How to Calculate Lead Time for Stitching Process?

Question

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The question I am going to ask is the basic of the manufacturing industry. 

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Answer

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While I was reading this question, suddenly one of my real-life stories came to my mind. A similar question I have faced 7 years ago. I appeared in a job interview for a production planning job. The interviewer gave me a problem and I had to solve that. Problem was as following 

“Suppose you receive an order of 5000 pieces. you have two sewing lines to run the style and average production per day per line is 400 pieces. Calculate how many days it would take to complete stitching?”

I was in a hurry to answer the question. I thought it is a very simple question. It can be solved easily by mathematical calculation. On a paper, I did a simple calculation and replied it would be 7 days.

My calculation was:

Production per day from two lines = 400 X 2 = 800 pieces

Total production requirement is 5000 pieces

So, production time in days would be = (5000/800) = 6.25 days and rounded off 7 days.

My answer was wrong.

The interviewer explained to me that I can’t complete the order in 7 days. 

First reason, from day one of production loading, I would not get the production of 400 pieces from each line. So I need to consider production build-up time (the learning curve).

The second reason, I need to add extra cutting quantity to the order to find the total quantity to be stitched. (Normaly, 2-5% extra cutting is done by garment factories).

Considering these two parameters, I need to calculate production time for that order. That was my story. Unfortunately, I did not get that job. Let me explain answers to today’s questions. 

Answer to Question #1. 

The same principle you need to apply when you calculate the Lead Time of the Stitching Process for your orders. Here is the lead time calculation procedure –

Step#1. Find Production build up time. I mean how much time a line takes to reach pick production. This is also known as a learning curve. If you know the learning curve, you can calculate day-wise production up to pick production.

Step#2. Calculate the average production capacity of your line daily from the garment SAM and average line efficiency.

Step#3. As said above, if you cut extra quantity add that to your total production quantity.

Step#4. Finally, calculate the lead time adding days required to complete stitching the full order.

Example: 

Assume that you got an order of 4,000 pieces. No extra cutting would be done. On the 4th day after loading a style, you reach to pick production. Based on learning curve production on 

• Day 1- 100 pieces
• Day 2- 250 pieces
• Day 3- 450 pieces 
• Day 4 and onward  – 500 pieces. 

So in the initial 3 days,  you will get production of 800 pieces and later daily projected production would be 500 pieces/day

So the stitching lead time for 4000 piece order would be –

= (3 days+(4000-800)/500) days 

= 3+6.4 days = 10 days (rounded off)

Day wise production in initial days (production build-up days) to be calculated from the learning curve.

Answer to Question #2. 

Production build-up time varies product to product. And secondly, the daily production capacity of different products (shirt, trousers or t-shirts) would be different as their standard time (SAM) are different. So yes, you need to consider product categories while calculating the production lead time of order.

Related article: How to calculate production capacity of a line?