5 Bears -Whirlwind Tooling 1

Sequential facing passes; each newly machined surface is then rotated 90 degrees into the fixed jaw. The face mill is the best choice for one-pass precision. Be sure your mill head is trammed correctly.	The fixture base began as a length of mild steel pulled right from my overflowing scrap bin. My guess is that it is probably C1018, definitely cold-roll. Cold-rolled steel is usually not the best choice for a jig, as the stresses in the steel, which are produced during the drawing process, often reveal themselves during machining. Especially so if the steel is milled into an assymetrical shape. If this doesn't make sense, try this experiment - Mount a rectangular piece of CRS into your mill vise, of dimensions 1/4" thick, 3/4" wide, and 6" long. Apply the end mill and reduce the 3/4" width to 3/8". When removed from the vise, you'll find that the original edge is now quite curved, often visibly so. CRS maintains stresses in the "skin". When the portion of steel bearing the stresses is milled away, the now-released stresses on the other side of the steel are free to act, warping the work. Cast iron is usually the best choice for fixturing due to it's stability. Hot roll, unlike cold roll, is relatively stable, but often the HR steel comes with an annoying and abrasive oxided surface which must be machined or ground off. For short runs, when the volume of parts to be created is rather low, a jig can be made of brass or aluminum, but these do not have the durability of steel. Again, even though this is cold-rolled, mild steel, since the shape of the bar is both thick and symmetrical, the work will not warp to any degree. The dimensions of the fixture base is now 22mm square by 76mm long, fresh off the mill.
The high finish and accuracy of the spigot is obtained with fine turning, + abrasives. Here's how it's done.	Two very precise "spigots" were turned from O1 tool steel, which will remain in its annealed state. Here, spigot #1 is being parted. Note that the spigot has two diameters, a major OD and a minor OD. The major OD measures 0.4995" (imperial), while the minor OD is 7.99mm, +0.00mm, -0.01mm - in other words, a very accurate turning just shy of 8mm in diameter. The large portion of the two spigots will be inserted into two borings in the square steel block. Before parting, the bottom of the spigot (to the left in this photo) has a small taper filed into place to ease insertion into the block. The junction of small to large (8mm to 0.500") spigot diameters has a small clearance undercut, done with a 1/16" parting tool. The top of the spigot (to the right in this photo) was also gently rounded with a needle file, in a fashion similar to the spigot base. Finally, the top of the spigot was drilled and tapped 8-32.
Two carefully drilled, bored, then reamed, 1/2" holes.	The steel base, after the initial truing operation, was carefully mounted in a 6" Kurt vise. The two holes as shown were machined exactly 51.50mm apart, and just as exactly 0.400" from the far side of the block, the side which will be against the fixed vise jaw during the future MOP. (Note: The correct pin spacing is 47.50mm, not 51.50mm) Notice I said the holes were machined, not simply drilled. If I drilled these holes 0.500" dia., 51.50mm apart, the odds that the spigots would actually be exactly 51.50 mm apart when installed would be slim, due to minute deflections of the drill. The holes were first drilled 15/32" (0.469"). A tiny boring bar was installed into my boring head, and the holes were carefully opened to 0.495" by boring, not drilling. Using a boring head to open the holes to this dimension creates truly round holes, produces a bore perpendicular to the upper base surface (no lateral drift), and finally, boring ensures that the holes are properly spaced 51.50mm apart. Boring in a mill or lathe is a powerful, effective technique which can correct any number of problems which are the natural product of twist drills. Drills are not precision cutting tools. The holes were finished off with a 0.500" reamer. They could have been bored to that dimension, but in this case, that would be a time-consuming, unnecessary operation.
Hmmm, what is it for??	To finish the fixture, two holes were drilled and tapped through the sides of the base for a pair of FHCS. The spigots were a light press fit into their respective holes, with the major spigot diameter shoulder being a few thousandths below the level of the base stock. On the sides of the base block, I stamped the important 0.400" dimension, as well as a marking specifying which side of the base will ultimately face the fixed jaw of the CNC mill vise. Doing so allows me to locate the edge of the vise, or the edge of the base (it doesn't matter which), thus setting the spigot exactly 0.400 from that datum. In use, the slave rod blank will be drilled for the two pins, and a bronze bush pressed into place. After installation on the fixture, a pair of SHCS plus two small but accurate pressure disks (a fancy term for washers), will hold the rod blank firmly against the base. The CNC cutting will take two cycles, each identical to the other. The first cycle will mill and contour the upper half of the rod. With the mill paused, the rod will be flipped over, so thet the "back" half can receive the same contouring.