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Old 03-20-2017, 06:58 PM   #1 (permalink)
 
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Default Purpose of Multi start threads?

So, I've looked through the library and searched the forums. I don't exactly know the benefits of multi start threads. Saw the article from Skiprat on how to cut them on the metal lathe but that really doesn't explain why they are better.

I know they cut multiple threads at once when making them (or I think they do...). I know they are primarily used on the cap to body threads and no where else (usually). That is all I can gather.

I know and like a minimum amount of twists to secure the cap on the body as well, but can't you just achieve this by shortening up the threads on the inside of the cap?

Sorry if this has been beaten to death, but looked and searched and couldn't find anything.

Lewis
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Old 03-20-2017, 07:40 PM   #2 (permalink)
 
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So I found this diagram on the web. Seems to indicate that the threads are stronger and the cap will turn farther in one turn. I don't see how the threads are stronger though. Is that correct?
Multi Start Threading
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Old 03-20-2017, 07:59 PM   #3 (permalink)
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One turn of a single thread has only one thread engaged. A 1/4 turn of multi-thread has as much or more holding strength.

And you are correct. Multi-thread allows the capping and uncapping quicker. Who wants to turn the cap 3 or 4 revolutions to put on and take off. Multi- threading allows the same to be done in 1 turn or so.
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Old 03-20-2017, 08:43 PM   #4 (permalink)
 
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Also great if you are trying to line up a grain pattern or a segmented pattern on both the cap and barrel. Gives you a better chance of doing this.
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Old 03-20-2017, 09:06 PM   #5 (permalink)
 
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I've been wondering how to do that on kitless. Haven't used any material with grain lines on both cap and body for that reason.

OK, one more reason for multi start. Have to keep my eyes peeled for a group buy or classifieds section!

Lewis
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Old 03-20-2017, 11:10 PM   #6 (permalink)
 
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Lewis,
It's actually easier to end up with a predictable cap-body alignment using single start. Something--usually a shoulder on the barrel that meets a shoulder in the cap, or a shoulder in the cap that meets the nib end of the grip section--stops the cap from turning any farther. By trimming that something back a little at a time, you can adjust how the barrel and cap mate up radially. With multi-start threads, you have a 1 in 3 or 4 chance of getting it right when you put the cap on. Brian Gray at Edison Pens has a couple of lines of pens with silver overlay on both the cap and barrel and those pens all have single start threads.

Hope this makes sense,

Bill
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Old 03-21-2017, 01:15 AM   #7 (permalink)
 
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The other consideration not mentioned is that having the cap go on faster also means that it comes off faster , frequently in your pocket , with predictable results .
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Old 03-21-2017, 03:11 AM   #8 (permalink)
 
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I believe it's all about physics. A multi start thread of say two threads has a lesser helix angle than a single thread. Ditto for a three or even four thread. The helix angle is in affect, a wedge. Imagine a wedge under a door. A sharp angle wedge will easily be pushed under the door and slip under with less required force than a wedge with less angle. For arguments sake, lets say the average person applies X amount of torque on any cap. That amount of torque on a single thread would easily split the cap on a traditional old pen cap. The same amount of torque applied to multi threads has less and less potential to crack a cap, the more threads there are. Unfortunately 4 threads seems to be the limit as it simply comes off on its own.

Now let's take it to the extreme. Add so many threads that the helix angle is reduced to zero and what do you get? You get a splined shaft. It can handle loads of torque because there is no wedge, but of course would be useless in keeping the cap on...
The fact that even numbers align is a lucky coincidence.
Anyway, that my theory and I'm sticking to it.
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Old 03-21-2017, 03:47 AM   #9 (permalink)
 
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Quote:
Originally Posted by skiprat View Post
I believe it's all about physics. A multi start thread of say two threads has a lesser helix angle than a single thread. Ditto for a three or even four thread. The helix angle is in affect, a wedge. Imagine a wedge under a door. A sharp angle wedge will easily be pushed under the door and slip under with less required force than a wedge with less angle. For arguments sake, lets say the average person applies X amount of torque on any cap. That amount of torque on a single thread would easily split the cap on a traditional old pen cap. The same amount of torque applied to multi threads has less and less potential to crack a cap, the more threads there are. Unfortunately 4 threads seems to be the limit as it simply comes off on its own.

Now let's take it to the extreme. Add so many threads that the helix angle is reduced to zero and what do you get? You get a splined shaft. It can handle loads of torque because there is no wedge, but of course would be useless in keeping the cap on...
The fact that even numbers align is a lucky coincidence.
Anyway, that my theory and I'm sticking to it.

Class dismissed. Skip at the blackboard


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Old 03-21-2017, 05:12 AM   #10 (permalink)
 
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Good information! I didn't think of the shoulder issue either. I have been trying to get the cap to barely clear the body without showing any gap around the body but not actually touching the body.
I suppose because I am trying to keep the body thin since a lot of my friends like thin pens. I could still use the shoulder as mentioned on the inside of a cap though to stop threads at a certain spot if I use material with grain.

Love this website

Lewis
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