Crankshaft Part III
All right, so I made it out to the shop today and made some progress on the little beast. After the last episode of cutting too much off of the crankshaft, I decided that I would do my homework and have a procedure written down and follow it during the next attempt (this one). I also found a couple of errors in my approach that would have bit me later.
PPP = PPP P_$$ Poor Planning = P_$$ Poor Performance
I checked the plans against the actual measurements on the crankcase and bushing. What I discovered is that the thrust face needed to have .032" taken off. I chucked the crankcase in the lathe and checked the back for run-out. First time was a charm! Cutting the bushing was straight forward, and it wound up dead on.
Checking the run-out on the crankcase with a dial indicator
Next up is machining a fixture to hold the crank for machining the business end. Maryak, I ripped this idea from your project! Hope ya don't mind!
The reason I chose to do this was to protect the newly polished main journal. My first thought was to use the four jaw to get the offset, but after seeing the way Maryak did his crank, I decided to have a go at it myself. The fixture only took about ten minutes to bang out, and I was on my way.
To make the fixture, I chucked up a piece of 1.750" aluminum bar stock about 4" long and faced the end and center drilled a locating hole. I flipped it and faced the opposite side. The piece was taken to the mill where it was squared up in the vise and the center was located. I then move the table the distance needed (radius of the stroke) and drilled/reamed a hole for the crankshaft main journal to be inserted. I then drilled a 1/4" opposite of the center-line through the part to relieve the stress from flexing during clamping. A slitting saw finished the fixture up after flipping it on its side in the vise.
In the three jaw chuck, it is hard to have repeatability due to the run out they typically have. To reduce the problem, I have identified each of the jaws on my three jaw with a different color on each. If I have a need to remove a part and put it back for further operations, I just make the part with the same colors at each jaw and then when it goes back I can have it clocked in the chuck as it was previously. In my experience with my chuck, I get about .001" run-out when doing this. The crank fixture was identified this way and I did a test run on the previous crank we'll call Shorty. I chucked Shorty up and did a cut to make sure the fixture would indeed hold the crank solidly,and also pulled it out and chucked it back up several times to verify the run-out. It was either dead on or at the worst .001" TIR.
This might be a trick to save some time with changing between the three and four jaw chucks. Keep in mind that it doesn't eliminate the run-out issue, it just keeps it close. If your task requires a high degree of precision, then I suggest you use the four jaw for those purposes. Use this trick at your own risk!!
The trial run on the crank fixture using Shorty
Next is making the infamous crankshaft. Fortunately, I have plenty of material and should a fourth, fifth, or even sixth attempt need to be taken at getting a good crankshaft, I wouldn't have to buy anything. And off we go. I chucked up a piece of bar stock in the three jaw and faced the end and center drilled the nose. I went ahead and drilled/tapped the hole (1/4 x 24) while I was there.
Next I started the mass removal of metal. It took moderate cuts until I got about half way there, then I progressively used lighter and lighter cuts as I approached the final dimension. This was to avoid any chance of warping the crank from too much force from the cutting tool and the reduce the amount of heat being generated in the part. I was concerned about a potential for warp-age, so I stopped about .050" from the final dimension and let it cool off for a few hours.
Turning the nose of the crankshaft
After the part cooled off, I finished the cut to .002" oversize. I employed the hillbilly tool post grinder to take the part to the final dimension (actually about .0002" oversize). I then polished the main journal with 800 grit Emory cloth and oil.
The hillbilly tool post grinder
This has worked out pretty well for getting a good finish on a part. A couple of things to keep in mind if you try it... Make sure you dress your stone before grinding the part. They are not very concentric! And don't try to take a heavy cut. The shaft on the bit will flex and the Dremel tool doesn't have enough torque to turn the stone with a heavy bite. I have found that .001" - .0015" is about all it wants. And protect your machine! Butcher paper is what I use.
So far so good. I was at the same place last time when things went awry! :
The next step is to cut the stock off and put the nose in the fixture for the next operations. And, this time, I got it right!
The crank was placed in the fixture and chucked up in the lathe. I carefully faced it off and then drilled a locating hole for the live center. The business end was then roughed out to size, but this was a slow go. I didn't want to take chances of the crank moving within the fixture and making my next post
Crankshaft IV. So light cuts were the order and I eased it down close to the dimension.
The crank in the fixture backed up with the live center
She's slowly getting there
Now we're real close, just need to do some final shape cuts and finish it up
And then Mr. Murphy makes his appearance... As I am making my shaping cuts, I notice that the live center is dancing very slightly back and forth. At this point I know the the crank has moved in the fixture. So I stop and take a break before exploding into a ginormous mushroom cloud. As I was contemplating which one of you were about to be the recipients of a mill and lathe, the "You Dumb-***" fairy popped in my head... All I had to do is loosen the chuck, then relocate the crank pin back to center and all would be well.
That's exactly what I did and moved on. I didn't have any more issues with movement going forward. The finish cuts were made and the rod journal was polished. I drilled the hole in the rod journal for the pin that will turn the rotary valve and verified all of the measurements. I missed the thickness of the counter weight by .001". Everything else hit dead on.
Success! Here is the crankshaft with the crankcase and cylinder
And here is the Shorty with the new crank
So that conclude this episode of
Crankshaft Part III
Next up for the crankshaft will be milling the angles on the crank disk, but I believe that will wait until I get the connecting rod and piston made. I am planning on doing a proper balance job on it. And I am looking at the rod journal throw and am thinking that I will be needing to notch out the crankcase to make way for the con rod.
It doesn't look like much, but that was a day!! Everyone's support and comments are appreciated. Finishing this piece today was a milestone, and I fell like I have now made it to the top of the mountain and will have some momentum to carry through the remaining part of the project. Stay tuned!!