Wheel Crossing Jig
I have described my attempts at wheel-making, and filing the crossings by hand. The final spoke shapes were less than perfect; filing by hand and eye takes some practice. To assist in this work, I decided to make a crossing jig. In J. Malcolm Wild's book, "Wheel and Pinion Cutting in Horology," he demonstrates the use of a home made jig for filing watch wheel crossings (spokes, rim and hub). He provides a photo of the components and a line diagram of it's function. It consists of a hardened steel workholder which holds the wheel and acts as the guide for filing the rim. It is drilled for indexing the work in the tool. A brass frame holds the steel workholder and has an adjustable hardened steel straightedge to guide the filing of the spokes.
I started with a workholder. A different one will need to be made for each size wheel, however, they are reusable for making the same wheel again. I started with 1-9/32" diameter oil hardening (O-1) drill rod. Hacksawing this diameter of tool steel would be quite the chore, fortunately the Hand-I-Hack was on hand to do the hard work. A disc of about 5mm thickness was sawed off. It was then faced on both sides using the six-jaw bezel chuck to hold it on the lathe. It was faced down to 3.5mm in thickness, and then center drilled for reference.
The tailstock center is used with the reference to set the disc up on the faceplate. It was drilled open to allow for boring the disc to an inside diameter of 17.5mm (center wheel rim diameter). It was then counterbored to the full wheel diameter plus a little clearance (~20.7) and to a depth equal to the wheel thickness plus clearance (~0.75mm).
The work was transferred to the three-jaw chuck and turned to final diameter (32mm). With the headstock locked, five indexing positions were drilled (1/16") into the periphery.
The disc was then suspended on a length of wire and hardened by heating with a propane torch and plunging into oil. It was cleaned up with a brass-wire brush and left fully hard for use.
Another wheel holder was made for the third wheel dimensions.
The frame is made from brass rod. To start, a piece of 2-3/8" (~60.3 mm) diameter 360-type brass rod was mounted in the saw to cut off a section about 7.5mm thick.
The brass disc is held in the 3-jaw chuck with jaws reversed to face both sides and true the disc to a uniform thickness. An exact thickness did not seem critical, and ultimately was about 7mm. The disc was drilled and bored to 1/4" to fit an arbor, in this case a Levin grinding wheel arbor. This allows the outside diameter to be turned true and chamfer the corners 45 degrees as well. The disc was then moved back to the 3-jaw chuck to continue boring open to about 26mm and finally counterbored 32mm to accept the wheel holder made above.
The work is kept in the chuck and transferred to the dividing head which is mounted on the cross slide with the riser to bring onto lathe center. Angular graduations were engraved onto the surface of the frame using a 1/8" engraving bit to a depth of about 0.15mm. Looking at the photo in Mr. Wild's book, an arc of 20º is provided starting at 5º from the center line in each quadrant. Thirteen graduations would provide spaces of 1-2/3º. That seemed to be an odd choice, but grads of 1º would be too tightly spaced, and 1-2/3º is still divisible into 5º, so marks are present for 5, 10, 15, 20, and 25º with two graduations in between. The dividing head is setup for indexing 216, which is 5/27th of a rotation of the crank for each increment. A center line was engraved first, and reference marks at 90º were made, followed by the 52 angular markings.
Positions for the screws to attach the straight edge were drilled (2.5mm) at 15 degrees from the center line in each quadrant and then countersunk. With the holes drilled, the work can be removed from the chuck and the holes tapped M3x0.5.
The brass frame is then mounted in the machine vice and setup to drill the position for the indexing pin. It was drilled 1.9mm and reamed 2.0mm.
The pin was turned from 4mm O-1 drill rod. The 1/16" diameter tip and 2mm shaft were turned first. The handle portion was chamfered 45 degrees. The 60 degree center on the tip was filed off and then rounded off with a jewelers cup bur. The pin was then transferred to the dividing head held in its collet to mill five slots in the handle for easy gripping during use. A 1/16" ball end mill was used. It was then hardened in oil and tempered to a purple-blue on a bed of brass swarf.
Screws for the straight edge were turned from 5mm O-1 drill rod. A length of 8.4mm was turned to 3mm and then threaded M3x0.5 using a die. They were parted off to have a 2mm wide head, and then slotted with a 0.023" saw. Finally, they were polished, hardened in oil, and tempered.
The straight edge was made from a piece of 1/2" x 1/16" oil-hardening gauge plate. Not having any in the stock bin, I reused a piece that had been previously made into a clamp. It has two 5mm holes that will ultimately be sawed off. For use on the jig, four 3mm holes were drilled for passing the screws made above. The holes were connected using a jeweler saw and filed to shape. The straight edges were coarsely sawed out and then finished on the lathe by holding in the vice and using an small endmill.
The outer ends of the straight edge were sawn and filed to shape. It was polished on emery paper and the straight edges stoned with an Arkansas slip stone. It was then hardened in oil and the polishing repeated. It was left dead hard for use.
The primary components of the jig can now be assembled.