Prototype Model

The machine rolls plasticine from 15 mm to 1mm in three passes, from 15 mm to 10 mm, then 10 mm to 5mm, then 5 mm to 1 mm. It is driven by DC motors and forces are measured through a load cell. Specifications for the parts can be seen below.

Summary Table

Parts Table

Motor Selection

The selected motor was the compact round-face DC motor. It operates at 12 V, with a maximum rpm of 3456 and a starting torque of 1.13 N m. At continuous operation, it runs at 3000 rpm at a torque of 0.15 N m. The full torque-rpm graph is shown below.

This motor was chosen because it can be used to satisfy the requirements. With a safety factor of 2 included, the calculated torque necessary was 2.52 Nm. With a gear ratio of 1:18, we can decrease the rpm to a much more manageable 167 rpm, while providing a torque of 2.55 N m. By using this motor, the reasonable gear ratio of 1:17 provides enough torque at a manageable rpm to function within the machine.


The motor will be held in place through a support stand that fits the motor. It will be powered by a 12 V battery supply. The motor will be connected to a gear through a set screw, which will then be connected to a very large gear with 17x more teeth.

Load Cell Selection

Moving forward we needed to identify the appropriate load cell applicable to our design specifications. We have chosen a tension/compression load cell. Specifically the HBM Z16A. Due to the forces of setting various heights on a metal slab we want to ensure that the pressures exerted will not exceed the capabilities of our design. We have identified areas prone to failure in the previously mentioned analysis. The compression load cell we have chosen is based upon the fitment within our design. It will be suspended and attached to the top roller. In addition, the maximum capacities, which are 7.5 tons & 15 tons, will be sufficient in this type of application. The exterior of the load cell is stainless steel which will prove beneficial in the type of conditions it will be placed in. The load cell is able to withstand loading of up to 10 V/m, which ensures the processing of the metal slab will be adequate.

Calculations

Simulations

In engineering, it is important to run tests on a design to predict real world behavior. Running simulations allow engineers to investigate product properties and test in-depth designs. It is part of the prototyping and testing phase to acquire detailed information for a polished finished product.


In a simulation, two major aspects that we want to look for is where in our machine stress and strain occurs if we apply a load in a certain direction. These two properties are something we use to understand where a component may be most susceptible to failure. By definition, stress is the pressure exerted at point or over an area, and strain is the measurement of how much an object deforms relative to its original state. Both are responses to applied loads and are measurements needed to determine failure criteria. Simulations are very useful because we are able to simulate simple and complex situations to determine the location of maximum stress and yielding properties.


Stress Equation: F/A

Strain: ΔL/L


Before we run simulations, boundary conditions need to be set. We use these to define our inputs such as the quantity and direction of applied loads and at what location. In our project, there are several ways that we can apply loads: concentrated loads at a single point, loads on a line or edge, distributed loads, and torque. In order to proceed with the simulating process, it is necessary to define what constraints are applied. In other words, we must define what points or faces are fixed and what bodies are free. It is important to consider whether these constraints and loads be applied on the individual component or should it apply to the surrounding parts as well.

Because of a lack of local computing power, we were not able to complete a full simulation of our device. However, we were able to make simplifying assumptions to reduce the computational load on our devices. For our required compressive force of 450 N, we split this force into 4 loads in these respective locations and directions:

  1. 450 N upwards on the inside of the load cell holding ring of the left frame.

  2. 450 N upwards on the inside of the load cell holding ring of the right frame.

  3. 450 N downwards on the bottom half of the main roller holding hole of the left frame.

  4. 450 N downwards on the bottom half of the main roller holding hole of the right frame.


Fixing every bottom surface of the frames that would touch the ground or table surface, we were able to run the simulation and record a range of stresses in our frame. We probed 3 areas of interest, the middle thinnest part of the frame, the point of highest stress, the top of the frame, and the hole in the frame that holds the main roller, as we expect these to undergo the most stress. Our results are as follows:


  1. Load Cell Contact: 9.58 MPa

  2. Middle of Frame: 2.00 MPa

  3. Roller Hole: 1.58 MPa