Metrology
Metrology
Got a Match?
EN-US

Got a Match?
George Schuetz, Mahr Federal Inc.

 Producing precision spools and sleeves (e.g., for fluid pumps) and other pairs of parts with matching inside and outside diameters can be among the trickiest of quality issues.  It’s very easy for an engineer to specify that the outside diameter (O.D.) of the one and the inside diameter (I.D.) of the other must be within .000025” -- and less easy for a machinist to accomplish.

 You can spend days trying to tighten up your processes to meet those specs consistently, and chances are you’ll still end up tossing or reworking a high percentage of your production.  The solution to the problem may lie not in the process, but rather, at the QC end.  Maybe you can loosen up on your process, and measure your way out of the problem with match gaging.

 When an engineer specifies tolerances for spool and sleeve diameters, what is his real concern?  Is it absolute dimension?  Likely not.  In an application like a fluid pump, or a fuel injector, it’s the clearance between the two parts that determines how well it functions.  That’s where match gaging comes in.  Match gaging doesn’t measure diameters:  it measures the clearance (or interference) between two parts.  It can be a tremendous time- and work-saver where a desired amount of clearance or interference is required.

 In its simplest form, match gaging uses an air gage with a two-legged manifold, one leading to an air plug, the other to an air ring.  To measure a match, place the O.D. part (spool) in the ring and the I.D. part (sleeve) on the plug.  The gage indicates the total of the clearances between the two parts and their respective fixtures.

 Match gages are available in a wide range of configurations from simple manual gages to highly complex, fully automatic, multi-measurement gages costing many thousands of dollars.  In terms of accuracy, they are available with 50 millionths resolution and a range of 0.003” at the coarsest, to 5 millionths resolution and a range of 0.0003” at the finest.

 The greatest advantage of match gaging is that it allows you to produce matching parts at extremely tight clearance tolerances without actually having to achieve the same level of precision in the machining process itself.  To see how that works, let’s look at a set of parts with a nominal size of 1.0000” and where engineering has calculated that optimal performance requires a clearance tolerance of between 100 uinches and 200 uinches.

 There are basically three ways of achieving this desired fit between matched components.  The first involves controlling the size of both parts to assure an accurate match and full interchangeability.  This method tightens the manufacturing process severely and necessitates machining to a plus tolerance on the O.D. part and a minus tolerance on the I.D. part, as follows:

Outside Diameter  Size Tolerance
1.000100”  +0.000050”/-0.000000”
  
Inside Diameter  Size Tolerance
1.000000”  +0.000000”/-0.000050”            

 The second method is to control the size of one component where the other is predetermined by a previous process.  Typically, the O.D. is the fixed measurement (in this case, 1.0000” +/-0.0001”) and the I.D. is then machined to fit.  Air gaging is used to assure the 100 uinch to 200 uinch clearance between parts.  This method usually requires maintaining a substantial inventory of O.D. parts and often involves a cumbersome measuring process.

 Match gaging gives us a third alternative: we can machine both components, allowing the tolerances to vary as they will, and use the match gage to select matched sets.  Thus, the O.D. is machined to 1.0000” +/-0.0005” and the I.D. to 1.0000” +/-0.0005”.  Assuming a normal distribution curve from our machining process, we can use a match gage (often automatic) to select sets which fall within our 100 uinch tolerance range.

 Some users may elaborate on this process.  For example, a manufacturer of fuel injectors had a clearance tolerance range of 20 uinches between barrel and spool.  They were unable to control the machining process to produce either part consistently within that range, but a fully automatic match gaging setup now allows them to stage, match, assemble and package 700 pairs per hour.  Feedback from the gage is used to control the processes, letting the distribution of spool O.D.s move up or down to accommodate an overabundance of barrel I.D.s  at one end of the scale or the other.  Both ranges are allowed to float, chasing each other up and down the scale to assure enough matching spools and barrels to maintain production rates.