High Speed Spindle for Milling Machine

Low spindle speeds are the bane of a home-shop machinist. Typical milling machine spindle speeds rarely exceed 2000 rpm for heavy duty geared machines. The top speed for my mill is 1250 rpm, which is great for boring large holes or facing big pieces of steel, but to machine a piece of aluminium at the typical 300 sfm requires a 20mm diameter cutter!

The spindle speeds are typically increased by changing the gearing / belt ratios, adding a "spindle speeder" (see below) or attaching an auxiliary spindle. A collection of good ideas can be found at CNC Cookbook. (The blog is also a great read).

I propose to modify my mill to have a high-speed spindle in a slightly unconventional way...

A large majority of the parts I make require cutting tools <10mm, so a spindle with a speed range of 6000-20000 rpm would have great utility, especially with carbide tools and softer materials.

The existing mill spindle comes in a cylindrical cartridge with involute rack teeth milled into the side to allow Z-direction movement. The O.D. is 75mm.
<Insert image of spindle removed here>

I propose to swap this entire cartridge for a similar cartridge holding a high-speed spindle. This allows the Z-direction mechanism to be reused, with no loss of Z-clearance. No X or Y travel is lost either (as is the case with offset auxiliary spindles).

These spindles from China abound on eBay for a very reasonable price.

The specifications are: 

Size: 65mm diameter, 180mm length

Power: 1.5kW 

Collet: ER11 (1-7mm)

Max Speed: 24000 rpm (at 400 Hz drive) 

Runout: <0.005 mm (to be tested!) 

Cooling: Forced air

These spindles can conveniently be driven from single phase 240VAC using a VFD, which also provides speed control. I bought a generic HuanYang 1.5kW VFD off eBay for this. One of the first things I did was to crack it open to inspect the quality of the electronics inside Cheap plastics aside, the quality seemed reasonable. There was also copious amounts of PCB lacquer ... higher leakage insulation?

Very few of these generic VFDs include a potentiometer to vary the drive frequency, despite having a dedicated space on the front panel for it. Inspecting the PCB reveals that it should be fairly straightforward to add one. 

<Insert Front panel PCB image here> 

A 75mm OD, 65mm ID sleeve will  be made to "adapt" this spindle to the milling machine. The rack teeth will be machined using a Module 2 involute cutter. A clamp will secure the spindle inside the sleeve, as well as index it to the axis of the cartridge. 

<insert CAD image here> 

Still to come....

• CAD of spindle adaptor
• Machining of gear rack and clamping block 
• Routing of power connections.