In our inbox, the most common starting point is not a clean STEP file. It is a photo of a hand-drawn sketch, a scanned paper drawing from a machine built fifteen years ago, or a single sheet metal sample a customer wants to copy and improve. For small and mid-sized manufacturers this is normal: there is no in-house structural engineer to produce flat patterns, legacy equipment drawings are lost or exist only as paper archives, and subcontract shops refuse to quote without production documentation. The project then sits still for weeks. Our job at Sanjun is to remove that bottleneck. We take whatever you already have and convert it into a manufacturable drawing package, at no engineering charge.
Key takeaways
- A missing drawing is not a blocker: a hand sketch, a photo, or one physical sample is enough for us to start engineering.
- Reverse engineering is more than scanning — K-factor, bend allowance and material behaviour decide whether the flat pattern actually fits.
- Hardware must be measured in: PEM studs, standoffs, nuts and hinges belong on the drawing, not improvised on the shop floor.
- DFM is folded into the drawing stage so the design matches tooling we already own instead of demanding custom tooling.
- You receive a complete package: BOM, exploded assembly view, and a 2D inspection drawing with critical dimensions and tolerances.
- MOQ 1 piece, first article in 7-10 working days, and a quote within 24 hours once we understand the scope.
Why Projects Stall Before They Start
Most delays we see have nothing to do with cutting or bending capacity. They happen earlier, at the documentation stage, and they usually fall into three patterns.
- No dedicated structural engineer. Small and mid-sized teams know exactly what the enclosure must do, but nobody on staff can produce a proper flat pattern or a STEP file. The idea is clear; the file that a shop can cut from does not exist.
- Lost or paper-only archives. Equipment that has run for a decade often has no digital model at all. The original supplier is gone, the drawing is a folded A2 sheet in a cabinet, and the only reliable reference is the installed unit itself.
- Subcontractors will not quote. Without production drawings a shop cannot commit to a price or a lead time, so the RFQ circulates, the answers stay vague, and delivery dates slip before a single part is made.
The Real Cost of Building Straight From a Rough Sketch
Sending a rough sketch to the floor feels fast. In our experience it is the most expensive way to start, because the drawing carries none of the decisions that make a bent part fit.
- Missing fundamentals. A sketch rarely states plate thickness, bend relief, or internal clearance. Those omissions become tolerance stack-up: each flange is slightly off, and the assembly closes short or gaps open.
- Bad reverse-engineered geometry. Copying a sample without accurate bend data produces parts that interfere in assembly and mounting holes that land millimetres away from their mates. Our sheet metal tolerance guide explains where those deviations come from.
- Trial-and-error loops. Every physical iteration costs material, machine time and engineering attention. Costs climb and the launch date moves, usually for problems that a digital check would have caught in an hour.
Reverse Engineering a Physical Sample
When a customer ships us an existing part, we follow a fixed sequence rather than eyeballing it with calipers.
- Scan the geometry. 3D or laser scanning captures complex outer shapes, cutout locations and edge profiles in one pass, including surfaces that are awkward to measure by hand.
- Calibrate the bend behaviour. A scan gives us the formed shape, not the flat pattern. We set the K-factor and bend compensation against the actual plate thickness, bend radius and forming behaviour of the material, then unwrap the model. Get this wrong and the flanges are short or long no matter how accurate the scan was.
- Map the hardware. PEM studs, standoffs, cage nuts and hinge positions are measured and placed in the model with their real hole sizes and centre distances, so the assembly stack-up is correct on the first build.
Our sheet metal bending guide covers how we set bend parameters.
From Sketch to a Rigid 3D Model
With only a sketch, we build forward instead of reverse.
- Fix the envelope. Overall dimensions, mounting envelope and door or cover openings become a rigid base model that everything else hangs from.
- Add application features. Ventilation louvers, rain hoods, cable entry glands, grounding points and mounting interfaces are added according to where the enclosure will actually live — outdoor, wall-mounted, or inside a cabinet.
- Check interference and clearance digitally. Before any plate is cut we check clearances against wiring harnesses, breakers, heat sinks and fan assemblies inside the model. A collision found here costs a few mouse clicks; found later it costs a rework.
DFM Folded Into the Drawing, Not Added Afterwards
We review manufacturability while the model is still open, because changes are free on screen and expensive in steel.
- Bend radii and relief slots are aligned to the press brake tooling we already run, which removes the need for special-purpose tooling and keeps unit cost predictable.
- Welded joints are redesigned where it improves consistency: tongue-and-groove lap joints that self-locate, or weld seams moved to positions the operator can reach and repeat.
- Flat patterns are laid out for material yield before cutting, which decides how much sheet you pay for on every unit. See our nesting and material yield guide.
The Engineering Package You Receive
| What you provide | What we deliver |
|---|---|
| A hand sketch, a photo, or a scanned paper drawing | A rigid 3D model plus production-ready 2D drawings |
| A physical sample or installed unit | Scanned geometry, calibrated flat patterns, hardware map |
| Overall dimensions and application notes | Application features: louvers, rain hood, cable entry, mounting interfaces |
| Your hardware preferences | A full BOM: plate thickness, fasteners, hinges, seals |
| Nothing at all yet | Free CAD drafting and a free DFM review before quoting |
Beyond the table, the package always includes an exploded view showing how panels, doors and sub-assemblies relate, and a dimensioned inspection drawing that marks critical dimensions, tolerances and inspection requirements so incoming parts can be verified against the same standard we build to.
From First Article to Volume
We do not ask you to commit to volume before you have held a part.
- Prototype at MOQ 1. One piece is enough to validate fit, finish and assembly.
- Build it in 7-10 working days. Fiber laser cutting plus CNC bending, with surface finishing applied when the application needs it.
- Freeze the process. Once you approve the sample, we lock the cutting paths, bend sequence, welding fixtures and inspection criteria, then scale to volume with the same recipe.
Send us a sketch, a photo of the installed unit, or the sample itself. We will return a quote within 24 hours, produce the CAD drawings for you free of charge, and run a free DFM review before a single part is cut.