The Bridgeport Series I Standard Knee Mill is the single most ubiquitous manual milling, drilling, and boring machine in the world. It can be found in job shops across the globe serving in countless capacities.

The ultimate small mill is a Deckel FP1. Weight around 1,200 lbs...takes up about half the space of a Bridgeport mill but is more rigid, has integral power feeds on X and Z axis, vertical and horizontal spindles, and many cool accessories avaliable, incuding integral tilt/swivel/rotary tables.

Over the years Bridgeport has made a number of different size Series I vertical milling machines. They range in size d from 1/3 hp to 2 hp and table sizes from 32" to 48". Here's are a couple links to brochures listing the different sizes the Series I J head machines came in.

It became known in the following decades for small and medium-sized vertical milling machines, with a form of quill equipped multiple-speed vertical milling head with a ram-on-turret mounting over a knee-and-column base.