Hi All,
Moving on with the stack casting machining as I had previously machined (radiused) the bottom stack flair to conform to the radius of the smokebox shell. However, I was not quite satisfied with the resultant fit between the two parts and I knew I wanted to have and should have a perfect fit here. So I re-set up the stack on the angle plate and remachined the radius.
In retrospect, there is a hard way and easier way to do this and of course I went ahead and did it the hard way again! I will describe the easier way if I ever need to do something like this again but I never describe anything which I have not actually done myself.
So, some "improvements" over the way I did it the first time.
Photo 129 shows the close fitting setting rig which was carefully lathe turned to be a push fit over the body of the boring head. I used the ring to dead align the body and the offset slide of the boring head to bring everything on to dead centre as this gave me an exact "zero" position when setting the cutter radius. The sixty degree turned plug seen in photo 130 fits the centre hole of the slide and is used to pick up the zero position (the exact end) of the 6" engraved rule as seen in photo 131. Good lighting and an eye loupe were used to pick up the end of the rule as seen in the photo. As this operation really needs 4 hands and I only have two, the 6" rule is first set flat to the table of the mill and two dead equal sized tee slot plugs were used to align the rule dead parallel to the tee slot. The rule also needs to be clamped the the table of the mill as it cannot shift or move at all for this operation to work. I do not have any digital aids so this is purely a mechanical setup.
The outside diameter of the smokebox shell was then carefully measured and the setting radius on the boring bar was 1/2 of this measured diameter which turned out to be 2-47/64" exactly (2.734") from the previously picked up "zero" position. The boring bar was placed in the horizontal cross slot of the boring bar slide, very lightly clamped and the tip of the boring bar set to this radius.
How is shown in photo 132. Using a square set from the front edge of the table of the mill (needed 4 hands again) hence the large steel block sitting on the blade of the square to keep it flat, the 2-47/64" graduation on the scale was picked up with good lighting and my trusty eye loupe splitting the graduation line in half. Lots of fiddling here so one needs both patience and determination and check, check, check. The smaller squared steel block seen in photo 132 rests against the outer edge of the blade of the square at exactly the split 2-47/64" graduation and the boring bar was then gently tapped on its end until the tip of the bar just very lightly grazed the end of the smaller block. Everything was then tightened and checked to see that nothing had moved.
Photos 133 and 134 you can ignore as it was used to find the centre of the saddle radius in relation to the stack bore as one has to be aligned (coincident) with the other and not offset one with the other. This has to be determined also as we are creating a partial arc here which is quite different (and much more critical) than using the boring head just to bore a circular hole. It really didn't work very well so in the end I had to rely on the cut and try method. So, as machining proceeded, I would place the smokebox shell against the stack and see how the two parts mated (or didn't) and make adjustments accordingly. I got away with it as the parts did mate perfectly in the end but it was mostly luck. However, there is a better way which I will briefly describe in principle.
This better way needs an extra mill accessory which you may or may not have (I have it). Once can employ a semi universal Brown and Sharpe type dividing head, the body of which will rotate from the horizontal to the vertical all in the same plane which is what is needed. The stack casting would first need to be carefully set in the 3 jaw chuck of this assembly and centered as close as possible to the bore or to the OD of the casting. It is not easy to do this because of its shape but, with care, it can be done. The assembly is first rotated vertical to machine the central bore. However, it is best not to rely on the units graduations alone for this critical alignment. I also have a ground test bar which can be held in a collet in the spindle of the mill to ensure verticality to a very high degree.
After machining (boring ) the central bore to size, the assembly is then rotated to the horizontal position for the machining of the bottom flair (radius) on the casting. This will ensure concentricity of the two curved surfaces. Many times I have unscrewed the chuck and the workpiece from a machines spindle and replaced it later without losing concentricity if your machine's spindle is in good condition. This way one can employ another ground test bar and indicator to ensure dead level without having to rely on the units graduations directly when conditions are critical. The pre setting shown in photos 129-132 still has to be done. This is what I will do in the future but I haven't actually done it to date this way so there may be hidden flaws; so beware, don't take my word as gospel.
Photos 135-137 show the results and I am extremely pleased with what I now see but it really was a very tense time.
to be continued.
