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Hammer Drilling Jig
#3000885 07/10/20 12:15 AM
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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:

[Linked Image]

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.

[Linked Image]

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.

[Linked Image]

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.

[Linked Image]

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.

[Linked Image]

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.


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Re: Hammer Drilling Jig
BDB #3000922 07/10/20 03:31 AM
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Very interesting, BDB!

Re: Hammer Drilling Jig
BDB #3000961 07/10/20 06:26 AM
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thanks, BDB

Re: Hammer Drilling Jig
BDB #3001022 07/10/20 10:05 AM
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BDB,

Nice work! Yes the centering is crucial. Nice fix on that.

Though I used to bore my own hammers I almost always use Ronsen hammers and the cost of having Ray Negron do it for me is insignificant in comparison. If I bore them, then I have to taper them too and prepare the tails myself, and I pay extra to do special stuff to the tails. He does an accurate job according to the specs I send him. Well worth it to me.

Peter Grey Piano Doctor


Peter W. Grey, RPT
New Hampshire Seacoast
www.seacoastpianodoctor.com
pianodoctor57@gmail.com
https://www.youtube.com/watch?v=PK0T7_I_nV8
Re: Hammer Drilling Jig
P W Grey #3001039 07/10/20 10:56 AM
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Shaping and tapering the tails is easy on a ShopSmith with the sanding attachments. As I said, I wanted to bore the hammers quickly enough so it competes with the supplier boring them. It takes less than an hour to drill a set with this jig, and the results are more accurate than some of the hammers I had received in the past. The hardest part is keeping the hammers in order!


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Re: Hammer Drilling Jig
BDB #3001113 07/10/20 01:45 PM
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Thanks

Re: Hammer Drilling Jig
BDB #3001284 07/10/20 07:25 PM
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Originally Posted by BDB
The hardest part is keeping the hammers in order!

Yes, I pick out what extras I need to and then number them straight out of the box. That helps a lot when they end up all over the floor by accident.

Peter Grey Piano Doctor


Peter W. Grey, RPT
New Hampshire Seacoast
www.seacoastpianodoctor.com
pianodoctor57@gmail.com
https://www.youtube.com/watch?v=PK0T7_I_nV8
Re: Hammer Drilling Jig
BDB #3001288 07/10/20 07:36 PM
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I cut the boxes they come in down the middle, lengthwise, and sort them by sections (bass, tenor, low treble, high treble) into those boxes. It makes them easy to grab. I put them into one end of the box, and put them back starting at the other end. That also helps me figure when to change the drilling angle.


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Re: Hammer Drilling Jig
BDB #3001420 07/11/20 09:36 AM
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I will note that every factory boring jig I have seen has been a horizontal slide.


Craig Hair
Hampshire Piano
Chesterfield, MA
Conservative Piano Restoration
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