Key takeaways
- Function, datum, tolerance and finish are the shared questions in every process review.
- The process-specific questions are about how the shape is created, not about the drawing style.
- A part that moves from one route to another needs the transition features, such as stock and fixturing, reviewed again.
- A DFM review is a written record of decisions, not a conversation at the quotation stage.
One checklist, three route choices
Machining, casting and extrusion are different ways to create a shape, but they share a design review. In every case the part has to fit, carry its load, be measurable and be made at the required volume. The first pass of a DFM review should answer those questions before the process is chosen.
The mistake is to start with the process and then force the design into it. A part that was drawn for machining may have pockets that a casting cannot release, and a part drawn for casting may have walls that a machining route cannot reach economically. Review the function first, then test the shape against each route.
This combined checklist is a way to do that in one meeting. It does not replace the supplier's process review, but it gives the supplier a better starting point and makes the trade-offs visible.
The shared questions
State the function of each feature. A face that locates a mating part, a bore that carries a shaft and a surface that only looks flat need different tolerances. If every feature is drawn to the same tight tolerance, the supplier has to choose between an expensive route and a non-conformance.
Choose the datum scheme before the process. The datum should reflect how the part is located in assembly and how it will be held for inspection. A datum that only exists on the drawing and cannot be fixtured will move the problem to the shop floor.
Define the finish where it matters. A sealing face, a sliding bore and a cosmetic cover have different surface requirements. State the parameter, the value and the method. A general note to make the part smooth is not a specification.
Tolerance is a cost decision
Every tight tolerance should have a reason in the assembly. If two features are tightly toleranced but never interact, the tolerance is probably buying cost without buying function.
Machined parts
For a machined part, the review is about access, rigidity and the number of setups. A deep pocket needs a tool that can reach the bottom without rubbing. A thin wall deflects under clamping and cutting force. A feature on five faces may need five setups, or a five-axis machine, and both change the price.
Check the corner radii against the cutter. An internal corner cannot be sharper than the tool, and a small radius at the bottom of a deep pocket forces a long, thin tool. Check the thread reliefs, the undercuts and the distance from a hole to a wall.
State the material condition and the heat treatment. A pre-hardened material machines differently from an annealed one, and a stress relief between roughing and finishing can prevent movement. If the part is thin, ask for the fixture and the clamping plan before the first cut.
Cast parts
For a casting, the review is about filling, feeding and release. Check the wall thickness for uniformity. A thick section next to a thin one creates a hot spot and a shrink risk. A sharp internal corner concentrates stress and can crack during stripping.
Check the draft on every vertical face and the parting line position. A parting line on a sealing face or a machined datum adds cost and risk. Check the core access: a closed internal cavity needs a core that can be removed, and a deep narrow core can deflect or break.
Decide the machining allowance with the foundry. Too little stock leaves a defect on the finished face; too much stock adds cycle time and can expose porosity. Mark the critical machined faces and the zones where porosity is not acceptable.
Extruded parts
For an extrusion, the review is about the profile. The shape must have a uniform wall, a balanced section and no closed hollow that needs a mandrel unless the supplier can support it. A profile with a thin fin that stands alone will distort as it leaves the die.
Check the aspect ratio of the fins and the tongue ratio of any leg. A long, thin projection cools at a different rate from the body and can twist. Check the die cost against the order quantity: an extrusion die is usually cheaper than a casting tool, but it is still a fixed cost.
Decide the cut length, the tolerance on the cut and the flatness after cutting. An extrusion is straight in the run direction, but a cut can release stress and bow the part. If the part is machined after extrusion, say which faces are used for location.
Where the checklist changes by route
The shared questions stay the same, but the answers and the red flags change. The table is a review aid, not a set of hard limits. Every limit depends on the alloy, the process and the supplier.
| Design question | Machined | Cast | Extruded |
|---|---|---|---|
| How is the shape made? | Remove material from stock | Fill a mould or die | Push metal through a die |
| Main geometry risk | Tool access and rigidity | Filling, feeding and draft | Profile balance and distortion |
| Wall thickness | Thin walls deflect | Uniform walls avoid hot spots | Uniform walls flow better |
| Internal corner | Cutter radius sets the limit | Fillet reduces stress and shrinkage | Die radius and stress |
| Holes and pockets | Any position with tool access | Formed by core or later machined | Along the profile or later machined |
| Typical finish | As machined, then optional | As cast, then machined faces | As extruded, then cut or machined |
| Cost driver | Setups and cycle time | Tooling, yield and fettling | Die cost, profile speed and cut |
A hybrid route is common: a casting or extrusion provides the near-net shape, and machining creates the critical features.
The transition zone
When a part moves from casting or extrusion to machining, review the transition as its own design. The near-net shape must leave enough stock on the faces that will be cut, but not so much that the cycle time becomes the main cost. The datum for the first machining setup should be a surface that is stable and accessible.
Check the fixture. A casting with draft cannot be held in a vice without a custom jaw or a fixture. An extrusion with a thin fin cannot be clamped on the fin. A machined part with a thin floor may need a support underneath during the final cut.
Check the distortion after each step. Stress relief, straightening, anodising and heat treatment can all move the part. If a tight tolerance is measured before these steps, the final part may not pass.
Run the review as a written record
A DFM review should produce a list of decisions, not a set of opinions. Record the feature, the issue, the decision and the owner. If a tolerance is relaxed, record why. If a feature is kept, record the cost or risk that comes with it.
Invite the supplier's process engineer, not only the salesperson. The person who runs the machine or the die can see a problem that the quotation will not show. Ask for a sample or a simulation where the risk is high.
Close the review before the tooling is released. A change after the die or the fixture is built is far more expensive than a change on the drawing. The checklist is there to make that conversation happen early.
Frequently asked questions
Can one DFM checklist cover machining, casting and extrusion?
Yes for the shared questions of function, datum, tolerance and finish. The process-specific sections still need to be reviewed by someone who knows that process, because the failure modes are different.
What is the most common DFM mistake?
Tightening every tolerance instead of the ones that affect function. That forces an expensive process, slows inspection and hides the features that actually matter.
When should the supplier join the review?
Before the drawing is frozen and before the tooling is released. A process engineer can catch a draft, access or fixture problem while a drawing change is still cheap.
How do I review a part that may be cast and then machined?
Review both stages together. Check the casting for draft, walls and stock, then check the machining for datum, access and distortion. The two reviews share the same datum scheme.
Sources and further reading
- ISO 2768-1:1989, General tolerances, linear and angular dimensions
- ISO 8062-3, Geometrical product specifications for moulded parts, casting tolerances
- ASM Handbook, Volume 16, Machining
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Want a combined DFM review on a new part?
Send the drawing and the intended process. The engineering desk will reply with the shared questions, the route risks and the changes to settle before tooling.
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