Metrology
Metrology
Gage to Part, or Part to Gage: That is the Question
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Gage to Part, or Part to Gage: That is the Question
George Schuetz, Mahr Federal Inc.


  There are many questions to ask when deciding what gage to choose for an application, but whether to bring the gage to the part or the part to the gage is one of the most fundamental. And the decision is influenced by a host of factors.

  In a highly productive manufacturing process where inspection time is critical it is important to bring the inspection process right to the point of manufacture. Here, it is not uncommon to bring the gage to the part while it is still held in the chuck of the machine. For tight tolerance bore checks, the gage of choice is apt to be a fixed plug mechanical gage or a piece of air tooling: both are portable gages that can be easily brought to the machine.

  For parts flowing through the shop, from machine to machine, it may be more convenient to inspect the part on a bench fixture along the way. In this case, bench stands, ID/OD gages, and accessories can be readily positioned to bring the part to the gage.

  An incoming inspection might have a variety of parts coming through its doors. In most cases, because of the variety of parts and dimensions to be checked, portable gaging, or bringing the gage to the part, is usually the best solution. However tolerances or complexity of the parts' dimensions may dictate a bench set-up to achieve the required level of precision.

  Part size is also an important consideration. A small widget being produced on a turning center can easily be measured with a hand gage at the machine, or taken off and brought to a bench gage. Speed, complexity of the check, and operator skill probably dictate which type of gaging operation works best. But what if you are trying to measure a precision bore on a transmission housing for a tank, or the surface finish and bearing surface of a cylinder bore on a large 6-cylinder diesel engine? In most cases the gage of choice is the portable one dedicated to the task and easy to use by the operator.

  There are numerous gage designs to allow for this fast point-of-manufacture check.

  But there are always exceptions to these practices where a new twist needs to be taken. Take the case of the 6-cylinder head of the diesel engine. There could be numerous reasons why a dedicated portable gage may not be the right choice. Maybe the volume can't justify the cost. Or maybe there are so many surface finish checks required or they are so complex that a portable gage simply can't do the inspection. This is when a hybrid solution may be appropriate. There are in fact three concepts that can be used.

  The first is to make the part portable and easily maneuverable so that the areas of concern can be brought to the gage for inspection. Consider a cylinder block or head that weighs nearly 500 pounds. This would be much too heavy to be easily brought to the gage unless special fixturing is designed to easily and precisely move the part to the gage. This can be done effortlessly with an air activated, floating fixture, combined with a balanced rotation fixture to get the part surfaces parallel to the surface finish probe.

                                          

  Or, if the part is too large to be moved with a special fixture, then a fixed bench gage can be made portable using similar concepts. In this case, the large part is staged on a granite surface plate and the surface inspection system maneuvered around the part on an air cushion. With enough axial and rotational adjustment, the probe can be precisely aligned to the part.