Assembly-ready fabricated metal parts

A line that stops because one bracket will not bolt up costs far more per minute than the bracket cost to make. Yet most sheet metal is still bought part by part, inspected part by part and shipped part by part, with the interface between two parts left to chance and to the fitter holding a reamer. Assembly-ready fabricated metal parts are a different purchase: you are buying the condition of the interface, not the shape of the component.

The distinction matters because the two are measured differently. A component is accepted against its own drawing. An interface is accepted against the clearance the fastener provides, and that clearance is finite. This page sets out the arithmetic that decides whether a kit bolts up, and then the packing discipline that decides whether it bolts up in the right order.

The bending and welded assemblies hub collects the rest of this cluster, from single-part bend limits to fixture design, and is the right place to start if you are still choosing a route.

Assembly-ready means the worst-case offset fits inside the clearance

A part is assembly-ready when the worst-case positional error between any two mating features is smaller than the clearance available at the fastener. Nothing else in the definition is measurable.

For a bolted joint the clearance is simply the difference between the hole and the bolt:

c = D_hole − D_bolt

An M8 bolt in a 9.0 mm hole gives 1.0 mm of clearance. An M8 in a 9.5 mm hole gives 1.5 mm. An M8 in an 8.5 mm hole gives 0.5 mm, which is a location fit rather than a clearance fit and behaves like one.

The next term is the error the two parts bring. If the mating parts are each held to ±0.5 mm — the typical sheet metal tolerance — the worst-case relative offset is 1.0 mm, and the root-sum-square estimate is 0.71 mm. Worst case is the number to design against for a joint that must assemble first time on a line without a fitter, because the tails of the distribution are what stop the line.

So an M8 in a 9.0 mm hole is exactly at the limit: 1.0 mm of clearance consumed by 1.0 mm of error, with zero margin. Any weld pull, any powder film in the bore, any gauge variation and the joint becomes a hammer-and-drift job. A 9.5 mm hole leaves 0.5 mm, which is why the same joint is specified differently by two companies who both claim ±0.5 mm capability.

The numbers that make assembly-ready fabricated metal parts bolt up

The table is a starting point, not a specification. Every row has to be checked against the assembly it feeds. The two diagonal rows are the ones most often got wrong, because teams carry a diagonal callout from a square frame onto a long one and assume the sensitivity carried with it.

Why the diagonal check is weaker than it looks

For a rectangular opening of width w and height h, the diagonal is D = √(w² + h²). Differentiating gives the sensitivity directly:

ΔD ≈ (w · Δw + h · Δh) / D

Take a 1,000 × 1,000 mm frame, so D = 1,414.2 mm. If the width grows 1 mm and the height shrinks 1 mm, the diagonal changes by (1000 × 1 − 1000 × 1)/1414.2 … which is zero on a square, because the two errors cancel. That is the wrong test. The right test is a corner offset: if one corner moves 1 mm along the width, the adjacent diagonal grows and the other shrinks by 1000 × 1 / 1414.2 = 0.71 mm each, giving a 1.41 mm difference between the two diagonals.

Now take a 1,000 × 600 mm frame, so D = 1,166.2 mm. The same 1 mm corner offset gives 1000 × 1 / 1166.2 = 0.86 mm per diagonal, or 1.72 mm of difference. Check the algebra the other way and the weakness appears: a frame built 1 mm wide and 1 mm short changes its diagonal by (1000 − 600)/1166.2 = 0.34 mm. A long, shallow frame is nearly deaf to the diagonal tape, because most of the diagonal comes from the long side, which barely changes.

That does not make the diagonal useless. It makes it a check that has to be sized to the aspect ratio, and it means a 1,000 × 600 welded frame needs a squareness measurement at the corners alongside the tape. A digital angle finder at the corner, or a machined square offered to the inside, will find in 30 seconds what the tape will not find at all.

Matched pairs and the kit that travels together

The second half of assembly-ready supply is identity. A left-hand and a right-hand bracket that are individually within tolerance but not interchangeable as a pair will still stop a line, because the operator has to find the pair that works.

Three rules do most of the work.

One tool, one nesting. Mirror pairs are cut mirrored in the same nesting so that thickness, grain direction and coating batch match, then formed with the mirrored bend programme from the same stored setup. A pair formed on two different days from two different coils will assemble, but it will not be identical, and on a visible product that difference is the complaint.

One revision per crate. Mixed revision levels in one crate is the single most common cause of a line-side reject that the supplier cannot reproduce, because the rejected part is from the superseded revision and the rest of the crate is not. Quarantine superseded parts physically, not by status flag.

One crate per build unit. Packing by part number optimises the supplier's dispatch. Packing by build unit optimises the customer's line. A build unit that needs 14 part numbers and 40 pieces in a defined sequence should arrive as one crate with a sequence sheet inside, and the pieces bagged in the order they are consumed.

Marking is the cheap enforcement. Part number and revision on every piece, set position on every pair, and the crate label carrying the build unit reference. A laser mark costs nothing per part once the nesting is set; a sticker costs a person and a mistake.

The supplier interface article covers the commercial side of consolidating these kits onto one purchase order. The dimensional side is covered by welded assembly tolerances, which is where the post-weld stack is worked through.

Coating changes the hole you drew

Powder coat is a film, and a film inside a bore reduces the hole. At 70 µm per face on the bore wall, the diameter falls by up to 0.14 mm. In practice coverage inside a small hole is lower than on a flat face because the electrostatic field is shielded, so the real loss is often 0.05–0.10 mm — but it is not zero, and it is not predictable enough to spend.

Two consequences follow. First, a joint that was designed at exactly 1.0 mm of clearance at bare metal is under 1.0 mm once coated. Second, a hole that is used for location rather than fastening must be masked, or the mating part must be located from a feature that is not coated.

The ordering rule is simple: size the clearance at the finished condition, then decide which holes get masked. Masking is a manual operation and belongs in the quotation, not in the rework. The paint shop capability sets out the pretreatment and booth route, and the DFT gauge and cross-hatch adhesion test are the two checks that confirm the film is what the drawing asked for.

What Ganesh actually runs for assembly-ready work

Assembly-ready kits are fabricated across the five units in Mumbai (Vasai) and Nagpur under ISO 9001:2015 and ISO 14001:2015. Cutting runs on the 12 kW fibre laser with a 40 ft × 16 ft bed, forming on the 200 T, 5-axis CNC press brake with a 4,450 mm bed and Delem control to 10 mm, and welding on three CLOOS robotic MIG cells plus more than 30 manual MIG/MAG, spot and SPM stations. Welding fixtures are designed and built in the in-house tool room, so the datum pins and the checking fixture come from the same drawing as the part.

Coating stays in the same system: seven-tank pretreatment, a 16 ft × 16 ft shot blast chamber, an 18 ft × 14 ft electrostatic powder booth, a wet paint booth and a curing oven. That matters for this purchase because most assembly-ready failures are caused by a film thickness nobody measured, applied by a job worker who never saw the mating part.

Customer work in this shape includes elevator frame sets for TKE and Eros Elevators, canopy and base frame kits for Mahindra Powerol, material handling assemblies for Godrej & Boyce and coach fittings for Indian Railways. Ganesh has manufactured sheet metal since 1992. PPAP and first article inspection are available, and the first article for a kit is signed against the interfaces, not only the components.

The welded assembly capability page carries the machine list and the fixture practice; the elevator frame assemblies page shows the kind of matched set this article describes.

What to put on the drawing and the purchase order

• Mating part numbers, with the interface dimensions dimensioned from a shared datum rather than from each part's own edge.
• Fastener size and hole size for every joint, so the clearance can be calculated rather than assumed.
• Positional tolerance on the datum holes, and whether the joint is located by a bolt or a pin.
• Diagonal or squareness callout, with the measuring method named.
• Coating specification with DFT, and a mask list for any hole that locates rather than fastens.
• Left and right hand identification, and whether the pair must be supplied as a set.
• Build unit reference and the consumption sequence, if the kit is to be packed per unit.
• Revision level, applied to every piece and to the crate label.

The RFQ checklist is a shorter version of this list for a first enquiry. Build-to-print manufacture explains what a supplier does with the pack once it arrives.

When assembly-ready supply is the wrong ask

It is the wrong ask when the customer's own line does the fitting and welding, because then the interface is created at the customer's station and the supplier's tolerance is only one input among several. It is wrong for one-off and prototype quantities, where the cost of matched sets, sequence packing and interface inspection cannot be spread over enough pieces to be worth it. It is wrong when the location datum lives in the customer's fixture rather than on the drawing, because no supplier can hold a datum they cannot see.

It is also the wrong framing when the volumes are small and the part count is high. At that scale, the right answer is usually to consolidate the parts into a welded sub-assembly and buy one item, which moves the interface inside the supplier and removes it from the line entirely.

Send the assembly drawing pack, the mating part numbers, the fastener sizes and the build sequence, or request a quotation with the pack attached. Engineering reviews the clearance stack and the datum strategy before quoting, and flags the joints that will not close at the stated tolerances — which is cheaper to hear before the purchase order than after the first shipment.