Should You Gage to Part or Part to Gage?

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.

Columns From: 7/18/2012 Modern Machine Shop,

Editor's Commentary

From the monthly column: Quality Gaging Tips
In a highly productive manufacturing process in which 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 machine’s chuck. 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 brought to the machine.
 
For parts flowing through the shop from machine to machine, it might be more convenient to inspect the part on a bench fixture. 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, bringing the gage to the part (portable gaging) is the best solution to check a wide variety of parts and dimensions. However, tight tolerances or complex part dimensions may dictate a bench setup to achieve the required level of precision.
 
Part size is also an important consideration. For example, a small widget produced on a turning center can be measured with a hand gage at the machine or taken off and brought to a bench gage. The speed and complexity of the check and operator skill will probably dictate which type of gaging operation works best. However, if you are measuring a precision bore on a transmission housing for a tank or the surface finish and bearing surface of a cylinder bore on a large six-cylinder diesel engine, the gage of choice is the portable one dedicated to the task and easy to use for the operator.
There are numerous gage designs that enable fast point-of-manufacture checks.
 
Yet, there are always exceptions to these practices where a new twist needs to be taken. For example, there could be numerous reasons why a dedicated portable gage might not be the right choice for a six-cylinder head of a diesel engine. Maybe the volume can’t justify the cost. Maybe there are too many surface-finish checks required. Or, maybe they are so complex that a portable gage can’t do the inspection. This is when a hybrid solution may be appropriate. There are, in fact, three solutions that can be used.
 
The first is to make the part portable and easy to maneuver 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 too heavy to be brought to the gage unless special fixturing is designed to 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 is maneuvered around the part on an air cushion. With enough axial and rotational adjustment, the probe can be precisely aligned to the part.
 
In situations in which the part is so large that it barely fits on the surface plate, the texture measuring system can be placed on a specially designed cart and rolled over to the surface plate and part.
 

With the flexibility and sophistication of today’s measuring systems, it’s possible to get to just about any location.  

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