The +/+ and -/- are typically used with fitting cylindrical parts to one another using the following types:
• RC = Running or Sliding Clearance Fit
• LC = Locational Clearance Fit
• LT = Transition Clearance or Interference Fit
• LN = Locational Interference Fit
• FN = Force or Shrink Fit
see also:
ASME B4.1 - 1967 Preferred Limits and Fits for Cylindrical Parts
ASME B4.2 - 1978 Preferred Metric Limits and Fits
the purpose of ISO 286-1/2 and ASME B4.2 is one for using a system of common tools, production methods, and gages. This is the description taken from the ASME B4.2 catalog:
"This standard describes the ISO system of limits and fits for mating parts as it is approved for general engineering usage in the United States of America. It establishes: (1) the designation symbols used to define specific dimensional limits on drawings, material stock, related tools, gages,
etc., (2) the preferred basic sizes (first and second choices), (3) the preferred tolerance zones (first, second and third choices), (4) the preferred limits and fits for sizes (first choice only) up to and including 500 millimeters, and (5) definitions of related terms."
for example, say we want a Ø45mm shaft/bearing assembly that requires a good positional fit but also needs to slide for assembling and for allowing movement during operation. Now from our preferred fit tables (ISO/ASME) we find that an RC3 fit (H7/f6) is a common choice for such function and should also prove economical for our manufacturing.
on the drawing, we can easily show this as:
Hole Ø45 H7
Shaft Ø45 f6
now you are asked by the inexperienced shop, what is H7/f6? so you add the tolerances:
Hole Ø45 H7 (+0.025/+0)
Shaft Ø45 f6 (-0.025/-0.041)
well, there is that nasty +/+, -/- again which nobody seems to like, so we can nominalize it to give:
Hole Ø45.0125 +/-0.0125
Shaft Ø44.967 +/-0.008
I think, it should prove quite easy to find tooling and gages (common) for a Ø45H7/f6 connection, but I would think that one should expect a few blank looks and likely additional costs if they inquired about a drill bit/reaming set and gaging for a Ø45.0125 +/-0.0125 hole and a roller burnishing tool and gaging for a Ø44.967 +/-0.008 shaft.
