Whew.... its been a good week. After visiting with John, Tom and John, the newly found gang of local model engine machinists, I was chuffed when got back in the shop on Thursday and began to tackle a job I'd never done, using some tools I hardly ever use and some new tricks yet to be discovered.
I needed a crank shaft and this Duclos guy said it needed to be a solid one piece version. To make things even more challenging, I've decided to add a couple of items to his design, one of which the motor will need to drive. This meant instead of ending up with an abbreviated "L" shaped crank, I'd be making a more difficult but more conventional -U- type with a one inch throw. Did I mention that one length of the shaft on this thing is nearly a full 6 inches long or that it has to align within .001 across 3 different bushings?
Since I had exactly no hot roll flat stock, I wound up using a 1 inch piece of Drill Rod....Silver Steel, for you UK guys. The long shafts would have to be turned to .375 +.002/-.000. so we're talking seemingly endless interrupted cuts over a nearly 10 inch steel rod, with no going over the mark allowed. No pressure..... for a first time attempt..... eh?
After a bit of head scratching, I broke out the dreaded 4 jaw chuck. In fairness, I only dreaded the 4 jaw because it was so difficult to change over when I was using the mini lathe. I'd learned any number of work around tricks to avoid the hassles of installation. The new C4 lathe has adequate room to easily access the chuck so that excuse was no longer valid. I decided to conquer my bias and get used to using the darned thing.
It took a bit of experimentation to get a feel for how much the jaws moved when adjusted, but my trusty Dial indicator and DTI soon had the 1 inch Drill Rod centered and then properly offset for cutting the specified dimensions. The cutting began with the long 6 inch shaft section. I've seen photos of any number of crashed crankshaft projects, so I wanted all the support I could get on the chuck end of things. That long shaft looked much longer when the cutting tool began to draw first blood.
Interrupted cuts can be a little unpredictable, even when working in close to the chuck. Step out a ways from that safe haven and things can go pear shaped real fast and real bad if you get too aggressive. I kept the cuts fairly light until the interrupt was down to a minimum. I can tell you there was a sigh of relief when the cutter stopped thumping and began cutting a nice smooth 1/2 inch round shaft.
It's always good to spot a moment where things are still fixable. This photo shows just such a moment. With plenty of metal yet to be removed to reach the .375 goal, I stopped off the machine and broke out the Micrometer to check the setup for any taper. All was well with a measured taper of .0013. Still time to make a correction if needed, however I chose to leave the problem for the polishing stage that would come later.
Once the long section was turned down to dimension, the short section near the chuck was undertaken. It was a much more stable work zone and the cut went quickly. I learned that the dials on this lathe are a lot more trust worthy than my smaller one. Less backlash and the additional mass greatly add to the confidence levels.
Once removed from the lathe, a quick trip to the bad saw removed the large end pieces so the shaft could be test fitted to the Engine body and the support. This also allowed me to prove my modifications would still work in the blind well described earlier in thus thread. The good news is all things are as the should be. The crank lobes have close but adequate clearance and the two bushings are dead on.
The next cut began with me holding my breath. The interrupted cuts were taken in light passes until things began to decrease in violence. The thin shafts were not making me feel any better about my chances of success, but I kept cutting, thinking each pass could be the one that turned things into a pretzel. Here is my solution to the problem.
Since the shafts on this crank were so long, I chose to use a trick I learned on this board some time ago. Rather than try to trust the project to the 4 jaw and end up with no way to support the long section, I made up an offset turning jig so that I could work close to the chuck and still keep the other end supported. this would give me a bit more stability which would hopefully prevent a crash and burn. I would be working within a confined space, not too different from doing a cut off and making some potentially nasty interrupted cuts with a lot less metal keeping things together.
The jig was made on the mill and the hole was placed at .500 from the center. The screws were added to lock things down and worked like a champ. The long end was supported with a similar jig which I center drilled to give the center a purchase and prevent any chance of the end whipping about under the stress of the thumping and bumping such a cut creates. The screws have small brass tabs to prevent the shaft from being scarred.
Once again the loss of the infernal thumping signaled that the cutter was now past the gap and cutting a nice smooth round. The sense of relief was strong as I began to move in on the desired dimension of .375.
A bit of filing edges and some clean up on the lathe were then the order of the hour. The crank looked cool inside the engine body and proved to be completely usable as modified.... so far....(grin).
Steve
