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.

 

[PaulJackson]

One possible senario... incomplete albeit for target boundary basics
Paul

 

[KENAT]

bxbzq do you have a copy of ASME Y14.38? It may be of use.

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[bxbzq]

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?

 

[bxbzq]

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.

 

[drawoh]

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

 

[KENAT]

Sorry, yes Y14.8 is the casting standard - appologies.

Do you have a contact at your foundry you could talk to about it, obviously they'll be a bit biased but they may be more used to seeing this kind of thing.

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[PaulJackson]

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

 

[PaulJackson]

Oops,

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.

 

[ewh]

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

 

[PaulJackson]

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.

 

[bxbzq]

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.

 

[bxbzq]

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.

 

[KENAT]

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.

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[PaulJackson]

See attached

 

[PaulJackson]

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???

 

[SDETERS]

How are you holding the the casting to machine it?

 

[bxbzq]

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

 

[bxbzq]

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.

 

[PaulJackson]

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