Milling Basics
Part of Machine Tools
Introduction to milling operations — cutting flat surfaces, slots, and gear teeth using rotating cutters.
Why This Matters
A lathe makes cylinders; a milling machine makes flat surfaces and prismatic shapes. Together, these two machines can produce virtually any metal part. Without milling capability, flat surfaces must be achieved by filing, scraping, or planing — slow, skilled hand operations. A milling machine does in minutes what takes hours by hand, and does it more accurately.
In a rebuilding scenario, milling capability is needed for gear teeth, key slots, flat faces on castings, dovetail guides, and the precision flat surfaces that hold everything else in alignment. Even a crude improvised milling setup — a rotating cutter held in a drill press with a cross-slide table — dramatically expands what can be made.
This article covers the principles of milling, the types of milling cutters achievable without industrial supply chains, setup procedures, and the basic operations every mill operator must know.
How Milling Works
In milling, a rotating cutter with multiple cutting edges removes material from a stationary (but traversing) workpiece. The cutter’s teeth take chips as the work is fed past them. Each tooth takes a chip of roughly uniform thickness, determined by feed rate and number of teeth.
Climb milling vs. conventional milling: In conventional milling, the cutter rotation is opposite to the feed direction — the tooth enters the cut thin and exits thick. This tends to pull the workpiece up off the table and requires more force, but is more forgiving of backlash in the feed screws. In climb milling, the tooth enters thick and exits thin, giving better finish and less heat but requiring little backlash in the machine. For lightly built improvised mills, conventional milling is safer.
Peripheral milling vs. face milling: Peripheral milling uses the side of the cutter to cut; face milling uses the end. Peripheral milling produces a flat surface parallel to the cutter axis; face milling produces a surface perpendicular to it. Most operations combine both.
Types of Milling Cutters
Milling cutters can be made in a forge and grinding shop given tool steel stock:
End mills: Cylindrical cutters with cutting teeth on both the end and sides. Used for pocketing, slotting, and profiling. The simplest to forge — a cylinder of tool steel with relief ground at the teeth angles. Two-flute end mills are easiest to make and work well in soft materials.
Side-and-face cutters: Disc-shaped cutters with teeth on the periphery and both sides, used for cutting slots and shoulders. Mount on an arbor through the center hole.
Form cutters: Ground to a specific profile (gear tooth form, dovetail angle, thread form). The cutter is passed across the work to produce the corresponding form in the workpiece. Critical for gear cutting.
Fly cutters: The simplest milling cutter — a single boring bar with one tool-steel insert, mounted in a flywheel-like holder. Cuts slowly (one tooth, one chip per revolution) but is trivial to make from any turning or boring tool. Adequate for facing flat surfaces and light profiling.
The Improvised Milling Setup
Without a purpose-built milling machine, milling can be done with:
Lathe milling attachment: A cross-slide table mounted on the carriage, with the work held in a vertical orientation and the cutter in the spindle. Functional for light cuts on small work.
Drill press with cross-slide table: The drill press holds the cutter; a machinists cross-slide vise on the table moves the work. This works for light cuts — end mill work in aluminum or bronze — but the drill press spindle lacks the rigidity and precision of a true milling spindle.
Purpose-built horizontal mill: A powered horizontal arbor with the work table below it. This is the form of the earliest milling machines and is simpler to build than a vertical mill.
Setting Up for Milling
Tramming the head: The spindle axis must be perpendicular to the table in both planes (front-to-back and side-to-side). Check by sweeping a dial indicator mounted in the spindle through a large circle on the table — the indicator should read zero variation throughout the sweep.
Indicating the vise: The milling vise must have its fixed jaw parallel to the table feed direction, otherwise slot milling will produce tapered slots. Indicate by sweeping a dial indicator along the fixed jaw face while traversing the table.
Work holding: Milling forces are substantial and multi-directional. Use a milling vise with the work against the fixed jaw, or clamp directly to the table with T-bolts, clamps, and step blocks. Never rely on friction alone — the cutter will pull the work out.
Basic Operations
Facing a surface flat: Mount a fly cutter or face mill. Lower the cutter until it just touches the work surface, then raise the table 0.5mm (or your desired depth) and traverse the work across the cutter in a series of overlapping passes. Each pass should overlap the previous by about half the cutter diameter for even finish.
Cutting a slot: Use an end mill equal to the slot width or a side-and-face cutter on an arbor. Lower the cutter to full slot depth in steps (0.5-1mm per pass in steel), traversing the length. Clean up sides with a final pass at full depth.
Step milling: Use the side of an end mill to cut a shoulder. Set depth of step with the quill or table; set horizontal position with the cross-slide. One pass at full depth, feeding toward the shoulder.
Gear cutting: Mount a form cutter of the correct involute profile. Index the blank using a dividing head (or an improvised indexing plate) to rotate the blank exactly one tooth spacing between cuts. Take consistent depth cuts on each tooth space until the full circumference is cut.