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The project consists of a branch cutting attachment than can be attached to a DeWALT 18V cordless impact gun to allow the user to cut tree branches up to 1-3/8" in diameter without exerting excessive stress or becoming fatigued during use. Current cutting loppers on the market require the user to exert up to 100 lb. of force to cut a branch this size but the adapter will allow the user to do this same task by simply pulling a trigger.
This project was designed for my grandfather who about 5 years ago underwent open heart surgery and has since then found it difficult to use conventional branch loppers. This project will allow him and my grandmother the ability to maintain their 5+ acres of land in Kenmore Washington.
The attachment had a list of requirements that I had set that I felt would allow the cutter to be very useful for my grandparents. They were:
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Weighs 10 lbs. or less that when added to the 5 lb. impact gun would weigh 15 lbs. or less which is the weight lifting limit for a person recovering from open heart surgery.
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Has the ability to cut through a 1-3/8" diameter tree branch which is the rated diameter a set of commercial non-ratcheting loppers is rated at.
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Has an optional handle to offset the weigh being supported by a single arm to reduce fatigue during use.
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Is made of materials that can be exposed to outdoor conditions and not corrode.
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Is less than 1 ft. in length and 3-1/2" in width which is the width of the attached battery back.
The cutting attachment is considered a success because my grandparents were able to use the cutter to cut branches of 1-3/8" in diameter and did not become fatigued during its use.
Introduction
Construction
The adapter has been machined primarily from 6061-T6 aluminum for its strength to weight ratio and its resistance to the elements as it is an outdoor tool. The adapter is a fully bolted construction with stainless steel fasteners to allow the adapter to be taken apart to clean and grease the gears. It also allows the user the ability to sharpen the blades or to replace a blade if it were to become damaged during use. The cutting blade and anvil are originally from a pair of Felco 210-60A loppers and have been modified to fit. They have a curved profile which decreases the force needed to make a cut and allows for easy replacement if needed. The wormgear set was purchased from Boston Gear a large gear manufacturer to keep the manufacturing cost down. The worm is made of hardened steel and the wormgear of bronze and provides a 5:1 torque increase to allow the impact gun to produce the torque needed to be able to cut the branches.
Project Testing
The cutting attachment was tested in real life scenarios by the people the attachment was intended for, real homeowners. The attachment was tested under 5 testing plans;
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The weight of the entire cutting tool to ensure it is under the 15 lb. weight limit that I set.
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The time it takes the user to attach the cutter to the impact gun and also to remove it. the estimated time is under 5 minutes with basic tools.
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A weathering test in which the attachment will be exposed to the elements for 7 days to see how well it does if left outside.
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A speed to cutting test to see how fast the user can cut 20 branches of around 1-3/8" in diameter.
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A test to see how many cuts the cutter can make on a fully charged battery on branches around 1" in diameter.
Cost and Budget
The initial budget that was set for this project was $500. This budget was set before any analysis was performed and was made using a gear set available off of Amazon.com. Through analysis it was determined that these gears would not suffice and a larger set would have to be used. The analysis led me to choose a gear set from Boston Gear at a price of $573.16. This cost exceeded the initial budget but was necessary to allow the attachment to perform as it was intended. The final project cost came to $845.83 for all materials and hardware to build a working cutting attachment.
Project Schedule
The estimated total hours for the project from beginning to end was 243.8 hours. This included the writing of the proposal paper, analysis of the attachment parts, construction of the attachment, project testing, and project close-out. I spent a total of 335.2 hours on this project. The project schedule went over on time compared to the estimate as the writing of the proposal, the analysis of the parts, and the construction of the attachment parts took longer than I initially estimated. Even though the project has gone over on time, through project time management all of the steps of this project have finished on time.
Design/ Analysis
The scope of work for this project included:
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The cutting attachment to fit onto the existing DeWALT cordless impact gun.
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The incorporation of replacement blades off of existing production loppers so replacement blades are easily obtained in the event the blade or anvil is damaged during use.
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An optional handle that could be attached or removed without tools to help alleviate the weight being supported by a single arm if the user desired.
Project Scope
Each part of the cutting attachment was designed and analyzed to ensure it would handle the wear, tear, and abuse an outdoor yard tool sees on a daily basis. Because this tool is intended to be used outside I choose to use 6061-T6 aluminum for all the frame and gear mounting parts. 6061 aluminum is a light weight and strong material that is also resistant to heavy corrosion that could weaken the tool if left outdoors for long periods of time. The parts that are not made of aluminum such as the gear set and mounting late were coated in products to help protect them from the elements to help ensure the tool will last. All of the parts were analyzed under either the forces produced by the gear set or by an external force that the user may put on the tool to free it from a branch if it were to become stuck during use. I ensured through my analysis that these forces with the desired safety factor did not exceed the ultimate strengths of the materials being used for the specific part being analyzed.
The blade set that I choose to use for the cutting attachment comes from a pair of Felco 210-60A 24" long branch loppers. Through testing I found that these loppers require the user to exert 100 lbs. of force on the handles to cut through a branch with a 1-3/8" diameter. With this value I could then determine my required gear ratio to allow the attachment to complete this same task.
I calculated that the blades require 150 ft-lbs. of torque to cut through a 1-3/8" tree branch. The DeWALT cordless impact gun I used had a rating from the manufacturer of 97.5 ft-lb. Due to the age of the gun I estimated that my impact gun has 60% of this value which is 56.5 ft-lb. of torque. Using a 5:1 gear ratio available through Boston Gear with a torque limit rating large enough to complete the task, I estimated that the tool would produce 282.5 ft-lb. of torque. Due to inefficiencies in the gear set and variations of torque output through the use of the gun I felt this torque would suffice.
Once I had this torque value I could calculate the radial, axial, and tangential loads on each of the gears and use these loads to calculate the stresses on the other parts of the attachment that the gears are acting on.
The theoretical loads that frame was analyzed under was a 50 ft-lb moment applied perpindicularly downward at the impact guns handle. The second theoretical load that I analyzed the frame of the tool under was a 100 lb. force applied along the length of the impact gun and attachment. These forces were chosen to simulate the user freeing the tool from a branch if it were to become stuck.
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