minus minus tolerance 2

[bloodydecks]

Hello,

I need some help and hope someone can help us.

We recieved a drawing that has a diameter called out like with the tolerance set as minus minus.

-0.02
Ø4.2 -0.04

 

[KENAT]

As others have mentioned, it's an ISO type deal.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[GMIracing]

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.

 

[321GO]

pmarc,

Quote
"There is a whole norm (ISO 286) that specifies:
- standard tolerances for certain ranges of nominal dimensions depending on the tolerance class"

That's not correct IMHO,
ISO 286 is only used for tolerancing shaft fits, nothing more.

Din-ISO 2768 (part 1) specifies tolerances for certain ranges of nominal dimensions (not ISO 286).

And as others have stated, it is fairly common practice in Europe.

 

[pmarc]

321GO,

You are right - this was my mistake.

 

[juergenwt]

321GO - DIN/ISO 286 is used for all fits, not just shafts.
DIN/ISO combines the former DIN specifications completely:
DIN 7150 T1 = ISO tol. and fits for length dim. 1 to 500mm
DIN 7151 = ISO Basic tol. for length dim. 1 to 500mm
DIN 7152 = Creation of tol. fields 1 to 500mm.
DIN 7160 = ISO - deviations for O.D. (shaft) 1 to 500mm.
DIN 7161 = ISO - deviations for I.D. (bore) 1 to 500mm.

DIN/ISO 286 replaces the following DIN specs in part:
DIN 7172 T1, T2, T3 and DIN 7182 T1.