head
The topmost, foremost, or leading part.
head: the business end of a machine tool
In machining, a head is the assembly that holds, positions, and drives the cutting tool or the workpiece during operation. On a lathe, the headstock clamps and rotates the part; on a milling machine, the spindle head holds the cutter and controls its speed and feed. The head is where the actual cutting happens, making it the most mechanically stressed and thermally loaded component of the machine.
Different machine types have different head designs matched to their function. A drill press has a simple spindle head that moves up and down; a CNC machining center has a complex turret head that can hold a dozen or more tools and index them automatically. A boring head, used to enlarge existing holes with precision, is itself a specialized spindle that mounts into a larger machine. Each variant trades simplicity for capability.
Heat and alignment matter most
Heads generate serious heat during continuous cutting, especially at high spindle speeds (often 3000 to 10000 rpm for steel, much higher for aluminum or composites). Thermal growth, even a few thousandths of an inch, throws off tolerances. Quality heads use precision spindle bearings, often angular contact or hydrostatic types, to minimize runout (the wobble of the spindle axis). Runout of just 0.001 inch becomes visible and costly on precision work.
The head mounts to the machine base through rigid bolts and often has ways or slots to allow movement along the machine's axes. Wear in these interfaces, or in the spindle bearings themselves, shows up as chatter marks on the finished part, a rippled surface that signals the head is flexing or vibrating. Skilled machinists can often diagnose spindle problems by ear and feel before they wreck a job.
Rebuild and repair of heads is common in shops. Spindle bearings can be replaced, ways can be scraped flat again, and taper pulleys or belt drives can be rebalanced. A worn head that cannot hold tolerance anymore is the typical point at which a machine gets a full overhaul or replacement, since the head is the heart of repeatability.