Perfect Gaging in an Imperfect World

Perfect Gaging in an Imperfect World

George Schuetz, Mahr Inc.


  It is certainly not news that, more and more, dedicated gages are being placed at the point of manufacture.  Tight-tolerance measurements that were once performed in a semi-clean room by a trained inspection technician are now being done right next to the machine, often by the machinist.  But just because shop floor gaging has become commonplace, doesn't mean that just any gage can be taken onto the shop floor.  To assure good gage performance, there are a number of specifications and care issues which need to be addressed.

  Is the gage designed to help the user get good measurements?  A gage with good Gage Repeatability and Reproducibility (GR&R) numbers will generate repeatable measurements for anyone who's trained to use it properly.  Technique or "feel" should have minimal impact on results.

  Gages with good GR&R are typically very robust.  Part alignment is designed into the gage to make sure the part is held the same way every time and eliminate the effects of operator influence on part positioning.  Bench gages usually outperform handheld gages in both respects.

  Is the gage designed for the rigors of the shop floor environment?  Gages designed for laboratory use often cannot cope with the dust and oil present on the shop floor.  Some features commonly found on good shop floor gages include: careful sealing or shielding against contaminants; smooth surfaces without nooks and crannies that are difficult to clean; and sloping surfaces or overhangs designed to direct dust and fluids away from the display.  (Try to distinguish between swoopy-looking cabinets that just look good, and those that are truly functional.)  If all of these are absent, one can often add years to a gage's useful life by installing it in a simple Lexan enclosure with a hinged door, or even by protecting it with a simple vinyl cover when it’s not in use.

  Is the gage easy to operate?  Machinists like gages that operate like their CNC machines; once it has been programmed, you push a button, and the machine runs, cuts a feature, and is ready for the next part.  Gaging should be simple too, requiring as few steps as possible to generate results.  If a variety of parts is to be measured on the same gage, it should allow for quick, easy changeovers.  Electronic gaging amplifiers, computer-controlled gages (such as surface finish and geometry gages), and even some digital indicators are programmable, so that the user only has to select the proper program and push a button in order to perform the measurements appropriate to a particular part.

  No matter how well protected it is against contamination, if a gage is used on dirty parts, or in a dirty environment, it will get dirty.  At the end of every shift, wipe down the master and place it in its storage box.  Wipe down the gage, and inspect it for loose parts: contacts, reference surfaces, locking knobs, posts, arms, etc.  Do this every day, and you will probably prolong its life by years or at least, you will make it easier for the calibration department to check it out and verify its operation.

  Pretend for a moment that you have just installed a new planer in your basement woodshop.  Glowing with pride, you set it up, adjust it, and then, just for fun, you make a big pile of shavings.  And next?  I'll bet you clean it off carefully, maybe oil the iron posts, and promise yourself that you'll always follow the recommended maintenance procedures. 

  Not a woodworker?  Then you probably treat your golf clubs, boat, Harley, or flower-arranging equipment with the same pride of ownership.  So why not your gages, which are far more precise than any of these, and deserve far more attention.