From DDL Wiki
Executive Summary
Customer Needs and Stakeholders
The post hole digger is a common landscaping equipment. The primary customer need in a post hole digger is to have the ability to inexpensively dig a hole effectively and efficiently for landscaping and various other purposes (such as deploying a fence, erecting a wall, installing lamp posts, etc..). Because this is a heavy duty tool that must undergo large amounts of stresses from breaking soil, the first and foremost requirement is reliability and durability. Oftentimes, digging a hole is simply the foremost and critical step in the grand scheme of a project, and thus the customer will not want to experience a setback in a faulty post hole digger, causing a delay in the entire project. In addition, operating the post hole digger requires constant lifting and carrying, and thus the tool must be of relatively manageable weight to prevent consumer fatigue and discomfort. To reduce weight, the shaft of the post hole digger will oftentimes be made of wood, and in more advanced designs, fiberglass.
Product Function/Evaluation
A total of two different post hole digger designs are examined. One is the most basic standard post hole digger manufactured by True Temper, and the other is a more complex post hole digger that utilizes a compound action, also manufactured by True Temper.
In all designs, the post hole digger consists of two main shovel blades hinged in a jaw-like arrangement, the shovel blades are open when the handles are closed, and close when the handles are pulled apart.
In operation, the post hole digger is initially thrust into the ground while the handles are in the closed position, allowing the shovel blades to effectively draw in soil and earth. Once the shovel blades are fully embedded into the soil, the handles are drawn apart, causing the blades to swivel shut, grasping the soil. Oftentimes for dense earth, the handles must be open and closed a number of times to effectively break the soil to be grasped. Once the handle is shut, the operator will lift up the post hole digger with the captured soil, leaving a round, deep hole in the ground.
Assembly
| Part Number | Name | Qty. | Weight | Function | Material | Maunfacturing Process | Image |
|
| S1
| Wooden handle
| 2
| 33.1 oz
| Allows user to operate tool
| Wood (Pine)
| Automated CNC turning
| 
|
| S2
| Carriage Bolt
| 4
| 0.7 oz
| Attaches wooden handle to digging head
| Steel
| Upset forging
| 
|
| S3
| Large washer
| 4
| 0.1 oz
| Prevents carriage bolt head from being embedded in wooden handle
| Steel
| Stamping
| 
|
| S4
| Digging head
| 2
| 22.5 oz
| Provides means of digging into dirt, sizing the hole, and removing dirt
| Steel
| Stamping
|   
|
| S5
| Pivot bolt
| 1
| 0.8 oz
| Joins two handle/digging head assemblies
| Steel
| Upset forging
| 
|
| S6
| Sleeve
| 1
| 0.2 oz
| Protects pivot pin
| Steel
| Roll forming
| 
|
| S7
| M8 washer
| 5
| 0.05 oz
| Increases surface area over which compressive force of the tightened bolt and nut is applied, placed between nut and and surface to prevent damage to surface when nut is tightened
| Steel
| Stamping
| 
|
| S8
| M8 x 1.25 Lock nut with nylon insert
| 5
| 0.1 oz
| Threads onto bolt and tightened to join parts together. Nylon insert prevents nut from loosening
| Steel
| Forging
| 
|
| Part Number | Name | Qty. | Weight | Function | Material | Maunfacturing Process | Image |
|
| C1
| Grip
| 2
| 2.2 oz
| Provides comfortable surface for gripping the tool, protects end of fiberglass handle
| Plastic
| Extrusion
| 
|
| C2
| End cap
| 2
| 0.05 oz
| Stiffens open end of fiberglass handle, provides surafce area for grip to attach over end of otherwise hollow handle
| Plastic
| Injection molding
| 
|
| C3
| Fiberglass handle
| 2
| --
| Allows user to operate tool
| Fiberglass with protective coating
| ?
| 
|
| C4
| Wooden handle insert
| 2
| --
| Reinforces fiberglass handle where handle attaches to connecting arm
| Wood
| Automated CNC turning
| 
|
| C5
| Connecting arm
| 2
| -- oz
| Extends length of handle to digging head, bent to particular shape to allow for compund lever action for opening and closing digger heads
| Steel
| Roll forming, swaging, and bending
| 
|
| C6
| Spacing bracket
| 1
| 1.9 oz
| Maintains spacing of handles when the tool is in the closed position
| Steel
| Stamping
| 
|
| C7
| Handle bolt
| 4
| 0.7 oz
| Attaches Fiberglass handle to Connecting arm
| Steel
| Upset forging
| 
|
| C8
| Handle washer
| 6
| 0.2 oz
| Increases surface area over which compressive force of Handle bolt is applied, protects Fiberglass handle from damage that would caused by tightening/rotating nut or bolt
| Steel
| Stamping
| 
|
| C9
| M8 x 1.25 Lock nut with nylon insert
| 7
| 0.1 oz
| Threads onto bolt and tightened to join parts together. Nylon insert prevents nut from loosening
| Steel
| Forging
| 
|
| C10
| M8 washer
| 3
| 0.05 oz
| Increases surface area over which compressive force of the tightened bolt and nut is applied, placed between nut and surface and/or bolt head and surface to prevent damage to surface when nut is tightened
| Steel
| Stamping
| 
|
| C11
| Bushing
| 1
| 0.7 oz
| Used to align two Connecting arms and create a low friction surface for Connecting arms to pivot about
| Brass
| Roll forming
| 
|
| C12
| Plastic spacer
| 1
| 0.05 oz
| Placed concentric with Bushing between two connecting arms to provide low friction surface and prevent the arms from wearing against one another
| Plastic
| Injection molding
| 
|
| C12
| Scissor pin bolt
| 1
| 0.3 oz
| Joins two handle assemblies together, passes though Bushing to create main joint around which handles pivot
| Steel
| Upset forging
| 
|
| C13
| M10 washer
| 1
| 0.1 oz
| Increases surface area over which compressive force of the tightened bolt and nut is applied, placed between nut and surface and/or bolt head and surface to prevent damage to surface when nut is tightened
| Steel
| Stamping
| 
|
| C14
| M10 lock nut with nylon insert
| 1
| 1.3 oz
| Threads onto scissor pin bolt and tightened to join connecting arms together. Nylon insert prevents nut from loosening
| Steel
| Forging
| 
|
| C15
| Digging head
| 2
| 26.3 oz
| Provides means of digging into dirt, sizing the hole, and removing dirt
| Steel
| Stamping
|   
|
| C16
| Digging head connecting bolt
| 2
| 0.8 oz
| Joins connecting arm to digging head, allows connecting arm and digging head to pivot
| Steel
| Upset forging
| 
|
| C17
| Digging head pivot bolt
| 1
| 0.8 oz
| Connects two digging heads together, creates pivot point around which digging heads open and close
| Steel
| Upset forging
| 
|
| C18
| Large plastic spacer
| 2
| 0.1 oz
| Placed on pivot bolt to align two digging heads and protect pivot bolt
| Plastic
| Injection molding
| 
|
| C19
| Small plastic spacer
| 1
| 0.05 oz
| Placed on pivot bolt to align two digging heads and protect pivot bolt
| Plastic
| Injection molding
| 
|
DFMA
FMEA
DFE
Mechanical Analysis
