Monday, September 6, 2021

Shop Update on the 1 by 1 - Part 7

It's hard to believe I spent the last 10 days since my update working on a part that looks much simpler than the side plates I just finished.  I've been working on the engine's crankshaft.  The crankshaft is the part of the engine that drives the piston up and down in the combustion chamber usually done by offset sections from the shaft.  This can get really complex in a multi-piston engine, but in a single cylinder engine like this one, they have a long shaft, with one smaller section offset from that which becomes the portion the piston rides on. 

This is the one I'm building.  The big trick is that these are made from a rectangular bar of metal, (1144 alloy stressproof steel in this case) that has to be turned on two different axes, the long section and the short section.  The two long shafts at the top and the small one at the bottom are all 0.375" diameter; the two rectangles joining them will remain rectangles.  The top half of the rectangular bar has a gap in it that's 7/16" across; the bottom half is nothing but gap - it's all cut away except for that inch (1.064") with a round bar in it.  Once that section is turned, the rest is cut off.

The first trick, though, is that 1144 steel is hard to find in rectangular bars and is more often sold as round (cylindrical) bars.  That bar has to be made into a smaller rectangle.  That task took up several hours last week.  The original plans suggested making the bar 1/32 (.0313) oversized on all four sides, but I opted for .025" The final rectangular bar has to be 1.000 by 0.500"; so mine needed to be cut to 1.050 by 0.550 and some experimenting in CAD told me that a 1-1/4" diameter bar would be fine. 

This is the bar after being cut to size.  By the way, I actually threw away more steel in those chips you see everywhere, 7.77 cubic inches, than I ended up with.  The volume of the 1.25" diameter, foot long bar to start with was 14.73 cubic inches.  The volume of the what's left was 6.96 cu. in. 

Once the bar is cut to size, I did the usual trim and de-burr of all the edges, and then put the two center spots in both ends that will hold the piece so it gets turned to the proper sizes and shapes.  This is how that was done - verifying zero on the left and after the work on the right.  It was just about all I could do with my mill to put the center marks on the ends of an 8" long bar. The head was just about at the top of its column.

My first step is going to be turning the offset little 3/8 diameter, 7/16 wide small section of the crankshaft.  How is that done?

The technique is called turning between centers.  Those marks I cut into the ends of the bar are exactly 0.500" inch apart and both of those are the centers the two sections of the crankshaft are turned around.  The lathe has tools called a live or dead center, each one of which will go into one of those cone-shaped holes. I wasted several hours to see if I could get by without a tool I don't have and found I really needed that, too. 

Faced with that, I decided to make my own version.  The tool is called a lathe dog, and it acts to transfer the power from the lathe motor driving the chuck by clamping the work and either having a pin or shaft that is straight and gets pushed on by bolt on the lathe, or it has a bent shaft that the lathe chuck presses on.  If you have no idea what I'm describing, here's a set you can look at.  Or you can see both kinds at tool seller MSC Direct.  I almost ordered that set from Amazon but (1) don't need a whole set, and (2) wasn't sure they'd fit on my lathe.  Besides, it would be next week before I could get them.

I had the idea I could make a decent lathe dog out of a 2" diameter piece of steel or aluminum, about an inch long.  Make a 1.5" diameter hole in the middle.  Cut away everything that doesn't look like a lathe dog.  My immediate problem was that while I had a cutoff piece of 2" aluminum, it wasn't long enough. I eventually found a piece of pipe in my junk stock that was smaller than 2" diameter but big enough to work, so today I made it into the lathe dog.  It's some sort of soft, gummy aluminum alloy, maybe the stuff they make shower curtains out of.   

I've done a little test cut with it and it worked fine.  It changed position a little, and if I had some steel or something that would resist stresses more than this one, I'd make a replacement.  Within a few days, I'll know if it held up to the stresses of making this crankshaft.


  1. The "Term Project" I had in the College machine shop was to build a little "paper punch" requiring a crankshaft.

    Took me two weeks to machine it!

    1. That makes you an elite machinist. Because we said so. ;-)

  2. Sometimes the metric system looks pretty good :-) I've seen many a failed project over a fractional mistake.

    1. Since everything I do on my mill is by CNC, I tend to make moves in multiples of .025. It's just easier to do the 25, 50, 75, 100 sequence than adding fractions. Those go to four places too often. Besides, the CNC controller doesn't handle fractions, only decimals. Everything I have is calibrated in thousandths anyway.

  3. I think I could (barely) understand this enough to see how it would work via manual machining. All I know about gcode is that there is something called gcode. There's a mental first step I haven't yet embraced enough to try 3d printing or modern cnc.
    I'm excited to see you DIY your own one-lunger, though. I have very fond memories of pumping an air handle for 10 minutes to start an old cast-iron beast in my first non-rowed boat as a kid.