In the majority of cases, the fastening of parts on checking fixture is made as follows: the supporting of the part is made with a support block and a clamp, the centring with a conic centring pin in a circular hole, and the anti-rotation with a diamond-shaped pin in a slot. Yet, to do this fastening correctly, one of the most important information we need is the tolerance of the circular hole, and most of all, of the slot. Tolerancing a slot, however, is not always that easy. Fortunately, Tec Ease has prepared a video of GD&T explaining how to tolerance correctly a slot, providing us once more with useful information for the designing of parts and checking fixtures.

Here is what this video says:

“Controlling slots and other irregular shapes has always been a challenge. The standard uses two ways to tolerance slots. The first one works for slots only. The length and width of the slot are toleranced as two features of size. The position tolerance, with the word “boundary”, is applied to each position tolerance.
The size limits are controlling the size and the form of the slot, although what is actually allowed on the radia is a little fuzzy.
The position tolerance controls the location and the orientation of the slots to the datum reference frame.
The other method would work for any internal and external shape, including slots.
A profile tolerance controls the size and form of the slot all around. One position tolerance controls the location and orientation of the slot to the datum reference frame.
In this case, there is an inner boundary that the slot may not violate. The slot can move left, right, up or down, and even tip, as long as it does not come inside this boundary, which is probably the space needed by the mating part.
I like the approach that works for all your regular shapes.”

Other related articles:

• GD&T - Datum Features must be Referenced to be Used