Casting Datums
Casting Datums
(OP)
I am currently preparing to do a machining drawing from a cast housing and trying to establish the datums. I have taken a class on GD&T some time ago and know enough to be dangerous, but do not use is that often, unfortunately so does my boss. I would like to set up the primary, secondary, and tertiary datums as it would mate to other parts, via parrallel jaws, etc. Im being told that it is better to cast datum features and basic dimension to datum targets/points because of the draft and that the machinist can fixture it better and that you have to start somewhere so the machinist can machine an adequate surface for the datums. Can you share some points on how to establish datums on a casting when there is only one flat surface (housing bottom) on the part. Any input will be appreciated.
Thanks
Thanks





RE: Casting Datums
Heckler![[americanflag] americanflag](https://www.tipmaster.com/images/americanflag.gif)
Sr. Mechanical Engineer
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This post contains no political overtones or undertones for that matter and in no way represents the poster's political agenda.
RE: Casting Datums
Heckler![[americanflag] americanflag](https://www.tipmaster.com/images/americanflag.gif)
Sr. Mechanical Engineer
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This post contains no political overtones or undertones for that matter and in no way represents the poster's political agenda.
RE: Casting Datums
RE: Casting Datums
Heckler![[americanflag] americanflag](https://www.tipmaster.com/images/americanflag.gif)
Sr. Mechanical Engineer
o
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This post contains no political overtones or undertones for that matter and in no way represents the poster's political agenda.
RE: Casting Datums
RE: Casting Datums
RE: Casting Datums
Heckler![[americanflag] americanflag](https://www.tipmaster.com/images/americanflag.gif)
Sr. Mechanical Engineer
o
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This post contains no political overtones or undertones for that matter and in no way represents the poster's political agenda.
RE: Casting Datums
RE: Casting Datums
Sand or other prototype castings are usually balanced manually to prevent thin/thick walls from inhibiting function. That does not mean that the eventual process "say die casting for instance" should not be supported by the product design specifications that you are specifying...it just means that alternate or special exceptions will be requirements will govern the alternate prototype casting processes.
If the "oil pan, cover, or whatever" is sand casted it will naturally exhibit blow, shift, and contour variations that are less probable than those in production die casting processes. The die-cast targets are typically ignored so that critical clearances and material thicknesses can be maintained in the prototype process .
Considering the eventual production process the casting datum targets or "3-2-1" registry “if I may speculate about the function” in this case should primarily reflect the functional considerations of the assembly. If the sealing surface could be cast flat enough or the clamp load could be such that the gasket material would maintain compression I would not even machine the gasket surface and risk exposing porosity… furthermore I would include a channel or inner chamfer could to retain excess RTV. I would establish three or possibly “4 clamped" datum targets "as primary" on the gasket surface…acknowledging that this surface would bend under clamp force …naturally it would be machined for early sand-cast prototypes. Next I would establish three "two on one side...fore and aft...and one on the opposite side middle… movable equalizing targets as the secondary datum targets and finally one or “two equalizing” targets longitudinally as the tertiary.
The clamp fixture would cycle rapidly with minimal force -to- maximum force to precisely constrain the “die casting” for the installation of all machined features including dowel holes that mated with expandable split roll pins, all fastener clearance holes and all other solenoid or electrical lead connection holes.
The point is that the casting target designations should reflect/insure both functional and process registry design considerations to insure minimum variation in functional clearance and min/max material outcomes.
Paul