In ISO 5459-2011 there were some new symbols added to the standard for geometric tolerances. Some of them are of limited use outside a few special instances, but some can be quite handy in certain circumstances. One of these is the >< modifier, signifying using a datum to lock the orientation, but not the position. This can be useful in situations where we want to measure the distance to something which is a poor reference for orientation, because it’s so small. Here’s a simple example:
We want the hole to be 45 mm from the top of the flange, but the flange is quite narrow. A small change in the angle of the top flange would not affect the function, but it would move the tolerance zone for the position tolerance a lot, and we could end up with a part that is outside tolerance, even though it’s quite functional. To combat this, we can use another, more stable datum to lock the orientation of the hole, and still use the [A] datum to lock the distance. The result is as illustrated below:
The indication with >< specifies that the datum [B] should only be used to lock the orientation, not the location, of the tolerance zone. The location is then locked by the next datum, [A]. This way we can still measure the location of the hole from datum [A], without the problem of the angle of the tolerance zone varying wildly due to small shifts in [A].
This symbol can be quite useful, but note that adoption of these things tend to take time in the industry and even though it has been in the standard since 2011, it has still not found widespread use and your suppliers may be unfamiliar with it. Be prepared to educate them, and point to the 2011 edition of ISO 5459 for reference.
