Has anyone made their own servos from DC Motors?

Has anyone made servos (that work well) from DC motors? I am not
sure but unless I am missing something, it seems very possible to do
and affordable too.

There are a host of 90 Volt DC treadmill motors out there for very
cheap ($20.00 ~ $50.00). While the claimed HP is 2.5, I suspect
they are more close to 3/4 ~ 1 HP, which should be fine for small
servos on a home shop machine.

Most of these motors have a small shaft on the back end which is
usually for a fan blade of some sort. This could be used to attach
a rotary encoder with relative ease. Rotary encoders can be
purchase new from US Digital for around $35.00.

This gets you a motor and encoder for less than $100.00

Next up is an enclosure of sorts for the motor to make it suitable
for use in a metalworking shop. This could be any form of aluminum
from square tubing to round tubing (if the id matches the od of the
motor). Incorporating a flange for mounting should also be pretty
easy to do. I suspect this would add another $40.00 or so.

In the end, you could have a decent sized servo for around $140.00
give or take a few bucks.

I don't know what else is different about a servo than from what was
just described (I am sure they are little more heavy duty for
industrial use). The electricals is something I am unsure of.
Encoders are encoders - that isn't much of a problem. But not all
DC motors are the same (I am guessing).

Has anyone done this and how did it work out for you? Any
suggestions on what type of motor to use or avoid? What about a
servo driver, Gecko or other?

TIA

Chris