Chairvana’s Table Assembly Operations:
Chairvana Inc. manufactures metal tables and chairs at its plant in Paxton, Illinois. Recently hired general manager, Bob Behar, is concerned about the large inventories and manufacturing inefficiencies in the plant. The Paxton plant has two separate assembly lines – one for chairs and one for tables. Demand for chairs is significantly larger than tables, so Bob decides to tackle the table assembly process first. Bob has recently hired a summer intern, Eileen Mayer, who he asks to study the table assembly line and suggest ways to improve the process. Having recently completed a course on operations management, Eileen decides to create a value stream map for the process. Over a period of several days she collects the following information about the process.
The Assembly Line: The assembly line operates one 8-hour shift per day, 5 days a week.
The Product: Chairvana makes one type of folding table. The table has two bent metal tubes that form the legs, and a formed sheet-metal top.
Demand for the tables is about 1000 tables per week.
Raw material storage: Raw material for the table assembly line consists of the formed sheet-metal tops which are manufactured in the Sheetmetal forming department elsewhere in the plant, and steel tubes for the legs. The tabletops are made in batches of 1000 and brought to the assembly area and on average the amount of table top inventory is 500.
Cut tubes for the legs are delivered by steel supplier, Steely Hollow, every week in batches of 2000 tubes, so there are on average 1000 tubes in inventory. (Since each tube has 2 legs that is equivalent to 500 table bases.) In addition, there are hinge pins, fasteners, rubber feet and cardboard boxes that are required. These are ordered in large bulk quantities and stored at the respective assembly stations. Because these other parts are purchased in bulk externally, Eileen decides that she will make a note of them but not analyze them further.
The Manufacturing Process:To maintain correct alignment during assembly,at each stage of the manufacturing process, processing is done on tube pairs i.e. two tubes at a time since a table requires two bent table legs.
Hole punching: Holes for the hinge pins are punched in the tubes. There is no set-up time for this operation. Eileen observes that the punching station can produce 60 hole-punched tubes every hour. She notes that there are, on average, approximately 750 pairs of hole-punched tubes in a work-in-process storage area waiting for the bending operation
Leg Bending: In this operation, the hole-punched metal tubes are bent in the shape of an U. There is no set-up required for this operation. The bending operation requires 4 minute per tube pair and there are two bending stations that work in parallel. There are, on average, 750 pairs of bent legs waiting for the next processing step
Hinge Assembly: A pair of bent U-tubes are assembled to form the folding base of the table by inserting hinged pins. There are two hinge assembly stations and assembly, and each table base requires, approximately, 2 mins for hinge assembly. There are no setups required. Assembled bases are sent to Footer assembly on roller conveyors as soon as they are assembled and there is no intermediate inventory.
Footer Assembly:Rubber footers are attached to the assembled bases and it takes 30 seconds to do this operation. There is only one station for footer assembly. Eileen observes that, on average, there are about 800 assembled table bases waiting for final assembly.
Table Assembly: The table top is mated with the table base at the final assembly station. The operation requires careful adjustments and takes 6 minutes per table. There are three assembly stations. Assembled tables are transported immediately on a rolling conveyor to packing to avoid damage to the table.
Packaging: The assembled table is packed in a cardboard box for shipping. Each table requires approximately 90 seconds to be boxed and there are two packaging stations. Packaged tables are shipped to BigMart twice a week. On average, therefore, there are about 250 packaged tables waiting to be shipped.
Planning and Scheduling:To understand how the different stations received instructions about their daily schedules, Eileen spent a day with the production planning department. She learnt that BigMart sends a monthly forecast of demand for tables electronically on the 25th of each month. In addition, a firm order for the week’s requirement is faxed to Chairvana on the previous Wednesday. Chairvana’s, planning department then looks at inventories and creates a work order for the week and a paper schedule is given to each station, except the Footer assembly and Packaging station. These two stations do not need a schedule since they immediately process whatever is sent to them by the previous process step. The planning department also sends a weekly order for tubes to Steely Hollow, and table-tops to the Sheetmetal forming plant.
To assist Eileen please answer the following questions. For all questions consider table equivalents – a table equivalent is 2 table legs.
1. Draw a Value Stream Map for the table assembly line. Show all activity steps, material flows, information flows and inventories. Draw a timeline showing processing times and waiting times.
(You may use any drawing package including powerpoint to create a legible chart. You can also use a package such as Visio or Lucidcharts to draw your chart. You will need to make a free account to use Lucidcharts software if you choose to use it. https://www.lucidchart.com/pages/examples/value-stream-mapping) (6 points)
2. Calculate the Takt time in minutes for the table assembly process. (1 point)
3. For each step of the process, calculate the cycle time in minutes and the capacity utilization (2 points)
4. Calculate the total lead time and total processing time in days, hours and minutes. (1 point)
