Milling technique: with-the-grain, against-the-grain and plane milling
How the cutting tool meets the material — and why it determines surface, tool life and safety
With milling, the tool rotates while the workpiece is clamped and is guided past. Because the mill has multiple teeth that alternate in engagement, milling is both versatile and complex: the same mill can make surfaces pockets grooves contours and steps. On the journeyman's exam for CNC technician, a milling task is the second of two individual manufacturing tasks. Understanding how the teeth meet the material is key to beautiful surfaces and long tool life.
§Downwind or against the wind
A key distinction is the direction the tooth moves relative to the feed. In climb milling the tooth moves in the same direction as the feed and the chip starts thick and ends thin. In conventional milling the tooth opposes the feed and the chip starts thin and ends thick. Climb usually gives better surface and longer tool life because the tooth cuts cleanly in while conventional causes the tooth to rub and slide at the start of the cut which wears more and can harden the surface.
| Down-wind milling | Against-grain milling | |
|---|---|---|
| Chips | Thick → thin | Thin → thick |
| Surface | Better | Worse, can rub |
| Standby time | Længere | Shorter |
| Requirement | Rigid machine without deflection | Tolerates fog better |
On modern CNC machines without play in the table, climb milling is almost always preferred. On older, manual machines with play in the spindles, climb milling could be dangerous because the play could cause the cutter to 'draw' the workpiece in — therefore, historically, one learned conventional milling. Know your machine and you know what you can allow yourself.
§Plane milling and feed per tooth
Ved planfræsning fjerner en bred fræser materiale for at skabe en plan flade. Her arbejder man med tilspænding pr. tand (fz), fordi det er den enkelte tands spånbelastning, der betyder noget. Tilspændingshastigheden bliver Vf = fz × z × n, hvor z er antal tænder. En fræser med flere tænder kan derfor køre hurtigere frem ved samme spånbelastning. For lav fz får tænderne til at gnide; for høj overbelaster dem. Ved planfræsning har man ofte fræseren delvist i indgreb og placerer den lidt ude af centrum for at styre, hvordan tænderne går ind og ud.
§Full or partial intervention
How large a part of the milling cutter's diameter is in the material (the cutting angle) matters a lot for load and heat. Full engagement — for example in a closed slot — puts heavy load on the cutter and traps the heat. Therefore, modern CAM strategies often use a smaller, constant lateral feed (adaptive or trochoidal milling) where the cutter only takes a thin slice at a time, but can in return run deep and fast. This spares both tool and machine and utilises the full cutting edge length.
- 01Prefer feed on rigid CNC machines without chatter
- 02Calculate the feed as fz × z × n in milling
- 03Avoid unnecessary full engagement—it keeps heat in the cut.
- 04Place the surface planer slightly off-centre for softer entry and exit
- 05Choose tooth count based on material: fewer teeth for soft, sticky items
“Mill with the machine, not against it: right direction, right chip, right surface.”