When Is Accuracy More Important Than Repeatability in Manufacturing Automation, And Vice Versa
An automated indexing system – whether it consists of high-speed rotary table, conveyor-based linear indexer or a combination of both – within a manufacturing operation attempts to optimize two variables during a pick-and-place or workholding application:
Accuracy: the act of positioning the indexer in the theoretically perfect position as it moves from station to station
Repeatability: the indexer’s ability to bring the tooling back to position one after it has completed its specific indexing functions
As the indexing operation commences, there are four possible outcomes vis a vis accuracy vs. repeatability. Here is an explanation of each, from least to most desirable:
Not Accurate/Not Repeatable: The indexing station stops are not near the bull’s-eye and not close together; the worst-case performance for an indexing operation
Accurate/Not Repeatable: Indexing stops are on or near the bull’s-eye, but not consistently in the same spot
Not Accurate/Repeatable: Increasing stops are at the same place, but not on or near enough to the bull’s-eye
Accurate/Repeatable: The ideal outcome: stops are always on the bull’s-eye along with always being close together
So, knowing that there can be some level of variance in both accuracy and repeatability during an indexing operation leads to this question: Are there times when accuracy is more important than repeatability in manufacturing automation, and vice versa? In general, achieving an optimized level of repeatability is more important than accuracy because every indexing operation that functions properly must start from the same exact spot at station one.
However, there are instances where accuracy is more important. An example where accuracy is super-critical is the production of syringes where the needle must be installed in the exactly perfect spot within the syringe body. These types of applications – while acknowledging that repeatability is also an important component of the indexing operation – demand the highest level of accuracy possible.
On the other hand, there are instances where repeatability is more important. An example in this realm is a welding operation. Sure, the weld must be made in the proper spot, but there is more forgiveness if it is off-center by a small amount; in this case, what’s most important is the repeatability of the welding tool arriving in the right area at the right time.
In the end, while achieving the highest levels of both accuracy and repeatability are the main goal, there are some indexing applications where it is better to be more accurate than repeatable, and vice versa. It is the needs of the user and specific indexing operation that will determine whether there can be even the slightest inconsistency in either accuracy or repeatability.
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