Startup Interview Question: Ensuring Part Flatness

Mechanical Engineering
1

How to design a machine that would ensure the part that it makes is flat on each side?

2

I would focus on Two main things

  • Tolerances
    focus on making sure the work table and wise have suitable tolerances

  • Stiffness
    When operating a machine and drilling if the mill has insufficient stiffness and deflects either from external forces applied on the mill or internal forces from matching operations the whole table could deflect leading to flatness issues.
    You will need to estimate the worst case for some of the loads you could see, and do a stiffness matrix to understand how sensitive the machine is to loads.

3

First I would ask a clarifying question:

  1. CNC? Injection molding? Sheet Metal forming?
  2. Is flatness the main thing we are optimizing for? Not yield?

I’ll assume I am designing a CNC and it is working on aluminum parts.

Things that come to mind:

  1. Spindle stiffness: The stiffer the spindle assembly, the less it will fall out of line as it cuts the part. To start looking further I would draw an FBD of all parts associated with the fixturing and cutting, looking for places to optimize for stiffness. I would take my idealized model into a simple FEA, identify weak points and consider changing some parts to a stiffer metal like steel or changing the structure.
  2. Alignment tooling. We can incorporate alignment tools into a standard tool changer such as laser alignment systems, edge finders, eccentric bars, digital protractors. These tools will help us inspect the part after the first run to ensure flatness.
  3. Spindle speed. The faster the spindle spins, the more the end mill will vibrate and deflect during cutting operations. Designing the spindle to have an upper limit would boost