Material and Process Selection

Three key components of the rolling machine were chosen: the rollers, the frame, and the connector (crankshaft). These primary components were analyzed to choose the material and the manufacturing process that should be used for the full-sized machine as well as for the prototype.

Rollers

In rolling mills, the material selection of the rollers is extremely important. The material determines what rolling load, rolling strength, and rolling torque is feasible for the machine to achieve. Roll life and wear are also important and directly related to the chosen material.

We know our full scale example machine deforms steel, and thus the properties (mainly strength, hardness, and toughness) must be that of steel or higher performing. Comparing to steel requirements of Young’s Modulus 200 GPa and strength 100 MPa minimizing cost, our Ashby Chart guidelines then look like so:

Ideally, we would use cast iron, but this fails the Young’s modulus test in the last chart of being above 200 GPa. Thus our full scale machine would probably use a carbon forged steel roller. The choice of carbon as an alloying element increases its hardness and wear resistance, but decreases its resistance to shock. Luckily, our process does not induce large shocks or vibrations. Ideally, these rollers would be forged, as it is cheaper than sand casting and the rollers are basic shapes. Also, forging’s impact hardens and makes the steel stronger as well.

With regard to small scale manufacturing, forging isn’t reasonable, so our rollers will likely be CNC or cut out on a lathe. Our metal options available are aluminum and steel, but steel remains much cheaper to produce our part out of. While steel is harder to work with than aluminum in terms of processing in a machine shop, we do not consider that as one of our major manufacturing constraints, and prioritize cost above that. Therefore, we will choose carbon steel from the machine shop’s selection for our rollers. Since we will not be spraying water or lubricant on our machine to quickly cool the product (as the plasticine will be a working room temperature), we do not have to look too much to coating our rollers for corrosion resistance. There is, however, a possibility we would want to heat treat our steel if possible to harden it and decrease wear.

Frame

The frame of the hot rolling mill is made of several parts welded or riveted together. The purpose of this complex structure is to hold and protect the internal components of the machine. In the process of selecting material for the frame, we need to consider properties such as strength and thermal properties and cost. In referencing the Ashby Charts used for the rollers, carbon forged steel is the best option as it has a high Young’s Modulus (200 Gpa) and strength (300-100 Mpa). In addition when considering thermal properties, we need the material to have a low thermal expansion constant because the frame is present in the metalworking process. It is important the dimensions of the frame do not change in the presence of heat. Stainless steel would also be a viable option, however it is a lot more costly per unit volume than compared to carbon forged steel. Production wise, the frame of the full scale machine is most likely produced by forging its parts or sand casting them. We would expect it to be forged in this case because of the cheaper price of mass producing these parts for the frame. Working the material in the process of forging also strengthens it, which is a bonus.

In regards to small scale production, the frame will be produced using a CNC machine. In addition, since we are using plasticine for our samples as opposed to super heated steel, our selection criteria with respect to our smaller model changes as we do not have to consider thermal expansion coefficient anymore. In our material selection, we had aluminum and steel as two of our main options due to their availability at the machine shop we would be working in. Aluminum is much less dense and an easier metal to work with. In other words, it is lighter and would take less time to produce. However, steel is better in terms of strength and more cost efficient than aluminum. In our analysis, we decided that the strength and cost efficiency is much more relevant in the perspective of our small scale production than the weight and time efficiency, so therefore we will select steel as our material for the frame.

Connector (crankshaft)

The crankshaft is responsible for absorbing forces from motor and having minimal vibration when rotating. It must also be long-lasting, with minimal corrosion and it should be cost effective.

Some important properties of the crankshaft that are not covered in overall demands/specifications:

1. 1. Mechanical

      1. Toughness - Connector must be able to absorb forces.

      2. Durability - Connector must be long-lasting.

      3. Stiffness - Connector must be able to resist deformation from forces applied to it.

      4. Shear Modulus - Connector must be able to withstand shear forces.

      5. Young’s Modulus - Connector must have adequate ratio of stress/strain.

      6. Wear Resistance - Connector must endure wear from components attached to it and forces applied to it.

      7. 
         

1. 2. Chemical

      1. Corrosion Resistance - Connector must be able to resist corrosion during manufacturing processes of metals.

1. 3. General

      1. Price - Connector manufacturing must be cost-effective

      2. Price - Connector material must be cost-effective

1. 4. Manufacturing

      1. Manufacturability - Ease of manufacturing especially with respect to cost.

1. 5. Physical

      1. Mass - Connector must not have excessive weight (Create interference and vibrational forces during rotation)

We chose cast steel as the material for the connector. The connector transmits energy from the motor to the rollers. The connector must be strong to absorb the energy, durable for industrial applications, and be a cost effective material. We chose the cast manufacturing process in contrast to forged billet because it is more practical in this application. An equivalent material we will use for our prototype is a high density 3D-printed connector, or an aluminum piece manufactured using a CNC mill. This is because we need a lightweight option that can be manufactured with the tools we have and be used in our prototype.

Transitioning to Small Scale

As mentioned in the above sections, our material specifications and processes are for a full scale rolling machine. For a small scale prototype, the alternative material/process is described after each section above.

In summary:

• Rollers: Use carbon steel cut on lathe with CNC if necessary

• Frame: Use carbon steel manufactured using metal shop tools and a CNC mill

• Crankshaft: Use high density 3D printed part or CNC milled aluminum