Choosing the press on its whole cost

By this point Press 4’s choice has been tested from every side. The crank press’s force curve, its flywheel’s energy, its frame’s stiffness, the hydraulic press’s speed and its oil’s stored energy, the die heights, the feeding and the stopping time have each been checked against MP-40 and AB-75. What is left is to bring the checks together, cost each option over its life, decide, and then prove on delivery that the press bought is the press specified.

This chapter sets out the three options, scores them against the five numbers and the safety requirements, works out their ten-year costs, records the decision and the reasons for it, lists the acceptance tests, and ends with a cost comparison that mixed prices with and without GST.

The press’s whole workload

MP-40 and AB-75 set Press 4’s specification because they are its most demanding jobs: MP-40 for speed, stiffness and die space, AB-75 for energy and guarding. They are not its whole workload. Press 4 replaces the shop’s older 250 kN press, which runs about 1,200 hours a year on a mix of the shop’s smaller cutting and forming dies, and those jobs move to Press 4 too. The cost comparison below uses 1,200 hours a year.

That matters to the decision. On their own, MP-40 and AB-75 need only about 33 press hours a year between them (11 for MP-40 at 120 strokes a minute, 22 for AB-75 on a shuttle feed), and no shop would buy a press for 33 hours a year; it would run them on an existing press or send them out. A press is a decision about a shop’s whole mix of work, tested against its hardest jobs.

The three options

All three options are illustrations, rated at 400 kN, with prices excluding GST.

• Mechanical gap-frame press, rated at 3 mm, 60 mm stroke, 40 to 120 strokes a minute, with the maker’s tie rods (Chapter 5), a tonnage monitor with a force-curve envelope (Chapter 3) and a brake monitor (Chapter 9): A$135,000, plus A$18,500 for the tie rods and monitors.
• Hydraulic press, short stroke, four-column frame, 63 L/min pump, with damping cylinders for blanking (Chapter 6): A$118,000, plus A$8,000 for the damping.
• Servo press, gap frame with tie rods, programmable motion and pendulum mode, monitoring built in: A$215,000.

Each needs the same coil line for MP-40, decoiler, straightener and servo feed (Chapter 8), at A$48,000, and the same shuttle feed and fixed guarding for AB-75 (Chapters 8 and 9), at A$22,000.

Figure 1Press 4's options against the five numbers and safety

Requirement	Mechanical	Hydraulic	Servo
Force at height	184 kN at 25 mm against AB-75's 55 kN	full force anywhere	crank curve, set by motor torque
Energy per stroke	7,896 J stored at full speed; AB-75 at 10% slowdown	no per-stroke limit	motor and drive ratings
MP-40 speed	120 a minute	about 70 a minute	120 or more in pendulum mode
Stiffness for a 0.15 mm clearance	0.10 mm tilt with tie rods	four-column, stiff	tie rods, similar to mechanical
Breakthrough	frame snap-through, about 24 J	oil column, damped	can slow before the cut
Shut height and bolster	240 to 300 mm; riser for MP-40	to suit	to suit
Guarding	fixed guards; shuttle for AB-75	same	same

Ten-year costs

The comparison adds the capital cost to ten years of maintenance and energy. As illustrations, maintenance is taken as A$3,000 a year for the mechanical press, A$4,500 for the hydraulic, with its oil, filters, seals and hoses, and A$3,500 for the servo; average electrical demand while running as 3.0 kW, 8.0 kW and 2.5 kW respectively, at 1,200 hours a year and A$0.30 per kWh.

• Mechanical: capital A$223,500; A$4,080 a year; ten years A$264,300.
• Hydraulic: capital A$196,000; A$7,380 a year; ten years A$269,800.
• Servo: capital A$285,000; A$4,400 a year; ten years A$329,000.

All figures exclude GST. Including GST at 10%, the mechanical press’s capital cost is A$245,850; a GST-registered business can generally claim the GST back as a credit, which is why business comparisons are made excluding it. The Australian Taxation Office’s guidance, or the business’s adviser, settles how that applies in a particular case.

The mechanical and hydraulic options are within A$5,500 of each other over ten years, about 2%, which is well inside the uncertainty of the estimates. The servo press costs about A$65,000 more, a premium for capability that Press 4’s jobs do not need.

How sensitive the answer is

A comparison this close should be tested against the assumptions most likely to be wrong. The press’s hours are the obvious one, because they drive the energy and, through wear, the maintenance. Recalculating the ten-year totals at 600 and 2,000 hours a year, with the same maintenance figures and energy rates:

• at 600 hours, the hydraulic option is the cheaper, by about A$3,500: mechanical A$258,900, hydraulic A$255,400;
• at 1,200 hours, the mechanical option is the cheaper, by A$5,500;
• at 2,000 hours, the mechanical option is the cheaper, by about A$17,500: A$271,500 against A$289,000.

The hydraulic press’s lower price wins when it runs little; the mechanical press’s lower running cost wins when it runs a lot. At the shop’s expected 1,200 hours, the two are close enough that cost alone cannot decide. The servo press is the dearest at every level of use tested, by A$65,000 to A$69,000.

Other assumptions deserve the same treatment: the maintenance figures, which are estimates for a press the shop has not yet owned; the energy tariff; and the price of the coil line and shuttle feed, which are the same for every option and so do not change the ranking, but do change the total the business must find. A comparison sheet that shows the result at a low, expected and high value of each uncertain input is more honest than one that shows a single total to the dollar.

Figure 2Ten-year cost by press hours a year, excluding GST

Hours a year	Mechanical	Hydraulic	Servo	Cheapest
600	A$258,900	A$255,400	A$324,500	hydraulic, by A$3,500
1,200	A$264,300	A$269,800	A$329,000	mechanical, by A$5,500
2,000	A$271,500	A$289,000	A$335,000	mechanical, by A$17,500

Figure 3Ten-year cost of each option, excluding GST

Mechanical gap frame with tie rodsA$264,300

Hydraulic, short stroke, dampedA$269,800

ServoA$329,000

The decision

With costs this close, the decision rests on the work. The shop chose the mechanical press, for four reasons it recorded.

1. Speed for cutting work. Most of the 1,200 hours are cutting dies like MP-40, which run at 120 strokes a minute on the mechanical press and about 70 on the hydraulic.
2. Breakthrough. The mechanical frame with tie rods stores about 24 J at MP-40’s load; the hydraulic option needs damping to deal with its oil column, and the shop has no experience of maintaining it.
3. Margin. The mechanical press carries AB-75 with a 10% slowdown at full speed and 55 kN against 190 kN at its highest working point. The checks that failed in earlier chapters, the pressure pad, the low speed setting, the parallels, the feed angle, were errors of use, each now covered by a setting sheet.
4. Familiarity. The shop’s setters and maintainers know mechanical presses, and its setting procedures, monitoring and brake checks are written for them.

It recorded the hydraulic press as the better choice if forming work grows, especially work with long pressure pads or deep forms, and noted that the decision should be revisited if the shop’s mix moves that way. A choice made on close costs should say what would change it.

What the numbers decided

It is worth listing what each chapter’s check contributed, because the specification that went to the supplier was built from them.

• Force curve (Chapter 3): a rating point of 3 mm, a force-curve envelope on the tonnage monitor, and no pressure pad on AB-75.
• Energy (Chapter 4): the highest speed setting for single-stroke forming, and AB-90’s 2,600 J as a future job the flywheel can just carry.
• Frame (Chapter 5): tie rods, bringing MP-40’s tilt to 0.10 mm.
• Drive (Chapter 6): damping if the hydraulic option were chosen; a record of why it was not.
• Die space (Chapter 7): a 50 mm riser with aligned drop-through openings, and a standard die height for the shop’s other dies.
• Feeding (Chapter 8): a servo feed with a 240° feed angle and pilot release, and a shuttle feed for AB-75.
• Stopping and guarding (Chapter 9): a part-revolution clutch-brake, a brake monitor, fixed guards for the coil-fed die and a shuttle-fed, fixed-guarded tool area for AB-75.

None of those lines would have appeared on a request for “a 40 tonne press”.

Acceptance tests

A press is accepted on delivery against its specification, not on the strength of its paperwork. Press 4’s acceptance plan, agreed with the supplier before the order, includes:

• rating and force curve: a load test with the tonnage monitor, at the rating point and above it, checking the monitor’s calibration and envelope;
• ram parallelism and frame deflection: dial readings across the bolster with the ram at the bottom, unloaded and under a test load;
• shut height, adjustment and stroke: measured against the specification;
• stopping time: measured with an analyser at the speeds to be used, and the brake monitor’s limit set from it;
• guarding and controls: verified by a competent person against the applicable Australian standards and the shop’s risk assessment, including the interlocks, the shuttle feed’s guarding and the coil line’s fixed guards;
• run-off with both dies: MP-40 at 120 strokes a minute for a full coil and AB-75 in single strokes at the highest speed setting, with first-off and last-off inspection, tonnage readings and burr checks.

The run-off is the final check of every number in this book at once. A press that passes it has been shown to run the jobs it was bought for.

Figure 4Press 4's acceptance tests before payment is released

• Do: Load test at and above the rating point, monitor calibrated400 kN at 3 mm; envelope set
• Do: Ram parallelism and deflection under loaddial readings across the bolster
• Do: Shut height, adjustment and stroke measured240 to 300 mm; 60 mm
• Do: Stopping time measured; brake monitor limit setat the speeds to be used
• Do: Guarding and controls verified by a competent personto Australian standards and the risk assessment
• Do: Run-off with both dies, first-off and last-off inspectedMP-40 for a full coil; AB-75 in single strokes

Installation

A press’s specification stops at its feet, and its installation is part of the purchase. Press 4’s plan covers:

• the floor: the press’s weight, plus the reaction of its working loads and the vibration of blanking, carried by a foundation or a floor slab checked for it by an engineer. A gap press bolted to an ordinary factory slab can crack it and walk across it;
• isolation mounts, which reduce the vibration passed to the building and to nearby machines and inspection equipment;
• power and air: a supply sized for the motor’s starting current, and clean, dry compressed air at the pressure the clutch-brake needs, because low air pressure slows the clutch and the brake;
• noise: blanking is loud, and WorkSafe Victoria’s guidance includes noise barriers and hearing protection among a press’s controls. The coil line and the press’s position are planned with the shop’s noise assessment;
• space: room around the press for the coil line, the shuttle feed, die changes and maintenance, with the guards able to open fully.

Training and procedures

The errors in this book were mostly errors of use, and the fix in each case was a written procedure and the training to follow it. Before Press 4 ran production, the shop wrote or revised setting sheets for MP-40 and AB-75, covering the riser, the centre-of-load mark, the speed setting, the feed angle and the release, and trained its setters on them; added the press’s daily checks of interlocks, clutch and brake to the operators’ start-up routine; and scheduled the press’s stopping-time measurements, monitor calibrations and inspections. A press is chosen once; it is run safely every day only if the people running it know its numbers.

After the purchase

The press’s numbers do not stay fixed. The brake wears and its stopping time grows (Chapter 9); the clutch’s air system, the gibs and the connecting rod’s bearings wear and the ram’s parallelism drifts; the tonnage monitor’s calibration ages. The shop’s maintenance plan for Press 4 schedules each of those checks at the intervals the maker and the inspector advise, and keeps the results with the press’s records, as WorkSafe Western Australia’s guidance recommends for inspection and maintenance. The numbers in this book are as true in the press’s tenth year as in its first only if they are measured again.

The error: a comparison that mixed GST

This is an illustration. The first cost comparison circulated to management showed the hydraulic press at A$129,800 and the mechanical at A$135,000. The hydraulic supplier’s quote had included GST and the others had not, and the sheet had copied each quote’s headline figure. On that basis, the hydraulic press’s capital advantage looked like A$5,200; in fact it was A$17,000, since its price excluding GST was A$118,000.

The same mixing carried through to the ten-year totals. The sheet showed the hydraulic option as about A$17,300 dearer than the mechanical over ten years; on consistent figures it was A$5,500 dearer. The decision did not change, because the reasons for it were not about cost, but the margin presented to management was three times too large, and a decision made on cost alone could have gone the wrong way.

The finance officer who reviewed the sheet found it by checking each quote’s GST statement. The shop’s template for capital comparisons now has a column stating whether each source figure includes GST, converts every figure to excluding GST before any arithmetic, and states GST separately at the end.

Figure 5The hydraulic option's figures, as first shown and as corrected

Figure	As first shown	Corrected, excluding GST
Hydraulic press price	A$129,800, including GST	A$118,000
Capital advantage over the mechanical press	A$5,200	A$17,000
Ten-year cost against the mechanical press	A$17,300 dearer	A$5,500 dearer

Exercise: more forming work

Suppose the shop’s mix changes so that 600 of Press 4’s 1,200 hours a year are forming work that would run at the same rate on either press, and the hydraulic press’s ability to carry pressure pads would save an illustrative A$4,000 a year in die costs and rework compared with designing around the mechanical press’s force curve. Recalculate the ten-year difference between the mechanical and hydraulic options.

Worked answer. The ten-year totals were A$264,300 for the mechanical press and A$269,800 for the hydraulic, a difference of A$5,500 in the mechanical press’s favour.

• The hydraulic option’s saving of A$4,000 a year for ten years is A$40,000.
• Its ten-year cost becomes 269,800 − 40,000 = A$229,800, now A$34,500 less than the mechanical press’s.

On those figures the hydraulic press would be the better buy, as long as the cutting work that remains can live with its lower speed and its damped breakthrough. That is the condition the shop wrote into its decision: the choice holds for the present mix, and a change in the mix is the trigger to revisit it.