High Speed Secondary Spindle

I kept coming across articles about folks that were attaching small high speed spindles to their mill head for various reasons, so I finally decided to create a page whose focus is this phenomenon. The reasons for doing it make sense. Most mills in home shops are doing good to hit 4000 rpm. Mine tops out at 3200 rpm if I change out the motor and 1600 rpm otherwise. The combination of cutting aluminum, which has very high surface speeds, using carbide cutters, which takes the speeds up further, and trying to use small cutters for accurate 3D profiling of fine detail or engraving all add up. I define a small cutter, BTW, as anything 1/4" or less, which is what I've seen on the web as the crossover point for HSM (high speed machining).

High speed spindle attached to mill head...

Using my copy of ME Consultant Pro to look at feeds and speeds, I came up with some interesting scenarios that reinforce my thoughts below that a high speed spindle would be helpful. ME Pro suggests a spindle rpm of 20,390 (!) and a feedrate of 17 ipm when cutting aluminum with a 1/8" cutter. No way your average HSM can touch those numbers! Except, ME Pro will also say the horsepower needed is only about 1/10 HP. That combination of high rpms and low horsepower required sounds like something a small high speed spindle could do.

There are a variety of alternate spindles you could strap on your quill. The Bosch Roto-Zip-like tools, the Proxxon high-end Dremel tool, as well as high end die grinders from outfits like Makita will get you anywhere from 15,000 to 30,000 rpms. I've seen folks using air powered spindles, there are expensive commercial spindles available (watch for them on eBay), and there are also devices called spindle speeders/increasers/multipliers that are just gearboxes that attach to your existing spindle and give more speed for smaller cutters.

Here is a typical thread on this subject from CNCZone.

Sample Pictures of Other's Slave Spindles

Attaching an air grinder as a spindle for engraving...

30,000 rpm air spindle by MacroTechnologies...

This fellow made a router holder for his CNC knee mill that uses a #40 taper shell mill holder on one side and a shaft with a linear bearing on the other to support the router. The linear bearing is necessary because the router goes up and down in Z as the quill moves:

You can see it works pretty good!

I like the idea that you can quickly detach this rig from the mill by popping out the shell mill holder and undoing the clamp for the linear bearing. For my mill, I wouldn't be using the quill, so the shaft would just clamp to the mill head directly for support.

This is the first time I've seen it done on a manual mill:

He's using a small 1.5mm endmill to cut a hex shaped hole in the workpiece rather than broaching.

Install the high speed spindle inside the main spindle with a flexshaft

Here is one of the cleverest ideas yet. This fellow built his high speed spindle so it can be installed inside the main spindle:

It seems to me there has to be enough room in an R8 spindle to do something similar. Tres clever!

I hear good things about those Bosch Colts...

Proxon on a Tormach. I'd be worried about the air cooling holes that close to flood coolant!