Getting The Most From Your Classics

If you are thinking of ways to economize these days, you may want to consider coaxing more performance out of your existing gages. Many of the gages being built today are refinements of gages built 50 years ago. They may have originally been designed for use with a 0.0001-inch indicator, but over the years it

Columns From: 7/26/2008 Modern Machine Shop,

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If you are thinking of ways to economize these days, you may want to consider coaxing more performance out of your existing gages.

Many of the gages being built today are refinements of gages built 50 years ago. They may have originally been designed for use with a 0.0001-inch indicator, but over the years it is likely that the tolerances on these have not stayed the same—what was once a 0.001-inch tolerance is now a 0.0005-inch tolerance or even tighter. Rather than buy new gaging, it may be possible to meet the tighter tolerance demand by improving your old “classic” gages. You can do this if you have the right foundation on which to build.

With good gage design you can’t go against physics. Simple, basic principles are the foundation of good gaging. These principles include all those characteristics we look for in a precision gage today: mass for stability; rigid frames and strong joints for stiffness; a sensitive contact in line with reference anvil; and flat, parallel surfaces. If your “classics” have all the basics of high-performance gages, then there are a number of things you can do to increase their performance.

The first step in meeting a tighter tolerance is to replace the dial indicator with higher-resolution readouts—usually an LVDT or digital probe. In this case, we can replace the 0.0001-inch grad dial with a digital readout having 10 microinches. Normally, this would not be recommended. However, on some ID, OD or snap gages, the effect of the increased resolution can be tested. If the result meets the Gage Repeatability and Reproducibility (GR&R) requirements, you’re good to go.

Once we have a higher resolution readout, we can start to think about all the little steps in the measurement process that can cost 0.0001 inch here or there, and in the end, provide the best performance possible. Here is a list of steps to squeeze the best performance out of the gaging:

  • Replacement of the readout with a high-resolution device is important, but it serves no purpose if the readout and probe are not calibrated. Always verify the electronics, and if possible, calibrate the complete measuring system—the gage, probe and readout system—with a set of min, zero and max masters.
  • Cleanliness has always been critical, but when it comes to high-resolution gaging, it is a must.
  • The temperatures of part, gage and master should always be the same.
  • When using similar gages throughout the shop, take steps to make sure the gage operating procedures are as identical as possible. Items for improvement include the following:
  1. Operator training should be the same for all users.
  2. Operators should load the part the same way and standardize a procedure for seating the part and checking
    repeatability.
  3. Gages with adjustable tables for setting the angle during use should all be set the same.
  4. The gaging force of all the gages should be set to the same pressure—strong enough to secure the part, but light enough to prevent deflection.
  5. Gaging jaws and contacts should be designed to the same length, width and material.
  6. The same style master should be used for setting the gages.
  7. A checklist or process should be followed to ensure that all of the correct information has been programmed if master deviations, offsets or multipliers are part of the setup process for the readout device.
  8. Contacts should be in good condition with no flats, nicks or worn spots. Also, they should be in line with each other and free-from sticking.
  9. A regular and thorough gage inspection and calibration procedure should take place to maintain top performance.

No rocket science is involved in this suggested list of steps to improve performance, and there may be more steps depending on the application. The suggestions are just the basics of good gaging practice. If you start with these, you may just see those 0.0001-inch errors fade away, allowing those old, “classic” gages to really rock.

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