Cutting Tools Chicago aka General Cutting Tools in conjunction with Greenfield Tap and Die explains the proper use and care for your gages. Limit-type thread plug and ring gages are important tools in any Quality Control program. These gages are used to determine whether a product’s threads fall within maximum and minimum functional size limits.
Product size limits are expressed in terms of pitch diameter, which is theoretical. Limit-type gages check the “functional” size of the threads, or the capability of product threads to assemble with mating parts, and the degree of tightness or looseness of the fit. Functional size takes into account not only the pitch diameter, but also the effects of deviations in other factors such as lead, angle, roundness, and helical path.
Decisions regarding whether to accept or reject a product are made based on its functional size. Obtaining an accurate functional size evaluation depends upon three key factors: 1) the accuracy of the gages used, 2) proper care of the gages, and 3) correct use of the gages.
The initial accuracy of the gage is the responsibility of the gage manufacturer, who should manufacture the gage within specified allowable tolerances. Maintaining gage accuracy then becomes the responsibility of the user and involves properly caring for and correctly using the gages.
Properly Caring for Gages
Gages should be handled with care to avoid damage and premature wear. Mishandling gages can result in unnecessary costs, both for replacement gages and, more importantly, for improperly rejected products. Gages should also be calibrated regularly to ensure they are within acceptable tolerances.
Use the following tips to properly maintain your gages:
- Keep gages clean and lightly oiled, if possible, both during use and when in storage. Light oil will prevent rusting, greatly increase wear life, and help to prevent “loading” or “smearing” of product material onto the gage threads. (In some applications, using oil on a workpiece might not be acceptable, so be sure to check the material specifications before using an oiled gage.) Using Seal Peal plastic dip on gage threads in storage is also acceptable.
- Protect gages from nicks, jams, and other damage by providing a protective box in the work area.
Using Gages Correctly
Correctly using gages will not only extend their life but also yield accurate results when evaluating threads. Refer to the following usage tips:
- Clean parts before gaging. Remove any dirt or chips so the gage can check product size unaffected by foreign material.
- Align and start threads carefully. Cross-threading can damage the end threads of both the product and the gage.
- Gages are not sizing tools! They must travel over the full length of the product threads or, in the case of blind holes or imperfect run-out threads, they should travel as much of the length required to enable assembly.
- GO gages must enter or be entered by product threads without undue force or pressure.
- NO GO gages have limitations on entry. It is usually preferable that they do not enter; however, some entry is generally allowable and the limitations vary for different gage standards. For inch threads, NO GO entry is limited to not more than 3 turns; for metric threads, NO GO entry is limited to not more than 2 turns.
- Don’t force the gage. Use reasonable light pressure only.
- Use thread ring gages equally from both faces. We often find that ring gages submitted for re-inspection or reconditioning are tapered from wear only on the stamped side. The ring gage is designed to be used from either face, and alternating use from each end will prolong wear life.
By properly using and caring for your gages, you can achieve a number of sound, economical benefits:
Prolonged wear life
Extended periods between calibrations
More reliable results in evaluating product threads
Acceptance of the maximum size range of the product tolerance. Thus, preventing defective threads from reaching the market