# Why are mass-produced metal parts machined by rotating the part against a stationary tool?

There's a TV programme called "How It's Made" which often shows automation of mass-produced machined parts. Very often, it seems that metal parts are machined (especially drilled) by rotating the part against a stationary tool rather than holding the part and drilling into it.

What is the advantage of doing it this way? I've never been able to work it out.

• are you asking about machine lathing or drilling holes into an item by rotating the item & not the drill bit?
– Fred
Jul 1, 2015 at 12:54
• @Fred Certainly the drilling aspect is the main thing - but sometimes there will be multiple tools that all act one-after-another on the piece as it spins. Jul 1, 2015 at 13:25

You can be sure the resulting surface is round.

If you used a router and rotated the bit around the piece then you would get inaccuracies due to the movement being harder to control.

Using a lathe-like setup the cutting tool is just a chisel that is cheaper to replace and refurbish when it's worn down than a router bit.

• I think you may have misunderstood the question. I'm talking about where a stationary drill-bit drills a hole in e.g. the centre of a steel rod but achieves it by rotating the rod while the bit is held stationary. Jul 1, 2015 at 13:57
• @Lefty the same thing - it ensures that the resulting part has strict rotational symmetry, and the resulting hole is centered on the same rotational axis, even if the drill bit would be positioned a tiny bit off of that center. If the only action was a single hole, then that wouldn't matter much, but as soon as you want to make multiple machining actions around a single axis, a lathe-like setup is the way to go. Jul 1, 2015 at 15:05
• Light Bulb! Yes, I understand. So the part spins, the drill bores at the centre of the axis of the lathe. If the part wasn't centred properly we essentially define a new centre and all further operations work from that same centre - which usually involves trimming the piece from the outside. Therefore we know that both these operations used the same axis and the "wall" of the piece now has a totally uniform thickness. Is that about right? Jul 1, 2015 at 15:38

Maintaining a co-linear hole/feature with the axis of rotation of the part. Automatic lathes with bar feeders have also been around for multiple decades. These machines can be "programmed" to make that particular part, in high speed, without any need for an operator. Milling or drilling that feature in a round part (which is more difficult to hold in such a set up) would require much more to set up square and maintain concentric to the outside diameter. These machines can also be set up to run autonomously (CNC) however they would require a robotic arm to load and un load parts into the machine.

Another possibility to consider is that the part is on a moving track and can then proceed to the next station / operation.