The followings are three methods for reducing the machining cost of sand castings.
1) Reduce the Amount of Different Machined Planes
A major cost contributor is the amount of time it takes for set-ups and programming a CNC to machine. So, try to consolidate machining surfaces to the least amount of planes as possible.
2) Reduce Surface Finish Requirement
Ever heard the phrase, "Don't Over-Engineer"? Often a print will require a surface finish that requires grinding operations. This can add a lot of cost to the machining operations of a casting, especially in the prototype to low volume quantity range since this will normally need to be done manually. Grinding may be necessary for some projects, but more often then not the surface finish from general machining is sufficient.
3) Consider the reach of the Cutting Tool
A major design consideration needs to be given to the ability of a cutter to reach a machined surface or hole. There needs to be clearance from the cutter and walls that are near the machined target. This obstruction can be perpendicular or parallel to the required path of the cutter.
Either the area cannot not be machined or the part will have to be moved several times, which requires more set-ups and labor time, for a specific way in the tool holder to allow for the proper machining.
If there is improper reach, such as needing to put an extender on the cutter for deep cuts, then issues with vibration may occur and it could also impact the speed of the cutter. This means more time in the machine and more labor time, so the costs keep adding up.