Cutting data in practice: cutting speed, revolutions and feed rate
The three figures that determine whether the object becomes beautiful the tool holds and the machine does not sing
Regardless of whether you are turning a shaft or face-milling a flange, the result stands or falls with three numbers: how quickly the cutting tool moves through the material (cutting speed Vc), how many revolutions the spindle runs (n), and how much the tool moves per revolution or per tooth (feed rate). Choose wrong and the surface becomes rough, the cutter burns out or the machine begins to vibrate. Being able to set and correct cutting data is one of the central competency goals of CNC technician training — and something you are assessed on at the journeyman test.
§Cutting speed and revolutions are connected
Skærehastigheden Vc måles i meter pr. minut og fortæller, hvor langt et punkt på skæret rejser gennem materialet på ét minut. Men maskinen indstilles i omdrejninger pr. minut. De to hænger sammen gennem diameteren. Sammenhængen er Vc = π × D × n / 1000, hvor D er diameteren i millimeter (emnets diameter ved drejning, værktøjets diameter ved fræsning og boring) og n er omdrejningstallet. Vil du finde omdrejningstallet ud fra en ønsket skærehastighed, vender du formlen om: n = Vc × 1000 / (π × D).
§Clamping: mm per revolution or per tooth
Ved drejning angives tilspændingen som mm pr. omdrejning (f). Tilspændingshastigheden — hvor hurtigt værktøjet vandrer langs emnet — bliver dermed Vf = f × n. Ved fræsning arbejder man i stedet med tilspænding pr. tand (fz), fordi fræseren har flere skær. Her er tilspændingshastigheden Vf = fz × z × n, hvor z er antallet af tænder. Spånbelastningen fz er afgørende: for lille, og skæret gnider i stedet for at skære (og bliver hurtigt sløvt); for stor, og du overbelaster skæret og maskinen.
§Scrubbing and finishing are two different worlds
With roughing you want to remove material quickly: large cutting depth and large feed but lower cutting speed to spare the tool. With finishing you want a beautiful accurate surface: small cutting depth and feed but higher cutting speed. Indicative cutting speeds depend on both tool material and workpiece material — and they are always fetched from tool supplier tables or cutting data tables in trade theory never from memory. For rough orientation Danish trade theory for steel typically gives low values for high-speed steel (HSS) and significantly higher for carbide.
| Operation | Cutting depth | Clamping | Cutting speed |
|---|---|---|---|
| Scrubbing | Large | Large | Lower |
| Deletion | Small | Small | Higher |
§Standby time: the fourth variable
Cutting speed is the parameter that wears tools the fastest. If you want the cut to last longer, you reduce the cutting speed — supplier tables often state a correction factor for desired tool life. If you lower the cutting speed by, say, about 15%, you can typically double the time before the cut needs to be changed. It is a classic trade-off: high cutting speed gives fast production, but more expensive tool consumption and risk of sudden breakage in the middle of a workpiece.
- 01Vc too high: the cutting tool becomes too hot, wears quickly and can burn off
- 02Vc too low: uneconomical long processing time and risk of built-up edge on the cutting tool
- 03Clamping too low: the cutter rubs, becomes dull and gives poor surface
- 04Clamping too high: the cutter is overloaded and the machine can vibrate (clicking)
“Cutting data is not numbers you memorise — it's numbers you look up, calculate and adjust until the sound is right.”