Sunday, October 2, 2016

Min and Max Gauges

No, not the kind that go in earlobes.  In the early days of the gun industry, to make standard part sizes on the way to interchangeable parts, gun makes constructed their version of go/no-go gauges.  A maximum size hole (for example, for a gunlock side plate) would allow parts to fall through if it was not too big; a minimum size hole was just a little too small for the part to drop through it.  I'm not sure how they were used in combination, but I am about to start using them for ScopeRoller rather than measure every part with a micrometer.  My thought (and perhaps the way they did it) is to stack the maximum gauge above the minimum gauge with enough room between for the part to slide out between the gauges made of 1/4" aluminum or acetal.  If the part falls through maximum gauge but not through the underlying minimum gauge you turn the combination on its side and the part falls out.  For ScopeRoller I am going to make the maximum gauge .01" larger than needed, and minimum gauge .01" smaller and hold them together with 4 1/4" bolts at the corner of each gauge.  This should give me parts +-.01".


Minicapt said...

Or you drill the big hole next to the small hole in an easily handled piece of stock; paint a green tag over the big hole and a red tag over the small one. Then you can perform the test with two hand movements: try green, then red.


Will said...

Min/max dimensions are never arbitrarily arrived at. There needs to be concrete reason(s) for both of them, and they are not normally the specified dimensions given to the production people to make the parts for you. Those are typically a tighter spec, with acceptable tolerances stated.

What you ultimately want to avoid is "tolerance stacking", where parts individually meet spec, but a collection of parts eventually creates a size problem that makes the assembly non-functional, without some sort of engineering intervention. This condition is fairly common, hard to diagnose, and eventually very embarrassing to some engineer(s). It can be hard to discover when there are multiple parts, and lots of variable specs from different batches of parts (different vendors, usually).
CAD/CAM should make this design mistake happen less often, but what it tends to do is leave you with a larger amount of unusable parts, because they were all made exactly the same, and the total stack doesn't work.