Print considerations for castings vs fabricated parts

[NDBM_]

Hello all,

I'm very experienced with designing and making drawings for fabricated metal parts (cut, machine, form, join, etc), but now I've stumbled into a situation where I need to design a cast component. I used many resources to make sure that the design itself is manufacturable: draft angles, parting line geometry, uniform thickness, features to encourage smooth mold fill. So I'm not too worried about that aspect.

The drawing on the other hand gives me pause. The geometry definition isn't a problem, I have a lot of GD&T experience from supplying big OEM's in the past. It's all the fiddly ancillary notes that I'm worried about. From my prior work, I know that the devil is in the details when you send drawings to a supplier.

In a perfect world, I'd collaborate directly with a supplier to understand their requirements and capabilities, but these are prelim drawings to go out for quotes. Even so, I don't want to spend a month going back and forth trying to get quotes because I forgot to specify surface finish or something along those lines.


Can anyone give some tips as to what "special" info a caster needs vs a fabricated or machined part? A pile of example drawings would be spectacular, I learn fastest by synthesizing examples.

Edit: I just found ASME Y14.8 and requested they purchase it, but advice from experienced people is always best!

 

[greenimi]

One of the methods (not necessary the "best" one however) is to send them the model and all of those details

It's all the fiddly ancillary notes that I'm worried about. From my prior work, I know that the devil is in the details when you send drawings to a supplier."

will be covered by the notes such as "general profile"

Unless otherwise specified all features are within profile |XXX| to a |specified datum reference frame DRF A|B|C|

More or less like this

NOTES (UNLESS OTHERWISE SPECIFIED):
1. CAD MODEL ___________ REV. __ IS REQUIRED TO COMPLETE PRODUCT DEFINITION.

2. DIRECTLY-TOLERANCED DIMENSIONS AND BASIC DIMENSIONS DEFINED ON THE DRAWING TAKE PRECEDENCE OVER DIMENSIONAL DATA DEFINED BY THE MODEL. OBTAIN ALL OTHER DIMENSIONAL DATA FROM THE MODEL. THE MODEL REPRESENTS BASIC DIMENSIONAL DATA
UNLESS OTHERWISE SPECIFIED.

3. APPLICABLE STANDARDS:
ASME Y14.41-2012 or 2019 APPLIES TO DATASET.
ASME Y14.5-2009, or 2018 APPLIES TO DIMENSIONING AND TOLERANCING.
4. profile |XXX|A|B|C| APPLIES TO ALL UNTOLERANCED SURFACES.
5. UNITS: INCHES/MM

Make a simplified drawing and refer the model to it.
Have drafted features included in the applicable model and define them with +DFT, -DFT, DFT INCL,

 

[jassco]

Hi, NDBM_:

It is not what your vendor needs. You should focus on what you will need.

You should have a 3D model to show the machined version. You should make sure that the cast part has enough stocks to be removed.

And of course, you need to make sure it is feasible.

Best regards,

Alex

 

[NDBM_]

That's good advice thank you. I always used to roll my eyes when I got a drawing from big customers with "Overall features surface profile of X.X" on it like that, but it probably does make sense in a situation like this. I'm not experienced with the process, so put a worst case overall tolerance on it, dimension the critical features, ref the model, and then hash out details with the supplier.

 

[NDBM_]

Thanks for the good perspective jassco. I entirely agree with you. Although one of the things we "need" is to get it done inexpensively on a short lead time . If I can give extra info that makes a suppliers life easier without making a difference to our design requirements, then I expect a supplier's price to reflect that to a certain extent. Also, in this case a casting supplier might actually know what I need more than I do when it comes to manufacturability concerns.

 

[geesaman.d]

First, try to understand which casting process. I work regularly with investment cast stainless, sand cast ductile and cast iron, and some 'no-bake' sand castings. Each one has different costs, part size range, and accuracy.

Second, leave plenty of space for clearance against as-cast surfaces. I don't find it strictly necessary to model the draft angles or know the split planes as long as you have adequate allowance built into the design. Unlike fabrications, where a fixture or simple +0/-x tolerance will prevent an interference, a casting is generally made to a nominal and variations occur in both directions from nominal.

We provide .igs or .stp files of our as-cast component design. We include machining allowances. They add the draft angles and parting lines.

Have realistic expectations for distortion. Unequal cross sections will cool unevenly and of course they will warp. Stainless warps more than steel and iron.

Know your critical (high stress in my case) areas and convey those to the foundry in some way. Or at least figure out where they plan to place the parting lines so you know where the casting will vary from the 3D model. So for example, if you have a thin, rounded edge that needs a very consistent shape, you don't want them to put the parting line there because there will be manual grinding there.

Don't forget to specify hardness limits or other important criteria. I had some rough times when a foundry made some Class 30 cast iron so hard that it could barely be machined. That's when I learned that "Class 30" is 30ksi minimum (no maximum, no hardness controls) and this foundry poured us glassy hard parts. It met the as-written definition of class 30 but it was awful to work with. I'm sure the original domestic foundries were providing Class 30 plus keeping hardness in a range not explicitly called out on the prints. (The joys of reproducing legacy designs and constantly changing vendors)

That's all I dare type without knowing your specifics.

 

[jassco]

Hi, NDBM_:

You need to understand how this cast part will be POSITIONED AND CLAMPED during subsequent machining operation. Datum selection will affect how you design this part. You need to make sure it is designed for manufacturing economically.

Best regards,

Alex

 

[3DDave]

Among things to note are where the gates and risers are allowable, radiographic or other inspection requirements, any heat treating needed.

You are better off if the supplier is back-and-forth than if the are not. The biggest problems I have seen are from the ones with no questions - you will get what's on the drawing, but it may not be what you expect.

They have seen and dealt with places that are unfamiliar with the process and can help better define a usable part, from alloy to parting lines to draft angles.

It sounds, geometry wise, you are well prepared. Have fun.

 

[CWB1]

If you dont have an internal casting process-engineer/specialist then I'd recommend inviting a foundry rep to your design reviews before drafting anything. I also highly recommend modeling not only the rough casting and finished part but also the tooling. Removing a shell/negative of your rough casting model from a solid block gives you an initial cope&drag, and you'll often see issues in the tooling design that are not easily spotted in the part. Also, if you can do a preliminary casting flow analysis then do so.

For prints, you'll obviously want one of the rough casting but likely a second (or more) for additional ops. Foundries will not only remove flash and break edges but also often do rough mill/drill/other ops so inclusions and other quality concerns are readily easily inspected. Be sure to understand every process step, set realistic tolerances, and draft every detail - the guy running the belt and angle grinders removing flash needs details/dims/tols too. Be sure to include applicable notes on cleanliness and preservation (oil, cosmoline, etc). Also, be careful with tolerancing/stacks to allow a healthy range of draft angles for different processes/products - I've seen many castings that were needlessly expensive bc an ignorant engineer required +/- 1/2*.

As mentioned above, dont change prints to accommodate specific suppliers. If a supplier suggests a change specific to them then use the deviation process.

 

[jassco]

Hi, NDBM_:

Unless you are an expert on casting processes and are very skillful in using your 3D software, you should focus on minimum and maximum material removals. You want to make sure you have enough stock to remove, but you don't want to remove too much material. Your manufacture engineer should be able to tell you how many passes your CNC machine need to remove the stocks. That way, you will be protected. Leave other things to your vendors.

I used to design complicated hot forging components. It is rather complicated to determine parting lines/faces let alone warping of the part. It is better to leave them to professionals.

You just need to show these 3 things below to your vendors, and you will be able to get good quotes.

1) Minimum stock removal;
2) Maximum stock removal;
3) How you plan to fixture your part during subsequent operations.

Best regards,

Alex