CNC Milling

What can CNC milling make?

Face milling cuts mainly with the end corners of the cutter and leaves flat surfaces or flat-bottomed cavities, and peripheral milling cuts along the cutter’s circumference and suits slots and threads (source 1). In practice that means brackets and mounting plates, fixture plates, housings, manifolds and billet aluminum parts cut from solid stock.

What should a milled part’s print show?

A round cutter leaves a radius in every inside vertical corner (source 2), so draw the inside radius the part can live with rather than a sharp corner, and put each pocket depth on the drawing so the quote is written for the part as drawn.

Tapped holes: an inch thread is called out as the nominal diameter and threads per inch, followed by the series (UNC coarse or UNF fine) and the class; class 2 is the most common, 2A for external threads and 2B for internal (source 3). Cutting Tool Engineering’s working definition counts tapping as deep when the thread depth is at least 1 1/2 times the tap’s nominal diameter, and in a blind hole the depth left past the threaded section is what makes room for the tap’s chamfer and its chips (source 4). A thread that must reach the bottom of a blind hole takes an intermediate tap and then a bottoming tap (source 5), so write the thread depth on the print and say if it runs to the bottom.

Critical dimensions and tolerances belong on the drawing; CNC machining tolerances explains how to choose a band, and CNC design mistakes covers corner radii, wall thickness and pocket depth, and more features to check before you send a file. A finish called out on the print is quoted with the part.

Milling, turning, wire EDM or grinding?

Most round parts are turning jobs, and a flat, slot or cross-hole added to one is quoted with it on CNC turning. Punch and die profiles in hard metals are a wire EDM job: that process is used to make punches, tools and dies from hard metals that are difficult to machine with other methods (source 6). A surface that has to be ground belongs to precision grinding.

Sources

All accessed 30 September 2026. Where a figure here differs from your print, the print governs.

1. Wikipedia, "Milling (machining)": en.wikipedia.org/wiki/Milling_(machining) (milling is machining with rotary cutters; face milling and peripheral milling; slots, flat surfaces, threads and flat-bottomed cavities).

2. Greg Paulsen, "Top Tips on Designing Parts for CNC Machining", Machine Design, 8 May 2023: machinedesign.com/automation-iiot/article/21265541/top-tips-on-designing-parts-for-cnc-machining (interior vertical walls must have a radius because a round tool removes the material).

3. Wikipedia, "Unified Thread Standard": en.wikipedia.org/wiki/Unified_Thread_Standard (inch thread designation, UNC and UNF, classes 2A and 2B most common).

4. Lee Carroll, "Tapping Deep Holes: Guide to Chip Evacuation and Tool Life", Cutting Tool Engineering, 1 August 1995: ctemag.com/articles/tapping-deep-hole (a working definition of deep-hole tapping as thread depth of at least 1 1/2 times the tap’s nominal diameter; depth past the threaded section of a blind hole for the tap’s chamfer and chips).

5. Wikipedia, "Tap and die": en.wikipedia.org/wiki/Tap_and_die (threads to the bottom of a blind hole: plug tap, then bottoming tap).

6. Wikipedia, "Electrical discharge machining": en.wikipedia.org/wiki/Electrical_discharge_machining (wire EDM used for punches, tools and dies from hard metals that are difficult to machine with other methods).