Build-to-print sheet metal: drawing to dispatch

Any fabricator can produce one good sample. The question a build-to-print contract actually answers is whether the four-hundredth assembly will match the sample when the coil supplier changes, the operator changes and the coating line has been through a bath change. That is a question about records, not about machines. Build-to-print sheet metal manufacturing from drawing to dispatch is therefore a sequence of gates, not a sequence of departments.

The distance between a drawing and a dispatch note is crossed at nine points. At each one, either a record is created or the gate was not really passed. A supplier that can show you the nine records for an existing job is a different proposition from one that can only show you a sample. The bending and welded assemblies hub holds the rest of this cluster, including the route-level articles on quotation, first article and production transfer.

The nine gates and the record each one leaves

The weakest link in most supplier audits is gate 6. A first-off sheet that lists only the five dimensions someone thought were important will pass a batch with a 2 mm error in a feature nobody measured. The fix is not more inspection — it is a first-off sheet generated from the drawing's own dimension list, with every callout either measured or explicitly waived in writing.

Gates 1 to 3: what has to be settled before a price means anything

A drawing arrives with a revision letter, a material callout, a finish note and a tolerance block. It rarely arrives with the things that decide the route: the annual quantity, the release pattern, the assembly the part fits into, and which features actually control fit.

The DFM review is where those are surfaced. It is not a courtesy. It is the point at which a 1.5 mm hole in 4 mm steel is flagged as a drilling operation rather than a laser cut, a bend is flagged as unreachable with the standard punch, and a weld symbol is flagged as ambiguous because it does not say which side is welded.

Ganesh reviews the drawing against laser, press, brake, weld and coating capability before quoting, and returns comments on the features that will cost money later. The manufacturability review checklist lists the seventeen things that review covers, and the guide to drawing for quotation sets out what a quotation needs in order to be comparable at all.

A quotation that is not comparable is worse than a high price. If one supplier prices a weld fixture and another assumes the customer will supply it, the difference will surface as a first-batch delay.

Gates 4 to 6: the route card, the fixture and the first-off

Process planning turns the quotation into a route card: cut, deburr, form, weld, coat, inspect, pack, with the inspection point marked at the operation that can make a defect unrecoverable. That is usually after forming and before welding, because a bend error found after welding costs the weld as well.

The bend programme is where a small arithmetic error becomes a large fit problem. Bend allowance for a 90° bend is (π/180) × angle × (inside radius + K × thickness). For 3 mm steel, R3 and K = 0.4: 0.01745 × 90 × (3 + 1.2) = 6.59 mm of neutral-axis arc. A flat pattern cut to 6.09 mm instead of 6.59 mm produces a flange 0.5 mm short, and on a closing flange that is a 0.5 mm gap at every joint on the assembly. The bend allowance calculator does the same arithmetic per material.

Tooling and fixtures come next, and the record matters more than the tool. A welding fixture without a number is a fixture that will be rebuilt from memory in two years, to a different standard, by whoever is on the floor. Ganesh designs and builds press tools, progressive dies, and welding and checking fixtures in its own tool room, and each one carries a number that appears on the route card.

Gates 7 and 8: first article, then the run

First article inspection is the gate that decides whether the programme is repeatable. It should contain:

1. Every drawing dimension with the measured value, the nominal and the tolerance.
2. Material grade, heat number and mill certificate.
3. The tool, programme or fixture number that produced the part.
4. For welded assemblies, the weld sequence and the diagonal measurement.
5. For coated parts, the DFT readings and the adhesion test result.
6. Who measured it, with what instrument, and the calibration status.

A first article that passes on dimensions but omits the tool number has proved the part, not the process. When the tool is replaced or modified, there is nothing to compare against.

For a welded assembly, add the diagonal. Two uprights at 2,000 mm with a 4 mm diagonal difference will fight the door or cover at installation, and the part dimensions can all be inside tolerance while the diagonal is not. Ganesh captures assembly diagonals on checking fixtures before coating, because a coated assembly cannot be corrected without stripping the film.

Coating is a gate in its own right. Seven-tank pretreatment, then the 18 ft × 14 ft powder booth, then cure, then DFT and cross-hatch adhesion. A batch that passes dimensionally and fails adhesion has to be stripped and recoated, which on a welded assembly usually means scrapping the weld as well.

Gates 9 and the documents that travel with the parts

What ships with the parts is part of the product. A carton with no inspection record is a carton the buyer cannot defend when the assembly line rejects it three weeks later.

A complete shipment package carries the packing list with quantity per box and per revision, the inspection record for the batch, the material certificate where the drawing requires it, the FAI reference, and a deviation note for anything accepted against a written concession. Ganesh packs for domestic dispatch and for export, with protection suited to the handling the route will see.

The second order is the real test

A first article proves the route works once. The second order proves whether the route survives a coil change, a die regrind, a coating bath change or an operator who has never run the job.

Three records do most of the work at that point. The first is the measured thickness on the material actually in the store, because a coil at the low end of the thickness tolerance bends to a different angle than the one the programme was proved on. The second is the tool condition log: a punch that has been reground changes the hole it produces, and a die that has been hard-faced changes the edge. The third is the approved sample, sealed, with the revision it was made to written on it.

Ganesh holds the approved sample and the first article record against the revision that was quoted, not against whatever revision is current. When a repeat order arrives, the register is checked before the route card is released. That single step catches most revision drift before a single sheet is cut.

For long-running programmes, ask for the maintenance interval in the quotation and hold the supplier to it. A progressive die due for regrind at 50,000 strokes that has run 80,000 will produce an out-of-tolerance profile, and the batch will be rejected after the fact — which is the expensive way to discover a maintenance schedule.

Assembly tolerances are the only ones that matter

A build-to-print programme can hold every part tolerance and still fail at the customer's line, because the parts stack. Three parts each at +0.5 mm can close a 1.5 mm gap into an interference fit, and no single part is out of tolerance.

The way to avoid that is to agree the assembly tolerance at the quotation stage, not at the first rejection. If the drawing is a single part, ask what it bolts to and how much clearance the joint has. If the drawing is an assembly, agree which dimensions are critical to function and hold those tightly while letting the rest run on the general tolerance block.

Ganesh works to ±0.5 mm typical for sheet metal parts, tighter on critical features by agreement. That number is a starting position, not a capability ceiling, and the agreement has to name the features.

How Ganesh Multitech runs build-to-print sheet metal manufacturing

Ganesh has manufactured sheet metal since 1992, across five units in Mumbai (Vasai) and Nagpur, with more than 200 people and ISO 9001:2015 and ISO 14001:2015 certification. The route for a typical assembly runs through a 12 kW fibre laser on a 40 ft × 16 ft bed, mechanical presses from 20 T to 300 T where volume justifies a tool, a 200 T 5-axis brake with a 4,450 mm bed and 10 mm capacity, three CLOOS robotic MIG cells and 30+ manual stations, seven-tank pretreatment and powder coating, then inspection and packing.

The bending capability and the finished coated assemblies route are the two stages where a build-to-print programme most often shows its quality, because a 4,450 mm bend and a 70 µm film are both visible in the finished assembly.

Customer work spans Mahindra and Mahindra Powerol, Godrej & Boyce, TKE, Eros Elevators, Minda, Gunnebo and Indian Railways. That spread matters for a new programme: a supplier already running automotive PPAP, lift components and railway fittings has the documentation systems in place before the first order is released.

What to put on the purchase order

• Drawing revision and the date it was received.
• Annual quantity and the release pattern, with the minimum batch size.
• Which operations are in scope: cut only, cut and form, welded assembly, coated assembly, packed kit.
• Inspection class and the record required per shipment.
• Tooling ownership, storage location and maintenance responsibility.
• Material grade and whether substitution is permitted.
• Packing standard and any export documentation.
• Escalation contact for a quality issue and the response time expected.

When build-to-print is the wrong relationship

Build-to-print is wrong when the buyer expects the supplier to redesign the part. A build-to-print supplier reports a manufacturability problem and quotes the change; it does not own the design intent. If the drawing is a concept, the programme needs a design partner, not a fabricator.

It is also the wrong model for a part that will be revised every quarter. Each revision re-triggers gates 1 to 7, and a programme that runs them monthly is paying for first article repeatedly. In that case, keep the part on a flexible cut-and-bend route until the design settles, then freeze it and build the fixtures.

Two companion guides go deeper on the gates either side of this one: prototyping through to serial production covers the transition into repeat supply, and quotation of a welded assembly covers the cost build-up once the route is fixed.

Send the drawing with its revision, the annual quantity, the operations you want in scope and the inspection record you need. The engineering team will respond with the route, the gates that apply, the tooling scope and a piece price, with each assumption stated so the quote can be checked line by line.