The Quarter Scale Merlin's camshafts seem like perfect candidates for being built up similarly to the cam in Jerry Howell's V-4. The camshafts are almost ten inches long with diameters over most of their lengths on the order of a quarter inch. One of the photos shows a sketch of the cam blank that will be used in both heads. There are clearance grooves on each side of the bearings, and a narrow integral thrust bearing at the rear end limits the shaft's forward/aft movement. The necked down sections between the lobes are required to clear the valve spring assemblies. The rear of each shaft will later be fitted with a sprocket drive adapter, and I added a hex at the front of each shaft per John's suggestion to help with indexing the cams during final assembly.
I originally thought I would machine six spools for each cam with each spool having an intake and exhaust lobe pair separated by a turned bearing. I planned to bore these spools for a close slip fit on a length of known-straight drill rod. Done this way, the cam lobes could be easily machined with the spools set vertically in a simple fixture on a 3-axis mill. A difficulty with this built-up approach, though, is that a scheme is needed to accurately align the spools on the shaft before the Loctite sets up. If the parts are properly prepared this can occur almost immediately. Being roller cams I wouldn't expect the load on the Loctite'd spools to come anywhere near the shear strength of the adhesive.
I decided, though, to first try my hand at machining a test blank from a single piece of steel, mainly to see if I could do it. I considered more than half the battle being the machining of the cam blanks with the same fit that I had with the drill rods in my cam block assemblies. To machine the actual cam lobes I'll most likely later create a CNC program similar to the one I used to machine the crankshaft counterweights. My concerns with a single workpiece approach included warping as well as my own ability to stay focused throughout the lengthy and unforgiving machining required to complete the entire part. If the test part wasn't reasonably successful, I still had the option of making the cams as built-up assemblies.
I started with a length of ground/polished 5/8" diameter Stressproof that I ordered from Speedy Metals. I bought enough (surprisingly inexpensive) material for two camshafts plus a spare for about $15 plus shipping.
On paper I divided the cam blank design into six 1-1/2" long segments, and I made six worksheets with all dimensions referenced to the same lathe z=0 point at the rear of the camshaft. I kept the pertinent sheet immediately above my lathe at eye level to help keep me focused. My plan was to chuck the workpiece into the 5C collet chuck in my manual lathe and incrementally pull it through the collet just far enough to machine a single section at a time. The run-out of my lathe's chuck and collet combination is pretty lousy at almost four thousandths, and so I used a reference line scribed along the side of the rod to maintain a consistent alignment as the workpiece was pulled through the collet. I was careful to always tighten the chuck using the same key socket, and I used a dial indicator to verify the runout near the far-end of the workpiece after each repositioning. After every other repositioning I also found it necessary to pull the workpiece out of the collet and the collet out of the chuck and clean the chips off everything before reassembling.
Even though it wasn't initially needed, I started out using a live center in the tailstock to prevent the workpiece from whipping around as more of it was pulled through the collet. The rear end of the cam required some machining for a sprocket drive adapter, and so this was done on the mill before starting the lathe work. Part of this machining included the reaming of a locating recess where the tailstock center would wind up, and so I turned a center-drilled button to insert into this recess. My live center has a run-out of 1-1/2 thousandths, but in this particular set-up the tailstock is used only for stabilizing the far end of the workpiece and, so there's no first order effect on turning accuracy.
As a precaution I decided to turn the bearings a thousandth under the diameter of the test rod that currently fits my cam block assemblies, because I didn't know what to expect for machining-induced warpage. The completed ten inch blank will be somewhat flexible, and with seven bearings supporting it, the assembly itself might tend to gracefully straighten out a slight bow without binding. I also added generous fillets to the blank on each side of every lobe in hopes of reducing the tendency of the workpiece to warp during machining. I had no science to justify this, just gut instinct.
Most of the machining between the lobes was done with a full-radius Iscar grooving insert (GIP 2.39-1.20). This .093" wide cutter has sufficient relief to cut side-to-side chatter-free in steel for a depth of cut up to .010". I used this insert on my 18 cylinder build, but being designed for larger lathes it was necessary to modify its toolholder to fit my 1/2" Aloris clone tool post. Combined with the lathe's power feed, I found this insert capable of mirror finishes in Stressproof. The only polishing I had to do was typically to remove a few tenths with 1000 grit folded paper.
The machining of the blank's numerous features was, as expected, pretty stressful and required a lot of concentration. I spent nearly ten hours bent over the lathe making the test cam blank. I had to spread the machining time over a couple days to accommodate my knees, back, and frequent bouts with brain fog. A real disadvantage of this single workpiece approach is that a momentary lapse of attention can spoil a lot of invested time. To be honest, I'm not sure I can come up a single advantage of this approach over a built-up version.
The TIR of the finished blank, measured at its center between two vee-blocks, was only 1-1/2 thousandths - a very unexpected surprise. The blank fit nicely in my cam block assembly and spun so freely that I was left thinking that the extra thousandth bearing clearance that I had added wasn't necessary after all.
I ran into an issue, though, when I installed the rocker arms. For purely cosmetic reasons I had increased the width of the cam lobes shown in the drawings. What I hadn't realized was that the cam lobes actually run inside the rocker arm slots for a portion of their rotation. After going back and modeling this section of the assembly I realized that the stock assembly had been designed for only .007" radial clearance between the peak of each cam lobe and a portion of the rocker body inside the slot. When I machined the cam blanks I left an excess radial stock of .010" on the unfinished lobe disks, and this created interference. As I discovered, there's very little leeway in the design of the cam block assemblies, and the tight clearances between its numerous parts needs to be carefully followed. I was able to salvage my test blank by re-machining the widths of the lobe blanks. Since these were facing operations, they didn't affect the TIR of the blank even though I no longer had a reference line and had to use a different collet for the re-work. I left the excess radial stock, though, for the lobe profiling operations later.
With the test cam feeling like a success, I continued on and machined two more cam blanks from the material I had left. For these 'production' blanks, I didn't add the extra thousandth bearing clearance that I had used for the test blank. The TIR of the next two blanks measured .002", and even without the extra thousandth bearing clearance, both blanks turned freely in both assemblies. Except for the bearing clearances of the test part, all three cam blanks are essentially identical, and so I ended with an unexpected spare. The next step is to figure out how to fixture the blanks so the lobes can be machined without screwing them up. - Terry
Are they kind of builders notes with pictures or set of plans only? Are you modelling in SW as needed or just when required? 
