Offset Turning

As discussed in the last post, I need the beam engine cylinder head partially complete as part of my plan for drilling the various matching holes in the cylinder. This is a fairly straightforward turning of a 1/4″ projection on one end of a 5/8″ round bar. The fun part is that the projection is offset from the center of the bar by 3/32″. I thought it would be fun to show how I get the part set up in the lathe to achieve that.

Firstly, I blue and mark up the end of the length of 3/4″ brass bar. (It was actually .748″) Knowing the diameter of the bar, I measure the height of the bar sitting in a vee block on the surface plate. Subtracting half the diameter from that gives me the height of center.

I use the scriber on the end of the height gauge probe to scratch a line on the bar. I then rotate the bar and scratch a line twice more. If I haven’t messed up anything, all three lines should cross in the same place and that is the bar center. I then scratch a line 3/32 below center as perpendicular to one of the center lines as I can manage and center punch that point.

Now comes the magic part. I put the bar in the four jaw chuck and I set up my wiggler in the tail stock with the point in the center punched mark. I then adjust the chuck jaws until the pointer stops moving when I manually rotate the chuck. (Some people can just put an indicator on the bar and move the bar via chuck adjustment. I one try at that was an unqualified failure. This method works for me.)

Lastly, I set a dial indicator against the pointer and and refine things until it shows no movement.

Visually, success is demonstrated when I power up the lathe. The center mark should not be moving even as the actual bar gallops around. This is the ML7 going flat out at about 1100 RPM. It really is in focus! 🙂

Lastly, here is the finished product. Brass is like aluminum in that is comes out nice and shiny and looks really good.

I just need to drill the appropriate hole through the projection, and then I can re-chuck the bar in the 3-jaw chuck, turn it down to 5/8″ and part it off.

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