datum features for casting 1

[bxbzq]

Hello,
I attached a draft showing my part, machining datums already established. Given the machining datum references, I'm now interested in the datum features for casting, especially primary and tertiary datum feature selection. The questions are listed in draft.
Any comments are welcome.

 

[bxbzq]

Paul,
If the surface you put AA on and it's opposite surface (at the right end) are all machined off, how would you set up datum targets?
We actually have parts like this.

 

[PaulJackson]

bxbzq,
You could use recessed target areas on the casting that are not removed when the surface is machined... but you have to make them friendly to section pulls if die casting. Or in this part you could change AA to be VEE contacts with maximized separation on the conic surface (constraining 4 DOF) and then add protruding tabs @ AB1/AC1 and @ AB2/AC2 with mix of movable and fixed target constriants to arrest the two remaining DOF (rotation about the axis[moveable]and translation along the axis[fixed]). I would only entertain this strategy if it was still economical to separate the casting tolerance constraints from the machining tolerance constraints... if not just tolerance all remaining cast surfaces from [A|B|C].
Paul

 

[bxbzq]

Like this, if the targets X are movable horizontally in your vee type primary strategy?

 

[pmarc]

bxbzq,

Are you sure that your datum targets Z1 and Z2 will do the job in establishing primary datum axis? Or is primary datum axis going to be derived from datum targets Z and Y? If the later, then Z and Y should be placed in the same compartment of FCFs separated by dash symbol. In general I recommend reading para. 4.24.10 and looking at figs. 4-50 through 4-52.

Are targets Z not on the part?

Is current datum targets scheme constraining the casting translationally in axial direction?

Also if your intention is to have datum targets X1 and X2 movable horizontally, I would say your print does not tell that. The fact that triangular portion of movable datum target symbol is shown horizontally does not automatically mean the simulators will move this way. According to the letter of the standard they will rather move along the lines connecting movable datum target symbol and true profile of the part. (What is the angle of the movement? I do not know, becasue your print does not tell it). If you want to change it, use the technique shown in fig. 4-47 or 4-49.

 

[PaulJackson]

Like this, if the targets X are movable horizontally in your vee type primary strategy?

No more like this... see attached

 

[bxbzq]

Paul,
Z targets are much better to stabilize the part. But, add a tab on the casting? Never thought about that. An example of Design for Manufacturing/Inspection/GD&T?

pmarc,
GD&T on casting is my weak area. I want some real life experience here. Our shop falls 200 years behind, I ain't no better.

 

[bxbzq]

Paul,

One more question, the surface B and C in my sketch are the surfaces mating with other parts. They should be two different datums on machining, why did you say "width should be declared secondary rather than one of the sides."?

 

[PaulJackson]

Bxbzq,
Why would you declare the surface opposing as [C] when it cannot constrain any DOF that B has not already constrained?
The only remaining DOF unconstrained is translation toward or away from your eyes in that view and [C] cannot constrain that! However one (or both) of your assembly pilot diameters can.
We should never use datum feature designations to identify important features... we use them to constrain DOF in a feature control frame.

Now about the width being the secondary rather than one side... what is functional? Would you like to keep the flange machining width more central to the casting DRF or would you rather one end be targeted (to its basic) or (to its limits) and the other vary with the OAL tolerance or does it matter at all... think functionally and design your tolerancing to reflect it... but also keep it as simple as you can. I thought that you might wonder about that... I am not advocating that strategy here I was just tying in to an earlier comment.
Paul

 

[bxbzq]

Paul,
My part is similar to fig.4-4 in '09 std. Each end is to mate with another part. I did not model bolt holes and many other features. The main difference here is my part is a half cone shape. Probably my sketch is too simplified that makes you guess the function, sorry for that.