Definition of CNC Machining Tolerances for Die Castings
The distinction matters because a drawing rarely calls for the same accuracy everywhere. Sealing faces, bearing bores, threaded holes and mating surfaces are machined; cosmetic surfaces and general body geometry are left as cast. A supplier reading your drawing should tell you which features it intends to machine and what it can hold on each.
JoinCast machines in-house at its Taichung plant rather than sending castings out, so the tolerance discussion happens with the people who cast the part. Taiwan die casters that combine casting and CNC work under one roof can adjust gating, wall thickness and machining stock together when a tolerance proves difficult to hold.
Why This Matters for Your Business
The second consequence is inspection cost. Tight tolerances on many features mean more measurement, more fixtures, and in some cases a CMM routine (not a hand gauge at the bench). That cost sits in your price whether or not it is itemized, so specifying tight tolerances on features that do not need them raises the price without improving the part.
It is worth checking whether a supplier's stated tolerance is a capability or a commitment. A shop that can reach plus or minus 0.02 mm on a good day is not the same as one that holds it across a production run and can show you the measurement data.
FAQ
How do I confirm that a supplier's CNC machining tolerances for die castings match my drawing?
Ask for CNC machining tolerances for die castings feature by feature rather than as a single headline figure. Send the drawing and request a marked-up copy showing which dimensions the supplier will machine, what it expects to hold on each, and which gauges or CMM routines it will use to prove it.A first article inspection report on the sample parts should then show actual measured values against your nominal, not a pass or fail stamp. Our quality inspection process at JoinCast sets out the measurement stages we run from incoming material through to final shipment, which gives you a sense of what to ask any supplier for.
Which features usually require CNC machining tolerances for die castings rather than as-cast accuracy?
CNC machining tolerances for die castings apply to the features that have to fit, seal or locate something else. Bearing bores, sealing faces, flange mating surfaces, threaded holes and datum pads are the usual candidates.General body geometry, ribs, cosmetic surfaces and draft faces are normally left as cast, since machining them adds cost without changing how the part works. The practical test is whether another component touches the feature. If it does, and the fit is controlled, it is probably a machined feature. Our CNC machining capabilities at JoinCast describe the machine types we use for these operations and the part sizes each one suits.
Does tightening CNC machining tolerances for die castings increase my unit price?
Tightening CNC machining tolerances for die castings raises price in three ways: slower cutting and lighter finishing passes, more measurement per part, and a higher scrap rate when a feature falls outside limit. The increase is rarely proportional.Going from plus or minus 0.1 mm to 0.05 mm on a bore is often close to free, while going from 0.05 mm to 0.01 mm can add a separate finishing operation and a dedicated fixture. Rather than specifying one tight tolerance across the drawing, mark the two or three features that control function and let the rest run at standard machining accuracy. Our die casting production process explains where machining stock is planned into the casting.
