Gotta say this does take the cake to my eye. Make it in a few seconds from parts you have on hand or hardware store and flawless in every way. This should really help everyone who wants to work on a threaded rod. Not the best application for what I'm doing bc it is fiddly when your cranking them out- but for 99% of the time when you have a handful of threads that need a mod this is a home run. To my mind this is brilliant.
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Turning threads off rod fine but now rod advances out of chuck.
- Thread starter helmbelly
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I apologize if I have offended. It was not my intention. There lots of ways to skin the cat. I offered one. As to being fiddly and time consuming, once you establish a rhythm. it goes surprisingly quickly. Setup/take down can be done in a minute or less. Weigh that against the time and effort to make a custom fixture and decide on a break even point.
As my byline suggests, I believe in using the right tool for the job. I have made custom tools or fixtures that have taken several hours to make where the actual usage only took a few seconds to complete but it was the right tool to complete the job and any other path would have resulted in a cobbled up mess. However, I always look for the easy way out and if I am not compromising quality, I'll take the shortest route to a solution.
If I were making a bunch of pieces, I would use a 5C collet chuck or ER collet chuck and a work stop. The collet chuck would afford better concentricity as well. Lacking those amenities, I would opt for copper or aluminum jaw liners in the three jaw chuck to protect the threads and use the method I described above to obtain a repeatable turned length.
The setup does have concentricity issues. But if I were concerned about concentricity, I wouldn't be using the three jaw chuck. I have seen/used nuts where the hex wasn't concentric with the threads. I would be less concerned about the clearance between the threads of the nut and the threaded rod as tightening the jam nut would force the contact points to the flanks of the thread. The back nut would be more an issue in that case because it is floating on the thread but unless one were working some distance from the chuck, the effect is minimal.
No worries, just defending my thread! The "breaking point" you mention is something Ive been wrestling with less and less the longer Ive worked in a shop, about 13 years now, many cobbled up messes as you say later. The less I knew about machining the more I thought I could get away with. As I found satisfaction in making something right I also found satisfaction in slowing down and enjoying the approach and sort of the respect involved in not just taking the extra step like it was a traffic direction but in learning what those extra steps are - almost like being curious how many extra steps there are to make it perfectly... I'm sorta exaggerating to make a point, but taking the slow road in the machine shop is always right if there is any doubt at all. it also happens to be where most of the fun is. That and picking the right radio station. The best times in the shop are when time doesn't matter, you're focused in, measuring, looking, calculating, planning a step ahead, a step adjacent and it my case when it's really an MVP performance I remember to don my goggles at the grinding or sanding wheel so I'm not itching my eyes for the rest of the darn night!
ps. what I mean by the slow road is exactly what you say, usually it's making an intermediate part to make the money part. For me it's usually workholding better but a lot of times it's realizing this (vice stop for instance) will not just help me now but it's gonna keep on giving and make a better shop. ofcourse I can also be guilty of the dark side of that coin which is rigging up a slick leather hammer holster when I'm trying to avoid getting the job done ;-)
But like you say it's a balancing act. yesterday I had to put a 1/4 thru-hole in a 3/8 tube in the mill vise. The parallels did not leave clearance for the bit to pass between them. I had to slide them both outboard in the vice so they fell out when unclamping - a mess which would lead to angry cursing all afternoon. Should I make a sacrificial center parallel or two thin alum parallels - either one requiring a whole pile of crap? Pondered a bit, then just hooked a wire to the holes on the parallels and ran it up over the mill ram so they stayed sat in the vise like Bob's your uncle and I felt like Edison Einstein for 10 mins.
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The problem with “3 threads” is that even if this resulted in a tapped hole whose strength were sufficient enough to make it a toss up between the bolt breaking or the threads pulling out ( I highly doubt this is sufficient to break the bolt) you’re still not factoring in the service factor.Since this thread is so long with lots of different suggestions/ thoughts, I figured I would add some of my own. It may or may not add to the OP original problem, but may help think about future projects.
The info about the smartest engineer who said 3 threads are needed for greatest strength, may be right, but only for a given size of thread.
I was alway told that generally, you only need the length of the thread equal to the diameter of the thread. But, when you start doing the actual calculations, you need to know a lot more variables. What is the materials type of threads. Aluminum, vs steel is going to make a big difference.
Fine thread, or course thread? Fine thread usually has a higher clamping force, and shear force, and thus a higher torque number too. But when you think about it closely, this is because the root of the threaded bolt is bigger in diameter, because the threads are not as deep as coarse ones.
What type of class fit. And are the threads in tolerance? The class of fit is basically how much metal to metal interlap contact is there theoretically. If I am single pointing threads, I can make over size the pilot hole size in the female thread but still get a tight fitting thread even though the root diameter is much bigger than tolerances call for. You would not realize it is not as strong a thread as normal.
A couple weeks ago, I was making a change on a part I was fabricating. I was reducing the length of thread engagement, and I needed to verify that I still had the needed strength. I needed to know the material, pitch of thread, diameter of thread, and the class of thread to make the calculation. Still then, I am still slightly over whelmed with the actual math needed to make the calculations. I am not a degreed engineer, I just play one at work.
But I work in hydraulics. If one of my threads fail, someone can get hurt. So we needed some math backup to quantify our changes were sufficient for job at hand.
Threads can be a huge rabbit hole to run down, realizing how many different variables it can have.
Oh, I have also been finding a lot of problems with store bought nuts lately. It seems quality control is a big problem in factories today. The threaded holes can be off .030” perpendicular to the nut end. I sometimes have to take a trueing cut on the nut, so it will tighten down properly. In order to do this correctly, I tighten it down on my threaded part so I know I am machining the face consistent to the thread. I can’t trust the threaded hole to be in alignment with the hex body. This is fun because I usually use Nylock inserted nuts, and I need to use the face I am facing off, for the locking to the threads. ( usually these nuts are 1-1/2” or 2” x 12tpi. A lot bigger than most of you deal with in home shops, but my problems can be the same, just on a bigger scale)
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The hole may have one bolt inserted in it and stay that way forever, which is a rarity. Most threaded holes have bolts run in/out several, to an extreme number of times over the life of the hole.
Only going one diameter or for gods sake “3 threads” makes no sense. You will deform and or wear those threads.
So, Why?
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The three thread idea most likely arises because when a bolt or screw is tightened to its design limits, the first thread bears the lion's share of the load due to the bolt stretch. Past three threads, additional threads contribute very little additional strength. For what it is worth, standard NC nuts have around five threads.
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What happens after the first thread pulls out of shape?
After the second?
I also view a threaded hole different than a nut, nuts are disposable, a stripped out threaded hole is a whole different level of repair.
I’ve rethreaded more holes than you could even fathom due to changeover guys giving the impact gun all the Ugg’a duggas. Thread it deeper, then go up a size, then plug it and drill and thread to the original size.
We gave up on helicoils decades ago as for our applications they would only last about as long as it took to make the repair.
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Browsed the engineering forum Eng-tips.com and found this factoid "The first thread in the nut (bearing face end)sustains a stress level 231%(fine thrd),179%(coarse thrd) of applied load." It drops off exponentially with each thread after. So roughly speaking the first three threads are doing 95% of the work. Pretty sure that's what Paul meant.
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