A visual tour through the process and methods used
By Bob Caletti
This project turned out to be quite a challenge but was very intriguing and rewarding. I thought it would be interesting for others to see what is entailed in a project like this and what new methods can be used in comb making or repair. I think the wire EDM (electrical discharge machine) process has other possibilities that could also be used in making a comb.
This is the music box as received.
Above is the base end of a comb that was damaged by a run. Evidentially the current owner’s father was making some adjustments to the governor when the spring was wound up. The current owner is the daughter and was in the other room when she heard a big bang and some expletives coming from her father. They thought that was the end of their music box that had been in the family for quite some time. She contacted me to see what could be done. I had repaired some other combs using the wire EDM process, but none as extensive as this one. I was willing to give it a go.
Here I am optically measuring the comb teeth centerlines and teeth edges using the digital readout on my milling machine. The teeth width changes as you move down the comb with the treble teeth being narrower. The best data for comb tip locations is from the scribe lines on the cylinder.
This shows how I optically measured the cylinder tracks scribed on the cylinder. These scribe lines are the most precise way to determine tooth locations.
This shows the digital readout on the milling machine used to display dimensions.
The cylinder box comb dimensions were put on this spreadsheet. The image at left is only a partial view of all the data collected. Here you can see the accuracy of the comb. This is a good check to make sure that no errors were made in determining the locations and teeth edges.
The comb tuning was measured before the damaged section was removed for tuning reference. Retuning a comb with so many teeth missing can be a real challenge.
This is a programmable heat treat oven used to harden the comb blank. A2 tool steel requires a very sophisticated heat treat profile. Also shown is the comb blank inside the fix- ture that is used to prevent distortion. The comb blank was wrapped in stainless steel foil to prevent oxidation. Note the large amounts of oxidation on the fixture plates.
This is the machined A2 air hardening tool steel comb blank and spacers after heat treating. Also shown are the fixture plates used to maintain flatness during heat treating.
Here is the CAD drawing used for wire EDM cutting of the comb section.
This is a picture of the wire EDM used to cut the hardened comb blank. Wire EDM is a process that uses an electrically charged wire to erode the material with extreme precision. It is especially suited for hardened materials and is used to make precision tooling.
Pictured is the wire EDM with the finished comb blank. The cutting process is done underwater and is very accurate with no burrs. Accuracy is on the order of 0.0001 inches; overkill for the comb, but it is distortion free. This machine can run a lights-out operation which means it can run at night automat- ically without anyone there.
Here is a different wire EDM used for making wire damper holes in the comb teeth anvils (.025 inches diameter).
This is a closeup of the wire EDM used for making holes in the comb teeth anvils.
Shown here is the original damaged comb section on left and bottom view of new comb section on right.
The new comb section is glued on to the base with high strength epoxy.
Here we have the original comb base with the damaged comb section removed and ready for the new finished comb section.
Shown is the finished comb with original section on the right and new comb section on the left (base end).
This is a view of the comb showing damper wire holes that were made in the anvils using the wire EDM
This is a view of the bottom side of finished and tuned comb.
Here is a view of the finished music box with the new comb section on the left (base end) not to be confused with the separate bell and drum comb section at the very end.
Now you see the completed music box.
The case was also refinished.
The original tune card was badly damaged. Here is the tune card after restoration (digital photo repaired
and reprinted).
Another part that was needed for this music box
This project gave me a perfect opportunity to use my new water jet cutting machine to make a male Geneva stop. My water jet is a bench-top machine called a Wazer Water Jet. I had a CAD drawing for a different size male Geneva stop that could be scaled to fit other music boxes because the geometry is the same. I took a measurement, scaled the part, and cut it on the Water Jet. It worked well, so now I know I can make any size Geneva if needed. I can also see many other opportunities for this water jet machine for making missing parts of all kinds in all types of materials.
A new male Geneva cut on the Wazer Water Jet Machine.