High speed spindles are complicated and pricy, as I've learned. KDN Tool & Automation Engineering will provide a belt drive for my MicroMark MicroLux #82710. KDN Tool & Automation (www.kdntool.com) is in the process of converting my mill to full CNC. The SIEG X2 can get up to 2,500 rpm with the stock gear drive. KDN Tool & Automation has an option, to belt drive the spindle, getting it up to 4,000 rpm. Going higher is not recommended.

I am not recommending that anyone do what follows!!! I'm only brainstorming.
You could purchase a Proxxon Micro Mill MF 70, from Tool-Switch for $300 + shipping and taxes. The Proxxon motor and spindle assembly, turns from 5,000 to 20,000 rpm. Proxxon is a quality made machine. The application for the MF 70 is "Small precision mill for lab, optician, jewelry, electronic, and model projects". I want to engrave. The maximum power is 100watts. So lets see if the Proxxon motor and spindle assembly can be adapted for engraving. Back of the envelope scoping calculations. I worked as an advanced development engineer up to 20 years ago. I switched to planning, marketing, and sales management - it paid a lot better. So I'm rusty on my Manufacturing Engineering (did that before becoming a Mechanical Engineer) -please feel free to correct me in my calculations.

Torque (N-m)=depth of cut (mm) X width of cut (mm) X feed rate (mm per tooth) X number of teeth X machineability factor (0.145 for aluminum, 0.435 for steel)
depth of cut (mm) 1 mm 0.039 in
width of cut (mm) 1 mm
feed rate (mm per tooth) 0.18 mm/tooth high speed steel cutter, on brass
number of teeth 2 a pointed engraving cutter
machineability factor 0.29 half way between aluminum and steel

Torque (N-m) 0.104 N-m (ref: one N-m = 0.74 foot-pound)

Next determine the torque of the Proxxon MF 70 spindle, assuming a constant power of 100 watts. (Who knows what the torque speed curve is)

Torque (N-m) = [Power (kw) X 9,550] /rpm
Power (kw) 0.1 0.1 0.1 0.1
rpm 20,000 15,000 10,000 5,000

Torque (N-m) 0.048 0.064 0.096 0.191

It looks like maybe somewhere between 5,000 and 10,000 rpm it might work, but not knowing the torque speed curve for the Proxxon MF 70 motor, it's only a guess. Well it was only an idea.

Lets look at price points for "high speed spindles"
Sherline (from DiscountCampus.com)
#33050 High Torque DC Motor & S/C Assembly $210 discount to $194.25
#33060 DC Motor & Headstock Assembly W-S/C $312 discount to $288.60
#33070 10,000 RPM Headstock, Motor and $402 discount to $371.85
Speed Controll Unit

Taig
#200-55CR Spindle Motor (1/4 hp, 3,400 rpm $110
Continous Duty

High end engraving machine spindles (I don't know if the price includes motor)
Spindle unit High-torque spindle unit 3,000 - 12,000 rpm
High-speed spindle unit 5,000 - 20,000 rpm
High-torque milling spindle $800
High-precision milling spindle $1,800
High-speed engraving spindle $1,700

Specialized 50,000 rpm "engraving" spindle and motor $5,000

Now, the article "High Speed Spindle Design and Construction" Engineering Research Center for Net Shape Manufacturing - The Ohio State University - by William Popoli, President, IBAG North America - on the web site MMS Online (http://www.mmsonline.com/articles/hsm9802.html) shows that amongst the many critical factors, the bearings play an important role. The article developes the knowledge required to select proper bearings.

Now I've got a spindle that came with the machine. What if Little Machine Shop were to develop replacement spindle bearings, good for 30,000 rpm, continuous duty, permanently lubricated, no forced water or air cooling. That were pressed into a SIEG X2 spindle casting (will need ceramic balls and specific design criteria for selection of the proper bearing, and don't want to brinell the raceways or crush the ceramic balls when pressing them into the casting and on to the spindle). So a shop needs to do this (tooling).

Now we can turn the $549.79 (including shipping) MicroMark MicroLux milling machine #82573 into a 30,000 rpm mill for the price of a few bearings. If you want to try this yourself Boca Bearing sells bearings one at a time (www.bocabearings.com). They may even work with you to develop a selection for your parameters.

But where's the motor? Good question. In all my searching there is very little out there about high speed spindle motors. In this day and age there must be all kinds of candidates out there. I'm not an Electrical Engineer, so is there anyone out there that can help? Assuming the formulas for spindle torque are correct, what is needed is a motor (not an air motor - too noisy) that can be controlled, continuous duty cycle (some "engraving" jobs could take 24 hours non stop), self cooling, and can run at 30,000 rpm and deliver quiet, reliable, smooth Torque.

Just brain storming, why can't a dc servo motor be used?

Whelen