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Preview: Drilling Holes with Boring Bars

July 26, 2012

Have you ever wondered how they bore a 10+ ft straight hole through a hull? Let’s take a trip to a few boatyards and let the the pros show you how it’s done.

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– [Voiceover] If you need to drill a long hole with precision, use a boring bar. This is a rigid steel rod with adjustable cutter protruding out, and at each end of the hole is guided by carefully placed bearings. The boring bar is made to spin between two bearings that hold it exactly in line, so the little cutting bit protruding out of the bar can scrape away the wood slowly, about 1/8 inch at a time with each pass. Boring bars come into play in finishing off the hole for propeller and rudder shafts. We’ll show this process on three different boats, the first one being the 83ft Fife schooner Adventuress at Rockport Marine.

– So what we’ve got set up here is a boring bar to drill the hole for our propeller shaft, through the hull. This is an offset propeller so it’s coming off the side of the hull instead of the centerline of the hull. And what we do is we start with an engine mock-up on the engine beds inside the boat. We know our angle off centerline and our angle down, from horizontal, and we drill a pilot hole through the hull in which we can run a string line through and get everything exactly where we want it. With that string line then we set up bearings that the boring bar will run in and get that all set up. So what we have here is, on the outside, is the outer bearing and then there’s another identical one on the inside. And this big pipe here is our boring bar. Here’s our cutting tip. This is, when you look at it, is pretty crude looking but it’s actually quite accurate. What we have is our cutter, which is mounted into a little piece of pipe that’s into our larger piece of pipe with a set-screw. So this, actually, this cutter can come in and out so when we start the hole it’s in quite a ways and you bore all the way through. And in this particular case we have about 28-in of wood we’re drilling through, and then once that’s cleared then we readjust and pull it out just another 1/16 or 1/8 inch, and then do it again, and it goes very slow because it’s only this little tip that’s cutting. And as you can see inside it leaves a pretty rough finish but it just scrapes and scrapes and goes all the way in and then out. And then once we reach close to our desired final diameter, we switch to a cutter that cuts a cleaner hole and sort of smooths it up. So now we’re on the inside of the boat and you can see the bar coming through. So here’s our inside bearing, identical to the outside bearing holding our pipe nice and true. He just finished that pass so that’s why he stopped. And you can see that we’ve made our cut through the inside. This is a backing block on the inside of the planking. This is an extra reinforcement for this frame because we are taking a bit of wood away from the frame. And then when this is all finished, there’s gonna be a bronze pipe that’s installed, that will fill this space and then the shaft will go through that bronze pipe.

– [Voiceover] On a smaller scale, over here at Brooklin Boat Yard, a similar boring bar setup cuts the shaft hole for the 26ft Gold Cup race boat Scotty 2 replica. The slim boring bar vibrates a little during it’s first few passes, so a small block of wood can be used to steady the spinning bar. And now we’ve moved over to Hylan & Associates and we’re watching Ellery use metal bearings instead of wood or plastic, for more precision and a faster-spinning boring bar. Molly B’s new backbone is built from an exceptionally tough-to-machine wood called Purpleheart. This calls for an extra-sharp cutter combined with shallow cuts made at high speed. As you’ve seen in all three examples big, portable electric drills power the boring bars, and it’s hard to beat them for convenience. To fit the drill’s chuck, the boring bar has to be lathe-turned at one end to reduce it’s diameter. Although no two situations are alike, the boring bar principle is the same. You need two bearings, the bar with it’s adjustable cutter, and the power to make it spin. Besides propeller shafts, boring bars can be set up for hose pipes and rudder stocks, and any other place where conventional drilling apparatus isn’t up to the task. So when you look at it, there’s really nothing boring about a boring bar.



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