This is a fully functional precision rotary stage combining a precision spindle with a brushless DC (BLDC) direct drive motor and a high resolution incremental angular encoder.  It is unusually compact as these things go, and it has a large (50 mm diam.) through hole in the center.  Use this for your critical metrology projects that do not require an air bearing spindle.  It is ready to go to work for you!

The stage is in excellent used condition.  While I have had it, I put less than 50 hours on it because I never did get to the point of using it for the application that I originally had in mind.

The spindle is a high quality moving element bearing type providing an asynchronous radial motion error in the less than 100 microinch range.  I have characterized the radial motion error of this spindle using an elaborate test setup. An introduction to this type of testing and to the test results can be accessed at:

while the summary report for this stage is at:

A supplementary report providing more details of the measurement, the data handling, and comparison with the ASME standard for such measurements is available at:

For this unit B the asynchronous radial motion error was found to be approximately 80 microinches p-p over a typical single turn.  This was measured at 1.5 inches above the top surface of the spindle. As motor turns are averaged, the residual asynchronous motion error decreases, leaving finally the synchronous motion error, which for this stage is less than 20 microinches radially, with about 10 microradians of tilt.

The drive motor specifications, as well as mechanical drawings for the stage, can be found on the Bell-Everman DDT rotary stages data sheet that can be accessed at:"

Suffice it to say here that the motor works well with many commonly available servo amplifiers for BLDC motors.  You can view a demonstration of this stage being driven with its internal motor at:

The encoder is a Renishaw RESR200 ring combined with a Renishaw RGH20 read head that provides a 2 microradian (slightly less, actually) angular resolution. Detailed specifications for the Renishaw hardware used in this stage can be accessed at: Encoder System installation guide.pdf DS [EN]_L-9517-9125-05-B.pdf angular encoder system data sheet.pdf angle encoder system installation guide.pdf

That the encoder works is demonstrated in a short video here:

The Bell-Everman data sheet linked to above was downloaded in January of 2011, about the time I obtained this stage.  It lists many other specifications for this stage, for instance, its angular positioning accuracy.  I note here that more recent data sheets from Bell-Everman, including the one that is available currently, provide some different specifications.  Another one that I downloaded in January of 2012 is here:

The tendency seems to be that some of the specifications have been made less stringent over time.  Certainly, some of the specifications are better explained in the later data sheets.

One of the unique properties of the MT version of the DDT200 is that the machined land on top of the stage can be made accurately parallel to the base of the stage.  This property appears to be an option on current stages, whereas it seems to have been a standard feature when this stage was produced.  While I have not characterized that property of this stage accurately, you can view an interesting video demonstration of the extremely small static axial runout and of the amazingly good parallelism between the top and bottom of the stage here:

Questions are welcomed.  I'm not offering a return, so be sure to carefully examine the documentation, view the videos, and ask questions!
On this item, to a U.S. buyer, I will provide free insured shipping by Priority Mail.  Other shipping can be arranged if you want, but the buyer will bear the cost.  A buyer from another country will have to deal with eBay about shipping, insurance, and import duties.