rates for all axes, which can seriously impact tool changes and movements in the work zone.

To discuss more about linear acceleration versus bell-shaped acceleration, if you compare how a traditional linear acceleration/deceleration works by ramping up in an even, long stroke. This causes jerking of the machine, which can be very rough on the machine. A bell-shaped acceleration/deceleration, on the other hand, starts slowly and then ramps up much more quickly, using inertia to its advantage. This keeps the axis moving gradually. It then speeds up much more quickly, avoiding jerking of the machine. It also slows more quickly and then tapers up, so it saves overall time to speed up and slow down, while providing a smoother transition to reduce how much the machine is stressed. But if you just look at the numbers, the acceleration/
deceleration doesn’t look as good on paper for the bell-shaped acceleration/deceleration machine than the linear.

Taking this into account, if you again compare the two machines and machine 2 uses bell-shaped acceleration while machine 1 uses linear, machine 2 actually provides the faster cycle time, even though it has the slower rapid rate and lower G of acceleration/deceleration. Again, buyer beware. You need to know what the numbers mean to see which machine will perform better. On paper, you might choose what you think is the faster machine, but end up with the slower machine.