GB527 Unit 2 Process Analysis Example

Why a Second Machining Fixture Adds 18 Percent, Not 41: A Unit 2 Process Analysis in an Irrigation Valve Plant

[Student Name]

Purdue University Global

GB527: Global Operations

Unit 2 Process Analysis

[Instructor Name]

[Date]

The plant, its stations and every figure are composite and illustrative; the sources cited are real.

The Question

Platte Valve Works machines and assembles valve bodies for center-pivot irrigation systems. Demand peaks from January to April, when dealers prepare systems before the irrigation season, and again in late summer, when farmers order retrofits. In peak months the plant needs about 140 valve assemblies a day and cannot reach it. A machine tool vendor has quoted a second fixture, a pallet that lets the operator load one valve body while the machining center works on another, at $38,000. The quote says that cutting the machining cycle from 7.2 to 5.1 minutes will raise output by 41 percent.

Exhibit 1: The Process Flow

Valve bodies move through six stations in sequence. Cycle times were measured per valve over two weeks of production:

Machining: 7.2 minutes.

Drilling and tapping: 6.1 minutes.

Assembly of regulator and seals: 5.8 minutes.

Actuator installation: 5.4 minutes.

Pressure test: 6.0 minutes.

Coating: 5.6 minutes.

Each station has one machine or work position. The plant runs two shifts with fifteen net working hours a day, or 900 minutes.

Exhibit 2: Capacity by Station

Dividing 900 minutes by each cycle time gives daily capacity:

Machining: 125.0 valves.

Drilling and tapping: 147.5.

Assembly: 155.2.

Actuator installation: 166.7.

Pressure test: 150.0.

Coating: 160.7.

The line can produce no more than its slowest station, so current capacity is 125 valves a day, set by machining. That explains why the plant falls short of 140 in peak months.

What the Fixture Does

With the second fixture, machining time per valve falls to 5.1 minutes, because loading happens while the machine cuts. Machining capacity rises to about 176.5 valves a day. The vendor's figure compares the two machining capacities: 176.5 against 125.0 is 41 percent more.

But the line does not run at machining's pace once machining is no longer the slowest station. Drilling and tapping, at 147.5 valves a day, becomes the constraint. The line's output rises from 125.0 to 147.5, an increase of 18 percent.

The fixture removes one bottleneck and reveals the next one, so its benefit is capped by the second-slowest station, not by its own speed.

Goldratt and Cox (2004) made this point the center of the theory of constraints: improving a station that is not the constraint does not increase output, and improving the constraint increases output only until another station limits it. The vendor measured the improvement at one station; the plant needs the improvement in the whole line.

Is 18 Percent Enough?

At 147.5 valves a day, the line exceeds the 140 needed in peak months, with a small margin. Utilization at 140 valves a day would be about 95 percent at drilling and tapping, 93 percent at pressure test, 90 percent at assembly and lower elsewhere.

High utilization has a cost the capacity table does not show. Hopp and Spearman (2011) show that queues at a station lengthen steeply, not gradually, as it runs closer to full load, and that variable cycle times make the effect worse. At 95 percent, small disruptions at drilling and tapping, such as a broken tap or a late casting, would build queues quickly. The Unit 1 post showed that waiting is already most of the plant's lead time, and running near full utilization would add to it.

What the Change Means for Lead Time

Little's law ties three quantities together: the valves sitting between stations, the daily rate at which finished valves leave, and the days each valve spends on the floor, which is the first divided by the second. Today the line holds about 380 valve bodies in process between stations, and it releases 125 a day, so a body spends about three working days in production. If throughput rises to 140 a day and work in process stays at 380, time in production falls to about 2.7 days. If the plant also caps work in process at 280 bodies, releasing castings only as the constraint pulls them, time in production falls to about two days. The fixture therefore helps lead time only if the plant also stops releasing work faster than the constraint can absorb it.

Value of the Fixture

In peak months, the fixture would let the plant make about 15 more valves a day, from 125 to 140, rather than turning orders away or working overtime. At a contribution of about $140 per valve assembly, that is about $2,100 a day, so the $38,000 fixture would pay for itself in about 18 peak working days. The case is strong, but it rests on the line reaching 140 a day, which depends on the next constraint.

The Next Constraint

Three options would raise drilling and tapping capacity:

A second set of tapping heads, reducing the cycle to about 5.0 minutes and raising capacity to about 180 valves a day. Pressure test, at 150.0, would then become the constraint.

Moving some tapping work to the machining center, which has spare capacity after the fixture. Tapping two of the six ports on the machining center would reduce drilling time and rebalance the line.

A third shift at drilling and tapping in peak months, adding capacity without capital but at overtime cost.

Rebalancing is the cheapest: it uses the machining center's new spare capacity and costs little beyond programming time. Heizer et al. (2020) describe line balancing as assigning tasks so that stations' cycle times are as close as possible, which reduces idle time and raises output without new equipment.

Recommendation

Buy the fixture, because machining is the current constraint and the fixture pays back in about a month of peak production. At the same time, move two tapping operations to the machining center, aiming to bring drilling and tapping to about 5.2 minutes, which would raise the line's capacity to about 150 valves a day, set then by pressure test. Measure cycle times again after both changes, since the figures here are averages and the next constraint should be confirmed on the floor.

Limits of the Analysis

The analysis uses average cycle times and assumes stations work independently. In practice, variation and the small buffers between stations will make actual output lower than the calculated capacity. It also assumes castings are always available; a late casting delivery would stop the line regardless of fixtures.

References

Goldratt, E. M., & Cox, J. (2004). The goal: A process of ongoing improvement (3rd rev. ed.). North River Press.

Heizer, J., Render, B., & Munson, C. (2020). Operations management: Sustainability and supply chain management (13th ed.). Pearson.

Hopp, W. J., & Spearman, M. L. (2011). Factory physics (3rd ed.). Waveland Press.