How to design to achieve the best alignment?

design diagram

When you have alignment pin holes in your model to fully fit and align another machined part, you need to mill important features onto the same surface as the alignment holes. This is because even turning over the material or replacing the end mill will bring the possibility of errors, so it is safer to mill the alignment holes and important features at one time using only the same end mill (as many as possible), so you can ensure them Correctly positioned relative to each other.

In the model shown in the figure above, the four alignment holes protrude from the top surface, but because they are not needed (there is no need to insert a pin from the top), they will not pass completely. The center circle is an "important feature", which is a cut out from the bottom of the block. The second and third pictures (top and bottom of the work, respectively) display better.

The problem with this design is that the alignment holes and important features are on opposite sides of the board, which means I need to turn the material over to mill out both. When I flip to do the other side, this flip will introduce errors, which is never good. Therefore, I decided to cut through the four alignment holes (see the figure above): this way, I can mill the alignment holes and the center feature without having to flip the part. Making them through holes is not necessary for the function of the part, because it only needs to be fitted with a positioning pin of about 3 mm to be connected to another board with the same alignment hole, but it is necessary for the machining of the part. This situation (and other similar situations) explains why not only must it be designed for this purpose, but it must also be designed for manufacturing.