Part 6 of the CNC Machining Mini-Course. Start with Part 1 if you're just joining.
---
In my shop, tooling was one of the biggest ongoing expenses — and one of the areas where knowledge paid back the most. Running the right tool at the right parameters extends tool life dramatically. Running the wrong tool, or running it wrong, burns through inserts and endmills faster than you'd believe.
This post covers what's actually doing the cutting, from material and geometry to coatings and holders.
---
The Two Big Categories: Solid Tools and Indexable Tooling
Solid tools are made from a single piece of material — usually carbide or high-speed steel. The cutting geometry is ground into the tool itself. When it wears out, you sharpen it (if possible) or replace the whole tool.
Indexable tooling uses replaceable inserts — small pieces of carbide clamped into a body. When the insert wears, you rotate it to a fresh edge (index it) or swap in a new one. The tool body lasts indefinitely; you're only replacing the insert.
For production shops, indexable tooling is usually more economical for larger tools. For small shops and hobbyists, solid carbide endmills are the workhorse.
---
Endmills
The endmill is the CNC mill's most versatile tool. It's a cylindrical cutter with cutting edges (flutes) that spiral along its length and cutting faces on the end and periphery.
Flute Count
2-flute endmills: Larger gullets (chip clearance space) between flutes. Better chip evacuation. Used for aluminum, soft materials, and slotting operations where chips need room to clear.
3-flute endmills: A hybrid — better chip clearing than 4-flute but more surface contact than 2-flute. Increasingly popular for aluminum, especially in high-efficiency machining.
4-flute endmills: More cutting edges, smoother finish, better rigidity. Better for steel, harder materials, and finishing passes. Smaller chip clearance — not ideal for gummy materials like aluminum at high chip loads.
The rule of thumb: 2-flute for aluminum roughing. 4-flute for steel and finishing. 3-flute when in doubt.
Geometry: Square, Ball, and Bull-Nose
Square endmill — flat end, sharp corner. Used for most milling operations: slots, pockets, contours. Creates sharp inside corners (limited by tool diameter) and flat floors.
Ball endmill — hemispherical tip. Used for 3D surface contouring, blending radii, and finishing sculpted surfaces. Can't make flat floors. The tip moves through curves in all directions — essential for mold work and organic shapes.
Bull-nose / corner radius endmill — flat end with a radius on the corner. The best of both worlds: flat floors with a controlled radius in the corner. The corner radius greatly increases tool life compared to a sharp square end. When you don't need a sharp inside corner, use a bull-nose.
Carbide vs. High-Speed Steel (HSS)
Carbide: Harder, more wear-resistant, allows higher speeds and feeds. More brittle — susceptible to chipping from impact. The standard for CNC work. Most endmills you'll buy for production use are solid carbide.
HSS (High-Speed Steel): Tougher, more forgiving of interrupted cuts and vibration. Slower speeds required. Much less expensive. Still used for drills, taps, reamers, and some specialty applications. Not ideal for high-production CNC milling.
Tool Coatings
Coatings dramatically affect tool performance, wear resistance, and what materials the tool works best in.
TiN (Titanium Nitride) — gold color: General purpose. Increases hardness and reduces friction. Good for mild steel, aluminum, and general use. The baseline coating.
TiAlN / AlTiN (Titanium Aluminum Nitride) — dark gray/violet): Excellent for harder steels, stainless, and high-temperature alloys. Performs better than TiN in dry and high-heat cuts. Common on endmills marketed for steel.
ZrN (Zirconium Nitride) — silver/gold: Very good for aluminum and non-ferrous materials. Low adhesion to aluminum (prevents built-up edge). If you're running a lot of aluminum, look for ZrN or AlTiN coatings.
DLC (Diamond-Like Carbon) — dark: Outstanding for non-ferrous, graphite, and composites. Very low friction. Expensive.
Uncoated bright carbide: Often preferred for aluminum. Aluminum can stick to coated tools at high temperatures; uncoated polished carbide with proper coolant/lubrication often outperforms coated in aluminum roughing.
---
Drills
You know what a drill is. CNC drills work the same way as hand drills but at higher precision, typically in a drill cycle (G81/G83) with controlled feed rates.
Jobber drills: The standard length drill you're used to. Fine for most holes.
Spot drills / center drills: Short, rigid tools for starting holes — the first operation before drilling. Creates a conical divot that guides the drill and prevents walking. Always spot drill before drilling with a jobber drill on a CNC.
Peck drills (G83): For deep holes (deeper than ~3x diameter), use a peck drilling cycle. The drill retracts partially on each pass to clear chips. Chips packed in a deep hole will snap the drill instantly.
Carbide drills: Much more rigid than HSS. Required for hard materials and high-production drilling. Expensive but worth it for production work.
---
Taps and Thread Milling
Taps cut internal threads — they're the tool that makes a threaded hole. Rigid tapping (G84) on a CNC is extremely fast and accurate but requires synchronizing spindle speed with feed rate precisely. Most modern Haas and Tormach machines do this well.
Thread mills are an alternative — a multi-point cutter that mills the thread profile in a helical path. Advantages: one tool can cut multiple thread sizes, can start and stop the thread at any depth, and if it breaks it doesn't seize in the hole (a broken tap is a nightmare to remove).
---
Indexable Tooling: Face Mills and Insert Holders
Face mills are large-diameter cutters (2"–6" typical) with replaceable carbide inserts. Used for rapidly surfacing flat stock — facing down the top of a block to get it flat and parallel. Face mills are fast material removers. A 4" face mill with multiple inserts removes far more material per pass than an endmill.
Insert geometry: Carbide inserts come in a huge variety of shapes, grades, and chip breakers. The grade determines hardness vs. toughness balance. The chip breaker geometry affects how chips form and evacuate. Your insert manufacturer's catalog will have a selection guide — use it.
Boring bars: Indexable tools for finishing holes to precise diameters. A drill gets you close. A boring bar gets you exact. Used when hole tolerance or surface finish matters.
---
Tool Holders
The tool holder connects the cutting tool to the machine spindle. This connection matters — slop in the holder means runout, vibration, poor finish, and shortened tool life.
ER collet chucks: The most common holder for mills. An ER collet (ER16, ER32, ER40 — the number relates to collet size range) grips the tool shank with high concentricity. Good runout spec, multiple sizes, inexpensive. Standard for Tormach and most benchtop machines.
CAT40 / BT40: Standard taper shanks for full-size vertical mills (Haas VF series uses CAT40). These are the toolholders that go into the machine's spindle. Within the CAT40 shank you'll typically find ER collets, set-screw holders, or hydraulic chucks.
Hydraulic chucks: Excellent runout, excellent vibration damping, high clamping force. Great for finishing work where surface quality matters. More expensive than collet chucks.
Shrink fit holders: The tool shank is thermally contracted into the holder — zero runout, extremely rigid, excellent for high-speed finishing. Requires a heating/cooling station to change tools. Used in high-precision production environments.
---
Tooling Brands Worth Knowing
Harvey Tool / Helical Solutions — Both owned by MSC, both excellent quality. Harvey specializes in exotic geometries (miniature tools, specials). Helical has excellent endmills for aluminum.
Kennametal — Major tooling manufacturer. Excellent inserts for steel and exotic alloys. Production-focused.
Sandvik Coromant — The Swedish tooling giant. Outstanding quality, outstanding prices. Industry standard for difficult materials and high-production work.
Seco Tools — Another major player, excellent for hard materials and high-temperature alloys.
Maritool / Destiny Tool — Good quality at more accessible prices, solid for small shops.
The dollar store endmills on Amazon: Don't. You'll spend more in scrapped parts and frustration than you saved on tools. Buy decent tools, run them at the right parameters, and they'll last.
---
One Thing I Wish I'd Known Sooner
The geometry of the tool matters as much as the material. I spent years just buying "carbide endmills for aluminum" without thinking about flute count, helix angle, or relief geometry. When I started paying attention to tool geometry — matching 3-flute variable helix endmills to aluminum, using high-rake geometry for gummy stainless — my surface finish improved dramatically and my tool life increased.
Don't just buy a tool. Understand what the geometry is doing and why. The manufacturer's tech data sheets exist for a reason.
Next up: feeds, speeds, and the math that ties all of this together.
— Dr. Scott