How do small diameter internal thread grinders typically work? Are the grinding stones profiled to look like thread mills? How long do the stones last before they need to be redressed?

Curious is all.

Zumba, Good question. I do not know myself, I'm sure somebody out there will tell us. I'm going to take a guess. Single layer plated CBN or diamond??? They are not dressed to size and shape after finishing in their mfg. process, only cleaned. We used them to finish the roots on turbine blades. As long as the wheels were high pressure cleaned with the coolant, they had long life for fairly small diameter wheels.

Nobody wants to tell us!

Good guess though, Richard. CBN would be pretty cool. What would you use to carve the thread profile in it? If it's conductive, I supposed an EDM would work quite nicely.

Zumba, Single layer plated wheels are steel, machined to form. The machined steel wheels are then electroplated with nickel. I dont know how the process is done but a layer of superabrasive is incorporated in the matrix one grain deep. Hence the name "single layer plated wheel". The plated wheel is then "dressed" with a diamond wheel. This is an extremely light dress to remove only some high spots on the grains of superabrasive. Unlike normal grinding, the grains of abrasive are not sacrificial. That is they are not pulled out of their bond, the grains stay put and the material being ground can be built up between the superabrasive grains. That is why the high pressure coolant is directed at the formed wheel to blow the swarf clear. During the grinding process, in which the abrasive lasts quite long, the grains gradually become dull. At that time, the formed wheel is sent back to the supplier who strips off the plating and may recycle the CBN or diamond abrasive grains. If the form is still good, it can be replated. In use, I imagine the wheel should be as large as possible in order to have as much abrasive as possible in the work zone. Does that help? Hope so.
Added note. check www.abrasive-tech.com

"Old school" ID thread grinders(Excello made a million of them) had a configuration very similar to an ID grinder like a Heald. Their table ran back and forth on a lead screw which determined pitch. The wheel spindle was mounted on a swivel base that allowed the wheel to be rotated to the helix angle of the thread being ground. The surface of this base was vertical and had a slide that allowed you to put the wheel back on center as a longer or shorter quill would change centerline. The dresser was fixed to the base and kept the "point " of the wheel in the same place. You could change wheels in the middle of a part,redress and press on only having to compensate for wheel diameter. Resin bond wheels were the norm. Small threads were a problem because of the RPM required for the tiny wheels. As most wheelheads were flat belt driven pulley diameters got pretty extreme sometimes. Water cooled wheelheads with oil mist lube were the only answer.