VERISURF
Published

Why QC Inspectors Should Consider Gaging Fixture Spring Rates

All fixtures for part gaging will have some amount of deflection or spring rate that can affect measurement accuracy. A better understanding of spring rate can enable quality control inspectors to detect and eliminate this possible error source.  

Share

The fixturing on which an indicating instrument is mounted often leads to gaging-error questions such as “what is spring rate?” and “how do you test for it?” Most fixtures are made of several component parts and are, in essence, a variation of the well-known C-frame. If the user is aware of the common problems that can affect the use of the C-frame and other fixture designs, it can be easy to detect and eliminate possible error sources.

All substances, regardless of their hardness, have some degree of elasticity. Small as it may be, elasticity is a real and vital condition to be considered in a precision gaging setup. One must accept that the frame arrangement inevitably will deflect from even the slightest pressure. This deflection becomes significant when it is great enough to affect calibration.

In fact, this deflection is also known as Hooke’s Law, as it was this physicist who determined that the amount a spring stretches varies with the amount of force applied. While there may be no springs in a micrometer or bench stand, in reality, the frame of the gage actually acts like a spring.

There are a few solutions: Increase the spring rate (stiffness) of the gage frame to the point in which deflection is no longer great enough to affect calibration, reduce the spring rate of the indicating system until the deflection becomes insignificant, or compensate for the deflection. Deflection can be compensated for with an acceptable degree of reliability, although virtual elimination is the preferable alternative.

The size of the part to be gaged also figures heavily into whether spring rate will be a large or small problem. Large frames designed to accommodate sizeable workpieces are much more susceptible to a spring deflection error than frames designed for small pieces.

In the example of a snap gage or bench stand, what is spring rate? Spring rate is the relationship of load, which is expressed in pounds, to the deflection it causes, which is expressed in inches. The less a gage frame will deflect under a given load the greater its spring rate.

Generally, the spring rate of the gage frame should be at least 100 times that of the indicating system. Here is a basic example:

  • A 1-pound weight on frame’s arm at the center line of the indicator's spindle deflects the arm 0.004 inch, as noted by a change in indicator reading with the work in place (see illustration).
  • The force applied to the spindle of the indicator itself will cause the same change in indicator reading, which should be no greater than 0.01 pound (about 1.5 grams).

How is this force measured? The force can be determined by placing a reasonably precise weight scale under the indicator and noting the change in scale reading as the indicator is moved to produce a 0.004-inch change in indicator reading. In simple terms, the spring-rate ratio between gage frame and indicating system may be calculated by dividing the spring rate of the gage frame by the spring rate of the indicating system. The answer, or ratio, should be 100 (or more):1 to ensure that deflection will be insignificant and not affect calibration.

For most hand tools and smaller bench fixtures, designs that have been in use for decades have demonstrated that unless there is some unusual set of circumstances, deflection is a very small potential source of error. In this case, trying to increase a level of tolerance out of the gage can push it to the limit in which spring rate would have an effect. However, with special applications, such as large parts and long posts/arms on fixtures, it may be wise to take a look at spring rate and deflection as a source of error.

VERISURF
DN Solutions
MMS Made in the USA
715 Series - 5-axis complete machining
Paperless Parts
Koma Precision
More blasting. Less part handling.
VERISURF
World Machine Tool Survey
TIMTOS
IMTS+
KraussMaffei

Related Content

How to Choose the Correct Measuring Tool for Any Application

There are many options to choose from when deciding on a dimensional measurement tool. Consider these application-based factors when selecting a measurement solution.

Read More

The Many Ways of Measuring Thickness

While it may seem to be a straightforward check, there are many approaches to measuring thickness that are determined by the requirements of the part.

Read More

Turning Fixed-Body Plug Gages Inside Out

Fixed-body mechanical plug gages provide fast, high-performance measurement for tight-tolerance holes.

Read More
Measurement

4 Ways to Establish Machine Accuracy

Understanding all the things that contribute to a machine’s full potential accuracy will inform what to prioritize when fine-tuning the machine.

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
Sponsored

Increasing Productivity with Digitalization and AI

Job shops are implementing automation and digitalization into workflows to eliminate set up time and increase repeatability in production.

Read More
Software

IMTS 2024: Trends & Takeaways From the Modern Machine Shop Editorial Team

The Modern Machine Shop editorial team highlights their takeaways from IMTS 2024 in a video recap.

Read More
VERISURF