I carved the bell out of a chunk of 3 1/4" diameter 303 stainless bar. I spent a few days working up a spreadsheet step-off chart to shape the outside curve of the bell and the inside curve of the bell. I chose to make the shape a section of an ellipse, since that looked most pleasing to me. And for the inside, I made two tables. The first for a rough cut with a boring bar, and the second for a smoother finishing pass with a 5/8” radius tool (I used a ball nose mill as a round boring bar).
Here I’m partway into the outside shaping. I used my modified parting tool for this. It was slow going. I only did 5-7 thou cuts (10-15 thou diameter) on each pass. Stainless is much harder to work with than brass. Who'd a thought?
Here’s a short excerpt of my step-off chart. It was many pages long. Note this is the “New” version, which fixed some errors in my original version. The inside curve charts came in “New,” “New Improved,” and “New Fixed Improved” versions
Here’s the completed rough shape of the outside of the bell.
Next, I blued up the outside of the bell and used some files and sandpaper to make the steps disappear. Unfortunately, this didn’t work as well as I’d hoped, as I just couldn’t get into the deeper center part of the top of the bell. I knew I was going to have to figure out some way to polish up the very center part after I cut it off, but at this point, I realized I’d have to use that method for the bulk of the outside of the bell.
Here I’ve completed boring out the basic inside shape. I kept this pass a few thou larger than my final pass. You’ll also notice a bizarre anomaly about two-thirds of the way toward the center. My insert had chipped and was making a terrible mess of things. It took me a while to figure that out. I was working from the inside out, as you usually do when boring. So the rough finish was deep inside the hole. Very hard to see what's going on down there. But once I realized what was happening, I put a new insert in place and continued. It worked much better with the new, sharp insert. I decided not to go over that lower part again, since I will be doing a second pass with the 5/8” end mill in just a bit anyway.
Also, you’ll notice that I fixed a rag around the bell in most of these pictures. Once the bell started taking shape, I was getting a TERRIBLE ring! Go figure!
But putting that rag in place helped immensely.
For the final finishing pass, it took some work to get the position of the 5/8" ball nose mill set properly, but I eventually did it. I used the 5/8" ball nose because that was the largest radius mill I had. Using the ball nose mill as a boring tool actually worked pretty well.
This picture is of about a fourth through the step-off chart. You can see the round lip indicating my progress toward the bottom.
Now the full pass with the round tool is completed. It is actually pretty smooth. Which is good. It’s hard to get a file in a deep concave hole like this. Which is why I chose to use the round tool for the final pass.
And after significant work with many grades of sandpaper, the inside of the bell looks respectable!
After that, I finally parted the bell from the parent stub. Then I made a little mandrel to hold the bell so I could work on the outside. This worked very well. I was able to get to almost the whole thing (except for the tiny bit under the head of the #5 SCHS that was holding it in place.) And after a lot of polishing, it looks pretty tolerable. It’s not Clickspring level work, but it passes my QC department's review. And fits (barely) within my level of patients .
Here’s what it looks like mounted on the clock:
Now I’m on to the strike works!
Kim
