Two Holes as Datum Feature - Complex Die Cast Dimensioning strategy

[filippoGDeT]

Dear All,

I'm currently looking to the best tolerance strategy for a complex Diecast part as shown in the enclosed simplified 2D sketch Let me better specify some background on my thread.
Supposing that I've a complex die casted part with 3 external holes on ear bracket that can be used by fixture (through locating pins) that will be used to adjust the part relative with it's own counterpart ( similar to that one showed).
I want to use these holes as they are accessible in every step of my process in my assembling line and this help to have the same Reference System for locating the others components will be mounted on it.
I guess that I can built a DRF as follow:
The flat surface that lie in the imaginary sheet will be the primary datum. The secondary datum is the hole numbered as 1 and the tertiary datum will be the hole numbered as 2 in the picture (that stop the clocking of the part ).
The first problem is that the second hole is not aligned with the view and the DRF will be thus rotated respect to the view. Can I specify a TED angular dimension to rotate the DRF ? Is that correct from GD&T rules?
The second problem is that I don't know if the second hole needs to be tolerated in position somehow in order to limits and compensate the tolerance will result from the manufacturing process.
There is a way that you guys know can be used in the fixture to reduce the positional tolerance of the second holes that could lead to a rotation around the first axis?
Furthermore, the smallest two holes in the part want to be located considering the DRF already created with positional tolerance defined by TED dimension ( all red dimension will be under square symbol)
Please help me to understand If my thought is correct.
Thank in advance.

Filippo

 

[KENAT]

ASME has a standard with guidance on dimensioning castings (or similar items like moldings) does Iso have anything similar that could help you?

https://www.asme.org/products/codes-standards/y148-2009-casting-forgings-molded-parts

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[drawoh]

filippoGDeT,

The standard way to fixture to holes is to use two pins. The first one is round and it locates in X and Y. The second one is diamond shaped, and it controls rotation, and allows for variation in the separation of the holes. The disadvantage of FOS[ ]datums is that there is clearance. Either your holes are very much more accurate than what you require for your subsequent manufacturing processes, or you need to account for MMC/MMB. Building datum targets into your casting is way less ambiguous.

Good GD&T practice is to dimension from your datum features. In SolidWorks, I have modelled parts, and then added views to the model that rotated my datum features into orthogonal positions. This allowed to me to do clear drawings.

--
JHG

 

[filippoGDeT]

Dear Both,

Thank you very much for your replies.

Drawoh, My intention is to realize the best tolerance strategy in order to dimension the part that will be located in the fixture ( only temporary ) during assembly operation through mounting holes.

Coming back to the diamond shaped and round pins location techniques, if I replace the round pins with a conical shaped pins, theoretical I can relax the hole tolerance for the first Datum. Am I correct?

In fact , the aim in my locating strategy is only to locate the theoretical axis that will refer all dimension on the part....the diamond shaped pins will allow my location tolerance to be larger but keeping , at the same time, good orientation around the first datum axis... Is my assumption correct to your eyes?

Thank a lot in advance.

Filippo R.

 

[drawoh]

filippoGDeT,

A conical pin will disable your primary datum, which I assume to be the bottom face.

The tertiary diamond pin allows you to control rotation of your part, even though the distance between your holes is not well controlled.

An alternate tertiary rotation control would be a straight edge, orthogonal to your coordinate system. I have no theoretical objection to a tertiary pin outside your coordinate system, but I am not the guy who has to build fixtures, or inspect this thing.

--
JHG

 

[SeasonLee]

The first problem is that the second hole is not aligned with the view and the DRF will be thus rotated respect to the view. Can I specify a TED angular dimension to rotate the DRF ? Is that correct from GD&T rules?

The tip from late professor Don Day will give you the replies you needed, here is the tip.

https://www.youtube.com/watch?v=qMn_Tx0Yx1c

Season

 

[3DDave]

What does the mating part do to locate this part? Consider using the same setup for inspection as you do for installation.