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datum features for casting

datum features for casting

datum features for casting

(OP)
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.

RE: datum features for casting

(OP)
Paul,

Thanks for the comments.
Secondary datum feature of machining is left or right end surface of the part, that is the function.

If I interpret correctly, you are trying to establish axis from datum target points AB1 and AB2. Datum target points AC1 and AC2 do the clocking. My question, can AB1 and AB2 establish an axis? And why are AC1 and AC2 movable radially, not vertically?

RE: datum features for casting

(OP)
KENAT,
I don't have y14.38, but i have read through 14.8, still could not figure out a casting datum scheme that I'm comfortable with.

RE: datum features for casting

bxbzq,

I recommend casting datum features in place on the casting, or otherwise, calling up datum targets. That way, everybody works off the same datums.

--
JHG

RE: datum features for casting

bxbzq,

My thoughts were to use the widest spread of target areas on one of the only "limited draft" flange surfaces for the primary datum and to register the outer-cone surface with only one "vee type" near its center to balance its error in size and conicity. I then modified that strategy to include two rotation targets that would be a proud of the "vee type" movable targets at some fixed radius.

The goal is to balance the casting, create a stable and repeatable DRF that remains post machining from which the machining DRF can be interrogated for conformance.

It was just my first impression of the task... not knowing the function or design outcome goals. It is incomplete and may not be functionally relevant what-so-ever. If either B or C is the functional machined secondary then perhaps both B and C are the functional secondary and the width should be declared secondary rather than one of the sides.

Paul

RE: datum features for casting

Oops,

Quote:

I then modified that strategy to include two rotation targets that would be a proud of the "vee type" movable targets at some fixed radius.
I meant at some fixed radius relative to the movable "vee type" secondary.

RE: datum features for casting

For castings which require precise machining, I have used 6 target areas to define the three primary datums(1" diameter - 3 for datum A, two for datum B and one for datum C) spotfaced into the raw casting, all mutually perpendicular, located to each other by basic dimensions and to appropriate cast features. These datums did not have a FCF associated and defaulted to established tooling/gaging tolerances. This allowed the machinist to have a precise foundation for his machinig operations, and worked quite well.

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

RE: datum features for casting

Quote:

My question, can AB1 and AB2 establish an axis? And why are AC1 and AC2 movable radially, not vertically?

I will answer your question with a question... An axis relative to what? The targets here are a system of registry areas and points the establish a DRF. The moveable targets AB1 and AC1 would be fixed together as would AB2 and AC2 at some included angle relative to one another and the "axis" or intersection of DRF planes would be determined by the system [AA|AB|AC] when all target areas are registered simultaneously. The axis you are referring to I suppose will be some fixed displacement above AC1 and AC2, equidistant from AB1 and AB2 and perpendicular to AA1, AA2, and AA3.

RE: datum features for casting

(OP)
It's more clear now. AB1 and AB2 establish second datum plane and AC1 and AC2 establish the third. Then, does the second datum plane pass through the intersection of AB1 and AB2 or does it pass through axis of cone? I'm trying to picture the DRF that will be origin of measurements.

RE: datum features for casting

(OP)
ewh,
thanks for sharing.

Paul,
By the way, I would be more comfortable to see a primary datum on casting tied directly to primary on machining. Similarly to secondary and tertiary.

RE: datum features for casting

bxbzq, while that would be preferred, since the actual datums are mutually perpendicular it's not essential. If you can get a more robust set up by switching the order of datum precedence between casting & machining I believe it's acceptable.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: datum features for casting

Quote:

Paul,
By the way, I would be more comfortable to see a primary datum on casting tied directly to primary on machining. Similarly to secondary and tertiary.

Not sure what you are saying here... I provided an example of profile tolerance applied to the machined primary from the casting DRF, likewise the secondary and tertiary???

RE: datum features for casting

How are you holding the the casting to machine it?

RE: datum features for casting

(OP)
Paul,
To be clear, does the horizontal datum plane in your DRF pass through AC1 and AC2?

RE: datum features for casting

(OP)
SDETERS,
The shop typically chooses surfaces that are convenient to them. But those surfaces have material to be removed, like both end surfaces, the dia ø180, ø158.9 in my sketch.

RE: datum features for casting

Quote:

Paul,
To be clear, does the horizontal datum plane in your DRF pass through AC1 and AC2
Yes... my sketch (which is just an example of datum target registry and tolerane linking to the machined features) was attempting to show one plane determined by contact with three areas [AA], one plane perpendicular to the first determined by contact with two moveable points [AB] (equidistant between the points), and one plane perpendicular to both determined by contact with two points [AC] that are moveable with their respective points [AB].

If the targets remain after machining it is typically best to replicate the fixture contact that is used to register the casting for its first machining. That registry strategy should provide stability for machining, locate and orient critical features/clearances and attempt to balance variation across the structure (in other words position the X0,Y0,Z0 near the center of mass).

If the casting targets are removed in machining then the remaining cast surfaces should be toleranced from the machined DRF (few like this scenario but it is necessary to eliminate the "trust me" claim).

Paul

RE: datum features for casting

(OP)
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.

RE: datum features for casting

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

RE: datum features for casting

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.

RE: datum features for casting

(OP)
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.

RE: datum features for casting

(OP)
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."?

RE: datum features for casting

Bxbzq,
Why would you declare the surface opposing [B] 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

RE: datum features for casting

(OP)
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.

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