GlossaryKind of term: Manufacturing termCutting speed and spindle speed
What are cutting speed and spindle speed?
Cutting speed vc is the speed of the cutting edge across the workpiece in m/min; spindle speed n follows from vc and the diameter at the cut.
Spindle speed n is the number of revolutions the spindle makes per minute. They are linked through the diameter at the cut: n = 1000 × vc / (π × D), with D in mm. In milling D is the cutter diameter; in turning it is the workpiece diameter where the tool is cutting. A ⌀10 mm cutter at 200 m/min runs at about 6,370 rpm; a ⌀20 mm cutter at the same vc at half that.
What do cutting speed and spindle speed depend on?
The right cutting speed depends on the workpiece material, the cutting material and its coating, coolant, engagement and stability. Tool suppliers give starting values per material group: aluminium such as EN AW-6082 takes a far higher vc than 1.4404 or 42CrMo4, and carbide a higher one than high-speed steel. A higher vc usually shortens tool life sharply; too low a vc can cause built-up edge in some materials. Empirical models such as Taylor's tool life equation describe that relationship within a limited operating range; they are approximations, not laws of nature. With a ball-nose cutter at a small depth of cut, the effective diameter is smaller than the nominal one, and so is the actual vc.
How do cutting speed and spindle speed work in practice?
Spindle speed is capped by the spindle. A small cutter sometimes cannot reach its cutting speed for that reason, and the feed rate then also falls short of the catalogue figure. In turning at constant surface speed the control adjusts spindle speed to the diameter; when facing towards the centre, the speed rises to the set limit, after which vc drops. Cutting speed affects machining time through spindle speed and feed rate, and tooling cost through tool life. Where the balance between the two lies depends on the hourly rate, the tool price and the time to change tools.
What are cutting speed and spindle speed often confused with?
Cutting speed is not feed rate: vc is the speed of the edge across the material, the feed rate vf how fast the tool or table advances, in mm/min. And spindle speed is not cutting speed: the same rpm gives a higher vc on a larger diameter.
How do you see cutting speed and spindle speed in Blake?
In Blake machining is its own line in the estimate, per piece and per batch, at the hourly rate of the chosen machine and labelled as an estimate. If the time differs from what your machines achieve with their cutting data, you correct the line with a reason; who changed what and when stays visible. More on estimating.
Frequently asked questions
How do you calculate spindle speed from cutting speed?
With n = 1000 × vc / (π × D), vc in m/min and D in mm. In milling D is the cutter diameter, in turning the workpiece diameter at the cut. The other way round: vc = π × D × n / 1000.
Why can't a small cutter reach its cutting speed?
Because the spindle speed it needs is above the machine's maximum. The spindle runs at its limit and the actual vc stays below the table value. The cut takes longer, unless a machine with a faster spindle is available.
What is constant surface speed in turning?
The control adjusts spindle speed to the diameter so that vc stays the same as the tool moves inwards. On small diameters the speed rises until it reaches the limit set in the program or on the machine.