This one from Grizzly:
http://www.grizzly.com/products/Flex-Shaft-Grinder/G9928
$64.95

Looks exactly like this one from Harbor Freight:
http://www.harborfreight.com/flexible-shaft-grinder-and-carver-40432.html
$44.95  (On sale from 49.95)

They both claim 15,000 RPM.  I have not been able to find the older ones that used to claim 18,000 RPM since I started looking at these for mill spindles a few weeks ago. 

I found the handpiece that came with the Harbor Fright one to be surprisingly good.  I highly doubt the motor puts out a true 1/4 HP however.  On an agressive cut or sudden turn when cutting surface increases it bogs down and makes the motor jump all over the place on its hook.  That's why I was considering trying to adpat the flexshaft to a router that definitely produces much more horsepower.  With apporpiate speed reuction pulley of course. 

A fixed solid mount for the motor instead of a hanging mount might help of course, but then it might just snap the shaft. 

Thanks for the links Bob.
I would try the Harbor Freight one. For engraving work your only doing very light cuts so HP is not a problem , any way even if it is not a true 1/4 HP. At least the motor dosen't work as hard as my Dumore and that one isn't cheap. I just flood the area with fluid, slow feedrates and let the cutter make slush....

I will add that before purchasing mine, i called them and asked if they had other handpieces and when they looked they found that you could order one with collet or chuck and i got both.

To fix the play you just lightly heat the end of the handpiece to loosen the epoxy / locktight / crazy glue and screw the end off. Then you shim
the play out by adding spacers. You test the handpiece by running it and if the bearings get hot reduce the spacer. I would guess that i have .0005" of end play. You need to reduce it or your engraving will look like crap or you'll break those nice graving cutters.

RICH

Just use your existing cutter to make yourself a set of mounts for the HF handpiece.  That's what I did.  Actually twice.  Once for a single cutter, and one to hold two of the HF handpieces so I could destroy work pieces two at a time.  

Thanks Rich, Bob et al, I have found a similar looking one here, lot more expensive than the USA but thats the way things are here
http://www.axminster.co.uk/axminster-axminster-heavy-duty-flexible-drive-unit-prod20091/

Hood

Hood,
Looks like a good choice and would recommend you get both hand pieces ( collet and drill ).
The drill chuck on mine was just as good as the Jacobs collet ( about .002"  runout at the jaws ) but it had a rough /bad / tight feel when tightening a the cutting bits in it.

Here is a link to a few holders i made for the hand peices. This way i can easily mount  them on the lathe , mill , and engraving machine.
http://www.machsupport.com/forum/index.php/topic,12484.msg88231.html#msg88231

RICH

HOLDER COMMENTS:
- Drill and ream block for holder OD
- make the holder such that it "springs" some to insert the holder ( the bearings are at the ends of the holder)
 that's why there are relief cuts in the block on mine
- with the holder mounted in the block, set up to machine the block so the base, sides, front & back are parallel with the handle center line
- do the base first and check with a piece of small ground stock inserted into the collet / chuck and check for runout at the tip of the bit /
  want it to be as good as possible ( match mark the handle and block for repeatable insertion of the handle into the block)

Just want to make later setup easy ie; can use a master square to set the holder.
If not done right you'll break small drills and end mills in a heart beat.
Naturally the less run out the better. Engraving line width will be determined by the runout, and should you do real fine stuff it becomes important.

FWIW,
RICH

Looks good Rich, will have to study when I get home, doing a dozen things at once here today

Hood

The drill chuck on mine was just as good as the Jacobs collet ( about .002"  runout at the jaws ) but it had a rough /bad / tight feel when tightening a the cutting bits in it.

I had that with my HF chuck keys.  One was better than the other so I took a look at them.  The center pin on one was longer than the other.  I ground it off and now both feel better.   I may grind it a little more and see if it feels perfect or not. 

I am using a Dumore 1/4HP flex shaft die grinder (22,000 RPM) mounted in an aluminum block that bolts to the side of the head. I have had this tool for about 30 years. It was expensive back when I purchased it new and now is somewhere around US$ 1,000 methinks. You might find one used, but I'll be taking mine to my grave as would most owners. Shown in operation here:
http://www.youtube.com/watch?v=r9Zf_5yHB1I

If runout and longevity are concerns, you would want to avoid the hobby level stuff like Fordome, etc. Use something that is intended for high speed grinding in a shop environment. An interesting thought would be to get a used Dumore tool post gringer and mount a 1/2"-20 mounted ER-16 collet holder on the end of the shaft. These grinders are pretty common on ebay. Runout would be the challenge with such a setup, I would imagine. Runout and dynamic balance are the critical issues in this application.

I built a mount and drive setup for a fellow to drive an industrial engraving spindle using a laminate trimmer motor thru a round belt setup, but I think that air turbines are the best solution. Note that these tools are very different from a vane type air die grinder or similar tools that are extremely noisy and require oiled air.

If I were inclined to go the 'mount-an-inexpensive-die-grinder' rout, I would probably use this one: http://cgi.ebay.com/Metabo-GE700-3-7-000-27-000rpm-6-2Amp-Die-Grinder-NEW-/330461681654?pt=LH_DefaultDomain_0 It has good reviews, a steel nose for mounting (very important), good speed and power (electronic torque comp), and comes with a 1/4" collet and most importantly for engraving, it has a factory 1/8" collet option available for it.

Here is a link to a promo video of turbine tools:

http://www.youtube.com/watch?v=SWovmvEwPR4

It seems to me that you would need to use a lot of shop air for engraving anyway to keep the tool cool and the swarf cleared away. It would be convenient if the turbine exhaust could be directed at the cutting tool. I don't know if that scheme is acceptable, but it would be zero cost air if it was.

Below is a link to the spindle that I made the mount and drive pulley setup for as mentioned in the previous post. The spindle was beautiful. Pure mechanical art:

http://www.artcotools.com/nsk-nakanishi-pulley-spindle-c-327-p-1-pr-17011.html