VERISURF
Published

Closing the Loop for Better Gaging

Gages and measuring instruments come in different shapes and sizes. Whether it be a snap gage, a bench stand, a surface-finish gage or even a form system, keeping the “measuring loop” as small or as short as possible is important for better gage performance.

Share

Most dimensional gages have some type of reference, which is the part of the gage that provides a hard stop and is the starting point from which the dimensional measurement is taken. For example, on a snap gage or bench stand, the reference is usually called a fixed or reference anvil since it tends to be flat. The reference point is where the measurement is started, and every contact gaging system has one.

Speed and increased performance requirements might necessitate the use of a comparative gage. In this case, another reference may be added. This helps eliminate both operator and gage variability errors, improving gage repeatability.

Directly opposite, and in line with the reference anvil, is the measuring instrument’s sensitive contact. This is the mechanism that does the actual comparative or direct sensing of the contact’s position. Take the case of a bench stand with a dial indicator: The contact — usually having a radius — is the actual tip of the indicator. For a micrometer, it's the sensitive anvil, and with a dial caliper, it's the sensitive jaw. But the message is that this sensing contact is the point that is really “feeling” the measurement.

In the fixed-reference anvil, the sensitive contact and backstop just can't live by themselves. They must be held together by the gage itself, and this is where our measuring loop comes into play. With a snap gage, it is the frame, reference(s) and indication device. A bench stand may have the base, post, an arm and indicating device

In these examples, the whole structure makes up the measuring loop. To get the best performance out of the measuring loop, keep it as small as possible along with being rigid and robust. For example, to make a length check on a part smaller than 10 mm, choose the smallest and most robust bench stand that has the capacity for this part. The larger a bench stand gets, the more “spring” or deflection might be seen in the posts and arms of the gage. Choosing a larger gage, say 10 times the capacity, would increase the measuring loop and potentially increase the chances of variation in part measurement due to flexing in the measuring loop.

As mentioned in the beginning, this not only applies to mechanical gaging but also surface and form measurement.

In the case of a skidded surface measurement, the sensitive contact now becomes the surface finish probe and its drive. The reference is the skidded probe and the measuring loop is created by the fixturing holding everything together. In most cases, the drive unit/probe can be directly placed on the part for the shortest measuring loop possible. However, sometimes certain applications call for measuring surfaces in deep bores. One solution for doing this would be to keep the drive outside the bore and use probe extensions to position the probe deep into the bore. But this, in effect, makes the measuring loop larger, opening the door for vibration and fixturing issues that can negatively influence the results. When possible, it would be best to position the drive and probe into the bore with special fixturing to close the loop and improve the results.

Finally, there are some form systems that have small part measuring capacities and others in which small- and large-volume parts can be measured. Certainly, the large systems have much more robust columns and arms employed. This is designed into the gage to account for both small and large measuring loops. For the best performance (since one can’t shrink the loop) the components in the loop must be made robust enough to produce the same performance for both small and large parts.

After reading this, the next time you look at your measuring and gaging instruments, you will have new insight into finding the measuring loop.

VERISURF
JTEKT
QualiChem Metalworking Fluids
VERISURF
MMS Made in the USA
715 Series - 5-axis complete machining
DN Solutions
World Machine Tool Survey
More blasting. Less part handling.
PMTS 2025 Register Now!
SolidCAM
Paperless Parts

Related Content

Workholding

Parts and Programs: Setup for Success

Tips for program and work setups that can simplify adjustments and troubleshooting.

Read More

6 Machine Shop Essentials to Stay Competitive

If you want to streamline production and be competitive in the industry, you will need far more than a standard three-axis CNC mill or two-axis CNC lathe and a few measuring tools.

Read More

The Link Between CNC Process Control and Powertrain Warranties

Ever since inventing the touch-trigger probe in 1972, Sir David McMurtry and his company Renishaw have been focused on achieving process control over its own manufacturing operations. That journey has had sweeping consequences for manufacturing at large.

Read More
Measurement

Ballbar Testing Benefits Low-Volume Manufacturing

Thanks to ballbar testing with a Renishaw QC20-W, the Autodesk Technology Centers now have more confidence in their machine tools.

Read More

Read Next

Sponsored

The Future of High Feed Milling in Modern Manufacturing

Achieve higher metal removal rates and enhanced predictability with ISCAR’s advanced high-feed milling tools — optimized for today’s competitive global market.

Read More

5 Rules of Thumb for Buying CNC Machine Tools

Use these tips to carefully plan your machine tool purchases and to avoid regretting your decision later.

Read More

Registration Now Open for the Precision Machining Technology Show (PMTS) 2025

The precision machining industry’s premier event returns to Cleveland, OH, April 1-3.   

Read More
VERISURF