A 100-Year-Old Measurement Tool That is Still Used Today

I will often scan through old documents as a method to get inspiration for topics for this column. As one can imagine, the basics of metrology were mostly developed in the early 20^th century but have been refined and controlled to get us where we are today. During this exploration, I realized that we are rapidly closing in on an important milestone for a mechanical transfer mechanism that was a breakthrough in high-precision measurement and, like the basics, is still used today for sub-micron or even nanometer resolution applications.

The device I am referring to is the reed mechanism, sometimes referred to as a parallelogram. In the early 20^th century, the real precision of gage block standards could not be realized without a high-precision comparator. As more precision parts were demanded for manufacturing, a higher level of measuring parts became necessary. There were high-precision instruments, but they were dedicated to the laboratory, difficult to use and sometimes unreliable.

The concept responsible for the next step, bringing more precision to the shop floor, was the reed mechanism. It was developed 100 years ago simultaneously by three different people and was first used in production by Western Electric. Despite the concept being over 100 years old, it is still used today in high-precision comparators for measuring and certifying gage blocks and other precision standards.

The reed mechanism is an ingeniously simple mechanical device for transferring or even amplifying small displacements — in the dimensional world, the displacement of the gaging sensor. It is based on a parallelogram shape with four straight sides. Each of the two pairs of opposing sides is of equal length and is parallel. The unique properties of the parallelogram have been applied extensively in industry to accurately transfer mechanical motion from one place to another. Perhaps the best-known application is the pantograph, a four-sided device used by engravers or navigators to reproduce an image or a straight parallel line.

In gages and gaging setups, simple reed mechanisms simulate the behavior of parallelograms to transfer motion from one component to another. One type of reed spring consists of two parallel blocks connected by two or more steel strips of equal size and stiffness to form a reed-type flexure linkage. One of the blocks is attached to a fixed surface. When a force is applied to the free block, the connection strips flex, resulting in a displacement of the movable block.

Some observers will note that when this movement occurs, the connecting strips bend ever so slightly and that, technically speaking, the parallelogram has been compromised. However, what is important is that fixed and moving blocks are still parallel and that the moving block is not deformed by the contact. So, nothing has been added or subtracted to the degree of motion transferred. For high-precision transfer of motion involving a range of a part of a micron, “reed spring mechanisms” can even be “EDM'd” from a solid piece of steel.

Now that we have the principle, what’s the big deal?

Most importantly, the reed mechanism is a friction-free transfer mechanism. Since no parts interact, the repeatability of the contacting block is as close to perfect as one can get. So, if one is looking for repeatability as an important criterion, there is nothing better. Thus, the reed mechanism can be used for these purposes:

• The reed spring can be manufactured into a micro precision sensor. The reed spring protects the valuable sensor, while its frictionless motion results in a repeatable nanometer measurement. With such high-performance repeatability, these are well suited for comparators or even used as a cost-effective, super-high-precision zeroing switch.
• The reed spring can be manufactured into a micro precision sensor. The reed spring protects the valuable sensor while its frictionless motion results in a repeatable micro-inch measurement.
• In a gaging situation where it may be necessary to protect the gaging indicator, the reed will accept all the side loading and not transfer it to the sensor. So, the reed switch itself takes all the pounding rather than the expensive sensor.

One could say there are certainly different ways of doing this motion transfer. Precision bearings and slides immediately come to mind. However, the reed spring is less expensive, and there is no moving contact between its components. This latter quality practically defies the laws of physics by resisting the onslaught of dirt and grease.

Being frictionless, the reed can sustain virtually millions of cycles without noticeable damage and maintain its high-performance repeatability. Perfect, in other words, for harsh shopfloor environments. The only downside is its limited degree of motion. But when talking about high-precision comparative measurement, there is nothing better than the 100-year-old reed mechanism.