casting and machining on seperate drawing

[bxbzq]

Hello,
In the company I work, we have separate casting and machining drawing. On the casting drawings, we use phantom lines on the view to show the machining outlines. One thing I have doubt is we attach datum feature symbols to some of these phantom lines to specify them as datum features. I'm wondering if this is legal because they are non-exist features on casting. Per para 4.24.13 in '09 std, "The datum feature symbol should be attached only to identifiable datum features.". '94 std has same statement. Any thoughts?

 

[MechNorth]

No, it doesn't make sense and isn't supported by the standards. On the other hand, cast material which is to be removed is sometimes shown on a machined component drawing using phantom lines, and the cast datums shown on those phantomed (removed) surfaces, with machined datums shown on the finished part.

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[KENAT]

bxbzq I'd take a look at Y14.38. Like you I don't believe machining datums should be shown on the casting drawing (though inversely casting datums should normally be shown on the machining drawing).

The subsequent machined surfaces may be indicated by phantom lines on the casting drawing as you do, I just don't believe it's appropriate to indicated them as datums.

(Mech North, I thought the casting datums were meant to remain on the machined part see ASME Y14.38-1996 4.3.1d.. Plus a quick look at 14.38 & I'm not sure I see phantom lines showing as cast surfaces on machined part drawings.)

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[ctopher]

I agree having separate drawings for casting and machining parts.
The casting drawing should not indicate machining data. One reason is if the machined part changes, the casting drawing may change.
If the part is cast at another company, they don't care about the machined data.
The machine drawing can call out cast surfaces if they are not machined.

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[bxbzq]

Thank you for the comments. Y14.8-2009 describes a combined drawing method, casting outline can be shown on machining drawing by phantom lines. But I was not aware casting datums are sometimes shown on machining drawing.

 

[fsincox]

In my opinion casting datums should be used like the old "start" dimensions they replaced, only once to set up the machined/functional datums and not really referenced again. I have seen lots of trouble caused by people constantly referencing back to cast datums in new and different frameworks whenever they needed an extra datum here or there.

 

[bxbzq]

Here comes another question about datums of casted parts. We always say datum selection is based on part functional assembly. For machined (finished) parts, this is relatively easy to figure out. But on casted parts, many times the functional features, mating surfaces for example, are only accessible on machined part (material removed). So in this case, what are the typical considerations when selecting datums for casted parts?

 

[KENAT]

bxbzq, you apparently have a copy of Y14.8 so take a closer look. Unless a lot changed in the -2009 standard it has the information you need, section 4 of my 96 version has a bunch of guidelines for datum target location at 4.3.1.

My basic understanding is that the machined datums should be related to the cast datums - hence the cast datums need to be shown in the machining drawing section 4.4 of the 96 edition. Set the datum targets up in such a way that the machined datums are easily derived from them - ideally opposite. You still try and capture some logical function - such as a minimum wall thickness or some such - but it may not be as clear/simple as in a typical machined part.

Sorry Ctopher but the foundry may indeed care about what surfaces are machined for several reasons, including where to put gates & risers etc. (if not explicitly stated on the drawing) and where to do the usual cast lot marking etc. Yes making sure the cast drawing gets kept up to date with the machining drawing can by tricky but there is justification in some cases.

Fsincox, if a full machined reference frame is set up then what you say is I believe correct, however if you have a casting with minimal machining can it not make sense to reference at least some or all of the cast datums? I had a part recently with minimal post casting machining - just a few holes and little 'trimming' and I just kept the cast datums. Arguably I could have established machined datums using selected holes but I think it would have been

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[MechNorth]

Kenat, as indicated by bxbzq, Y14.8 shows the phantoming of the cast surfaces. Cast datums can either remain on the final part, or be machined off. My preference is for them to remain if possible in case you want to set the part back up later for some clean-up work. Machined datum features should be controlled back to the cast datums as well as to the higher precedence machined datums. All other machined features should be controlled wrt the machined datums only. As-cast surfaces shold be controlled back to the cast datums.

Some considerations for cast datums are;
1) machine-off or leave on
2) surface conditions (roughness, form, shape, etc.)
3) stability of workpiece
4) stability of cast datum feature (a thin section may not be ideal as it may shift more during cooling)
5) proximity of machined datum features to the cast datum features

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[KENAT]

Sorry Mechnorth, but Y14.8-1996 4.3.1d seems pretty clear.

"Datum targets should be located as follows... ...(d) on features no subsequently altered or removed"

Of course it's a 'should' so you can argue just how strong of a 'suggestion' that is but there are very few "shall" statements in any of the standards any more so I generally take a 'should' as a 'shall' unless there's a really good reason to do otherwise.

As to the phantom lines showing the cast surface, I only see this for combined views. This is not the situation the OP says they are in since they explicitly mention separate casting and machining drawings - am I missing something?

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[MechNorth]

Kenat, as you indicated, "should", not "shall". Personally, I wish they would use definitive language more often so that things would be more clear, but that won't happen because you can't get 100% consensus in any industrial group. The reality, though, is that many designs set up cast datums on features that are machined off. As I said, not my preference, but definitely reflects a legal and common practice.
As for the last comment, I just posted an appropriate use...it's something we tend to do in this forum ;~}


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[KENAT]

Not really, I'd argue there's a distinction between machined part drawing and combined views but it's too late for me to put much effort into it.

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[MechNorth]

I believe you have a much more formal drafting background that I do, Kenat. I suspect you were more appropriately trained in the use of views, drawing types, etc. Unfortunately, I was not. I was trained under the "do the least work to convey the content" banner, though I pushed well beyond those limits long ago, to learn some better practices, and still do pick up tricks from you and others on the forum. Classical drafting practices aren't taught much anymore, and while most companies want those skillsets revived now, they aren't willing to pay for the apprenticeship costs associated with them; as a result, we get a mish-mash of drawing and view types on the same specification. That's where I'm coming from with my posts, above; it's what I see frequently in practice in many different companies and industrial sectors. One of the issues in this and all other forums is the distinction between standard and practice.

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[fsincox]

Ken,
Understood, I am talking of much more complicated parts where I see people reuse cast datums in entirely new frameworks. Not parts setup on cast datums and machine complete in one setting. I still prefer a functional definition of features if at all possible, though.

 

[looslib]

I like the idea of separate machine and casting drawings with both showing a phantom line view of the other information.
The casting house needs to know which surfaces are not machined if the cast lot is to be on a surface that is not machined away.
The machine shop needs to know which surfaces are not machined and also the amount of stock on the machined surfaces.

There may be the need for multiple views on the casting drawing if one casting is used to produce multiple finished parts.

Datums should be defined for each state of the process and only carried through if it is the same surface.


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[SDETERS]

Looslib How do you keep your phantom lines consitent between the two different drawings? So if the Machining Changes you have to change the RM drawing also?

I guess if you tolerance your Rm drawing and machining drawing to leave a min cleanup of .01 why do you need to show the machining features on the RM drawing. We produce two different drawings and or models which have their own respective drawings. The only thing copied from one to the other is showing the RM datums on the Machining drawings. Single source of truth.

Thanks

 

[KENAT]

MechNorth, sadly no not really formally trained in drafting though I take it as a compliment you though so.

Did serve a year - 18 months under a really good checker and learned a lot from him and from various other older, more experienced folks however very limited formal training.

In the 'finer things of life' like casting drawings I'm mostly self taught from referencing the standards.

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[MechNorth]

A strong mentorship with support of the standards often beats what is available in school anyway. I'm still saving my pennies to buy more of the standards for my collection.

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