This article considers the increased complexity of multi-planar designs as well as the importance of ensuring adequate space for cutters in machining.
Whenever considering machined products, consideration must be given towards planar geometry. You should consider the impact of multi-planar geometry on the end product machining cost. If features are placed on different planes unnecessarily, it will increase the time required to machine the part due to needing to reset the mill for each plane.
In the event that multi-planar features are required, you should attempt to locate these features on the same face of the part being machined. Locating features on the same face significantly reduces mill setup time as each face to be machined requires adjustments to the mill prior to starting.

Another vital consideration for machined product design is ensuring that there is adequate space for the cutter. Additionally, space should also be made for any clamping fixtures to be used. Without proper forethought, redesign will be necessary. Each redesign action required will substantially drive up tooling costs for the product.
Key points:
(a) Attempt to place machined surfaces on the same plane when at all possible.
(b) Attempt to place machined surfaces such that they are on the same side of the workpiece to minimize repeated setup times.
(c) Inspect design for impossible machining operations prior to finalizing.
By following what we just went over, you can reduce flow time and prevent impossible-to-machine products forcing expensive redesign.