Metrology
Metrology
Gaging Oldies But Goodies - A Review Of Some Things We Should Know By Now
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Gaging Oldies But Goodies -
A Review Of Some Things We Should Know By Now
George Schuetz – Mahr Federal Inc.

 Quality Gaging tips has been being published for a long time and we have covered many tools and techniques for assuring accurate, repeatable measurement and gaging in machine shops.  So I thought it would be appropriate to double back and talk about some oldies but goodies—things we should know all too well by now, but sometimes overlook or take for granted.  Here are a few of my personal favorites:

 10:1 Rule.  The history of this is dubious and you will never find this published in any standards document.  However it’s one of those “guidelines” that seems to make a lot of sense. Whenever possible, the measuring analog instrument should resolve to approximately one-tenth of the tolerance being measured.  So if the total tolerance spread is 0.01 mm (that is, ±0.005 mm), the smallest increment displayed on the gage should be 0.001 mm.  This amount of resolution allows you to make accurate judgments for borderline cases and makes it possible to observe trends within the tolerance band.  Many people know of and can explain this rule.  Yet it can still sneak up on the best of us and bite us in the leg with appalling frequency. 

 Simple SPC.  It’s amazing how many shops still don’t do SPC because they can’t afford digital gaging equipment, don’t have the right computers or software—name your favorite excuse. One of the first SPC manuals was published by Federal Products in 1945, long before the digital revolution.  It has gone through 14 printings.  Back then, simple SPC empowered machine operators to keep their processes in control by showing them how to make simple charts and other visuals that gave them regular feedback on how things were going. 

 SPC wasn’t conceived of as something you have to buy, but something easy you can do. SPC can be that simple, or it can be very complex.  By starting out on the simple side and taking just one step at a time, any shop can develop an SPC program to take it to the next level of manufacturing excellence.  The trick is to get started.  Later on, you can think about getting digital tools, more computers and software.

 SWIPE.  When gaging results don’t live up to expectations, it’s easy to blame the gage. However, this outlook will rarely get you on the right track.  A more helpful approach is to consider how good gaging practice encompasses a range of factors which can be summarized by a single acronym, SWIPE.  The letters represent Standard, Workpiece, Instrument, Personnel and Environment.  Study how each of these might contribute to the effective or ineffective use of gaging and you will be on the shortest path to setting up a solid gaging system or troubleshooting one that is currently shaky. 

 How to Use a Rule (scale).  It is impossible to overemphasize how important it is to remember the basics.  For example, one of the oldest and most pervasive of all measurement tools is the steel rule.  But when was the last time you received any training in how to use it?  The first grade?

“What’s there to know?” you ask.  Plenty.  Rule styles, for example: English or metric, rules with the zero point on the edge or inset a short way.  Avoiding parralax error.  Using a stop, even if it’s your thumb, for better alignment.  Better still, starting a measurement at a graduation instead of the edge.  Measuring similar parts from the same starting point and in the same direction for greater consistency.  The fine art of rotating the rule to get the longest dimension
(e.g., diameter ) across a hole.  Finally, when you need to use a more accurate measuring tool (See 10:1 rule above).  If basic skills are so important to using something as mundane as a rule, imagine the influence they have on something more complex like calipers, a CMM or laser interferometer.

 The truth is that no matter how technologically advanced our manufacturing processes have become, we will never outgrow our need for revisiting basic measurement.  From the
pyramids to the space station, they continue to be the foundation upon which anything of quality was ever built.