Tom, I own a Sherline boring head and it is a very light duty head. It does work, though, if you are careful to stay within its work envelope and account for the backlash in the screw (like most adjustable things we use). The problem for me is that I was always at the edge of that envelope so I went with the Criterion S 1-1/2A. There is no comparison between a Sherline head and a Criterion, even one as small as the S 1-1/2A. More mass, more precision and greater work envelope. In fact, you have to be sure to lower your speed when boring larger holes or your mill will walk off the table due to vibration!

The S 1-1/2A comes up on ebay from time to time and that is where I would look for one. It uses a 7/8" adapter that you can get from LMS to fit the 3/4-16 head of the Sherline mill. This is a good head but a little heavy when you bore out past 2" or more. They come in 2 versions to fit 3/8" and 1/2" shank tools and the 3/8" version is better due to the smaller mass of the boring bars.

Criterion also makes the Tinymite series, which are even smaller. The have an integral shank and so will mount with a collet. I seem to recall a very small square head they made, about the size of a quarter, but cannot remember the model number.

There are other makes/models of precision boring heads - Wolhaupter and others - but for the hobby guy a Criterion is a good choice. If you buy one on ebay the cost is not bad at all - mine cost $50.00!