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Over the past couple of weeks I've printed a few things that turned out to be mostly just for fun. This is one of them.
This sad, and slightly blurry speciman is what's left of what was originally the cap for a salt and pepper shaker set. My wife and I had that set since we got married and that would have been 43 years ago. During the autopsy the cause of death was determined to be tissue rejection. This was a Miller High Life press on cap that was living on a 7 ounce screw top PBR bottle. I guess it was only a matter of time until the cap succumbed.
But.... I have a 3D printer. I can rebuild him, better, stronger, faster - well better and stronger at least. A multi-start internal thread in the cap of unknown dimensions... How hard can it be? I can measure the pitch between the threads. I can measure the OD of the bottle above the threads - it's 25.5mm. I know that there's a section at the top of the bottle that's unthreaded. I know how tall I want to make the cap. I know that the threads rise 5mm from start to finish. I know that there are 4 starts to the threads - I counted'em on my fingers to be sure. I know that the major dimension of the threads is 27.5mm. I know that the threads rise 5mm in 90°, 4 starts so it's gotta be 90° right? That means they have to rise 20mm in a full turn, therefore I am making an M27.5x20 four start thread - piece-a-cake. It'll be fine.
This was my first attempt, looks pretty good doesn't it?
BUT, it didn't fit. Why not? Well I'm so glad you asked. Somebody, and I'm not going to name any names - but we are VERY closely related, used the minor dimension of the threads as the ID of the cap - then projected the threads inward. Definite whoopsie. Second attempt looked pretty much like the first. Except this time the un-threaded portion of the cap actually fit over the threads like I wanted, but the threads wouldn't screw together.
Wonder why not? The bottles got 4 starts, my cap's got 4 starts. The unthreaded portion of my cap fits over the threads like I wanted it to do. The threads on the bottle only go part of the way around - same as my cap. Ummmm... shouldn't my threads sorta resemble their threads? Aw CRAP! The duration of each thread is 120°, not 90°. Their threads rise 5mm in 120°, while mine rise 5mm in 90°. That means the threads should been M27.5x15, not M27.5x20. (The picture of the threads didn't turn out - SOMEBODY shook more than the camera could compensate for.) Ok, after the third or fouth attempt - who's counting anymore - I had a cap with printed threads that looked like this.
To end this long-winded story on a happy note, here's a shot of the PBR bottles proudly wearing their new screw top caps. YUP, one of the labels has a boo-boo - but I haven't been able to find a replacement 7 ounce PBR label of that vintage yet.
Don
This sad, and slightly blurry speciman is what's left of what was originally the cap for a salt and pepper shaker set. My wife and I had that set since we got married and that would have been 43 years ago. During the autopsy the cause of death was determined to be tissue rejection. This was a Miller High Life press on cap that was living on a 7 ounce screw top PBR bottle. I guess it was only a matter of time until the cap succumbed.
But.... I have a 3D printer. I can rebuild him, better, stronger, faster - well better and stronger at least. A multi-start internal thread in the cap of unknown dimensions... How hard can it be? I can measure the pitch between the threads. I can measure the OD of the bottle above the threads - it's 25.5mm. I know that there's a section at the top of the bottle that's unthreaded. I know how tall I want to make the cap. I know that the threads rise 5mm from start to finish. I know that there are 4 starts to the threads - I counted'em on my fingers to be sure. I know that the major dimension of the threads is 27.5mm. I know that the threads rise 5mm in 90°, 4 starts so it's gotta be 90° right? That means they have to rise 20mm in a full turn, therefore I am making an M27.5x20 four start thread - piece-a-cake. It'll be fine.
This was my first attempt, looks pretty good doesn't it?
BUT, it didn't fit. Why not? Well I'm so glad you asked. Somebody, and I'm not going to name any names - but we are VERY closely related, used the minor dimension of the threads as the ID of the cap - then projected the threads inward. Definite whoopsie. Second attempt looked pretty much like the first. Except this time the un-threaded portion of the cap actually fit over the threads like I wanted, but the threads wouldn't screw together.
Wonder why not? The bottles got 4 starts, my cap's got 4 starts. The unthreaded portion of my cap fits over the threads like I wanted it to do. The threads on the bottle only go part of the way around - same as my cap. Ummmm... shouldn't my threads sorta resemble their threads? Aw CRAP! The duration of each thread is 120°, not 90°. Their threads rise 5mm in 120°, while mine rise 5mm in 90°. That means the threads should been M27.5x15, not M27.5x20. (The picture of the threads didn't turn out - SOMEBODY shook more than the camera could compensate for.) Ok, after the third or fouth attempt - who's counting anymore - I had a cap with printed threads that looked like this.
To end this long-winded story on a happy note, here's a shot of the PBR bottles proudly wearing their new screw top caps. YUP, one of the labels has a boo-boo - but I haven't been able to find a replacement 7 ounce PBR label of that vintage yet.
Don
