DFM Guide
One-stop precision sheet metal manufacturing
The design guidance we send back with a quotation. Bend radius, hole-to-bend clearance, relief cuts, where tolerances should sit and what a coating will not cover — the points that decide how many operations a part needs and how much of the batch gets reworked.
Engineering notes and buying guides
Practical notes from the shop floor, written for the people who specify the parts.
Design for manufacturing, written by the people who cut the parts
Most of the cost of a sheet metal part is fixed before a machine is switched on. Bend radius, hole-to-bend clearance, relief cuts and where the tolerances are placed decide how many operations the part needs and how much of the batch gets reworked. What follows is the guidance we give at quotation — not a generic checklist, but the specific points where the drawings we receive most often create avoidable cost.
Bend radius and the minimum flange
A bend radius tighter than the material thickness forces a special tool and often cracks the outside of the bend. As a rule of thumb, keep the inside radius at or above one material thickness for mild steel and above that for stainless and aluminium, which spring back further. A flange shorter than the tooling needs cannot be formed at all — the part gets made in two operations or redesigned, and both cost more than moving the fold.
- Inside radius ≥ 1 × material thickness on mild steel; larger on stainless and aluminium
- Flange height sufficient for the tooling on the brake that will run the part
- Radius shared across folds, so one tool set covers the part
- Radius called out as inside or outside — the two are not interchangeable on a drawing
Hole-to-bend clearance
A hole or slot placed too close to a bend line will distort when the part is formed, because the material around it is dragged by the bend. The usual answer is to move the feature or to add a relief. Both are cheaper than inspecting a distorted hole, and both have to be decided before the part is cut.
- Hole edge at least 2 × material thickness from the bend line, plus the radius
- Slots perpendicular to a bend need more clearance than round holes
- Relief cuts at the ends of a bend to stop tearing at the corner
- Where clearance cannot be given, form the part first and put the feature in afterwards
Punching or laser, decided per feature rather than per part
The cheaper process is decided feature by feature. Turret punching has no kerf and no heat input, and it forms as it goes — louvres, embossing and hardware insertion happen in the same set-up. Laser cutting wins on thick material, on complex contours, and anywhere the edge has to be clean enough to weld. A part can reasonably be punched at one feature and cut at another, and the quote will say which route is assumed.
- Repetitive hole patterns and formed features: turret punching, 1,250 × 5,000 mm table
- Complex contours, thick material and weld-clean edges: fibre laser, ± 0.05 mm
- Parts above the punch table but within the envelope: laser, in one piece to 6,500 × 4,700 mm
- Hardware: pressed in line straight after punching, so no second operation is needed
Put the tolerances where they matter
A drawing with a tight general tolerance and loose critical dimensions is the most expensive version of the same part, because it applies precision to features nobody measures. State the datum, state the critical dimensions, and let the rest run to the general block. Laser cutting holds ± 0.05 mm across the whole bay, so the tight tolerances belong on bend-derived and assembly-critical dimensions instead.
- One datum, stated, with critical features referenced to it
- Critical dimensions individually toleranced rather than covered by a general note
- Bend-derived dimensions recognised — the tolerance stacks through each fold
- Flatness or parallelism stated per face, with the reference face named
What the coating will and will not hide
Powder coating is a finish, not a filler. It will not close a 1 mm dish in a large panel, it does not cover a sharp edge evenly, and it is an insulator — which matters wherever an earth path runs through a coated surface. A-surface parts are masked through fabrication for this reason, and flatness is checked before coating because afterwards is too late.
- Flatness and gap consistency controlled before coating, not after
- Earth studs welded to bare metal and masked, then pull-tested
- Coating thickness and UV stability specified for the exposure, not chosen by colour alone
- Cut edges treated, because an exposed edge is where corrosion starts
Tolerance capability by feature
| Laser-cut feature position | ± 0.05 mm, all three laser cutting machines, across the whole bay |
|---|---|
| Robotic welding | ± 0.08 mm repeatability on the robot welding cells, and rail-mounted robotic welding on the 8 m station |
| Bend angle — Salvagnini panel bender / TRUMPF press brake | ± 0.25° |
| Bend angle — conventional press brakes | ± 0.3 – 0.5° |
| HMI and device apertures | ± 0.2 mm, checked with a go/no-go gauge before coating |
| Aperture-critical instrument panels | ± 0.1 mm, cut in one set-up against a common datum |
| Rack frame squareness | ± 0.5 mm per metre, checked on the diagonals |
| Machined or ground faces | Flatness and parallelism to the figure stated on the drawing |
Before you send the drawing
Will you review my drawing before I place an order?
Yes — that is what the quotation is. A PDF drawing with tolerances is enough to quote, and a STEP or IGES model lets us check bend deductions and interference before anything is cut.
Do you charge for design-for-manufacturing feedback?
No. Where we can see a cheaper route — a feature that does not need to be held so tightly, a fold that could move 3 mm and save an operation — we would rather tell you at quotation than win the order on the expensive version.
What happens if the drawing conflicts with something you know will not work?
We raise it and wait. Building a part we know will not hold a tolerance, because the drawing says so, is not a service to anybody; we would rather resolve it on paper where it costs nothing.
Send your drawings and quantity — an engineer, not a sales script, comes back with a quotation covering material, tooling, surface finish and lead time.
Send your drawings and quantity — you will get a quotation with lead time, not a sales script.