Casting- Modeling

[pratyu]

I've come across a drawing where in there is no edge radii incorporated on the casting. My question is

Is it possible to cast a part with sharp internal edges? (without any edge or corner radius?)

 

[IRstuff]

"Sharp" to what criteria? Your edges are limited by the process by which the mold is made, are they not?

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[btrueblood]

Short answer to the OP question is yes, you can have sharp outside corner (edges) anywhere on a casting, but most typically where a parting line or mold/core join is made. Sharp internal inside corners can exist too, but should generally be avoided as they are stress risers in the cast part. As IRstuff said, it is limited by the process - for internal edges, the strength of the "core" dictates what you can do.

 

[rb1957]

but most likely the draftie forgot, or there's a drawing note ... "break all sharp edges", "min edge radius ..."

Quando Omni Flunkus Moritati

 

[WKTaylor]

Pratyu...

Be careful: There are external corner radii, internal corner radii, edge radii and fillet radii... Your term "internal edge" leaves me puzzled as to what You actually mean... just guessing I think You meant fillet radii.

ASME Y14.8 Castings, Forgings, and Molded Parts - Engineering Drawing and Related Documentation Practices clearly shows how a casting drawing should be made.... including definition of critical radii.

When drawings fail to specify necessary details that allow a "caster" [casting vendor] to make a satisfactory ["sound"] part, they usually refuse to make the casting without minimum mandatory radii all around. Corner, edge and fillet radii details seriously affect molten metal flow, solidification strains, etc... leading to distortion, laps, cold-shuts, cracks, voids, etc.


Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[pratyu]

All,

I'm a newbie to all this casting stuff... so, my understanding is pretty limited. Please let me know if my perception is wrong!!

IRStuff,
By the word Sharp, I meant, an edge without any fillet.

btrueblood,
Your reply was a good insight of the answer I was looking for!
I have another doubt, if the cast part can have sharp corners (internal or external), then the mold used for the same would also have sharp corners (if I'm not wrong!). How can the mold or pattern be manufactured without any blends? (either wax or steel..)

rb1957,
yes.. those notes were mentioned... but break sharp edges applicable for only the external corners (please correct me if I'm wrong!)
The general note was present but the drawing views did not show any sight of blends!

wktaylor,
Internal edge was the term, maybe i christened due to my limited knowledge!
My intent was referring to the edge formed by the two faces, which is internal i.e., the edge would require an addition of material for forming a blend, unlike the external edge which would require removal of material to form a blend.
Your explanation regarding the formation of flaws, I would agree. I came across a drawing that had sharp corners and I wouldn't want to leave it at that because of the general note which mentioned a general edge radius. The views in the drawing didn't reflect any resemblance to the presence of blend around the edges. So, it got me confused whether to include the blend or not!

I would appreciate if you can suggest how I can go about it!

 

[BrianGar]

A mould with sharp internal meets/edges/angles/etc can be made in wax easily, or in steel easy enough too depending how much money you have.

Wax - pour wax over the model to form tooling. Make/pick partline with card and clay for first mould strike. Look up this process if you dont know about it.

For steel, rough machine out mould cavity. Then, machine half the model from copper - you will be able to create sharp corners, and use this die to spark erode out the rest of the steel tool. This is called edm - electronic discharge machining. Maybe look that up too.

OR,

Cast oversize with a fillet radius of your smallest bull-nose cnc bit that you can get decent feed speeds from, and once cast, machine both faces down to form a square corner on part.

EDM is not cheap, and makes the cost of steel tooling even greater. If you can at all, and if you can gain access with a cnc, add a radius in there of whatever bit the machinist is intending to do his final pencil milling 3d profile operation with. Of course, if you want a very fine finish, you will have to EDM or polish anyway to rid tool of fine marks and/or attain/regain sharp corners.

You haven't mentioned if you are gravity casting, or die/pressure casting, or the material....
If gravity, the alloy may freeze off before it makes it all the way into a sharp corner - so going to all the trouble of make a sharp intersection may be wasted.
If its pressure casting, then yes, the alloy will make it into all details including corners and machining marks...

Is it alloy or a setting polymer...if its alloy consider investment casting since the tooling can be cheaper as it only needs to contain wax. If its a setting polymer, consider silicone/resin tooling.

Brian,

 

[KENAT]

It will depend on a number of factors, but some processes can give relatively sharp internal corners.

It's a convention for simplification of drawings to often leave off small 'break edge' or 'minimum internal radii' and just put notes covering it. For moldings & castings this can occasionally cause some confusion/issues - especially if the CAD model is used directly for making the mold.

What material & process are you looking at?

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[pratyu]

The material is a titanium alloy & process is investment casting.