The Drawing Review That Saves Both Sides a Sample Round
Datums first, tolerances second
A datum scheme built on cast surfaces that are themselves variable pushes every downstream tolerance onto the casting process. Datums should sit on surfaces the die controls well - parting-face-adjacent areas, machined pads, or features located by fixed cores. Ask where the casting will be held during machining and whether the datums match that fixturing.
Profile and flatness callouts on large cast faces should reflect what the die and thermal cycle realistically hold. A flatness call across a broad open face fights die distortion and thermal cycling; if the function allows it, break the call into zones or move it to a machined pad.
Notes that decide the quote
Flag which surfaces are machined and which stay as-cast, with the machining allowance shown rather than implied. Thread callouts should state whether holes are cast, drilled, or tapped, because each route has different scrap economics.
Inspection notes matter too: which features get CMM reports, which get gauged, and how porosity class or leak test is specified. A drawing that answers these questions gets an accurate quote; one that leaves them open gets an optimistic number and a surprise after T1.
FAQ
Why do datum choices matter so much on die castings?
Datums on variable cast surfaces transfer every tolerance to the casting process. Datums on machined pads or die-controlled areas keep tolerances where they can actually be held.
Can flatness be held across a large cast face?
Only within limits set by die distortion and thermal cycling. Broad flatness calls are better replaced by zoned calls or moved to machined pads where function allows.
What should be flagged on a die cast drawing before quoting?
Machined versus as-cast surfaces with allowances, thread manufacture routes, inspection scope, and any leak or porosity class. These drive both tooling design and unit cost.
Who should review the drawing?
Ideally foundry and machining together, before tooling starts. A joint review catches datum conflicts and unrealistic callouts for the cost of a meeting instead of a tooling change.
All figures above refer to standard product data sheets; final confirmation is subject to written quotation.