Manufacturing Processes — Machining

Machining has many different subcategories within itself, to name a few: lathe, milling and drilling. This article is all about what I learnt from the MITx 2.008x module on machining.

Let’s talk about CNC (Computer Numerically Controlled) lathes. You can use it for cutting, boring (also known as internal turning), turning (reducing the diameter of the workpiece), facing (reducing the length of the workpiece) and contour turning (turning but at different angles).

A few important terms are material removal rate (MRR), spindle speed, feed rate, rake angle, shear angle, relief angle, friction angle, shear strain and cutting force. All of these terms are interconnected in different ways, but first let’s briefly explain each term: Material Removal Rate — volume of material removed from a workpiece per unit of time Spindle Speed — the rotational frequency of a machine tool’s spindle, measured in revolutions per minute (RPM) Feed Rate — the velocity at which a cutting tool advances into or along a workpiece Rake Angle — Angle between the tool and the normal to the workpiece Shear Angle — Angle between the chip and the workpiece Relief Angle — Angle between the tool and the workpiece Cutting Force — Force with which the tool cuts the workpiece

Most of these terms are used to figure out the increase or decrease in the velocity of the cutting force. Hence, for an increase in cutting force, we need either a decrease in rake angle or an increase in spindle speed, feed rate or depth of cut.

While cutting, the wear and tear of the tool occurs, and this may be for a number of reasons. We can look into the hardness of the material of the tool, RPM, temperature or, in the modern age, if or if not there are coatings on the tool. Nowadays, we use different alloys, like TiN for low friction, TiCN for wear resistance, Al₂O₃ for high thermal stability and carbide for hardness.

Then there is cutting energy and power, power being force multiplied by velocity. There are many forces: thrust, shear, cutting and friction. To find the power, just multiply these forces with their respective velocities in the same direction. We can get more information from looking into the specific energies of all these powers by dividing power with material removal rate. Totally, there is 75% shear, 20% friction and 5% other negligible energies that sum up to 100%.

Furthermore, we can talk about milling. What is different about milling from turning? In milling, basically the tool rotates, while in turning the workpiece rotates. There are 2 types of milling machines (3-axis and 5-axis). There are also two ways to mill: the conventional way or the climb method. In conventional milling, the tool rotates in one direction, while the material is removed against the cutting rotation. While, in climb milling, the material is removed in the same direction of rotation, which for some reason gives a better finish to the workpiece.

Finally, a very important aspect of mechanical engineering and manufacturing in particular. Everything we try to build in the real world needs to be designed first, but some designs can’t be manufactured. Hence, there is a phrase called ‘design for manufacturing’. We need to understand the constraints of the machine in use and only give designs that are possible to be manufactured. Thus:

• We can’t be giving values like 0.627 mm; rather, give numbers like 0.625 mm.
• We can’t design models in which tools can’t access areas that need to be tinkered with
• We should avoid long narrow holes in our design
• We should avoid long thin sections
• We should avoid curved features to make it easy to hold
• We should add fillets as corners are tough to manufacture

This is all about machining.