Hole vs Shaft Based Fit

[Saravana Kumar]

Hello,


As per ISO,is it allowed to mention any fit without using "H (Hole Based)" or "h (Shaft Based)"?


For Example, Does 20F8g6 is a valid clearance fit as per ISO or not? or Is it must be either like 20H7g6 or 20G7h6 with h rule?

Thank You.

[Kelly_Bramble]

I suspect that you are referring to ISO 286...

Upper case letter designations are reserved for internal feature of sizes and lower case letter designation are reserved for external feature of sizes.

For Example, Does 20F8g6 is a valid clearance fit as per ISO or not?

Yes, it is - well almost is should be presented as:

20 F8/g6

or

20

20 is the basic or nominal size in mm, F8 is the hole tolerance grade of + 0.053 and - 0.02 and g6 is the shaft feature allowing + 0.007 and - 0.02.

There is a rule within ISO 286 allowing for the use of lower case designations when transmitting information of equipment with limited character sets - like a telex machine. However, one must show the correct designation on engineering drawings.

See Related:
http://www.engineersedge.com/calcula...tolerances.htm
http://www.engineersedge.com/calcula...tolerances.htm

[Saravana Kumar]

But I came thru that as per ISO 286, we MUST use either H(Hole Based) or h(Shaft Based) while mentioning a Fit. Because in the above example there is no "H" or "h". So, Is that(20F8/g6) correct as per ISO 286 or not?

[Kelly_Bramble]

Because in the above example there is no "H" or "h". So, Is that(20F8/g6) correct as per ISO 286 or not?

Yes! H is just a different tolerance class than F.

If you're reading ISO 286 Part I, paragraph 5.2.1 the use of H and h are just examples. They could have used any one of the tolerance classes - like P and p or D and r.

For holes, the tolerance classes range from A to ZC and for shafts a to zc - read ISO 286 5.1.2.1. All letters are used to define tolerance classes except I, i; L, I; 0, 0; Q, q; W, w.

Go look at the tolerance classes given within my links or ISO 286, Part II.