Its a chuck with three jaws..probably what is called self centering. .which answers to its purpose of holding round stuff for futher work… The benefit of self centering means stuff is held with the center of the material in a defined place….
He doesn't say that it is a 3-jaw independant just that it's a 3-jaw. If it is a self centering then, its use is as Jason states. The only variation of that I know of is the Grip Tru which is commonly ( although not exclusively) a 3- jaw chuck.. So Doug – is this a 3-jaw self centring chuck which which has an additional adjuster next to each jaw? If so what you have is a chuck to be cherished!
Here is a pic.
You will see that, alongside the normal hole for the chuck key is socket for a hexagon key. This allows the micro adjustment.. If this is like your chuck just reply here and I will post the adjustment instructions.
Could possibly be a 3-jaw combination chuck where each jaw can be adjusted separately and then a single key will move all three. Used for repetition of holding a round object off centre eg if you were making loads of eccentrics.
OK Michael – I'm in error here – I've never come across the combination chuck referenced by Jason so the only one to fit the bill, to my knowledge, was the Griptru. Hopefull we will be enlightened eventually. ( Although I've spent much of my life, fruitlessly, awaiting enlightenment)
Some old very experianced Machinist have observed this Chuck with their Jew on the floor, one said that he feels that he had seen one while in the Navy but only vague about it.
This I beleave to be American and may possibly be associated with a Rotary Indexer for possibly Ecentric and Cam Grinding/Cutting??
The only reason I use a three jaw chuck is for holding hex bar… I use a four jaw self centering for round and square bar and a four jaw independent for placing the workpiece where I want it rather than where the chuck wants it
I also use my four jaw self centering for holding hex. The only advantage my three jaw has is the larger hole in the centre for allowing larger diameter work to be held further inside the body of the chuck, rather than having to stick out.
Many Thanks to those gentleman that responded professionly and constructivly, I particuly apreciate the input of discussion from both Michael and JasonB eith a high 5 to Michael for the look at This, that forced me to disect the Chuck and solved the mistery.
You have to ask questions about its work holding accuracy, it is not only depended of the accuracy of the thread of each jaw, but also of the gearing, In a normal scroll chuck the accuracy of the gearing has no part ín the centering of the workpiece. Niko.