Locating Principle:Which one to prefer, Die Cast part 1

[filippoGDeT]

Hello Guys,

I need your help in order to clarify my doubts in term of Datum Setting.

Supposing that I've a complex part ( maybe Die Casted or Injection Molded)....In order to set a valid datum reference frame which is the best choice among the following three options?

- Use 2 holes as secondary and tertiary datum rispectively

- Use one hole (secondary datum) and one flat surface as tertiary datum.

- Use two flat surface as secondary and tertiary datums


Which one will ensure in your opinion best accurancy and reliability of part?

Thank you in advance for your help

Have a nice day

Filippo R.

 

[MikeHalloran]

It depends too much on too many things that aren't revealed yet.

Go ahead and take a guess, and make a drawing of the part, with tolerances and whatever dimensioning scheme you think is okay.

Then take it to a molder or a diecaster, in either case the guy who will make or subcontract the tooling, and has to stand behind the product it makes, and go over the part design in detail.
Bring prints to sketch on, red and yellow pencils, and have a markup session.
Do not bring your ego.
Leave with one of a pair of identical markups, both signed by both participants.



Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

By definition, the primary datum plane definition requires 3 non-colinear points. The secondary datum plane requires 2 points located on a line non-coincident to the primary datum plane. And the tertiary datum is a plane normal to the primary and secondary datums through just a single point.

With complex castings, it is common to use a targeting fixture at the foundry to precisely machine tooling datum features (holes, spotfaces, etc) on the casting that will be used to mount the casting for the initial machining operations. After there are accurate machined surfaces on the casting, the original datum points are no longer required.