Chapter 29.1 – Steam Dome On this engine, the Steam Dome's main goal in life is to cover the throttle and safety valves. It is made up of two major parts; the saddle, and the dome itself. And for the past several days I’ve been working on the Steam Dome Saddle. Not dramatically different than the saddle for the smoke stack, but it’s quite a bit bigger, which makes it more challenging. For one, it’s harder to hold on to. It’s a big, relatively thin round part. I spent some time trying to figure out how to do most of the work while it was still attached to the parent stock, but then I couldn’t figure out how to hold it and finish the backside at the same time. Actually, I could figure it out, but it didn’t do any good to have additional stock that needed to be cut off because I'd still have to hold the skinny puck to clean up the backside anyway, so it didn't buy me much. Consequently, I just decided to slice off the puck and go for it. It’s a 2 3/4" diameter by ~1 1/4" thick slide of 12L14.
The best method for holding that I came up with was the 4 Jaw. Luckily, one end was very perpendicular to the sides, so I used that to set things up against. I should have made a nice lathe spider to simplify this setup, but I didn’t. I used a stack of various parallels between the face of the chuck and the back of the part during setup (held in with some tape, so they wouldn’t fall out as I rotated the chuck). This worked to get things centered up and tightened in the chuck. Once that was done, I took out the stack of parallels before turning things on. Then I faced the end and turned the first feature on the saddle. This will become the ring that the steam dome registers on eventually.
Next, I drilled out the center in steps, up to 1” (my largest drill).
Then bored the hole out to 1 15/32” diameter – just large enough to slide over the valve body.
The last thing I needed to do in this setup was to make a 5/16” radius profile around the top edge of the saddle. My original idea was to use a 5/8” diameter ball end mill as a form tool. I tried this, but it was just very hard going. The ball end isn’t really a very good form tool. The cutting edge around the nose of the mill isn’t flat, it varies. Here’s a pic of the tip of the 5/8” ball end mill. You can see that the business edge of the mill isn’t flat, like a good form tool should be so the whole edge can’t be on the centerline of the cut. I set it at a bit of an angle to try and average the height difference so some would be a little above center and some a little below. I made a red line just above the cutting edge to highlight my dilemma.
When it became clear this would be too hard to do I made a step-off chart and approximated the 5/16” radius pattern, like so:
I tried the ball end mill again here, thinking that with most of the heavy cutting out of the way, I could use it to clean up the stair steps that were left. I used my fat blue Sharpie to color the whole thing so I could see how I was doing. However, even this proved to be too much for my poorly-shaped-make-do radius form tool. Guess this is just too much to ask of it, eh?
So, I went old school and used some files to clean up the stair-steps. At least a some. It’s not super clean but I’ve got a ton of filing on this part in my future anyway, so I’ll clean it up then.
The next setup was to reverse the part in the 4-jaw. I used the same “stack of parallels” trick to make sure the part was square with the jaw face and centered it up using the inside of the bore as the reference. Then I faced off the back side to get the part to the correct thickness of 1 - 1/16”
Finally, I bored out most of the hole another 1/16”. The boring went really well and I was getting a finish I was very pleased with. However, about halfway through the finishing pass a chip must have gotten stuck on the cutting edge because there’s a weird streak on the bore. You can barely feel it, but you can sure see it! It won’t matter in the slightest, but it made me kind of sad anyway.
That completed all the lathe work I could do using these setups.
Next, I’m going to have to make a faceplate setup to turn the inside face of the saddle. Last time I did this (for the smoke stack) I set it up on the rotary table and did it on the mill. Unfortunately, this part is too deep for me to do that way. I don’t have a mill that could reach all they way across it. The longest mill I have is just over 3” and I need a 2 3/4" reach! So using the boring bar on the lathe seems like the right way to go. And it will be a new experience for me since I’ve never used the big faceplate on the lathe before!
But before I can do that, I have figure out how to set it all up. So that’s the next task.
One of the things I need is a nice little plug that will fit in the hole on the saddle to bolt it in place. So I made that. My intention was to make it from some 12L14, but what pulled out of the 12L14 pile seemed more like cast iron. It turned easily but didn’t make chips like 12L14. It made little chips – almost dust much of the time – more like cast iron would do. Plus (and this should have been my first hint) the bar was a bizarre size. Bigger than 1.5”, but not 1.625”. It was something like 1.578” to 1.582”. Not very even round ether. So I think it was a cast bar (i.e. cast iron). So I relabeled it when I put it in the CI pile when I put it away.
Anyway, here’s the mounting plug for the saddle, just as I’m cutting it off.
I did check the size and even did a test fit on the part, but I was going for a nice close fit... I guess I got it too tight. I couldn’t even get it to fit all the way through the hole.
So, I used my little taig 3 jaw to hold the plug and carefully touched it with a file to shave a thou or so off.
And now it fits very nicely. Still a very good fit, but I can get it in and out (with some work).
Here’s what it looks like from the top.
So that brings my build up to date. Believe it or not, that’s what I’ve gotten done this week. Doesn’t seem like much, but it’s taken me some time to get here.
The next step will be to figure out that faceplate setup. I’ve got a picture in my mind, I just have to get there. And yes, it’s going to look a lot like the setup Kozo suggests, so don’t be too surprised! But even at that, I have to figure out how to get it all held in place at the right spot on the faceplate. So that’s my next challenge.
Thanks for looking in.
Kim
