Machine Design #87: Directional Materials — Face, Back, and Grain Must Follow the Part
Directional material must remain clear, manufacturable, searchable, inspectable, and maintainable when the boundary changes.
Function before naming
Write the function, input, expected result, failure symptom, acceptance limit, and measurement method before selecting a name, mark, material, or note. Assign an owner to every value.
Core checks
- Material direction and functional load: record value, source, method, owner, and pass/fail evidence.
- Face/back and surface condition: record value, source, method, owner, and pass/fail evidence.
- Grain or rolling direction on drawing: record value, source, method, owner, and pass/fail evidence.
- Cutting, forming, bending, and assembly orientation: record value, source, method, owner, and pass/fail evidence.
- Marking and supplier inspection: record value, source, method, owner, and pass/fail evidence.
- Scrap or rework rule when direction is wrong: record value, source, method, owner, and pass/fail evidence.
| Failure mode | Symptom | Verification |
|---|
| Ambiguous identity | Wrong part or record | Search and audit a real lot |
| Omitted interface | Purchase, assembly, or service error | Walk the real route |
| Documents out of sync | Correct name, wrong revision | Baseline every reference |
Four different kinds of direction, and mixing them means marking the wrong thing
When people say a material is directional, the designer, the shop and the inspector are often talking about four different things. Writing one general note for all four is the fastest way to have each party read it differently.
| Kind of direction | What it decides | How to state it so it can be checked |
|---|
| Rolling direction of sheet metal | Strength and toughness along each axis, whether a bend cracks | Arrow for the rolling direction on the flat pattern, with its relation to the bend lines |
| Surface grain (hairline, brushed, directional polish) | Appearance when panels sit next to each other | Arrow for the grain on the visible face, with the viewing condition |
| Front and back face of a sheet | Which face carries the protective film, which is coated, which is the datum | Name the face on the view, mark it in a place that does not affect function |
| Fibre or extrusion direction (composite, extruded plastic) | Strength and deformation per axis, direction of thermal movement | State the fibre or extrusion direction relative to the part axes |
The first three can appear on the same part and do not necessarily share the same axis — the grain on a hairline stainless panel is set by the surface finishing step, while the rolling direction is set by the stock. If the function needs both, both must be stated, and it must be checked whether that requirement is feasible with the stock being bought.
Bending: the relation between the bend line and the rolling direction
This is where material direction stops being about appearance and becomes about whether a part cracks. When the bend line is parallel to the rolling direction, the outer fibre of the bend is more likely to crack; when the bend line is perpendicular to the rolling direction, the material bends more favourably.
Consequences for sheet-metal design:
- A part with bends running in two perpendicular directions cannot be optimal for both.
Choose the bend that matters most functionally and state that priority.
- The harder the material and the thicker the sheet, the stronger the effect. The minimum
bend radius has to be taken from the supplier's table for the grade, the thickness and the bend direction.
- A cut edge left with burrs or with a rough fracture zone will crack much earlier when it
is on the tension side. The common sheet-metal mistakes in cutting and bending are in Machine Design #27.
Constraining the grain costs money, so state it only where there is a reason
Once the drawing locks the rolling direction, the shop loses the freedom to rotate the part during nesting. Nesting efficiency drops, scrap rises, and on an order with many small parts the difference shows up in the quotation. So classify it directly on the drawing:
- Required — parts under cyclic load, parts bent close to the minimum radius, visible
faces that sit next to each other in an assembly.
- Not constrained — lightly loaded parts, parts hidden inside the machine, parts with
no visible face.
Stating the second group explicitly as "rolling direction not constrained" is worth as much as stating the constraint: it tells the shop where it is free to optimise instead of having to guess or ask.
The marking has to survive until assembly
An arrow on the drawing only works while the part is next to the drawing. After cutting, a symmetric flat plate does not announce which face is the front. Three common methods, in order of robustness:
- Add an asymmetric geometric feature, an offset locating hole or a corner chamfer, so the
part only fits one way. This is the most reliable route because it does not depend on a person.
- Mark the part in a location that affects neither function nor a highly stressed area.
- Keep the protective film on the visible face until final assembly, and write that into
the documentation.
For appearance requirements, the word "good-looking" has to become something measurable: state the viewing angle, the distance and the light source used for inspection, and use a limit sample to align the acceptable level between the parties. The name and use of limit samples on a Japanese shop floor are covered in Technical Japanese #10.
Grain flow in forgings: something machining can cut through
In a forging the metal is plastically deformed, so the grains elongate along the direction of flow and create a grain flow. Grain flow following the shape of the part is why a forging is stronger than a part of the same shape cut from solid stock.
The point often missed: machining after forging can cut across the grain flow. The freshly cut surface then exposes grain ends, and in an area under cyclic load that is where a fatigue crack starts.
| Approach | Effect on grain flow |
|---|
| Forge close to final shape, finish machine only the mating faces | Grain flow follows the profile and the advantage of forging is kept |
| Forge a rough blank then machine heavily to shape | Grain flow is cut across in many places and most of the advantage is lost |
| Machine from rolled bar | Grain flow runs straight along the bar, not along the part shape |
Consequence for whoever issues the drawing: if forging was chosen for strength, the grain flow requirement in the critical area has to be stated, and the amount of machining there limited. Writing "forging" and letting the shop machine freely means paying for a forging and receiving a part barely better than machined from solid.
Castings: solidification direction and where porosity concentrates
A casting has no grain flow in the forging sense, but it has an equivalent in its consequences: the solidification direction. Metal cools from the outside inward and from thin sections toward thick ones, so the thick section solidifies last and is where shrinkage porosity concentrates.
Three things the designer decides:
- Uniform wall thickness, or changing gradually, so no isolated thick mass sits in the middle.
- Which areas are precision mating faces, since those should sit in an early-solidifying region, not
on a thick mass.
- Where risers and gates can go, if the function allows — this is discussed with the foundry, but the
geometry of the part largely decides how freely they can be placed.
For a casting under pressure or load, the acceptance level for internal defects and the inspection method have to be stated. They are requirements, not defaults.
Laminates and fibre-reinforced materials
In composites and laminates, fibre direction decides strength per axis far more sharply than in metals. Three things must be on the drawing:
- The sequence and orientation of each ply, not simply "composite".
- How cut edges are finished, because cutting exposes fibre ends and an open edge lets moisture in.
- Drilled holes: drilling severs fibres, so the area around a hole is markedly weaker. Load-carrying
parts should avoid holes in critical areas or reinforce around them.
MINATA release checklist
- [ ] Function, identity, boundary, and failure symptom are written.
- [ ] Ownership, interface, and mistake-proofing are clear.
- [ ] Six topic checks have evidence and pass/fail limits.
- [ ] Manufacturing, purchasing, inspection, and maintenance were tried.
- [ ] Revision, supplier, database, and lifecycle records agree.
Good engineering is an explicit chain that survives manufacture, operation, maintenance, and change. For directional material, evidence that identification and function remain reliable is the MINATA standard.
Frequently asked questions
Are the rolling direction and the surface grain the same thing?
No. The rolling direction comes from how the stock was rolled and affects mechanical properties; the surface grain comes from the finishing step and affects appearance. The two can differ on the same sheet, so if the function needs both, both must be stated.
A bent part cracks at the edge although the bend radius matches the table. Why?
Check three things: whether the bend line runs parallel to the rolling direction, whether the cut edge on the tension side still has burrs or a rough fracture zone, and whether the table being used matches the grade and the thickness. The minimum radius depends on the bend direction too, not on thickness alone.
Should the rolling direction be constrained on every sheet-metal part?
No. Constraining it lowers nesting efficiency and raises cost. Constrain it on parts under cyclic load, parts bent close to the limit, and visible faces that sit next to each other. State the rest explicitly as not constrained.
How do you stop an assembler fitting a part the wrong way round?
Prefer geometry: an offset locating hole or a chamfered corner so the part only goes in one way. Marking and protective film are a second layer, not the only layer, because both depend on human action.
How is an appearance requirement written so QC can check it?
State the observation conditions — distance, viewing angle, light source — and tie them to a limit sample agreed by both parties. Writing "surface must look clean and even" without an inspection condition produces a different result on every shift.
Frequently asked questions, continued
Is a forging always stronger than a machined part?
Not automatically. The advantage comes from grain flow following the shape of the part. If heavy machining after forging cuts across the grain flow in the loaded area, most of that advantage disappears.
Is writing "forging" on the drawing enough?
Not if forging was chosen for strength. State the grain flow requirement in the critical area and limit the machining allowance there. Without it you pay for a forging and receive a part barely better than one machined from solid.
Where does shrinkage porosity concentrate in a casting?
In the thickest section, because it solidifies last. The designer reduces the risk by keeping wall thickness uniform or changing gradually, and by placing precision mating faces in early-solidifying regions.
How is drilling composite different from drilling metal?
Drilling severs fibres, so the area around the hole is markedly weaker — unlike metal where a hole only reduces the section. Load-carrying parts should avoid holes in critical areas or reinforce around them.
What does a drawing need for a laminate?
The sequence and orientation of each ply, how cut edges are finished, and the requirements for drilled holes. Writing "composite" leaves almost everything that decides the mechanical properties open.
Conclusion
Directional Materials is not paperwork done to make a file look tidy. It is how intent becomes a result that can be manufactured, assembled and measured repeatedly. A good drawing does not need the designer standing beside it to explain it; the structure of the information has to do that work.
View all MINATA technical articles