11 Internal Logistics

Question 1

AGVs

Answer 1

automatic guided vehicles

Question 2

Logistics adds three types of value

Answer 2

• Time. A product is more valuable when you need it than a week later.
• Place. A product is more valuable where you are than far away in an unknown location.
• Presentation. A product presented to you for easy identification and access is more valuable than
  one mixed in a box with many other parts.

Question 3

Where the boundary is placed between Logistics and Production is a managerial decision.

Answer 3

While logistics activities take up much more space than production, they usu-
ally employ an order of magnitude fewer people.
Another key boundary is that between the plant and the rest of the world, which is usually materialized
in docks for receiving and shipping.

Question 4

internal logistics is often also called

Answer 4

in-plant logistics or dock-to-
dock logistics

Question 5

internal logistics dif-
fers from

Answer 5

the in-bound logistics of getting parts from suppliers and the outbound logistics (“shipping”) of
distributing finished goods in the way it is managed.

Internal logistics is under the control of one
organization.

Question 6

Many manufacturing organizations use only one way to do the work of logistics.

Answer 6

• Parts arrive in full truckloads from one supplier in each truck.
• Operators use forklifts to unload pallets from the truck.
• After the receiving operations, forklift operators put away pallets into single-deep pallet racks into
  any available slot, and log the location with a radio terminal.
• The Manufacturing Execution System (MES) or the Warehouse Management System (WMS)
  issues workorders or routing slips.
• The forklift operators retrieve full pallets from the warehouse and log this operation with a radio
  terminal.
• They deliver full pallets wherever they can find space to set them down near the destination pro-
  duction area.

The same pattern is applied to all items, regardless of required quantities or frequency of use. The logis-
tics organization is attached to this one-size fits-all approach, and its management is particularly con-
cerned that its members would not be able to deal with the complexity associated with a different
approach for each category of items.

-> There is no
one size that fits all in logistics.

Question 7

In-plant transportation

Answer 7

• Besides improving safety, restricting forklifts to marked aisles may be the key to massively increase
  the manufacturing density of the shop floor.
• Relocating a frequently used item may be enough to change its handling method from a forklift
  with a certified driver to a pallet jack hundred times cheaper that anyone can use.
• Changing the minimum in-plant transportation quantity from a pallet to a small bin may elimi-
  nate the returns of partial pallets to the warehouse for incoming materials, while simplifying and
  accelerating the movement of finished goods.

Question 8

In-plant transportation: decentralized manual palletizing with semi-automation retrofits may
be cheaper and more reliable than one central automatic palletizer fed through a network of conveyors.

Answer 8

Regardless of distance, the following needs to happen:
1 The minimum transportation quantity must accumulate at the point of origin.
2 The parts need to be prepared for transportation, which may entail, for example, placing them in
bins and palletizing the bins.
3 The part must be transported, for example by a person, a forklift with a driver, or conveyor.
4 At the destination, most parts must be prepared for production – that is, removed from pallets and
bins and possibly placed on lineside shelves or kitted.

Cutting the distance between two lines in half will make a difference, but integrating the two lines and
eliminating the transportation step will make a much larger difference.

Question 9

Many shop floors have no marked transportation aisles. This has three main consequences:

Answer 9

• Extra materials, pallets, fixtures, surplus equipment, and other items encroach upon and eventu-
  ally block transportation aisles.
• There are no restrictions on where vehicles are allowed to go, and production operators must keep
  looking over their shoulders to make sure no forklift is coming.
• Any visitor who is not familiar with the usual truck driving pattern is in high risk.

-> Aisle markings should be kept current and standardized.
-> manufacturing islands – where only production personnel work
-> transportation aisles that are reserved for vehicles

Question 10

In-plant milk runs
(there also exists inbound milk-run and outbound milk-run)

Answer 10

The need to move small quantities of a large number of items within plants with short, predictable lead
times and without multiplying transportation costs has driven manufacturers to implement so-called milk runs.
Milk runs are loops through a number of pickup and delivery points.

Key Features of a Milk Run:
* Fixed schedule and route.
* Multiple pickups/deliveries on the same trip.
* Just-in-time (JIT) material delivery to minimize on-site inventory.
* Reduces transportation costs by consolidating loads.

Question 11

Using forklifts for transportation

Answer 11

you need to answer the following questions:
1 Who orders the forklifts to take action?
2 How is a specific forklift assigned to each task?

The person requesting a trans-
port calls a central dispatcher who communicates by radio with the fleet of forklifts and sends the next available one

How many forklifts (100) are there and how much of the time (90%) are they occupied? -> Probability (all taxis occupied) = (90%)100 = 26 ppm = 0.0026%
AABER: wenn Probability (all forklifts occupied) = (90%)10 = 35% dass heisst ein drittel der Zeit ist keins frei.. das ist nicht genug
Entweder mehr Forklifts (dann auch mehr Personal) oder sie müssen weniger genutzt werden.. beides schlecht..

Pushcarts or trains of tow-carts traveling on fixed routes at fixed intervals, like city
buses and subways, picking up and dropping off quantities of materials that are usually less than full
pallet loads provide a cheaper, more reliable, and more predictable service.

Question 12

Water spider

Answer 12

A water spider is a person whose main job is to make sure that materials are available in a practical pre-
sentation where and when they are needed for production. Hence, a water spider is a job position rather
than a method. It serves assemblers on a production assembly line, who are usually organized in teams
from 4 to 10 members. The water spider is a fully cross-trained member of an assembly team and can
relieve any other members for short periods of time like bathroom breaks. A water spider is not just a
“floater” whose only role is to fill in for others. The water spider’s special duty is to prepare parts and kits for the other assemblers.
it requires the most senior, most versatile operator.

Question 13

supermarkets

Answer 13

The local stores from which water spiders pick are often called “supermarkets.” These supermarkets, however, are more like a household refrigera-
tor than a grocery stores.

Question 14

Runners, repeaters, and strangers

Answer 14

• Runners: Items without which nothing can be shipped.
• Strangers: Items without which most of the orders can be shipped. The threshold can vary.
• Repeaters: All other items.
• Runners: Dedicate easily accessible warehouse locations for each, and load them systematically on
  milk runs. Consignment and vendor-managed inventory are options for standard items like nuts,
  bolts, and washers. For product-specific runners, kanbans or the reorder point method may be
  useful if demand fluctuates.
• Repeaters: Dynamically assign storage locations like hotel rooms, and use kanbans to determine
  when to deliver them through milk runs to supermarkets. Then kit them for delivery to assembly.
• Strangers: Don’t order from suppliers until you have a customer order for products that use them.
  Set up a separate storage and retrieval system for these items.

Question 15

Pull systems

Answer 15

A pull system is one in which the transfer of materials out of one operation is
triggered by a signal from the next operation.

Question 16

The distinction between a pull and a push system

Answer 16

• In a pull system, parts do not move until the destination plant or production line signals that it is
  ready for them.
• In a push system, parts move as soon as they are ready, regardless of the conditions in the destina-
  tion plant or line.

Pull: Part/Lot moves when next operation is ready
Push: Part/Lot moves to next operation when ready
Pull: partial pallet stays
Push: entire shrink wrapped pallet moves

Question 17

Issuing pull signals

Answer 17

The downstream operation can communicate its needs upstream in many ways. An approach that pre-
dates lean manufacturing is the reorder point system, in which a replenishment request is issued when the
inventory on hand crosses a lower limit.
In the reorder point system, the quantity requested is intended
to bring the inventory level back up to a maximum quantity.
The reorder point is set at such a level that the remaining inventory is sufficient to cover the demand
during the replenishment lead time.

Question 18

The kinds of variability the pull system is designed to absorb, however, are as follows:

Answer 18

• Daily fluctuations around a fixed rate, such as are due to trucking times affected by traffic condi-
  tions, tool breakage on machines, or operators on short-term sickness leave.
• Small changes in the demand.

Question 19

Pull signals are usually not used alone but in conjunction with other systems that forecast activity and
generate explicit sequences and schedules for pacemaker lines. The pacemaker is most commonly final
assembly, from which pull signals are issued that drive the rest of the plant. Explicit sequencing and
scheduling is required in the following circumstances:

Answer 19

1 The line stops for setups and following the raw sequence of pull signals would require so many
setups that the line would not have the capacity to keep up.
2 The raw sequence of pull signals contains random variations in product mix that are better
smoothed out before passing the flow of signals onto upstream feeder lines and suppliers.

Question 20

Pull signals in dedicated versus mixed-flow lines

Answer 20

In a dedicated line – a line that always produce one specific product – the applicability of the pull system
depends on the stability of the demand.

In a mixed-flow line, it also depends on the relative length of production runs and the replenishment lead
time.

Question 21

Calculating the number of kanbans in a loop

Answer 21

The formulas in the literature are all variations on Little’s law, which says that, in a steady-
state, on the average, any input-process-output black box satisfies a simple formula:
student copy
Inventory = Throughput × Lead time
When pulled from a part or a bin, a kanban enters a process that takes it through a loop. It
moves to where it is attached to another part or bin, returns attached, and is pulled again.
Here, the parameters of Little’s law take on the following meaning:
* The Inventory is the number of kanbans in the loop.
* The Throughput = Demand/Bin quantity is the rate at which kanbans are pulled.
* The Lead Time is the time between two consecutive pulls on the same kanban.

Unless Lead Time ≥ Replenishment Lead Time, the number of kan-
bans cannot sustain the Throughput. How much larger it needs be is a question that Little’s
law does not answer because it is about means and not fluctuations around means.

Question 22

Replenishment Lead Time

Answer 22

The Replenishment Lead Time is how long it takes for a pulled kanban to be delivered
attached to a part or bin.

Question 23

Part presentation

Answer 23

The way parts are presented to operators impacts both productivity and quality.

Parts should be presented to assemblers unpacked, within arm’s reach, with their smallest dimensions
facing out, and oriented for easy installation.
The picking strategies of kitting and lineside supply
should not be viewed as mutually exclusive but used in combination.

Question 24

Key principles of part presentation

Answer 24

The following methods and ideas are applicable in a variety of circumstances:
* Single-piece presentation
* Removal of packaging materials before delivery
* Location within arm’s reach of the assembler
* Orientation
* Adjustments to specific part characteristics
* Matching quantities
* Containers with dunnage for counting
* Kitting versus lineside supply.

Question 25

Kitting versus lineside supply

Answer 25

Kitting and lineside supply are two major parts supply strategies that can be used in combination, but usually are not. Some plants are designed around kitting all parts prior to assembly; others, around bringing them individually to lineside stores at assembly stations. In reality, the better strategy is to combine both approaches on the same line.

Question 26

Logistics technology

Answer 26

Conveyor systems Forklifts and pallet jacks Pushcarts Tuggers and trains of tow-carts Automatic guided vehicles (AGVs) Part supply to assembly cells

Question 27

Best practice in conveyance

Answer 27

Following are some of the best practices and corresponding common mistakes in the area of conveyance systems: * Conveyors should not be WIP buffers. The ideal is one-piece flow. Some companies deliberately extend the path between stations by raising the parts and dropping them onto a downward spiral flow rack. The legitimate use for this technology is in coolers or freezers, but, in some plants, they are simply used to accumulate WIP between stations to protect the downstream station against failures in upstream equipment. * Conveyors should not separate upstream and downstream stations. When straight conveyors are used as WIP buffers, they pull upstream and downstream work stations so far apart that operators cannot help or relieve each other. * The conveyance system should not lose part orientation. It is common that a part coming out of a machine in a controlled orientation will be dropped onto a conveyor belt in a random orientation, and that money will be spent at a later stage to recover that orientation. * If a conveyor is used, assembly work should be done on it. Another mistake is to use a conveyor but to have assemblers take parts away from the conveyor to work on them and put them back on afterwards. If you’re using a conveyor, then the assembly work should take place on it. * Avoid cranes or having assembly units hang on chains. These devices do not accurately control positioning and orientation. They not only require operators to manually position dangling work- pieces but are also unsafe. * Avoid shared conveyance systems. If, in spite of the drawbacks, you use a crane, you don’t want to share it among multiple cells, because it makes the feasibility of a move within one cell contin- gent on what happens in other cells. * First-in-first-out (FIFO). The conveyance system should preserve the process sequence. While FIFO is often perceived as “fair,” the real reason to pursue it is quality. Any conveyance system that shuffles the parts reduces process transparency and makes quality problems more difficult to trace. This excludes hoppers, bowl feeders, or even stacks, which are last-in-first-out devices. * Takt time enforcement. In moving assembly lines, takt time is enforced by the movement of the conveyor. There are many types of conveyance systems that do not enforce the pace. This is acceptable, as long as other devices do. In automatic lines with takt times of 5 seconds or less, sen- sors can be used to draw the control system’s attention to takt time violations that are too small for people to detect, yet hurt the performance of the line. * Dedication. The conveyance system should be dedicated to the pair of stations between which it moves parts. * Preservation of part orientation. A conveyance system which keeps the product in a controlled orientation is superior to one that doesn’t, and this orientation may not be the same at two subse- quent stations. The unit, for example, may need to be rotated 180 degrees, and this is better done while it is moving between stations than after it arrives. * Unobtrusiveness. The conveyance system should be in the background, rendering a valuable ser- vice without drawing attention to itself, allowing assemblers to focus on assembly. If all a visitor sees when entering the shop floor is a complex network of conveyors and elevators, the convey- ance system fails this particular test. The conveyance system must not interfere with the work and not dwarf the actual assembly stations. * Safety. In an assembly plant, there may be more safety issues with conveyance than with anything else. * Low cost of acquisition, operation, configuration change, and maintenance. Conveyor networks are not only expensive to buy, install, operate, and maintain, but they are also inflexible.

Question 28

Forklifts

Answer 28

Forklifts are such common devices that they immediately come to mind in any discussion of in-plant transportation. They are versatile and powerful, but they are not without disadvantages: 1 They cost tens of thousands of dollars. 2 They can only be operated by specially trained drivers. 3 They are a safety hazard and must be constrained to run in designated areas. 4 They are appropriate for moving pallet-sized loads but not smaller quantities.