Knitting Calculations


Knitted fabric is made with the help of yarn loops. Yarn of different counts is used to produce fabric of different grammage. There is also a need to calculate optimum production of knitting machines. It is the job of knitting manager to do certain calculation for proper use of machines and production of fabric according to the demands of the customer. This chapter is aimed at explanation of different calculations.

Most suitable count for knitting machines

As it has been discussed in Chapter Two that needle hook has to take yarn to convert it into a loop and finally latch has to close the needle hook so that loop is properly held by the needle hook and ultimately this helps in passing new loop through the previously held loop. It is clear from this explanation that there should be a proper balance between needle hook size and the thickness of the yarn or filament. If the yarn is thicker than needle hook then there will a chance that needle hook will not able to hold this loop and consequently there will be a small hole in the fabric. If the situation is reverse, means yarn is thinner than the size of the needle hook then the fabric produced will look like a net. Both situations are not wanted. This situation demands a balance between needle hook size and count of yarn. It is worth to note that needle hook size depends upon the machine guage. Furthermore for different garments, fabric of different grammage is required. Every time knitter has to decide about the yarn count. There are many ways for the selection of proper count. In the following lines we will discuss most common methods to select count for different machines of different guage. It is also important to note that selection of yarn counts also depends upon the machine manufactures and type of machines, like, single and double knit machine. However a general guideline will be given hereunder.

As a thumb rule knitting experts prefer to use such knitting machine whose gauges is near to count of yarn (English count) i.e. for 20-gaugemachines most suitable yarn count is 20s. This rule is has certain limitations, like, for 28-gauge yarn of 26s to 30s is most suitable. But for very fine counts this rule is not applicable and also machines have maximum gauge 32. Normally fine counts are not used as such rather they are make double, like count 60s double, which means that net count is near to 30s. And this 60 double count is suitable for 30-gauge machine.
To solve this problem some authors have suggested following formulas.

For single Knitting Machine
Suitable count = G*G/18

For Double knitting machine
Suitable count= G*G/8.4
Where G is gauge of knitting machine

Some knitting machine manufacturers suggest a range of yarn count for their machine. There is another way to solve this problem and that is to take help from old record. Every firm is producing many types of fabrics and on the basis of experience they develop a database for ready reference. In the following line we give a table for guidance (table is under construction). One can get a ready reference from the table to produce fabric of certain grammage. We are also giving expected width of fabric after wet processing. This table can provide just a reference. Knitters have to decide by themselves after doing a trial production, since there are many more factors, which can affect yarn and gauge selection process.

Knitting Machine Parameters
Every knitting machine is made to fulfil certain demands of the customer. There are number of characteristics of machine which are intimated by the machine manufacturers while delivering the machine to customers/users. It is helpful for the user to be well aware about these parameters. Furthermore machine specifications are given in different unit. We will explain these parameters and will also give the conversion factors to convert parameters from one system to other.

Machine Gauge
As per Oxford Dictionary the word “gauge” is a noun and as well as verb. It is used to measure level of any thing or for an instrument to measure width, length or height of any thing. In knitting it is used to express the number of needle in a unit length of the needle bed. This needle bed may flat or circular. In double knit circular machine it is used for cylinder and as well as dial. Generally gauge is defined as number of needles per inch. According to German standard DIN 60917 (Iyer et al1995) alphabet “E “ is used to denote knitting machine gauge.

E = Number of needles
1 inch (25.4 mm)

Machine Pitch
As per German DIN 62125 (Iyer et al1995) the notation “gauge” is to be avoided in the future. Rather they prefer to use notation “pitch” for comparison purpose. Machine Pitch means the distance between the centres of two neighbouring needles. It is denoted with small “t”. It is given in mm.

Knitting Machine Production calculation

Before explaining the method to calculate the nominal production capacity of the knitting machine it is imperative to be well aware of count and denier system and one should also be familiar with the conversion factors. Yarn is sold and purchased in the form of cones and bags. Cones and bags have certain weights. Still in the international market yarn is sold in pounds not in kilograms. Bags are of 100 pounds, which is equal to 45.3697 kgs. Previously there were 40 cones in a bag but now there are bags available of 25 cones. In other words cones are of 2.5 pounds and four pounds. Big size cones are most suitable for knitting. When these cones are used in warping then there is a need to know the length of certain weight of yarn. And some time length is available and some one wants to know the weight of the yarn and in some cases count of the yarn is required. In the following lines we will give methods to calculate above-mentioned figures.
It is imperative to be well aware of count system. In the end of the book we have given different tables and explanation of different terms. Before going ahead students are asked to consult tables and explanation for better understanding of this chapter.

Relationship between count, length and weight of yarn
Length (in yards) = Count *840 *weight of yarn in pound
Count = Length / weight of yarn in pounds*840
Weight of yarn = length/count *840

Note: as per definition count is a relationship between length and weight of yarn. English count is defined as number of hanks in one pound. Hank means a certain length. It is different for different fibers. For details see tables given in the end of book. For explanation purpose we will use English count of cotton. For cotton length of hank is 840 yards. For other fibers use relevant length of hank

Examples:
Example:01
Calculate count of cotton yarn from the given data:
Weight of yarn = 2.68 pounds
Length of yarn = 33600 yard

Formula: Count = Length / weight of yarn in pounds*840
=33600/2.68*840
Answer =14.93s

Example :02
Calculate length of cotton yarn from the given data:
Weight of yarn = 3.5 pounds
Count = 40s

Formula: Length (in yards) = Count *840 *weight of yarn in pound
= 40*840*3.5
Answer =117600 Yards

Example :03
Calculate weight of cotton yarn from the given data:
Length of yarn = 40600 yards
Count = 30s

Formula:
Weight in pounds = Length of yarn in yards/ Count *840
= 40600/30*840
Answer = 1.61 pounds

Next examples are related to filament. Note that for filament we use direct system. In which most popular is denier. There are other units too. For detail consult the tables at the end of the book. Denier is number of grams per 9000 meters of filament.

For calculation related to denier we use following equations:

Length of filament in meters = Weight of filament in grams* 9000
Denier

Weight of filament in grams = Length in meters * denier 9000

Denier = 9000* Weight of filament in grams
Length of filament in meters

Example 4

Calculate length of polyester filament from the given data:
Weight 690 grams
Denier 75

Equation: Length of filament in meters = Weight of filament in grams* 9000
Denier
= 690 * 9000
75
Answer =82800 meters

Example 5

Calculate weigh of polyester filament from the given data:

Length in meters = 50900
Denier = 50

Equation: Weight of filament in grams = Length in meters * denier 9000

= 50900*50
9000
Answer =282.8 grams

Example 6
Calculate denier of polyester filament from the given data:

Length in meters = 550,000
Weight = 4.5 kgs (4500 grams)

Equation: Denier = 9000* Weight of filament in grams
Length of filament in meters

= 9000*4500
550,000

Answer= 73.66 Denier
Note: this calculation is up to two digits. For more accurate answers use calculation up to 9 digits.
Nominal Production of knitting machines
One very simple way to calculate knitting machine production by weighing the total production of one hour or one shift or one day. This will be most realistic production value but we cannot get knitting machine capacity in this way. There is a scientific way to calculate optimum production figure of any machine. This needs certain information and some calculation. In the following lines we will explain this method in detail and will give some example so that one can be familiar to this process. In the end we will give an equation to calculate the knitting capacity of the machine. In this method following information for production calculation are required:

• Machine Guage and Dia
• RPM Knitting Machine
• Yarn Count
• Stitch Length

From these figures we can calculate the length of yarn being used by the machine in one hour and then by converting this length into weight with the help of count given we can calculate the quantity of yarn being consumed by machine in one hour. This would be the optimum production of the machine. This optimum production can be converted into nominal production by multiplying it with efficiency. In the following pages we will explain this with few examples.

In the following pages we will explain the method to calculate nominal production capacity of knitting machine. It is commonly believed that we can run knitting machine up to 85% efficiency. However, by creating most suitable environment one can increase machine efficiency.

For this we need following figures:
Machine speed RPM
Machine guage
Machine Dia
Count/ denier of yarn being used
Stitch length

From the above-mentioned figures we can calculate the length of yarn being used in one revolution and if we know the length and count of yarn then it is quite easy to calculate weight of yarn (see Example: 03 for more details)

Example 07
Calculate nominal production of a single jersey-knitting machine per hour from the data given:
Machine Gauge 24
Machine Dia 30 inches
Number of Feeders 90
Machine RPM 26
Yarn Count 24
Stitch length 4 mm
Efficiency 85%
Solution:
Step one
First we will calculate number of needles and number of stitches produced in one revolution. This would help us in calculating the total length of yarn consumed in one revolution.
Number of needles = machine dia * gauge *  (3.14)
= 30* 24*3.14
=2260 (exact 2260.8 but needles are always in even number
so we will take nearest even figure)

Number of stitches produced in revolution
Every needle is making one stitch on every feeders because machine is producing single jersey fabric (full knit fabric).
Number of stitches produced in one revolution = Number of needles * number of feeders
= 2260*90
= 203400
This figure shows that machine is making 203400 stitches in one revolution.

Step Two
Length of stitch is 04 mm (stitch length is always calculated in metric system)
From this figure we can calculate yarn consumption in yards in one hour

Yarn Consumption (in yards) in one hour
= number of stitches * length of (mm) * RPM *60 (minutes)
1000(to convert mm into meters)

=203400 * 4 * 26 * 60
1000
= 1269216 meters or
= 1388015 yards

Step Three
In previous step we calculated quantity of yarn consumed in yards. We can easily calculate weight of this yarn while its count is known (see example 03).

Weight of cotton yarn = length of yarn
Count * 840

= 1388015
840 * 24
= 68.85 pounds or
= 31.23 Kilo grams
Efficiency 85% = 26.55 Kilo grams
Answer: this machine can produce 26.55 Kgs fabric in one hour at 85 % efficiency

Example 08
For Filament yarn
Calculate nominal production of a single jersey-knitting machine per hour from the data given:
Machine Gauge 28
Machine Dia 26 inches
Number of Feeders 120
Machine RPM 30
Yarn Denier 75
Stitch length 4.5 mm
Efficiency 85%
Solution:
Step one
First we will calculate number of needles and number of stitches produced in one revolution. This would help us in calculating the total length of yarn consumed in one revolution.
Number of needles = machine dia * gauge *  (3.14)
= 26* 28*3.14
=2286 (exact 2285.92 but needles are always in even number so we will take nearest even figure)

Number of stitches produced in revolution
Every needle is making one stitch on every feeder because machine is producing single jersey fabric (full knit fabric).
Number of stitches produced in one revolution = Number of needles * number of feeders
= 2286*120
= 274320
This figure shows that machine is making 274320 stitches in one evolution.

Step Two
Length of stitch is 04.5 mm (stitch length is always calculated in metric system)
From this figure we can calculate yarn consumption in yards in one hour

Yarn Consumption (in yards) in one hour
= number of stitches * length of (mm) * RPM *60 (minutes)
1000(to convert mm into meters)

=274320 * 4.5 * 30 * 60
1000
= 2221992 meters

Step Three
In previous step we calculated quantity of yarn consumed in yards. We can easily calculate weight of this yarn while its count/denier is known (see example 05).

Weight of filament in grams = Length in meters * denier 9000

= 2221992*75
9000
Answer =18516 grams or
=18.516 Kgs

Efficiency 85% = 18.516*85%
=15.74 Kgs

Answer: this machine can produce 15.74 Kgs fabric in one hour at 85 % efficiency

Note: if we are producing any textured fabric, like fleece, then we use two different yarns at different feeders and ultimately stitch length is also different. In such case we should calculate separately consumption of different yarn at different feeders. Following example will help in calculating production in case of use of more than one kind yarn.

Example 9
Calculate nominal production of a fleece-knitting machine per hour from the data given:
Machine Gauge 18
Machine Dia 30 inches
Number of Feeders for 60
Front yarn
Number of feeders 30
For loop yarn
Machine RPM 28
Yarn Count 26s for front
Yarn count for loop 16s
Stitch length of 4.5 mm
front yarn
Stitch length of 2.5 mm
Loop yarn
Efficiency 85%
Solution:
Step one
First we will calculate number of needles and number of stitches produced in one revolution. This would help us in calculating the total length of yarn consumed in one revolution.
Number of needles = machine dia * gauge *  (3.14)
= 30* 18*3.14
=1696 (exact 1695 but needles are always in even number
so we will take nearest even figure)

In this example we will calculate consumption of yarn in Kgs of both yarns and then we will add them to get final production per hour

Consumption of yarn for front knitting
Every needle is making one stitch on every feeder because machine is producing single jersey fabric (front of fleece).
Number of stitches produced in one revolution = Number of needles * number of feeders

= 1696*60
= 101760
This figure shows that machine is making 101760 stitches in one revolution.

Step Two
Length of stitch is 04.5 mm (stitch length is always calculated in metric system)
From this figure we can calculate yarn consumption in yards in one hour

Yarn Consumption (in yards) in one hour
= number of stitches * length of (mm) * RPM *60 (minutes)
1000(to convert mm into meters)

=101760 * 4.5 * 28 * 60
1000
= 769305 meters or
= 841312 yards

Step Three
In previous step we calculated quantity of yarn consumed in yards. We can easily calculate weight of this yarn while its count is known (see example 03).

Weight of cotton yarn = length of yarn
Count * 840

= 841312
840 * 30
= 38.52 pounds or
= 17.43 Kilo grams
Efficiency 85% = 14.85 Kilo grams
Answer: this machine will consume 14.85 Kgs of yarn to knit front of the fleece fabric in one hour at 85 % efficiency
Step Four
Yarn consumed for loop knitting (back of the fabric)
Every needle is making one stitch on every feeder because machine is producing single jersey fabric (front of fleece).
Number of stitches produced in one revolution = Number of needles * number of feeders

= 1696*30
= 50880
This figure shows that machine is making 50880 stitches in one revolution.

Note: that we have put 30 cones of course count for loops after every two feeders.

Step Five
Length of stitch is 2.5 mm (stitch length is always calculated in metric system)
From this figure we can calculate yarn consumption in yards in one hour

Yarn Consumption (in yards) in one hour
= number of stitches * length of (mm) * RPM *60 (minutes)
1000(to convert mm into meters)

=50880 * 2.5 * 28 * 60
1000
= 213696 meters or
= 233696 yards

Step Six
In previous step we calculated quantity of yarn consumed in yards. We can easily calculate weight of this yarn while its count is known (see example 03).

Weight of cotton yarn = length of yarn
Count * 840

= 233696
840 * 16
= 17.39 pounds or
= 7.89 Kilo grams
Efficiency 85% = 6.70 Kilo grams

Step Seven
Now we can add both yarn consumed
Yarn for front 14.85
Yarn for back 6.70
Total 21.55

This machine can produce 21.55 Kgs fabric in one hour at 85% efficiency

All above discussion to elaborate the way to calculate the optimum production of a knitting machine. We have develop a equation which is useful in evey situation to calculate the optimum production capacity of a knitting machine at 85% efficiency.

For cotton count

Production in one hour=

Gauge * Dia * 3.14 * RPM *60 * Stitch length (mm) *1.0936 * 1 * 85
1000 *840 * yarn count * 100

Grammage Expressions

Generally grammage is expressed in Grams per Meter Square (GSM) but in certain cases it is also expressed Ounces per Yard Square (OSY). People, particularly working in marketing and merchandising departments face problems in converting GSM into OSY. We will explain this conversion method with examples before that it is imperative to know the standard conversion factors of different measuring units. A complete conversion chart is given at the end of the book. One should be much familiar with these conversion factors.

Conversion of GSM (grams per square meter) into OSY (ounces per square yard)

250 GSM means that weight of one meter square fabric is 250 grams and 10 OSY means weight on one yard squares is 10 ounces. In the following lines we will explain the method of conversion from GSM to OSY and vice versa with the help of examples.

Example 10

Convert 10 OSY (ounces per square yard) into GSM (grams per square meter).

It means weight of one yard square is 10 ounces or
Weight of one square yard is 280 grams (one ounce is equal to 28 grams) or

Weight of one 0.836 meter square (one yard square is 0.836 meter square) is 280 grams or

Weight of one meter square = 280* 1
0.836

Answer = 344.9 grams per meter square

Example 11

Convert 250 GSM (grams per square meter) into OSY (ounces per square yard)

It means weight of one meter square is 250grams or

Weight of one square meter is 8.93 ounces (28 grams are equal to one ounce) or

Weight of 1.196 yard square (one meter square is equal to 1.196 yard square) is 8.93 or

Weight of one yard square = 8.93* 1
1.196

Answer = 7.47 ounces per yard square

Relation between length, width and grammage

It was observed during interaction with the people working in garment business that they face difficulties in calculation related to grammage, width and length of the fabric. In the following lines we will explain relationship among these factors with examples.

Example 10
Calculate weight of fabric from the given data.

Grammage 300 GSM
Width of fabric 35 inches (in tubular form)
Length of fabric 20 meters

First we will calculate area of the fabric

Area of fabric = Fabric length * fabric width

= 20 * 35*2 (since fabric is in tubular)
39.37 (one meter is equal to 39.37 inches)

= 35.6 meter square

Weight of one meter square is = 300 (GSM)
And weight of 35.6 meter square = 300*35.6
= 10680 grams or 10.680 Kgs

Example 13

Calculate GSM from the data given

Total Weight of fabric = 15.5 Kgs
Length of fabric = 35 meters
Width of fabric in open form = 65 inches

Solution:

First we will calculate area of the fabric

Fabric length = 35 meters
Fabric width = 65 inches or 1.65 meters
Fabric area = Length * width
=35 * 1.65
=57.75 meters square

Weight of 57.75 Meter square is 15.5 kgs or 15500 grams
So weight of one square meter = 15500/57075

= 268.39 grams per meter square of GSM of

the fabric

Calculation of different fibre percentage in knitted fabric

Normally fabrics are knitted with one kind of yarn but in some cases more than one type of yarn of different counts and combination (mixing of two different fibres) are used. One very common example is knitting of fleece fabric, which is knitted by using fine and course yarns, and one yarn is made of polyester and cotton. Another example is knitting of fabric by using spandex filament and cotton or pure polyester. In such condition there is a requirement to mention exact percentage of different fibres in the fabric. Supplier has to mention this ratio on label. In the following lines we discuss the methods to calculate such percentage with the help of examples.

Example
Find exact composition of different fibres in fleece fabric from the following data:
Yarn count front 30s 100 cotton
Yarn count for loop 20s 50:50 P/C
Consumption ratio Front: loop 2:1 (by weight)
Suppose for front we need 2Kg yarn and for loop we will be requiring 1 Kg yarn
Front yarn 2 KGS 100 % cotton Cotton 2000 grams
Loop yarn 1 Kg 50:50 P/C Cotton 500 grams and Polyester
500 grams
Exact Ratio

Cotton total 2.5 Kgs
Polyester 0.5 Kgs

Ratio:
Cotton: 83.33%
Polyester : 16.66

Ref:- http://munawarz321.blogspot.in/2008/07/knitting-calculations.html

Production Planning and Control


In any manufacturing enterprise production is the driving force to which most other functions react. This is particularly true with inventories; they exist because of the needs of production. In this chapter the relationship of production planning and control to work-in-process inventories is stressed.

Objectives of Production Planning Control

The ultimate objective of production planning and control, like that of all other manufacturing controls, is to contribute to the profits of the enterprise. As with inventory management and control, this is accomplished by keeping the customers satisfied through the meeting of delivery schedules. Specific objectives of production planning and control are to establish routes and schedules for work that will ensure the optimum utilization of materials, workers, and machines and to provide the means for ensuring the operation of the plant in accordance with these plans.

Production Planning and Control Functions

All of the four basic phases of control of manufacture are easily identified in production planning and control. The plan for the processing of materials through the plant is established by the functions of process planning, loading, and scheduling. The function of dispatching puts the plan into effect; that is, operations are started in accordance with the plant. Actual performance is then compared to the planned performance, and, when required, corrective action is taken. In some instances re-planning is necessary to ensure the effective utilization of the manufacturing facilities and personnel. Let us examine more closely each of these functions.

Process Planning (Routing)

The determination of where each operation on a component part, subassembly, or assembly is to be performed results in a route for the movement of a manufacturing lot through the factory.  Prior determination of these routes is the job of the manufacturing engineering function.

Loading

Once the route has been established, the work required can be loaded against the selected machine or workstation. The total time required to perform the operation is computed by multiplying the unit operation times given on the standard process sheet by the number of parts to be processed. This total time is then added to the work already planned for the workstation. This is the function of loading, and it results in a tabulated list or chart showing the planned utilization of the machines or workstations in the plant.

Scheduling

Scheduling is the last of the planning functions. It determines when an operation is to be performed, or when work is to be completed; the difference lies in the detail of the scheduling procedure. In a centralized control situation – where all process planning, loading, and scheduling  for the plant are done in a central office- the details of the schedule may specify the starting and finishing time for an operation. On the other hand, the central schedule may simply give a completion time for the work in a given department.

Combining Functions

While it is easy to define “where” as process planning, “how much work” as loading, and “when  as scheduling, in actual operations these three functions are often combined and performed concurrently. How far in advance routes, loads, and schedules should be established always presents an interesting problem. Obviously, it is desirable that a minimum of changes be made after schedules are established. This objective can be approached if the amount of work scheduled for the factory or department is equal to or slightly greater than the manufacturing cycle. For optimum control, it should never be less than the manufacturing cycle.

Dispatching

Authorizing the start of an operation on the shop floor is the function of dispatching. This function may be centralized or decentralized. Again using our machine-shop example, the departmental dispatcher would authorize the start of each of the three machine operations – three dispatch actions based on the foreman’s routing and scheduling of the work through his department. This is decentralized dispatching.

Reporting or Follow – up

The manufacturing activity of a plant is said to be “in control” when the actual performance is within the objectives of the planned performance. When jobs are started and completed on schedule, there should be very little, if any, concern about the meeting of commitments. Optimum operation of the plant, however, is attained only if the original plan has been carefully prepared to utilize the manufacturing facilities fully and effectively.

Corrective Action

This is the keystone of any production planning and control activity. A plant in which all manufacturing activity runs on schedule in all probability is not being scheduled to its optimum productive capacity. With an optimum schedule, manufacturing delays are the rule, not the exception.

Re-planning

Re-planning is not corrective action. Re-planning revise routes, loads, and schedules; a new plan is developed. In manufacturing this is often required. Changes in market conditions, manufacturing methods, or many other factors affecting the plant will often indicate that a new  manufacturing plan is needed.

Factors Affecting Production Planning and Control

The factors that affect the application of production planning and control to manufacturing are the same as the factors we have already discussed that affect inventory management and control. Let us briefly review these in relation to production planning and control.

Type of Product

Again, it is the complexity of the product that is important, not what the product is, except as this may in turn relate to the market being served. Production control procedures are much more complex and involve many more records in the manufacture of large steam turbine generator sets or locomotives to customer orders then in the production of large quantities of a standard product involving only a few component parts, such as electric blankets, steam irons, or similar small appliances.

Type of Manufacturing

This is probably the most influential factor in the control situation. For a large continuous manufacturing plant producing a standard product, we have already indicated that the routing was included in the planning of the plant layout.

Production Planning and Control Procedures

A detailed discussion of all the techniques and procedures of production planning and control is  beyond the scope of this book; many complete text books exist on the subject. We have already  indicated that planning and control practices will vary widely from plant to plant. Further the many ways in which of the functions might be carried out in practice were indicated earlier in this chapter.

Though no production control function can be entirely eliminated, the least control that results in  effective operation of the factory is the best control. It must be remembered that production planning and control systems should be tools of management. The objective is not an elaborate and detailed system of controls and records, but rather, the optimum operation of the plant for maximum profits.

Production Planning and Control Systems

Because production planning and control places an emphasis on the control of work-in-process, the system will in effect tie together all previous records and forms developed in all planning for the manufacture of the product.

Market forecast

The market forecast is discussed in Chapter 26. Its value to production planning and control is that it will indicate future trends in demand for manufactured product. Work shift policies, plans for an increase or decrease in manufacturing activity, or possible plant expansions may often be based upon the market forecasts and in turn affect the planning of the production planning and control group.

Sales Order

This is the second of the five classes of orders. It is a rewrite of the customer’ order specifying what has been purchased – product and quantity and authorizing shipment of the goods to the customer. Multiple copies are prepared and all interested functions are furnished a copy. Sales orders may be written by marketing, inventory control, or production control.

Stock Order

This third class of order is not always used. In the preceding paragraph we indicated how it may be used after sales order accumulate to an economical manufacturing lot. It is, of course, the principal order when manufacturing to stock. It will authorize production in anticipation of future sales.

Shop Order

This fourth class of order deals with the manufacture of component parts. Customer orders, sales orders, and stock orders are for the finished product. In the preceding chapters we discussed how,  by product explosion, the requirements are established for component parts to build assembled products.

Standard Process sheet

This form is prepared by process engineering and it is the source of basic data as to the type of machine to be used, the time required for processing and the sequence of operations in the manufacture of the product. Routing and scheduling of shop orders, as well as loading of workstations in advance of scheduling, depend on up-to-date standard process sheets being available to the production planning and control group.

Engineering Specifications

Blueprints and bills of materials are used by production planning and control when they become a component part of the packaged instructions issued to the shop through the control office. One good planning procedure is to accumulate all necessary data for a shop order in a single package the standard process sheet, the blueprint, the bill of material (if an assembly operation is involved), the route sheet, and possibly the schedule for the production of the order.

Route Sheet

This is the form on which the route of a shop order is indicated. In practice, this form is generally combined with one of the other forms in the system. For example, the shop order, the standard process sheet, and the route sheet are often one piece of paper- usually called the shop order or the manufacturing order.

Load Charts

These charts are prepared to show the productive capacity that has been “sold” – and at the same time the available productive capacity. These charts may be prepared for each workstation or machine in the plant, or they may be for groups of machines or departments.

Job Tickets

This is the fifth and last type of order in a manufacturing situation. Job tickets authorize the performance of individual operations in the manufacturing process.

Project Planning Methods

The production planning and control methods discussed thus far in this chapter deal primarily with the production of consumer or industrial products which could be considered to fall within the area of “repetitive manufacturing”. The products to be produced are often manufactured in quantities of more than one, and their total processing time can be measured in hours, or at most, days. The best –known methods that have been developed are CPM (for Critical Path Method) and PERT (for Program Evaluation and Review Technique). The original PERT technique is now considered, more accurately, PERT TIME, whereas a later development is known as PERT COST.

From the optimistic, most likely, and pessimistic times, the expected elapsed time (te) can be obtained by statistical techniques. The relationship of the three estimates to the expected elapsed time is given by the formula
image
Where a = optimistic time
b = pessimistic time
m = most likely time
It can be seen from the formula that the most likely time estimate is given four times as much weight as the optimistic and pessimistic estimates when computing the expected time.

Systems Analysis

As with other manufacturing control systems and procedures, production planning, and control lends itself to modern mechanization techniques such as machine accounting and use of  computers. Careful study of the control system through procedure analysis will indicate the  savings that may be effected by the utilization of modern equipment. These savings may be in the clerical help required in the administration of the system or in the advantages of quick compilation of data, which in turn results in up-to-date control data.

Production Planning and Control Organization

It should be obvious that there is no single pattern for the organization of the production planning and control activity. In many small plants the routing, loading, and scheduling functions may well be included in the duties of the operating line; the shop manager, superintended, and foremen. But it is difficult to combine day-to-day work with adequate planning, and as a result it is often more feasible to break away the production planning and control functions and assign them to qualified specialists. These groups should be organized as staff sections normally reporting to the top manufacturing executive.

Centralized Production Planning and Control

Centralization or decentralization of duties of the production control staff depends upon the design of the production planning and control system. In a completely centralized setup,  determination of shipping promises; analysis of sales, stock, and shop orders; preparation of  routes, load charts, and schedule charts; and dispatching of work to the shop complete with job tickets and all other necessary paper would be accomplished by a central production planning and control unit. In addition, as work is completed, a careful analysis of the actual performance would be made, and if corrective action were required, it would be initiated by this group.

Decentralized Production Planning and Control

We have discussed at great length that no matter how general the planning may be in a central office, the plan must eventually be developed into a detailed plan on the shop floor. Some companies are now endeavouring to make each foreman a manager of his own departmental operation. In these cases the foreman is furnished with a complete staff for the production planning and control of the activities in the department.

Planning Phase

We have already indicated in some details the duties involved in the production planning phase. Working from the basic data mentioned earlier, the personnel in this part of the activity routes and load and schedule charts.

Control Phase

The completed job ticket, or its equivalent, is the key to this phase of the production planning and control system. It is the means of reporting back from the shop floor that indicates that a job is completed; or if daily job tickets are turned in, the daily progress of a job can be determined.

Relation to Other Functions

Good relationships with all the other functions in the enterprise are essential to effective production planning and control. Full cooperation with the marketing group is necessary, particularly in view of the importance of market conditions and the goodwill of customers. Both product engineering and process engineering must keep production planning and control informed as to their plans to avoid the manufacture of goods either to incorrect specifications or by an improper method.

Measurement of Effectiveness

In determining the effectiveness of a production planning and control system, there are quite a few problems. The key criterion might well be whether or not shipping promises are being kept – the percentage of the order shipped on time. This, however, would not be a true criterion if excessive overtime of expediting costs were involved in getting any of these orders shipped. The cost of the control system in relation to the value of goods shipped is another possibility. Again, however, this may not be sound: if markets slump, a bad ratio will develop. Many good production planning and control systems have been discontinued because of “high costs” under these conditions- and have never revived after business picket up. In a study of benefits and costs of computerized production planning and control systems, Schroeder et al. list the following performance criteria by which production planning and control systems might be judged:
1. Inventory turnover
2. Delivery lead time
3. Percent of time meeting delivery promises
4. Percent of orders requiring “splits” because of unavailable material.
5. Number of expeditors
6. Average unit cost.

Production Planning and Scheduling Software for the Textile Industry


Introduction

As far as enterprise resource planning systems (ERP) are concerned, the textile industry may still be a manageable affair. But the moment you talk about developing a production planning and scheduling software for this industry, you are asking for a difficult task to be performed. Many a veteran has failed in attempting to achieve this feat.

Challenges

Some of the unique challenges posed by the textile industry to any production planning and scheduling software vendor are discussed here. These challenges can be grouped as raw material concerns, manufacturing lead time, manufacturing constraints, orders, and inventory.

Raw material concerns involve high raw material costs and seasonal raw material procurement cycles. Cotton, for example, is a seasonal commodity; therefore, the availability and price will change throughout the year. High raw material cost is another issue. Raw material costs may constitute as high as 60 to 70 percent of the total costs.

Manufacturing lead time can also pose challenges. Manufacturing lead times can be excessive, sometimes more than two months. This is because the raw cotton production process, for example, has to go through many processes and most of them have huge lead time requirements. Looms in particular take the most lead time. A loom machine can make only 500 meters of fabric in a day whereas typical order lengths are in the range of 25,000 to 50,000 meters range.

Most of the manufacturing processes also have high setup times. Quality analysis time also runs high as the finished cloth needs to be manually inspected for defects. Extra lead times also result due to unavoidable generation of inventory in the form of extra meters than the ordered lengths.

Another challenge involving manufacturing is that different processing speeds occur at different work centers, which must also be included in the equation. Dyeing machines, warping machines, spinning machines run at speeds of 30,000 to 50,000 meters of yarn per day but looms run at 500 meters of fabric per day. Because of this, there may be only 2 to 5 dyeing machines, but there may be as many as 500 looming machines in the same plant.

Likewise there are different processing requirements on the same production line. For example, up until the dyeing process, the manufacturing process fits orders that are big and similar. But at looming, the manufacturing process fits many smaller and varied orders. This poses a real challenge as fitting these two diametrically opposite requirements is next to impossible to do.

Another issue is the unpredictable generation of second quality textile and the fact that variations in color and shade is only known after the fabric has been woven and finished (though they are caused back at the dyeing stage). This can result in a lot of rejected material being processed unnecessarily, thus adding to manufacturing costs and processing time.

These manufacturing constraints ultimately impact customer orders. Because the production rates are very low on looms, customer orders are broken into smaller sub-orders, and the sub-orders are distributed to many looms to reduce the lead time for individual orders. However, variations in the color or shade from the order can also emerge, which, as explained earlier, are detected at the end of the entire process.

Not surprisingly, inventory is another challenge faced by the textile industry because there is a high generation of extra finished products. In addition to extra material resulting from second quality and color shade variations, extra yarn moves through the entire production cycle. Up to dyeing stage, the work-in-process (WIP) is in yarn form and the length of this yarn is fixed at the yarn making stage. It cannot be cut as per order lengths. These extra meters travel through the production cycle and end up as excess inventory, which is later adjusted in the next planning cycle. Consequently, plant capacity is inefficiently utilized due to unavoidable generation of extra meters—more than the lengths ordered.

After going through these constraints, it is obvious that it is difficult to develop production planning and scheduling software for the textile industry. Only a veteran who has in-depth industry knowledge as well as knowledge of how to tackle these constraints in the implementation can develop a software for planning and scheduling for the textile industry.

Dyeing versus Looming

It is very important to understand the different requirements at the dyeing and looming processes so a suitable planning and scheduling software can be suggested. The dyeing and looming processes are the true bottlenecks in the entire production cycle of all textile plants. Both dyeing and looming have high setup time, high production time, and high change overtime, but looms are far slower than dyeing machines. Looming is more like a warehouse with a lot of WIP inventory called grey stock and this grey stock is on many looming machines, in small quantities. Dyeing machines, however produce long sets of warp (dyed yarn). One set of warp can be produced by one dyeing machine in one day but the warp can only be consumed by at least 50 looming machines in one day. To keep the ability to produce many kinds of fabric, the manufacturers generally install many kinds of looming machines. All of these looms are fed by only 2 to 5 dyeing machines. Due to these factors the dyeing area is always hard-pressed to feed the looms with small lengths and different types of dyed yarn for the next work orders in line.

So dyeing machines are better suited to produce big quantities of dyed yarn of the same type, (e.g. same color and same number of ends). For example, if the ordered length of fabric is 25,000 meters and the order has been broken into 10 work orders at 10 looms, then it will take 5 days to finish the WIP at looming. This is if all the work orders are done simultaneously and speed of looms are 500 meters of fabric woven per day. A single dyeing machine will produce 25,000 meters of warp in half a day.

Solutions

These challenges in the textile industry can be met by conducting a profitable to promise analysis; grouping, breaking, and sequencing orders; and by routing WIPs. In the textile industry, orders are considered more like combinatorial meters rather than individual order meters, so the same type of orders can be grouped and sequenced to achieve production efficiency as well as reduce inventory creation. All WIPs can also considered the same way for the same purpose.

Profitable to Promise Analysis

Businesses in the textile industry mostly gets varied orders in terms of rate per meter, quantity, fabric type etc. Because of this, each order has to be evaluated on profitability, customer service levels and long and short term goals of the company. Profitable to promise analysis allows the business to find out if the particular order will be profitable to make by considering the costs of raw material, process, inventory, and other factors against the price the customer is willing to pay. Thus it can be seen that some orders will be a lot more profitable than other orders. This analysis is perfectly possible if you have the right software tool, which can provide you with this kind of information.

If raw material availability, machine capacity, and production lead time are known at the time of order taking, then it is possible to give a definite delivery date to the customer. This is known ascapable to promise. If we can also provide information about customer, production, inventory, stock out, material, and other overhead costs down to the item level, and then compare all incurred costs to the selling price, it will be possible to decide whether the incoming order should be taken and what priority it can be assigned, at the time the order is being taken. This functionality is very important for the textile industry.

In conjunction with above mentioned factors, a planning system that is also capable of grouping, breaking, and sequencing orders while it is doing total lead time calculations to determine a delivery date will solve many production planning problems. It will eliminate waste, reduce the generation of extra inventory, increase machine capacity utilization, increase customer service levels, eliminate stock out costs, and reduce production costs.

Grouping, Breaking, and Sequencing Orders

Grouping, breaking, and sequencing orders will also help to overcome textile production challenges. Group smaller orders at dyeing process. The same dyeing WIPs can be grouped so the generation of extra meters can be minimized. Break bigger orders into many smaller orders at dyeing, and sequence them with other orders. Loom areas typically have many kinds of loom machines which can produce different kinds of fabric but at very slow rates. If big orders of same material are continuously coming from dyeing, they will only go to a particular loom machine which can process it; other loom machines which cannot use these warps will go idle for want of material. Another way to minimize set up time is to sequence WIP orders with the same color at the dyeing process. This will minimize the set up time to change of color. Also, breaking individual orders into many parts will create many work orders for the same order at looming process. This will minimize lead times significantly at looming.

Also, look for already existing inventory in the form of extra meters at looms and finished stock in the inventory to allocate these meters against the matched fresh orders and plan for the remaining meters.