Several years ago, I had a set of hammers which were bored correctly for the tenor, but were the wrong distance for the bass. So I plugged the holes in the bass, and put together a jig to rebore them on my ShopSmith. It was okay for that, but I began to think about how it could be better. Being able to bore hammers would be nice, but I live in an area where the cost of living is really high, and so I had to come up with something that would do the job so quickly that it competes with getting the hammers pre-bored.
Besides the diameter, there are three dimensions involved in boring hammers:
1. The length of the bore, from the tip of the hammer to the center of the hole.
2. The tilt angle up or down.
3. The tilt angle left to right.
For a set of hammers, number 1 only changes when going between different levels of cross stringing, once or twice at most. Number 2 does not change at all. Number 3 changes often, so that had to be accommodated. The important factor is that in order to keep 1 and 2 from changing when 3 changes, the drill bit must go exactly through the center of the hammer molding.
The second factor was the hammers need to be changed quickly.
The ShopSmith as a horizontal boring machine only needs the hammer to be held in place, which I can do with my hand. No clamps!
So I came up with this:
This is how it looks from the top looking down. The clamps hold the back stop and the length stop in place. The back stop can be adjusted for the angle (if the hammer is tilted up or down from the shank, as on some uprights) and the length stop sets the length of bore. At the boring point, the vertical center of the hammer is on the centerline of the table, which is also the centerline of rotation for the trunnion.
Since the hammer is resting flat on the table, I can hold in in place with my fingers, so the hammers do not need to be clamped, which is the main time savings.
The cutout in front is for when the table needs to be tilted back, like when boring upright bass hammers, or tenor grand hammers.
The center of the drill is at the height of the center of the hammer molding, which is half the thickness of the hammer molding.
The center of the hammer is at the center of the axis of rotation of the trunnion. The left-to-right boring angle is set by tilting the table at the trunnion. Using the scale on the trunnion, it is easy to adjust the angle to within 1/2°. The important part of the design is that tilting the table does not change the center point of the hammer, so the drill always goes through that center point.
To use it, set the stop for the length, and the back stop for the front-to-back angle, if necessary. I use the depth stop on the ShopSmith to keep from drilling all the way through upright hammers, and from drilling deep into the back stop when drilling grand hammers.
It only takes a second or so to change the left-to-right angle using the trunnion. When going from the tenor to the bass hammers, the length stop needs to be changed, as well as the trunnion angle.
Markings on the table for the centerline of the hammer and hammer length make set-up faster.If you need bigger pictures, they are here.
This is so easy to use that I have used it to bore individual hammers.
I made this at the time ShopSmith was upgrading their table system, so I found someone who had upgraded his, and got his old table assembly for $25. (More than he wanted for it, but it was worth it for me.) Everything else was made from scraps of plywood.
It may not be so easy to find the trunnions now, but I am posting this to show the principles. If I were to make something like this now, I would start by making the table that tilts on an axis half the thickness of a hammer above it. I think this could be built using a pair of good-quality door hinges. A trunnion to indicate the tilt could be cut out of wood. The table does not have to tilt very far. I would make the table slide into the drill bit, probably with some pillow blocks and rods, with appropriate stops.