High Speed Spindle

Here in the RD workshop, consideration is being made on adding a one horsepower 24,000 RPM spindle to the Taig CNC Micro Mill. The standard spindle is a belt drive one quarter horsepower unit with a top RPM of 10,000.

The standard spindle and drive is quite adequate for most of the milling work the micro mill is asked to produce. The primary use for one of my mills is wax carving with as small as 0.003-inch milling bits. That size mill bit can best operate at the 10K RPM and higher ranges.

I did some comparison of speeds and feeds in G-Wizard from CNC Cookbook. Doubling the spindle RPM can almost double my rate of travel, within limits of tool deflection, which is not extreme in wax. It will certainly reduce my running time.

The new high-speed spindle is a bit pricy. I’ll know the final cost if I decide to proceed with this quest for rotational speed. The little Taig mill doesn’t need more than ¼ HP for most of the work it performs. The Spindle I am considering is 800 watts which is 1 HP. It requires a VFD (Variable Frequency Drive) that produces 3 phase, 110 volt output and can vary its frequency output from between about 100Hz to 400Hz. The frequency change is what varies the speed of the spindle.

I am a certified commercial VFD start-up/commissioning engineer and have a very good understanding of how VFD drives function. I have not seen the VFD for this spindle, but its (published) operational parameters are familiar.

The spindle may have an issue for some folks. It’s not actually an issue, however. The spindle has an ER11 collet holder rather than the Taig ER16 size. That means the largest nominal size tool the high-speed ER11 collet can hold is ¼ inch. The ER16 holds a 3/8 as its largest.

I call it a non-issue as the purpose for using the ER11 spindle is to spin small diameter tools at very high speeds. Not to be a universal replacement for the standard Taig ER16 spindle. Solution is, don’t throw away the stock spindle and motor.

The new spindle has the motor inside the housing which includes the four main bearings. The spindle motor and bearings require water cooling. The water also greatly reduces spindle noise. The standard Taig spindle and motor are very quiet, and I expect the HS water cooled spindle will not be any louder.

Water will be supplied by a small pump and a reservoir of water. I have no idea of the amount of heat the spindle will produce or the size of the reservoir required. That will be determined empirically. It will certainly NOT be consuming a continuous 800 watts. The work load produced by small micro bits is minuscule. The spindle will be running almost totally unloaded. The VFD will display the exact motor load.

I don’t need a one horsepower spindle. It’s like putting a 427 Hemi in a Ford Falcon. I would however, like to run a 24,000 RPM spindle. This project is simply an exercise to explore what a high speed, high power Taig micro mill can do. I think perhaps other folks will also be interested. The only custom parts required will be for the mounting of the spindle to the column.