Slave Rod Fixture for CNC Contouring
Wright Whirlwind Radial Engine 5 Bears Home

Here we go! The first of what will probably be 50+ installments of the Wright J5 Whirlwind!

Background: A few weeks ago, I made a posting to the CNC Zone. In it, I described a concept for a new radial engine which would make use of Saito cylinders and heads. Saito makes extremely nice 4-stroke engines for the R/C hobby market; you can read about them and see some pictures too by reading the thread. I am Swede, of course. ESjaavik is Einar Sjaavik, a very nice and extremely talented model engineer.with whom I had been corresponding with on and off for some time on CNC and other issues. When I discussed the Saito concept (not a new idea, it has been done before), he pointed me to the Whirlwind website. Score! The Whirlwind castings were still available! Einar is also making his own Whirlwind.

The decision to embrace the Whirlwind wasn't hard. When the engine first appeared in Strictly IC, I was awestruck. It was at a time when I did not think that I could handle such a project, so I mentally made a note... "Do this in about 20 years when you have a clue." Of course, I didn't order the available castings, and I thought that I'd missed out. I am glad to discover that I was wrong.

The Saito project is on the back burner for now. I retrieved my five copies of Strictly IC (#24 to #28, available at the SIC site as backissues) which contained the serialization by Mr. Olsryd, and carefully copied the plans at the library. I had a bit of an itch to make something, so I decided to machine this precision fixture to produce a certain Whirlwind component.

On fixtures and Jigs: Any complex project with repetitive parts (valves/rockers/rods, etc) or with awkward castings (heads), will benefit greatly from the creation of precision fixtures. Castings, especially, can be a real bear to hold accurately and firmly, without marring the workpiece. It has been said thousands of times - here it is again - the majority of your time spent with machining metals is on setup. In the past, I have had setups which consumed 2 or 3 hours of focused effort consummated with a 30 second swipe of an end mill. A well-engineered fixture eliminates edge-finding and setup, and for many parts, to be cut via CNC, fixturing is the only practical way to hold the part for the cut.

An example - on my Hodgson Radial, I had to produce 18 tool steel rocker arms. Without the small finger block and turning fixture, pictured here, each rocker would probably have consumed several hours. By devoting those same hours to these fixtures, I ultimately saved much tedious setup work. Note the size of the rocker clamped on the left edge of the finger block. The Socket Head Cap Screw (SHCS) next to it is only 1/4" diameter across! The rocker is small, and small often equates to tricky workholding.

Post Script: When this page was initially posted, I left the tool's identity a mystery. Apparently it wasn't too hard to figure out. Within a few hours, Raphael from the Netherlands had correctly guessed the function on the Guest Page... it is a fixture designed to firmly clamp an aluminum or steel slave connecting rod, 51.50mm between pin centers, for rod contouring operations in the CNC mill. Within a few more hours, Einar had emailed me, breaking the news that the updated prints call for 47.50mm between centers, so this effort here is wasted. I will leave it posted, as the new jig (yet to be made) will look the same with the exception of the 47.50mm pin spacing.

No more jigs and fixtures until I study the new plans!


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.

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