Only fools will go… step 7 - Case & Dial
The last installment finished with this image of the wax models for the case.
I was initially wary of casting after hearing stories of the problems of uneven shrinkage and air bubbles in the casting. But after talking to a local jeweler and the info from others on this forum at TZ I decided to take the chance. I had wax models made from my CAD files by stereo-lithography, 3.5% larger than the fact to account for shrinkage, and took these to a local caster who does a lot of jewelry work for casting in sterling silver.
This is what I received a couple of days later:
You can see the rough surface that comes from the stereo-lithography. This makes a lot of work to clean it up, but the process of stereo-lithography and lost wax casting is quite inexpensive in comparison with having a case milled from a block by CNC so for me that work is worth it. And yes there is uneven shrinkage and yes there are air bubbles in the casting, but it is all very manageable.
The case back is also a lot of work. Here are some pictures showing the evolution of the back from a piece of 1.5mm sterling silver stock to rough finished. The cross pieces are to position the movement and strengthen the top and bottom edges as I only have place for screws on the sides.
Now some drilling of the case, for the back screws and the strap spring bars.
I just can’t wait to put a strap on it and see how it looks.
To mill the opening for the front glass I made a jig to position the case and allow rotation around the centers of the top and bottom radii.
For the straight sides I simply clamped the jig to the XY table. For the arcs the jig is positioned by a brass axle and gently turned using a long handle to be able to accurately control the position.
Then comes polishing. Much more work than I expected. I first sanded to 600 grit, then used a blue silicon polisher in the Proxxon tool followed by jewelers rouge on a felt tool. I think that I will do a last step just before doing the protective plating with diamantine on a soft buff. But even without that the case shines really well. It is just about impossible to photograph ‘though, either just blaring white or reflecting the dark environment.
I made this dial in brass and then rhodium plated, but I am not satisfied with it. It is too shiny. I think that I will make another in silver to replace this one and finish it to a soft silvery white. But for the time being here are the steps of making this first dial and the procedure will be the same for the silver dial.
I am making the dial in 2 layers, a lower main dial and a second frame outside the area of the hands, which makes a second higher level. The main dial I want brushed radially from the two hand axles. The frame I will brush concentrically and engrave radial marks for each hour. I showed adding the dial feet to the dial already quite a while ago, which is the first step. I then did a rough polishing to remove most of the scratches from the dial surface. In retrospect I now see that this polishing was not enough. Under the right light it is still possible to see some slight parallel graining under the radial brushing.
Here is the main dial shellacked to a brass carrier with openings in the back for the dial feet and to allow the positioning needle of the lathe through to the dial itself for accurately positioning the dial. The brushing was done with a steel brush in a Proxxon tool while the dial plate rotated slowly on the lathe.
In order to be certain that the radial hour marks on the frame dial are positioned correctly I decided to make them on the lathe in a sort of modified guilloche procedure. I placed a formed graver sideways on the xy-carriage and cut by moving the graver to the front.
Finished main dial and frame ready for plating.
After rhodium plating, too shiny for my taste, unfortunately.
The decorated movement has now been running for a couple of weeks. I have been carrying it around with me in a movement holder in my pocket all this time to see how it holds up being moved around and with some shocks. It has been running at +3 seconds a day all this time. Sometimes one has to be lucky I guess. I haven’t touched the regulator even once since installing the balance.
And now for a first look at the movement in the case, still waiting for me to finish the hands and without the glasses front and back.
you have enjoyed this. Although it looks like I am just about done, there is
still lots to do to, finish the hands, steel movement parts need to be
finished, installing glasses, having case engraving done and last but not
least making a crown.
Copyright © 2006 Donald W. Corson