“Everything is a spring”. You’ve probably heard that expression before. How deep do you think your appreciation of that particular turn of phrase really is? You know who truly, viscerally groks this? Machinists.
As I’ve blathered on about at length previously, machine tools are all about precision. That’s easy to say, but where does precision really come from? In a word, rigidity. Machine tools do a seemingly magical thing. They remove quantities of steel (or other materials medieval humans would have killed for) with a slightly tougher piece of steel. The way they manage to do this is by applying the cutting tool to the material within a setup that is so rigid that the material has no choice but to yield. Furthermore, this cutting action is extremely precise because the tool moves as little as possible while doing so. It all comes down to rigidity. Let’s look at a basic turning setup.
Spring Forces Illustrated on a Lathe
The colored arrows show some of the major spring forces that a machinist will know and feel. First and foremost is the downward pressure on the tool bit (red). This is, in turn, trying to torque the rear of the toolpost upwards (green). The toolpost is then trying to torque the rear of that cross-slide downwards (blue), and the whole setup is torquing the far end of the carriage upwards on the ways (yellow). Meanwhile, the tool pressure is driving that tail stock (brown), and the spindle bearings (purple) upwards.
Everything is a spring, and these are but a few that you’ll find in the simplest of machine tools. If any of these sources of springiness fails to hold up its end of the job, the result will be chatter, imprecise cuts, or the machinist picking shards of parting tool out of her forehead. None of those are good outcomes.
The Machinist’s Rigidity Aresnal: Mass and a Perfect Fit
The weapons we have in this fight for rigidity are primarily two in number. First and foremost is cast iron. By sheer mass it can be made rigid inexpensively, and it is also good at absorbing harmonic vibrations which can make any rigidity problems elsewhere much worse. The second weapon is precision fitting parts. If we can minimize movement where the parts connect to each other, we eliminate sources of spring. This is difficult when those parts need to slide, but that’s why we have things like dovetails and adjustable gibs. These are features that allow parts to slide relative to each other in one dimension, while moving as little as possible in all other dimensions. It’s all about murdering the foul demon of spring that threatens the glorious and righteous kingdom of rigidity.
So, next time you hear the phrase “everything is a spring”, pour one out for your local machinist. Sure, the mechanical engineers will go on about it, but it’s the machinists who Really Know™.