Nightforce - Automated Thread Burnishing

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

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

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

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

Backlash
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.

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

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

Calibration
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
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.

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

Conclusions
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
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
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: Cons:
 * Directly outputs torque values
 * Packaging can be difficult
 * A torque sensor that can read the initial torque value generally cannot read the target torque value.

Motor Output
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: Cons:
 * No additional sensors required
 * Torque monitored cannot be correlated to an actual torque
 * Coding the control system could be more difficult

=Final Design= 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.

=Prototype Manufacturing= The first prototype was manufactured from February to April. A total of 50 hours of shop time was used.

=Prototype=

Control system uses Arduino and stepper motor drivers to control the system. The system is controlled by commands via the serial monitor.

The prototype is capable of: The prototype is nearly capable of: The prototype is not capable of:
 * Moving work through the machine
 * Measuring backlash
 * Ejecting rejected and accepted turrets at separate locations
 * Burnishing turrets
 * Clamp is provides inadequate force and will fail if applied for too long
 * System is incapable of sensing torque
 * Loading multiple turrets into a feed mechanism
 * Automation

=Areas of Improvement=
 * Implement fully automated control system
 * Implement torque sensing
 * Replace clamp actuator with air piston or actuator with higher duty cycle
 * Build an implement magazine system
 * Replace moving ejector servo with more robust model
 * Reduce motor vibration
 * Motor vibration may be reduced with control system upgrade

=Team Members=

=Additional Documentation=

Project Schedule

[[media:The longshots gantt fall2018.pdf]] [[media:The Longshots Gantt full year.pdf]]

Meeting Minutes

[[media:The Longshots Meeting Minutes Fall 2018.pdf]] [[media:The Longshots Meeting Minutes full year.pdf]]

Presentations

[[media:The Longshots First Design Review.pdf]] [[media:The Longshots Expo technical presentation.pdf]]

Client Interview