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Revision as of 23:01, 3 February 2014

Contents 1 Executive Summary 2 Major Stakeholders 2.1 Manufacturers 2.2 Distributors 2.3 Retailers 2.4 End Users 3 Use Study 3.1 Steps for Use 3.2 Our Experience Using Product 3.3 Other Customers' Experience Using Product 4 Mechanical Function 5 Assembly 6 Bill of Materials 7 Design for Manufacturing and Assembly (DFMA) 7.1 Manufacturing 7.2 Assembly 8 Design for the Environment (DFE) 8.1 Production 9 Failure Modes & Effects Analysis (FMEA) 10 Team Roles 11 Appendix 12 References

Executive Summary

Electric grass shears are a small, hand-held device which trim grass using oscillating blades. They are commonly used to trim the edge of a lawn, along a garden, or other places that a mower cannot reach. Electric grass shears offer an alternative to both string trimmers (aka weed whackers or weed eaters) and hand shears – all for cutting edges of lawns so they look nice and well-kept.

This report contains a detailed analysis of the use and design of Homelite electric grass shears, purchased for $30 from Home Depot. From our own use of the product, comments from existing users, and online reviews we found that from a user’s perspective the electric grass shears are overall very effective in trimming grass but have limited battery life (which also degrades over time), require the user to pull the trigger constantly during use as a safety feature (which can be half an hour or more), blades jam in thick grass, and blades dull over time.

findings from dfma, dfe, fmea

After analyzing how the product is used and problems with the current design, we have the following recommendations for improvement:

• Use a removable battery so that the user can switch to another fully charged battery if other is completely discharged
• Add a charge indicator to indicate battery charge status
• Remove excess housing material on back and bottom

Major Stakeholders

Manufacturers

• Low cost materials, parts, and manufacturing equipment
• Minimal assembly labor
• Reliable supply chain

Distributors

• Compact packaging to reduce shipping costs
• Lightweight to reduce shipping and material handling costs
• Durable to prevent damage during product shipping

Retailers

• Compact packaging to reduce shelf-space costs
• Aesthetically pleasing packaging and product to draw customer interest
• Good quality and performance to prevent returns and improve customer loyalty

End Users

• Function as expected – trim grass effectively and efficiently
• Reliable and long product life-time
• Safety features to prevent harm to user during use (very important since sharp, oscillating blades are on product)
• Easy to use
• Battery life to last user’s duration of grass trimming
• Ergonomic to prevent harm and injury to user’s hand
• Lightweight to prevent excessive strain on user’s muscles
• Compact for portability and small storage space
• Reasonable recharge time
• Easy to maintain
• Aesthetically pleasing

Use Study

Steps for Use

Initial Assembly

1. Connect charger to grass trimmer and let charge for 10 hours

2. Open the black bottom cover

3. Insert blades into latch

4. Close by sliding in black bottom cover

5. Slide the locking switch

Grass Trimming

1. Hold down the lock down button

2. Press the switch trigger

3. Bending down and holding the trimmer parallel to the ground, slide the trimmer over the grass

4. Let go of both the trigger and the lock down button to power off

Our Experience Using Product

The grass shears came in a rectangular, cardboard box. The box was plain and contained no padding. Included in the box were:

1. Main shears assembly
2. Attachable cutting blades
3. User manual
4. Charging unit.

Removing contents from the packaging was simple. The parts were all in working order; there was no visible damage.

The battery came in a very low (nearly uncharged) state. The instruction manual advised charging overnight, or at least 10 hours. The instruction manual also stated that the charger should be removed when the shears are fully charged. Unfortunately, the shears have no battery charge indicator. This is a fairly simple indicator, which is included on many consumer items. The lack of an indicator on this product is unfortunate. We charged the shears and began assembly. The only assembly required was the insertion of the cutting blades into the main assembly. We then began product testing.

The product is operated using a two-button mechanism- meaning, two buttons must be depressed, simultaneously, in order to actuate the cutting blades. The buttons are located at the top and bottom of the shear handle. This is a safety feature, used to avoid accidental operation of the cutting mechanism.

The product was used to cut moderately high grass, bushes and thin branches. The product cut through all three media, with ease. The largest branch tested was about 1/4” in diameter. Cutting of thicker branches may be feasible, but it not tested in this study. When cutting taller grass, it was harder to see the work that the shears were doing, because the cut grass fell on top of the shears and impeded view.

The battery life was tested by running continuously (not cutting any grass) and was found to be 25 minutes. The manufacturers claimed that the device would last for 1 to 2 hours which may be correct depending on how often the device is switched on an off.

Overall, the product accomplishes the job for which it was built- cutting grass and light brush, with precision. The price ($30) seems reasonable for the level of performance provided.

Other Customers' Experience Using Product

After discussing the product with customers who have used the product and reading online reviews, we have found that customers are generally satisfied with the performance of electric grass shears (in general, not restricted to the specific product analyzed in this report). They thought it functioned well and found it easy to use. However, were dissatisfied with its battery life, having to pull the trigger constantly during use, occasional jamming, blade dulling, and limited ability to cut thick and wet grass. Users consistently compared the product to its alternatives because the product is not a must-have - it is a nice-to-have. Compared to string trimmers, electric grass shears are more precise, less expensive, quieter, more compact, lighter, easier to use, and do not throw grass into gardens, but cannot cut as thick grass. Compared to hand shears, they are less strenuous for the user's hands and easier to use, but more expensive and require more storage space.

Electric grass shears also come in two other variants: Some are sold with long poles so that the user can stand while using the device (user gets less precision but is more comfortable). Others are sold with an additional attachment to replace the grass blades with hedge blades for trimming hedges and bushes.

Mechanical Function

The device uses a rechargeable battery to provide power to a motor. Recharging the device is accomplished by plugging the device into a wall outlet via a power adapter. When the user deactivates the safety and pulls the trigger, a switch supplies power to the motor. The motor in turn rotates a gear set. The gear shaft connects to a link mechanism that rotates one of the blades back and forth relative to the other (using a rotating cam in a slot).

Assembly

Bill of Materials

Bill of Materials
Part Number	Name	Quantity	Material	Mass (g)	Function	Image
1	Female Housing	1	Plastic	151	Row 1, column 6
2	Male Housing	1	Plastic	154	Row 1, column 6
3	Housing Screw	7	Aluminum	1	Row 1, column 6
4	Upper Blade	1	Steel	28	Row 1, column 6
5	Lower Blade	1	Steel	95	Row 1, column 6
6	Blade Rotation Pin	1	Aluminum	2	Row 1, column 6
7	Blade Rotation Gear	1	Plastic	4	Row 1, column 6
8	Blade Rotation Plate	1	Aluminum	3	Row 1, column 6
9	Blade Rotation Disc	1	Plastic	3	Row 1, column 6
10	Blade Rotation Guide	1	Plastic	1	Row 1, column 6
11	Blade Connection Pin	2	Aluminum	1	Row 1, column 6
12	Blade Casing	1	Plastic	14	Row 1, column 6
13	Blade Casing Screw	4	Aluminum	< 1	Row 1, column 6
15	Blade Cover	1	Plastic	14	Row 1, column 6
16	Blade Housing	1	Plastic	28	Row 1, column 6
17	Blade Housing Latch	1	Plastic	2	Row 1, column 6
18	Trigger	1	Plastic	7	Row 1, column 6
19	Safety Button	1	Plastic	5	Row 1, column 6
20	Safety Lock	1	Plastic	3	Row 1, column 6
21	Trigger Spring	1	Aluminum	< 1	Row 1, column 6
22	Safety Button Spring	1	Aluminum	< 1	Row 1, column 6
23	Safety Lock Spring	1	Aluminum	< 1	Row 1, column 6
24	Battery Half	2		88	Row 1, column 6
25	Battery Sheath	1	Plastic	< 1	Row 1, column 6
26	Battery Wire	2		4	Row 1, column 6
27	Motor Housing Shell	1	Aluminum	21	Row 1, column 6
28	Motor Housing Shell	1	Aluminum	?????????/	Row 1, column 6
29	Magnetic Coil	1	Aluminum	?????????	Row 1, column 6
30	Motor Housing Screw	2	Aluminum	< 1	Row 1, column 6
31	Motor Housing Clasp	1		??????	Row 1, column 6
32	Motor Housing Washer	1	Aluminum	< 1	Row 1, column 6
33	Motor Housing Cap	1	Aluminum	????????	Row 1, column 6
34	Magnetic Brush	1		??????????	Row 1, column 6
35	Motor Platform	1	Aluminum	20	Row 1, column 6
36	Motor Platform Pin	2	Steel	5	Row 1, column 6
37	Large Motor Gear	1	Plastic	20	Row 1, column 6
38	Medium Motor Gear	1	Plastic	4	Row 1, column 6
39	Small Motor Gear	1	Plastic	4	Row 1, column 6
40	Motor Gear Washer	2	Aluminum	< 1	Row 1, column 6
41	Charger Connection	1	Plastic	5	Row 1, column 6
42	Charger Connection Wire	2		2	Row 1, column 6
43	Charger Connection Wire Sheath	4	Plastic	< 1	Row 1, column 6
44	Charger Connection Wire Clasp	2	Aluminum	< 1	Row 1, column 6
45	Trigger Switch Housing	1	Plastic	2	Row 1, column 6
46	Trigger Switch Cover	1	Plastic	1	Row 1, column 6
47	Trigger Housing Wire	1		2	Row 1, column 6
48	Large Trigger Switch Tab	4	Aluminum	< 1	Row 1, column 6
49	Small Trigger Switch Tab	2	Plastic	< 1	Row 1, column 6
50	Charger Cable/Adapter	1		?????????	Row 1, column 6

Design for Manufacturing and Assembly (DFMA)

Tables 1-A and 2-A, found in the appendix below, show our findings of the DFMA guidelines of the grass trimmer. The main objective in this design seemed to be to make an affordable grass trimmer for the lower end of the market. This was done by using a simple design while allowing for lower quality manufacturing processes and materials. The assembly process most likely included low skill workers in an assembly line. Not many opportunities exist to make competing product cheaper as it is all ready a base model but some savings could come in reducing the size of the housing.

Manufacturing

Many components were injection molded plastics and it seems like most of the components which were not injection molded were outsourced. There were also many standardized components. The main flaw with this design is the overall shape of the design. Right now the housing is very bulky with a high mass. The main body of housing is in the shape of a rectangle with most of the components in the upper (handle) and left (motor/blades) of the housing. This could be reduced in both mass and size by moving all components to the upper and left of the housing and removing the housing material from the other two sides making an “L” shape.

Assembly

Assembly of this product could be done fairly easy by attaching most of the components directly to the housing and connecting the housing. This would most likely be done in an assembly line as mentioned above. Connection of the housing looks to be the most difficult part of the assembly however as it requires fitting two housing pieces together, each with multiple components attached, and then tightening several screws to complete assembly. This could be fixed by having all components attached to one side of the housing and by having the housing component attach to one another via snap fits.

Design for the Environment (DFE)

When designing for the environment, the main focus is to design a product that is environmentally friendly. The environmentally friendliness of a product is measured by the amount of greenhouse gases emitted during production and during the use of the product.

Looking at this product closely, it can be seen that environmental friendliness wasn’t priority during the design process. By putting environmental friendliness as the priority during a redesign, large strides can be made to make this product have less of an impact, such as using lithium ion batteries rather than the nickel-cadmium ones currently being used, and to use less plastic when making the shell/casing.

Looking at the above table it can be seen that the largest impact is made by the electrical power generation. This is most likely due to the needed 10 hour long charge after every use of the product.

Production

Looking at the Figures 2-A and 3-A in the Appendix below, it’s seen that the manufacturing of the grass shear and the electrical power consumed by the grass shear are two major factors in the product’s greenhouse gas emissions. In total, if there were to be a tax on GHG emissions, the product would incur a tax of $ 2.57 which is 8.6% of its total cost.

In order to reduce the emissions during use and production, a change that can be made would be to use a battery that requires less time charging since the current battery requires 10 hours of charging after every use. Reducing this would greatly reduce the GHG emissions since a majority comes from the use of electricity.

The uncertainty in the above table comes from the cutting fluid. The cutting fluid is used to lubricate the blades after each use. Trouble was had finding the appropriate sector to use for the cutting fluid. It can be approximated that most users with not, or very rarely lubricate the blades since it requires an additional purpose, and because of this the environmental impact of the cutting fluid manufacturing with be negligible.

Figure 1-A: "DFE Guidlines and Ideas for Improvement" goes into further detail of the DFE analysis which was completed. It can be seen in the Appendix below.

Failure Modes & Effects Analysis (FMEA)

Failure modes and effects analysis (FMEA) is a design tool- used to predict possible ways that a product may fail, consider the effects of a failure and prescribe preventative action against serious, negative outcomes.

Electric grass trimmers have a number of mechanical and electrical components that can fail. These include, the battery assembly, blades assembly, trigger assembly and outer shell (case). In the FMEA process, we analyzed the various failure modes of the product, both injurious and non-injurious. A few key ideas were attained. First, it was discovered that the severity of a failure mode does not necessarily indicate the overall RPN value. The most harmful failure mode of the grass trimmer is "breaking and shattering of blades", but the low probability of thick steel blades shattering makes the overall RPN score relatively moderate. The highest scoring failure modes- in terms of RPN- do not pose high injury risk. Instead, they result in loss of product functionality. These modes are "dulling of blades" and "inability of battery to hold charge." In both cases, the failure mode causes loss of product usability, moderate detectability and high probability of occurrence. In summary, the presence of exposed, moving blades results in the possibility for serious injury, but the most highest scoring failure modes, for the product itself, come from less injurious sources.

Figure 4-A and Table 3-A in the Appendix below goes into further detail about the FMEA analysis which was completed.

Team Roles

Humberto Gonzalez: DFE, Secretary

Alex Fry: FMEA, Product Testing

Jordan Zile: DFMA, Treasurer

Brian DeVincentis: Use Study

Cameron Hall: BOM

Appendix

Table 1-A: Design for Manufacturing
Design Objectives	Strengths	Weakness
Minimize Part Count	Overall simple design leads to fewer parts Outer housing is made of only two parts	Could use snap fits on housing to reduce screw count Could potentially use fewer gears and other parts in scissor assembly Trigger assembly could use fewer parts by moving location
Standardize Components	Standard screws, washers, wire, electrical connectors, gears, and springs were used	Instead of using multiple screw types, one type could be used Charger is not a commonly used type
Commonize Product Line	Injection molded parts lead to low marginal costs	Large investment for injection molding tooling
Standardize Design Features	Two housing parts are very similar in dimensions and could both be made with slight tooling alterations Both blades could be made from same sheet metal	Could have potentially made the two triggers into single part type
Keep Designs Simple	Simple overall design with no complicated features added (settings, etc…)	Outer housing design is overly complicated; mass shape could be reduced Scissor assembly could be made simpler
Multifunctional Parts	Housing protects inner components, provides support and placement for components, and is a handle for the user Bottom cover lock functions as both a clamp and a safety lock	Could eliminate switch housing and use overall housing to also support inner switch components
Ease of Fabrication	Used an easily injected molded plastic for many of the parts	Could have used only one types of plastic for all plastic parts but instead used 3 types
Avoid Tight Tolerances	Tolerances for component support in housings seemed to be adequate without having tight tolerances	Tolerances of where the triggers moved within the housing could have been better
Minimize Secondary & Finishing Operations	By injection molding housing, finish could be done when molded and not as separate process	Finish on brand lettering could be removed and placed on already used adhesive paper
Take advantage of Special Process Properties	Used grittiness of inside of housing mold tooling to allow for a better surface for the adhesive paper	N/A

Table 2-A: Design for Assembly
Design Objectives	Strengths	Weakness
Minimize Part Count	Housing is in just two parts	Could eliminate switch housing Could eliminate a gear in the motor/scissor assembly
Minimize Assembly Surfaces	N/A	Two housing assembly surfaces could be reduced to one
Use Sub-assemblies	Most parts and sub-assemblies are outsourced	N/A
Mistake-Proof	Grooves in housing ensures proper placement of most components	Trigger assembly is not mistake proof and could be assembled backwards
Minimize Fasteners	N/A	Could use snap fits instead of screws to connect housings
Minimize Handling	N/A	Components placed on two housing surfaces which makes it hard to put two surfaces together without parts misaligning
Minimize Assembly Direction	N/A	Wiring makes for harder assembly
Provide Unobstructed Access	Unobstructed access granted by having two housing components instead of one	N/A
Maximize Assembly Compliance	Chamfers and radii on inner housing surfaces allow for easier placement of components	N/A
Assembly Support Features	N/A	N/A

We used the following criteria, from Dieter and Schmidt's Engineering Design, in performing our FMEA.

Table 3-A: Failure Modes and Effects Analysis - Grass Trimmer
Item & Function	Failure Mode	Effects of Failure	S	Causes of Failure	O	Current Controls	D	RPN	Recommended Actions
Blade Assembly	Blades jam	Temporary or permanent loss of function	5	Particulate trapped between blades; Bending of blades due to misuse	7	Better blade lubrication; Tighter tolerances on blade separation	2	70	Easier method to clean and lubricate blades
	Blades break	Possible serious injury; permanent loss of function	8	Blades hit hard surface; misuse	2	Thicker blades; stronger material	2	32	The blades are already pretty resistant to breaking
	Blades dull	Inability to cut grass	5	Extended use; Cutting unintended materials	7	Stronger material; sharpen blades at steeper angle	4	140	The company could sharpen the blades to prevent rapid dulling (steeper sharpening angle), but this would reduce initial sharpness
Battery	Battery does not retain charge	Product cannot be used for extended periods of time	5	large number of recharge cycles; Improper charging methods	7	Provide higher quality batteries	4	140	Add a charging indicator to enable proper charging techniques
	Battery leaks	Loss of function; possibility of minor injury	6	Dropping shears; Impact to battery	2	Contain battery within housing	5	60	Cover battery assembly in protective coating
Outer Cover	Cover cracks	Aesthetic degradation; loss of functionality if seriously cracked	2	Misuse; Direct, hard impact	3	Case is a single piece of molded plastic	2	12	Stronger case material
Trigger mechanism	Trigger becomes stuck	Device runs uncontrolled; possible serious injury	7	Debris in mechanism; Misuse by fixing trigger shut	2	Two trigger mechanism prevents malfunction of one trigger	2	28	Include kill switch, in case of malfunction
	Trigger mechanism breaks	Temporary or permanent loss of functionality	4	Repeated, hard use; shocking impacts, which dislodge components	3	Secure trigger components in crevices of molded outer case	3	36	Change trigger component materials

References

Dieter, George Ellwood, and Linda C. Schmidt. Engineering design. McGraw-Hill Higher Education, 2009