Joining, hardware and assembly

Once parts are cut and folded, they are joined, fitted with hardware and assembled into a product. This stage is labour-heavy and varied: clinching seams, setting rivets, pressing nuts, hanging a door, fitting a gasket, running a water test, packing. It is easy to price as a lump, “assembly, about an hour”, and that lump is where quotes go most wrong in both directions, because it hides how many joints there are, how long each takes and which parts have already been priced elsewhere.

This chapter prices EN-700’s joining from a count of joints and a time per joint, then adds assembly, testing, inspection and packing as separate lines. It sets out the rule that each cost appears once, on one line. The running case finds that a first estimate added a joint cost that already included the fasteners’ prices to a material line that also included them, over-pricing the job by enough to risk losing it.

Count the joints

The joints come straight from the drawings, and Sheet Metal Joints by the Numbers gives EN-700’s joint schedule. For quoting, each kind of joint has a count per enclosure and a time per joint. This is an illustration, with the labour rate of A$95 an hour, about 2.6 cents a second:

Figure 1Joining labour per EN-700 enclosure, ex GST

	Count	Time	Labour
Clinches on seams	116	2 s each, 232 s	A$6.12
Blind rivets, brackets and hinge	16	5 s each, 80 s	A$2.11
Self-clinching nuts pressed	10	6 s each, 60 s	A$1.58
Earth stud pressed	1	6 s	A$0.16
Cable plate screws	6	10 s each, 60 s	A$1.58
Rain-hood seam grooved	1	30 s	A$0.79
Sealant applied to seams	5.8 m	20 s per metre, 116 s	A$3.06

The joining labour totals about 584 seconds, 9.7 minutes, about A$15.41 per enclosure. The joining equipment’s set-up, the clinching die, the riveting tool and the nut press, takes about half an hour per batch, about A$47.50, which Chapter 7 adds with the other set-ups.

Assembly

Assembly is everything that turns a set of joined parts into a finished enclosure: hanging the door on its hinge, fitting the gasket, fitting the customer’s latch, fixing the equipment plate, fitting the cable plate, checking the door’s fit and adjusting it. It is harder to count than joints, because it includes judgement and adjustment. A good estimate breaks it into steps and times each from experience or a trial assembly.

This is an illustration. EN-700’s assembly steps total about 25 minutes, about A$39.58, at A$95 an hour. The figure came from timing the prototype’s assembly and allowing for the faster times a team reaches on a batch, which Chapter 10 checks against the actual times.

Timing assembly from a trial

The most reliable assembly time comes from building one, watching the clock. EN-700’s prototype was assembled by a fitter while the estimator noted each step’s time, and the times were then adjusted for a batch: the prototype’s fitter was learning the product, and a batch team works from kitted parts with set-up tools.

Figure 2EN-700's assembly steps, batch times

1. 1Hang the doorHinge to frame and door, 6 minutes
2. 2Fit the gasketDoor-frame gasket cut and fitted, 4 minutes
3. 3Fit the latchCustomer-supplied latch, 3 minutes
4. 4Fix the equipment platePlate to brackets, screws to torque, 5 minutes
5. 5Fit the cable platePlate, gasket and screws, 3 minutes
6. 6Adjust and check the doorFit, closing force and seal, 4 minutes

Each step is something a reviewer can question: does hanging a door really take six minutes, is the latch’s fitting known, has anyone fitted this gasket before? A single figure of 25 minutes invites no such questions; a list of steps invites all of them, and the quote is better for the answers.

Kitting

Assembly runs faster when every part and fastener for each enclosure arrives at the bench together, in a kit, rather than being fetched from bins as needed. Kitting itself takes time, a few minutes per enclosure for the store, but it usually saves more at the bench than it costs and it reduces errors, such as the wrong rivet length that Sheet Metal Joints by the Numbers describes. A shop that kits should price the kitting as a small line; one that does not should allow a little more assembly time for fetching.

Rework at assembly

Some faults appear only at assembly: a door that does not close evenly, a hole that does not line up, a gasket that does not seal on the first try. Each costs time to correct, and a new product has more of them than a mature one. A quote for a new product can include a small assembly rework allowance, in this illustration about two minutes per enclosure for EN-700’s first batch, or fold it into the risk allowance of Chapter 9. Whatever the method, it should be based on the shop’s experience with similar products, and checked after the job.

Testing, inspection and packing

EN-700’s specification requires a water spray test on one enclosure in ten, which takes about 15 minutes including drying and inspection, so about 1.5 minutes, about A$2.38, per enclosure on average. Final inspection and packing in its carton take about 10 minutes, about A$15.83. These are small lines that are often forgotten because they produce nothing new, and on a product where they are required they are not optional.

Figure 3Joining, assembly, test, inspection and packing per enclosure, ex GST

1. Joining labourA$15.41 · 21%
2. AssemblyA$39.58 · 54%
3. Water test, 1 in 10A$2.38 · 3%
4. Inspection and packingA$15.83 · 22%

The four lines total about A$73.20 per enclosure, nearly twice the laser and the brake together, which is typical of a product with many parts and much fitting. In such products, assembly is where estimates most need care and where actual times most often differ from the quote, so it is also where Chapter 10’s comparison of quote and actual is most useful.

The error: fasteners counted twice

This is an illustration. The estimator’s first figure for joining used the joint costs from the fabricator’s earlier costing of EN-700’s joints, about A$21.70 per enclosure. Those costs included each fastener’s price and the sealant as well as the labour. The quote’s material line, in Chapter 2, already included about A$6.00 of fasteners and sealant as bought-in items.

The fasteners and sealant were therefore counted twice. The labour-only joining cost is about A$15.41, so the double-count added about 21.70 − 15.41 = A$6.29 per enclosure, about A$315 on 50, and, carried through the margin of Chapter 8, about A$393 on the quoted price. In a competitive tender, that is enough to lose the job to a shop that counted correctly.

The review found it by checking that every item in the bill of materials appeared once and only once across the quote’s lines: the sealant appeared both in the bought-in line and inside the joint costs. The fix was to define each line’s contents precisely, material and bought-in items in one place, labour by operation in another, and to use labour-only figures for operations.

Parts and labour on separate lines

The general rule from that error is to keep parts and labour on separate lines throughout a quote. Parts are priced from supplier quotes and the bill of materials; labour from counts, times and rates. A cost figure that mixes them, such as “cost per joint including fastener”, is useful for comparing methods, as Sheet Metal Joints by the Numbers does, but dangerous in a quote, because its parts can easily be counted again elsewhere. When a mixed figure must be used, its contents should be written beside it.

Hardware insertion before folding

Self-clinching nuts and studs are usually pressed into flat blanks before folding, when the press can reach them easily, as EN-700’s are. That sequence matters for the quote in two ways. It puts the pressing in the flat-part flow, where it is quick, rather than in assembly, where it would be awkward; and it means the hardware must be specified and available before cutting starts, which affects lead time. A drawing that requires hardware on a flange that the press cannot reach once folded may force a slower method, such as rivet nuts set by hand after folding, and the quote should reflect whichever method the production team will actually use.

Learning within a batch

Assembly times fall as a team repeats a product: the first enclosures in a batch take longer than the last, and the first batch longer than later ones. The effect is often described by a learning curve, in which each doubling of the number made reduces the time per unit by a fixed percentage. A quote for a new product should allow for the slower start, which can be large for a product with many fitting steps, either by using an average time that includes it or by adding time for the first few units. EN-700’s 25 minutes of assembly is the expected average across a batch of 50, from a prototype that took about 50 minutes; at 20 off, the average would be higher, because fewer units follow the slow first ones.

A benchmark for the labour

Adding up the labour minutes gives a figure that can be compared with experience. This is an illustration. EN-700’s people-time per enclosure, excluding the laser’s unattended cutting, is about 2 minutes of deburring, 7.4 minutes at the brake, 9.7 minutes of joining, 25 minutes of assembly, 1.5 minutes of testing and 10 minutes of inspection and packing: about 55.6 minutes, a little under an hour. The fabricator’s records for a similar enclosure it made last year show about 62 minutes per unit in practice. The estimate is close enough to be plausible and a little optimistic, which is worth knowing before the quote goes out; the estimator kept the figures but noted the comparison for the review of Chapter 9.

A benchmark like this cannot replace the detailed count, because the similar product is never quite the same. It catches the large errors, a missing operation or a time entered in hours instead of minutes, that the detailed count can hide.

The joining method is part of the price

The quote prices the joints the drawing specifies, but the estimator can see things the designer may not: that a seam specified as drilled rivets could be clinched, that a pressed nut would be cheaper than a tapped hole that will strip, that a hem could be replaced by a folded return. Sheet Metal Joints by the Numbers prices these alternatives. A quote that offers, alongside the price to the drawing, an alternative price for a suggested change, with its effect stated, gives the customer a better decision and the shop a reputation for helping. The alternative must be clearly separate from the main price, so that the customer cannot accidentally order the drawing at the alternative’s price.

One rate or several

EN-700’s estimate uses one labour rate, A$95 an hour, for all its operations. Some shops use different rates for different skills: a lower rate for packing and kitting, a higher one for skilled fitting, welding or programming. Several rates can make a quote more accurate, if the shop’s costs really differ by skill and the estimator assigns each task to the right rate. They can also make it more fragile, because every task must be classified and every rate kept current. For a small shop whose people move between tasks through the day, a single well-calculated rate, as Chapter 8 builds, is often the more honest figure.

Packing and delivery

Packing is part of the product. EN-700’s individual cartons, labels and pallet wrapping are priced as bought-in items and packing labour. Delivery to the customer’s site, Chapter 1’s question, is priced from a freight quote for the batch: in this illustration, a single truck delivery of 50 cartons on two pallets. A delivery charge per batch, like a set-up, costs more per enclosure in small batches. The quote should state whether delivery is included and to where, because “delivered” means different things to different people. Delivered to a loading dock, delivered to a store room up two flights of stairs and delivered and unpacked are three different services with three different costs. EN-700’s quote says “delivered on pallets to the customer’s loading dock, unloading by the customer”, which leaves no room for a different reading when the truck arrives.

Exercise: a water test on every enclosure

Suppose the customer asks for a water test on every enclosure instead of one in ten. Each test takes 15 minutes. What does that add per enclosure and for 50 off, and what else should the estimator consider?

Worked answer. At one in ten, the test costs about A$2.38 per enclosure. On every enclosure, 15 minutes at A$95 an hour costs about A$23.75 per enclosure, an increase of about A$21.37, about A$1,069 on 50 off before margin.

The estimator should also consider capacity: 50 tests of 15 minutes take 12.5 hours, more than a shift of one person’s time, and the drying space they need. A test station that sprays several enclosures at once might cut the time per enclosure, and is worth suggesting. The quote should state the testing level priced and offer the alternative, so that the customer can decide whether testing every unit is worth its price.