• Decide what characteristics to measure. If a part, with five different diameters, is cut on a lathe in one operation with the same tool, all diameters have to be qualified to prove the programming is correct. Once this is done, only the first and last diameters need to be gaged to maintain control. This requires only two custom gages.
• Discuss gaging requirements with the machine operator. Operators know what, how, and when to measure. Even if an application is similar to something that was done before, get feedback from the operator. Operators will know how well the gages work, how often repairs are required, and how process stock interferes with gages.
• Develop custom gaging specifications. Custom gaging requirements should include gage drawings and specifications. Drawings are used for gage maintenance and product changes. Whether documentation hard copy or electronic media, drawings should be complete and reproducible, match the company's standards, be drawn in full scale with the product drawn in phantom, meet the ANSI Y14.5 dimensioning and tolerancing standard, include assembly drawings of the gage and master, and include a bill of material.
• Establish general guidelines for gages and masters.  Specify carbide contacts for long life and scratch resistance. Master tolerances should be 10 percent or less than product tolerances and marked with the actual size. The master shape should simulate the product shape. The point of contact on the master should have the same angle as the product part.  Masters should be tagged if they weigh more than 25 pounds and have hoist rings if they weigh more than 50 pounds.
• Provide storage containers to protect gages and masters.
• Mark shop prints to show which dimension(s) to gage, including tolerances, including all holding fixtures, clamps, or arbors in place during gaging. Mark any unfinished stock that can obstruct the gage, and make note of environmental conditions.

To avoid designing a gaging system based on guesswork, gage designers need to know about the part and how manufacturers use them. Specify:

• Gage type—hand held or bench.
• Characteristics—note all the dimensions to be gaged at once.
• Gage readout—graduated scale, vernier, micrometer, dial, or digital indicator. 
• Whether a remote display or SPC interface is needed.
• How many cycles per day a gage will function.  Whether a certified set master traceable to the National Institute of Standards and Technology (NIST) is needed.
• How many gages are needed.

In the request for quote (RFQ):

• List the name and phone number of the person who specified the gage requirements. This person will later need to review and approve supplier concept drawings and arrange to have the gage and master inspected, working closely with the machine operator.
• Write a detailed RFQ to make sure the designer knows what you want.
• Ask peers in other companies to recommend a gage supplier.
• Cultivate a good relationship. You need a partner for gaging more than you need cheap gages.  Look at the total cost of the gaging system. This means looking at more than the purchase price of the gage. It includes the cost of calibrating the gage, setup, recalibration, maintenance, and time spent using the gage.

Insist that your purchasing department allow your input on the buy decision. If you have specified what you want and requested quote sketches, you will be able to show why you can't accept any tool that will not meet specs.

Specifications and feedback will enable a custom gagemaker to meet your needs consistently.

Written by MICHAEL LOVELL, Manager Special Gage
Division of The L.S. Starrett Co., Athol, MA.

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