Casting datum features 1

[greenimi]

I would like to know, from your perspective, which features should be assigned as DATUM FEATURES when trying to define casting drawings?
Otherwise stated, what would be some good selection criteria in order to successfully define such drawings (casting drawings)
Look like there are many choices and multiple candidates for datum features for those kind of drawings.

 

[Burunduk]

Seems like ASME Y14.8-2009 provides some useful guidance on this matter:

4.31 Datum Target Location
Datum targets should be located as follows:
(a) on features produced by one segment of a die or
pattern (see Fig. 4-2) except in the case of equalizing
datum targets (see Figs. 4-3 and 4-4)
(b) on features opposite machining cuts that establish
a subsequent machining datum reference frame (see
Fig. 4-5)
(c) on features not subject to processing variables, such
as parting lines, fl ash extensions, etc.
(d) on features not subsequently altered or removed
(e) with optimum spacing considering function and
producibility (see Fig. 4-4)
(f) on noncoplanar features where the area or location
requires one or more datum targets offset from the datum
plane (see Fig. 4-6)

 

[SDETERS]

Will This casting get machined? If so how are you holding the part during the machining process? We have a Good practice of retaining our cast datums after the machining process. We typically have the Raw Material datum Setup simulate the how the part will be held during machining.

 

[SeasonLee]

We have a Good practice of retaining our cast datums after the machining process. We typically have the Raw Material datum Setup simulate the how the part will be held during machining.

Machining on the casted part is very common, would you mind to share some practical examples before and after machining.
Thanks in advance.

Seasin

 

[greenimi]

I agree with SeasonLee.

SDETERS,

Could you, please share some practical exampless.
I see the ones provided on 14.8 (casting standard), but still would like to understand more.

 

[SDETERS]

I have made a really simple sample example. This was done really quickly no draft or fillets. We deal with aluminum die castings mostly. We also add material for datums and machine holding.
So this example I have 3 datum targets, for X, which is recessed from the top of the box which the top will need to be machined, and then we have datum hole Y which a machining pin will fit into, and Z a datum slot which a diamond pin will fit into on the machining fixture.

The machine part will have the top of the box machined off, this will become the primary A datum, two dowel pin holes will be machined which becomes datum features A and B.

Notice on the final machine part the raw material feature stay on the part, and we reference the machine datums back to the RM datums.

Does this help?



[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1628691194/tips/RAW_MATERIAL_DATUMS_039919_fchirj.pdf[/url][URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1628691200/tips/MACHINE_EXAMPLE_039920_jc194k.pdf[/url]

 

[greenimi]

SDETERS,

Yes, it does help very much.
The only thing I am questioning is the practice of using machined datum features and casting datum features in the SAME feature control frame.
A|Y|Z| and A|B|Z|,
A and B being machined datum features and Y and Z being casting datum features.

Why not X|Y|Z| ?

Or A|B| with no Z

 

[SDETERS]

When we are doing our tolerance stacks, we use some features that are as cast, and some that are machine. So we need to tolerance stack back through the machine datums, and back to the RM datums, then to the as cast features, which has profile or position back to the RM.

You could use AB with no Z if you wanted to, I think it depends what features you are using in your design.

We do the RM to Machine datums a bit different depending on the design intent and what the design and manufacturing requires. Like you mention and question.

 

[greenimi]

SDETERS,

Wouldn't the tolerance stackups be easier if the same type of datum features will be used? Otherwise stated, castings with castings / machined with machined.
Just curious how your stackup works "correctly" and why it is more meaningfull to do it accross functional features/ different from each other.

 

[SDETERS]

Imagine a shaft that goes through two different castings that are bolted together. Then this shaft needs to clear an as cast rib that is in the first main casting. We would identify the gap shaft to the as cast rib. Then we would have to tolerance stack From the rib to the RM datums, then Rm Datums to the machine datums, then to the bearing bores, through the bearings, through the shaft tolerances, and we end up back to the as cast rib. This would Complete the tolerance stack loop. Now if we are just stacking machine features we would only use the the machine datums like you mention. It will depend on what tolerance stack you are trying to figure out.