In a table saw, it is the attachment point of the Circular Saw 's Arbor to the underside of the table.
The primary adjustment point for aligning the saw blade to the miter slot on the top of the saw's table, the Trunnion also handles the adjustment of the angles of the Blade . The use of trunnions in a tilting-arbor saw allows the axis of blade-tilt rotation to be Coplanar with the tabletop. ("Coplanar" means "lying or occurring in the same plane. used of points, lines, or figures." From Dictionary.com, and Jeff Joslin.)
Historically -- as shown in the Oxford English Dictionary -- trunnion eenters the English language in Medieval times, where it means "tree trunk", and later cannon (the gun) anatomy; strangely the OED makes no mention trunnion making up part of the antomy of a table saw. Using Google Books research shows that for its meaning associated with table saws, trunnion only turns up in books and magazines in the 1940s, not before -- a scant four "hits". Google evidenlty has yet to digitize the Deltagram; the material below comes from my own set of Deltagrams
Online is this piece: Popular Mechanics Nov 1947. pages 218-222:-- Article in 1947 issue of Popular Mechanics shows woodworkers in an America still under WW II conditions -- recovering from a war economy, not yet entirely free of rationing policies -- how to build their own table saw, including fabricating the mechanisms such as trunnions.
Etymology:-- (copied verbatim)
trunnion (trun'yan) n. either of the two round projtions of a cannon. Before 1625, borrowed from French trognon core of fruit, stump, tree trunk, from Middle French troignon, from Latin truncus TRUNK, but fluenced by Old French moignon, with the same meaning, from Gallo-Romance *monionem, from Gaulish *moni- neck; compare Old Irish main neck, necklace, Middle Welsh mwn neck, Welsh mwnwgl neck, mwng MANE.Source: Barnhart Dictionary of Etymology, Robert K. Barnhart, editor New York: H. W. Wilson, 1988, page 249.
Table Saw Anatomy
In the table below, with a few editoral additions to accommodatre the quote to this entry is a word-by-word reprint of an exceptionally clear description of a table saw's under-the-table anatomy that comes from the source indicated that I consult frequently for both excellent descriptions of woodworking tools and expertly photgraphed images.
The internal components of a contractor-style table saw include the saw cradle (or carriage), trunnion and the arbor assembly, which is bolted to the underside of the saw table. The cradle supports the arbor assembly, which holds the arbor. The arbor assembly consists of a shaft held in place by bearings. The arbor has a pulley on one end for a V-belt, and the saw blade is held by a flange and nut on the other end. The cradle also holds the motor, which is in the back of the saw on the contractor design and below the cradle on the cabinet style. Two trunnions, one at each end of the assembly, align the cradle and the motor. They make it possible for the cradle to be tilted for bevel cuts.
[Two Cranks Adjust Blade Height and Blade Angle]
Two crank wheels change the height and angle of the blade. The tilt wheel on the side of the saw adjusts the angle of the blade to the table. The tilt mechanism generally has a worm gear, which engages a semicircular rack on the front trunnion. When the saw's tilt wheel is rotated, the angle of the cradle and blade changes. Two adjustable screw stops limit the range of trunnion travel and are usually set at 90 degrees and 45 degrees. Another hand wheel at the front of the saw drives another worm gear, which raises and lowers the blade.
Table saw design
Two styles of tilting arbor table saws predominate. The cabinet saw is a relatively large industrial saw with a solid base, or cabinet, and a correspondingly large and heavy tilt mechanism. The motor is enclosed in the base and often drives the arbor with three belts. The contractor's saw is lighter and has an open base. It has a lighter undercarriage, and the motor hangs off the back of the saw on one belt. Contractor-style saws usually cost about half of the price of a more substantial cabinet saw.
[Blade Tilt Direction]
There is an ongoing controversy over which way the blade should tilt. On most American, European, and Taiwanese saws the blade tilts to the right. On some American saws, however, the blade tilts to the left. No saw tilts both ways. Blade tilt direction has safety implications. Most people crosscut with the miter gauge in the left table slot, so it is best to have the blade tilt to the right for crosscutting bevels. However, most people rip with the fence on the right side of the blade, which makes the left-tilting saw better for ripping bevels, so the workpiece doesn't bind between the fence and the blade, and kick back. Neither tilt direction presents an insolvable problem. On right-tilting saws, you can move the rip fence to the left side of the blade when you make a bevel cut. With a left-tilting saw, use the right miter slot when crosscutting a bevel.
Source: Garrett Wade Company, Tools: A Complete Illustrated Encyclopedia New York: Simon and Schuster, 2001, pages 52-53
The image above comes from Willis H. Wagner, Modern Woodworking , 1974, p. 12-1.
Sources: Popular Mechanics Nov 1947, pages 218-222; Barnhart Dictionary of Etymology, Robert K. Barnhart, editor New York: H. W. Wilson, 1988, page 249. Willis H. Wagner, Modern Woodworking , 1974, page 12-1; Garrett Wade Company, Tools: A Complete Illustrated Encyclopedia New York: Simon and Schuster, 2001, pages 52-53;