Tapped holes in a casting

[jdg268]

Hello,
I apologize if this question has been asked before or if the answer is too simple to warrant a posting.

I have an aluminum casting with cored holes. (4 mounting holes 90deg apart, around the part's center). At our maching level print we are tapping all 4 holes. In reality, the tap will follow the hole's existing center. So, what is the best practice or acceptable method of dimenioning the tapped hole locations? Standard dimension, reference, basic???

Thanks in advance.

John Graham CSWP

Mechanical Design Engineer

 

[KENAT]

Do you really get adequate accuracy in location & size from the casting process to serve as the 'tap drill' holes for a pattern of fasteners?

If you truely are tapping the cored holes then I guess at most reference dimension.

KENAT, probably the least qualified checker you'll ever meet...

 

[ctopher]

The tap and hole would be one callout.
It is up to the machinist for the process.
Do a search here, it may have been discussed before.

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion

 

[btrueblood]

What Kenat said, only in +verbose mode:

We do a lot of castings here, and unless we're talking about non-critical threads, we never core the holes - we couldn't hold position tolerances if we did.

That said, your situation may be different. Several parts that I've designed require a test port thru a wall, tapped to 1/4-NPT; the location is uncritical other than to be roughly centered on a cast boss. The tapped hole is located on the machining print by a reference dimension, and the callout states "CENTERED ON CAST BOSS". You should be able to use a similar callout and reference dimensions for your as-cast pilot holes.

 

[jdg268]

btrueblood et al,

Thanks for your input. We like to core our initial holes because of porosity problems. We've found if we machine away too much then we're potentially left with weak threads because the aluminum doesn't always fill as desired.

With our die castings, we can get fairly accurate (+/-.005") hole locations and even tighter sizes. So we don't really have to further drill the hole.

Chris, in this case the tap and drill can't be one callout because a hole already exists. The drill/tapping bit will always want to follow the exising hole. Hence my question about which type of dimension to use.



Thanks

John Graham CSWP

Mechanical Design Engineer

 

[KENAT]

In that case, reference dimension for the location (technically you don't need it and normally they should be avoided but this is one case where they may be of benefit), just call out the thread and something like Btrues note - CENTERED ON EXISTING CAST HOLE or similar.

KENAT, probably the least qualified checker you'll ever meet...

 

[ewh]

I also agree with KENAT. If the as cast location is all that is required, either do not dimension the hole location, or use a reference dimension.

When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - [small]Thomas Jefferson [/small]