Nightforce - Automated Thread Burnishing

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Sponsors
  • Nightforce Optics[1]
Team Name The Longshots
Duration Fall 2018 - Spring 2019
Faculty Adviser
  • Dr. Dan Cordon
Mentor
  • Sarah Willis
Client
  • Pietro Boyd
Team Members
  • Jake Elliot
  • Morgan Kerby
  • Dave Rutherford
  • Tom White

Produce a machine to burnish riflescope turrets and check the backlash of those turrets using an automated process.


Problem Definition[edit]

Produce a machine to burnish riflescope turrets and check the backlash of those turrets using an automated process.

Background[edit]

The point of aim in a riflescope is adjusted using a leadscrew assembly called a turret. In order to maintain precision, the turrets need to have a backlash of approximately 0.0001 inches. This level of precision cannot reliably be machined, so the threads must be manufactured over-sized and then fit through a process called burnishing. Currently this process is done by hand, which is time consuming, inconsistent, and prone to failure. Using an automated machine can make this process faster and more accurate, resulting in less time spent and less turrets discarded.

Deliverables[edit]

The deliverable is a machine capable of burnishing the turrets and testing the backlash. This machine must be able to handle current turret models, and be upgradable to handle any future models.

Specifications[edit]

The machine must be able to fit on a workbench, with a maximum size of 2 ft by 2 ft. It must be easily moved if necessary, with a maximum weight of 50 pounds. It should be able to burnish turrets at a similar rate to the current process. It must also be user friendly, and any parts that need to be changed to burnish different turret models must be easily accessed and quickly changed. It should output a value for initial tightness of the threads for improvement in the manufacturing process.

Design Considerations[edit]

The machine must:

  • Be within budget
  • Be able to burnish turrets to specification
  • Have a method of testing backlash
  • Have a method of sensing backlash during burnishing
  • Have a method of moving turrets through the machine
  • Be able to burnish existing turret models

The machine should

  • Be buildable in the Gauss-Johnson machine shop
    • Some parts can be outsourced if necessary
  • Have a method of loading multiple turrets into the machine
  • Output data about initial torque requirements

Project Learning[edit]

For this project the team needed to learn a great deal of additional information. Here is some of what we researched and brainstormed.

Workflow[edit]

Workflow involves how to get the turrets into our burnishing machine and how to move the turrets from one station to another.

Loading Turrets Into Machine[edit]

Magazine Project Learning.png
Magazine

An angled chute that uses gravity to feed turrets into the machine. Workers load the magazine one turret at a time.
Pros:

  • Simple
  • Inexpensive
  • Easily swapped out for a different model

Cons:

  • Labor intensive to load
Hopper Project Learning.png
The turrets are dumped into the hopper which orients the turrets and feeds them into the machine.

Pros:

  • Little labor necessary to load

Cons:

  • Complicated
  • Expensive
  • Loading process could damage leadscrews

Moving Turrets Through the Machine[edit]

Turntable project learning.png
Turntable

Turrets are loaded into each slot in the turntable which then rotates through them through the machine.
Pros:

  • Completely controls the movement of work through the machine.

Cons:

  • Adds complexity by adding an additional motor and rotating component.
Racetrack project learning.png
Racetrack

Uses an angled chute to move the turrets through the machine which are stopped at each station by servo motors.
Pros:

  • Simpler as no motor or rotating assembly is necessary.

Cons:

  • Could become jammed as the turrets move through the machine.

Backlash[edit]

Backlash is the space between the threads of the leadscrew and the threads of the nut. Some space is necessary for movement, but too much will cause inaccuracy in our application.

Backlash Representation.png Visual representation of backlash.

Measurement Method[edit]

To accurately and precisely measure the backlash we have developed a method with six steps.

  1. Secure the turret to its holder
  2. Apply force to the bottom side of the leadscrew to take up any backlash
  3. Measure initial location
  4. Remove force from bottom side
  5. Apply test force to top side of leadscrew
  6. Measure final location

Tolerances[edit]

The tolerance required is around 0.0001 inches, but to precisely measure this we need an electronic sensor with a repeatability of less than 10% of this value.

Sensors[edit]

Two types of sensors exist which satisfy our requirements, contact displacement sensors and laser displacement sensors.

Contact sensor example.png
Contact sensors will require our system to move the sensor into contact, then apply the force.

Pros:

  • Lower cost

Cons:

  • Must be moved into contact with the leadscrew
Laser Sensor example.png
Laser displacement sensors will measure the top side of the leadscrew as the force is applied.

Pros:

  • Can be fixed in place

Cons:

  • Higher cost

Burnishing[edit]

Burnishing Theory[edit]

Objective is to burnish threads enough that they “feel good” but are not too tight to turn and also are not too loose.

Calibration[edit]

First we need to determine the range of torque values that “feel good.”
Using a torque sensor (or similar method) we will find a torque value that is ideal and calibrate our device to burnish until it reaches that torque value.

Burnishing process[edit]

Burnishing is done by screwing the lead screw in and out of the turret until the thread tightness reaches the “feels good” torque.
A drill press setup is attached to a stepper motor and a hex bit fits into the lead screw. The stepper motor will turn counter clockwise first and then clockwise until specified torque is reached.
A torque measurement will be incorporated so that any tight spots on the lead screw will be detected and the device will focus on that spot.
Torque sensing will be an important part of the burnishing process. The two options for sensing torque are attaching a torque sensor to the clamping mechanism, or sensing the power output of the burnishing motor and correlating that to torque.
There are 6 different sizes our device must burnish so 6 different settings will burnish differently for each size.

Burnishing Options[edit]

Direct Drive[edit]

The burnishing motor will be directly attached to the turret, removing any power transmission components such as belts or bearings.
Pros

  • Less variables - no belts or additional bearings to add variance.
  • Enables precise torque measurement - because there are less variables the stepper motor can be used to more accurately sense torque.

Cons

  • Space constraints - The motor needs to be mounted in line with the turret
  • Weight distribution - The weight of the motor could bear on the lead screw, causing excess backlash to be introduced during the burnishing process.

Orientation[edit]

Burnishing forces horizontal.png Horzontal

Leadscrew is positioned horizontally.
Pros

  • Weight of motor does not bear on leadscrew

Cons

  • More difficult workflow
Burnishing forces vertical.png Vertical

Leadscrew is positioned vertically.
Pros

  • Workflow is easier

Cons

  • Weight of motor could bear on leadscrew, thus requiring more complicated systems.

Conclusions[edit]

Direct drive is better for our overall system because we can eliminate a separate torque sensor.
Vertical burnishing is better for our over system because the workflow is easier to design.

Torque Sensing[edit]

Torque sensing will give the machine feedback as to the approximate backlash while it is burnishing the threads. There are two methods we researched for torque sensing were using a dedicated torque sensor, and correlating the output of the burnishing motor to torque.

Dedicated Torque Sensor[edit]

Torque sensor

The dedicated torque sensor will directly output a torque signal, which can be calibrated to any unit system.
These sensors have a resolution and maximum value of what they can read. Generally the higher the maximum value, the lower the resolution.

Pros:

  • Directly outputs torque values

Cons:

  • Packaging can be difficult
  • A torque sensor that can read the initial torque value generally cannot read the target torque value.








Motor Output[edit]

Sensing the motor output will give us a value that can be correlated to torque without the need for a dedicated sensor. The method we researched this is using pulse width modulation (PWM) and motor speed.

PWM is a speed and power output control for DC motors which controls the input voltage via rapid pulses. We can use this to measure toque by setting our stepper motor's speed with PWM and then monitoring the motor speed. A decrease in the motor speed correlates to an an increased load, giving us an effective method of monitoring torque.

Pros:

  • No additional sensors required

Cons:

  • Torque monitored cannot be correlated to an actual torque
  • Coding the control system could be more difficult

Final Design[edit]

The final design will consist of a machine which uses direct drive vertical burnishing with turntable workflow and magazine input, laser position sensor to measure backlash, and motor control to monitor torque.

Timeline for Spring Semester[edit]

January Finalize design

Start building prototype

February Finish prototype

Start testing and refinement

March Continue testing and refinement
April Finish testing and refinement

Present at Expo on April 26

May Finalize report and documentation

Validation[edit]

Team Members[edit]

Jake Elliot.jpg
Jake Elliot

Major: Mechanical Engineering
Hometown: Fruitland, ID
Graduation Date: May 2020
I was immediately interested in this project when I saw that the application was precision riflescopes. I would consider myself an avid hunter, and long range/precision optics have always been an area of personal interest. After learning more about the complexity of the project proposal, I was excited by its challenging nature, and ultimately decided that it was a perfect fit. I hope to find work as an engineer shortly after I graduate to begin gaining experience in the industry. Depending on where my career leads me, I would eventually like to design and manufacture quality suppressors for firearms.
Email: jake4173@vandals.uidaho.edu


Morgan Kerby.jpg
Morgan Kerby

Major: Mechanical Engineering
Hometown: Lenore, Idaho
Graduation Date: May 2019
I chose this project because of all the projects I felt like it would be the most enjoyable to work on. It was one of the more challenging projects and I am interested in guns, so it makes the project more of a personal interest. The products that will result from our design will be the best in the industry and that is really satisfying. My future career goals include working in the aerospace or agriculture industry designing and advancing technology.
Email: kerb7816@vandals.uidaho.edu


[[File: |180px|thumb|left]] Dave Rutherford

Major: Electrical Engineering
Hometown: Kent, WA
Graduation Date: May 2019
David Rutherford is an undergraduate in Electrical and Computer Engineering at the University of Idaho, working towards a Bachelor of Science degree in Electrical Engineering, with an emphasis on Signal Processing and Propagation. He decided to pursue this project because of his interest in serving in the United States Army, and the possibility to have a positive impact on equipment issued to those serving in USSOCOM and adjacent units. After graduation, he will begin service in the United States Army as a Second Lieutenant in Military Intelligence, branch detailing in Armor.
Email: ruth3312@vandals.uidaho.edu


[[File: |180px|thumb|left]] Tom White

Major: Mechanical Engineering
Hometown: Post Falls, ID
Graduation Date: May 2019
I chose this project because I it looked interesting and I thought it would be a good challenge. I am interested in guns and I was impressed with the quality of the scopes Nightforce brought to the fall 2017 career fair. After I graduate I plan on working in either mechanical design, or in manufacturing.
Email: whit8642@vandals.uidaho.edu


Additional Documentation[edit]

Project Schedule

media:The longshots gantt fall2018.pdf

Meeting Minutes

media:The Longshots Meeting Minutes Fall 2018.pdf

Presentations

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Client Interview

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