Cast It, Then Cut It — in That Order
What should be cored and what should be cut
Coring a hole saves material and machining time but leaves draft and a rougher wall, so a cored hole is usually a starting point for a drilled or reamed finish rather than the final feature. Threads are almost never cast; they are cut or formed after casting, because cast threads cannot hold a tolerance and the die features that form them wear quickly.
The practical rule is that anything functional gets machined and anything cosmetic can stay as-cast. A hole that only passes a cable can stay cored. A hole that locates a bearing cannot. Deciding this at DFM stage avoids the common situation where a cored hole has to be plugged and re-drilled.
Thread depth, bosses and clamping
Thread depth in a die casting is limited by the boss that surrounds it. A thin-walled boss will bulge or crack when the tap cuts, and the wall thickness needed depends on the alloy and the boss diameter. Short engagement is usually better than a deep thread in a thin wall, and a through-bolt with a nut is often the more robust answer where depth is constrained.
Clamping also matters: castings vary in thickness more than machined parts, so a fixture that clamps on an as-cast face can distort the part before the tool touches it. Clamp on machined datums or on defined pads, and specify which surfaces are datums on the drawing — otherwise the second operation inherits the variation of the casting.
FAQ
Can threads be cast directly into a die casting?
They are generally cut or formed after casting. Cast threads cannot hold tolerance reliably and the die features that form them wear quickly.
Why not leave every hole as-cast?
Cored holes carry draft and a rougher wall, so they suit clearance features but not locating or bearing features, which need machining.
What limits thread depth in a casting?
The boss wall around the thread. Too thin a wall bulges or cracks when tapped, so a shorter engagement or a through-bolt is often the better design.
Why does clamping matter for second operations?
Clamping on an as-cast face can distort the part before machining. Clamping on machined datums keeps the second operation consistent with the first.
All figures above refer to standard product data sheets; final confirmation is subject to written quotation.