How to do Line balancing using Operator Skill History?


Balancing of stitching line, Skill matrix

The prerequisite of this method is to have a skill matrix of sewing operators. Normally, at the time of line setting, operators are selected based on their experience on operations. The calculated skill level of the operators on the operations is not considered at all. As a result after couple of hours, high skilled operators start sitting idle and low skilled operators stuck with their work. Thus line becomes imbalanced and lot of productive time is lost as operators sit idle. To utilize operator’s maximum capacity, work allocation must be done based on operator’s potential performance level (efficiency) and work must be shared with operators who has excess capacity.

To get a balanced line from day one, select operators for the operations based on operator’s skill levels and capacity required for the task. In this article I will explain, how to use skill history to balance a line. Assume that you already have skill matrix of your operators. For example, a line produces Tee shirt and skill inventory of the operators are as following. Operator’s skill levels in various operations have been shown in the following table-1.

Table-1: Sample Skill Inventory

Work sharing between the operators may vary depending on production systems. I will explain line balancing method considering that we are balancing an assembly line (Progressive Bundle system). Assumptions:

  • Operators will work on single work station. 
  • Work will be shared or operator’s excess capacity will be utilized only where machine type matched and operator has performed that operation earlier (refer to operator skill inventory) 
  • No time delay when an operator switches from one operation to another. 
Step 1: Keep a copy of the skill matrix of all operators working in the sewing line where new style will be loaded. Or you can prepare it if you don’t have one.

Step 2: Collect Operation bulletin (OB) for the style to be loaded or is running on the line. Operation bulletin must have information such as Operation name, SMV of the operations, type of machine is used and hourly target from the line. Refer to following OB. Daily target output is 675 pieces in 8 hours shift and hourly target is 85 pieces.

Figure-1: A Basic Operation Bulletin (For a Tee for example only)

Step 3: Prepare a table on spread sheet with the headers as shown in the following Figure-2. Enter information to this sheet from your operation bulletin. Columns A, B, C and E to be filled from OB.
Figure-2: Line balancing calculation table

Step 4: Other columns and formula used for calculation are explained in the following paragraph. Insert formula in your spread sheet as explained here.

Output Needed per Hour (B): Target output per hour in pieces. Initially target output in each operation is 85 and when an operator worked in an operation produce part quantity, balance quantity will be the needed quantity for second operator doing the same operation.

Standard output (D): Potential production at 100% efficiency. D= 60/ SMV of an operation

Operator (F): Operator name to be entered after selection from skill inventory.

Potential performance (G): It represents operator’s efficiency level in the past on the listed operations. Information will be selected from skill history.

Hourly potential output (H): Number of pieces expected from the operator based on his/her past performance. H= Standard output X operator’s efficiency%

Time Available (I): Time available to an operator in hours at the time of starting of a task. If an operator spends 0.5 hours in an operation, he will have 0.5 hours for another operation.

Over and under production (J): It is the variation of potential output against target output. Negative (-) sign represents underproduction and no sign represent overproduction. J = (Hourly potential output – Hourly target production)

Time needed to make target production (K): Operators time needed to produce target output. K = (Hourly target production/ Hourly potential output)

Spare Time (L): Excess time than time required to produce target quantity. L = (Time available – Time need to make target production) 

Table-2: Filled format
Step 5: In this step you will learn how select an operator from skill inventory and allocate them for the operation one by one. All calculations are done for one hour. 

In the above sheet (Table -2) for the 1st operation (Mark Neck trim and run stitch) operator Radha has been selected as her efficiency is highest (81%) among 3 operators can perform this task. Based on her past experience (skill inventory) she can produce 88 pieces per hour. So she can meet the hourly production target and her spare time is 0.04 hour in an hour. With this much spare time we don’t give her any other job.

For the second operation (Join shoulders) operator Seema has been selected. Her potential efficiency to the job is 93% and she can make 124 pieces per hour. But we need only 85 pieces to maintain continuous feeding flow in the line. To make 85 pieces Seema needs to spend only 0.69 hour. Balance 0.31 hour can be allocated for other operation that can performed 4TOL machine. Her excess capacity will be used later where it is needed. 

Similar ways calculate and fill details for 3rd, 4th 5th and 6th operations. Anil who is allocated for operation “serge margin” has spare time of 0.45 hours and Nirmal who has been allocated for operation “Top stitch on neck seam” has spare time of 0.36 hours. 

For the 7th operation ‘Attach Sleeve’ we will utilize Anil’s spare time (0.45 hour) as he is working on similar type of machine (4TOL). At 77% efficiency Anil can produce 59 pieces if works for full hours (out of 85 pieces). In his spare time he can only produce 27 pieces. Balance quantity 58 pieces (85 – 27) need to be made by another operator. Here, Sunita (4TOL operator) is selected to make balance quantity. In an hour she can make 62 pieces (short fall was 58 pieces). Thus Anil and Sunita’s combined output meets hourly target. 

Roma, who is given operation “Sew side seam with labels”, can make 66 pieces in an hour. Balance 19 pieces will to be given to another operator. Seema, who is doing an operation ‘Join shoulder”, has spare time of 0.31 hours. As she is working similar machine type (4TOL), Roma’s work is shared with Seema. At 90% efficiency Seema can make 64 pieces and in her spare time (0.31 hour) she can make 20 pieces. Combined output of Roma and Seema meets the hourly production target. 

Similar ways 9th operation “Sleeve hem” allocated to Komal and his excess work has been shared with Nirmal. As Nirmal has spare time of 0.36 hours. 

The last operation has been allocated to Sivani, who can produce 91 pieces in one hour. 

In this way we can maximize utilization of operator’s excess capacity and balance a line better in a better way. Instead allocating 13 machines and 13 operators, the target quantity is achieved by 10 operators. By using this method, with balancing a line we can increase labor productivity and labor utilization percentage. 

I hope you have understood above explanation on line balancing using skill history. For further clarification you may write us or enter your comments below.

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Online Clothing Study: How to do Line balancing using Operator Skill History?
How to do Line balancing using Operator Skill History?
Balancing of stitching line, Skill matrix
Online Clothing Study
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