Spindle Speed Increasers: Electric, Air-Driven or Coolant-Driven?

4 hours ago
3 min read

A 2 mm end mill needs 20,000–30,000 rpm to reach its recommended cutting speed. Most machining centre spindles top out at 8,000–12,000 rpm — a gap of three to four times.

You cannot close that gap with parameters. You close it with a speed increaser that multiplies spindle rpm, or with a separate power source of its own. Three approaches exist, and their power sources are completely different.

Why small tools need high rpm

Cutting speed is a function of tool diameter and rpm. The smaller the diameter, the higher the rpm needed for the same surface speed.

Run a small tool below its designed cutting speed and you get abnormal tool wear, poor surface finish, and long cycle times. Many people blame the tool. Usually the machine simply cannot spin fast enough.

This shows up most in mould detail work, engraving, small-hole drilling and thread milling.

Three power sources

Mechanical (gear or belt) — multiplies spindle rpm by a fixed ratio, for example 3:1. A 5,000 rpm spindle gives 15,000 rpm at the tool. The machine spindle must turn.
Air turbine — driven by an external compressed-air line, spindle stationary. Requires plumbing air to the machine.
Coolant driven — driven by the machine's existing through-spindle coolant, spindle stationary. No external power source needed.

All three rely on an anti-rotation pin or block that holds the housing and stops it spinning with the taper.

Searching for an "electric speed increaser"?

This is a common wording gap. Speed increasers actually used on machining centres are rarely electric — electric means running a power cable, dealing with signals and heat, in an environment flooded with coolant.

What is widely used on machines is the air-driven and the coolant-driven type. If you searched for an electric speed increaser, the problem you are really solving is usually "my spindle rpm is not high enough" — and these two are what you want to look at.

Where the coolant-driven type wins

The biggest advantage is that it needs no external power. The machine already has through-spindle coolant; that coolant drives the turbine and flushes the cut at the same time, so drive and cooling happen together.

Two more practical benefits:

It can sit in the tool magazine and be changed by the ATC — managed like any other tool, with no manual intervention
The spindle does not turn — spindle bearings never have to run at high rpm, so their service life is less affected than with the mechanical type

The trade-off: it lives on the machine's coolant supply. Not enough coolant pressure, not enough rpm. That is the first thing to confirm when specifying one.

When it is not the right choice

No through-spindle coolant, or insufficient pressure — coolant-driven is out; consider the air type
Long reach required — a speed increaser trades reach for rpm; high-rpm units have very little extension and are not suited to deep cuts
Large tool diameter — big tools do not need high rpm; running them straight off the spindle is more efficient
Very high coolant temperature or heavy chip loading — turbine life is affected; filtration matters

What to confirm before quoting

Machine make, model and spindle interface (BT / HSK / PSC etc.)
Through-spindle coolant pressure and flow (this decides whether the coolant type is usable at all)
Whether ATC tool change is required
Tool shank diameter and clamping method
Target rpm
Operation (milling, drilling, thread milling, engraving) and workpiece material

Mongtec coolant-driven speed increasers

The Mongtec precision coolant-driven speed increaser is driven by cutting coolant at 15–70 bar. The manufacturer states a maximum of 55,000 rpm and 0.003 mm accuracy, with BT / HSK / ER / PSC interfaces, and it can be stored in the magazine and changed by the ATC. If your work also involves long reach or fixed-position tooling, see our precision boring bars and angle heads.

Not sure whether your machine's coolant supply is adequate? Send us the machine model and its through-spindle coolant spec and we can help you assess it.