Specifying Limits and Fits
But how to specify it? For specifying the limits and fits you decide the following three parameters:
Basic Size: Decide what should be the basic size of the shaft hole pair. It is the dimension to which tolerance are specified.
For example, 90~+mn~0.5 has the base dimension of 90.
The base dimensions of both the shaft and the hole should be same.
Fits: Decide what kind of fits you require for the shaft and hole pair. Is it clearance fit or interference fit? On the basis of this decision you will select the two alphabets (H and g of the previous example) of the limits and fits specification.
For specifying the hole you will use the capital letters from A to Z. And for specifying the shaft you will use the small letters from a to z.
If you require clearance fit, then use a capital alphabet between A to H (for specifying hole) and a small alphabet between a to h (for specifying the shaft).
If you require interference fit, then use a capital alphabet between H to Z (for specifying hole) and a small alphabet between h to z (for specifying the shaft).
For the hole base system, hole tolerance should be H and for the shaft base system the shaft tolerance must be h.
See the figure below for better clarity:
As you move from A to H (or a to h), the fundamental deviation will decrease, again if you move from H to Z (or h to z) the fundamental deviation will increase. So, select the shaft and hole tolerance symbols based upon your fundamental deviation requirements.
- Tolerance Grade: The numbers written after the each alphabet (7 and 6 for our previous example) signifies the tolerance grade. You can specify any number between 2 to 16. Lesser the numbers, tighter the tolerance zone. Tighter the tolerance zone, more precise machining operation is required. In other words lesser the tolerance grade costlier the machining process.
You have to use the ISO 286-2 table (Hole and Shaft Tolerances) for finding out the maximum and the minimum tolerance value for a specified basic size and tolerance grade. How? See the next example.