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Distinguished SOTW Member/Technician
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Discussion Starter #1
In a recent article I posted, "key fitting" Gordon wished to discuss the reasons why the end of a thread is undercut. This was in response to this statement from myself

...Once you've cut the thread, give it a tickle with a file or on the buffer just to remove and thread dags that build on the end of the thread, manufacturers undercut the end of a shank to prevent the need for this when mass producing threads and rods
A new thread could be worth while to discuss this

My view is simple, the main reason for undercutting the thread at the end of a rod / screw is simply to expediate manufacturing, in manufacturing a bolt machine cuts a taper on the front it cuts a thread relief at the back and then cuts the thread, by doing so, no dressing up is required to remove any raised thread areas, this means a bolt machine can knock out 10's of thousands of units a day with no human input other than loading fresh round / hex stock into the feeder bar.

I never personally thought there was any other reason for the thread being undercut other than the one I knew. I'm open to alternative reasons, There are advantages for sure, but I had never considered there to be an alternative reason for manufacturers and repairers undercutting.

Do others have different views, or do we have a bolt manufacturer here on the forum.
 

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Discussion Starter #2
Ive carried over this qoute from the key fitting thread

As far as I know, both manufacturers and technicians machine this shoulder mainly so that when the rod is screwed firmly into its post, the flat-face if this shoulder homes flat on the post, in a plane exactly at right angles to the rod's axis.
Im curious who here undercuts the shank of the rod when they make new replacement rods for instruments as a standard repair practice.

Hands up,

For those that are unsure what we are referring to Ive attached a picture

I know I don't undercut the back of the thread, when Im making new rods for keys. I cut the thread as far back as I need to, and then file off any raised burr from the process.

Im interested in how prevalent this practice is amongst other repairers.
 

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I learned to undercut threads (not in that scale, mine were 10mm+ in diameter), because
a) it looks neat, and neat work signals care.
b) it disrupts discontinuities and potentially safeguards against tearing and ripping. And the thread dimensions are the same throughout - no "half baked" threads near the end. Just as you punch a hole at the end of a slot:

c) it better prevents the female part, eg nut, from getting sore threads when turned past the intended end of the thread.

But this was <counts fingers> some 25 years ago, and we worked on bigger pieces. It probably doesn't really matter as long as the thread is long enough for a given purpose.
 

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Discussion Starter #4
They are valid reasons to undercut the thread, neatness always counts.

In your scenario the threads I believe would be used in a tension situation. If Im machining a thread to be used in tension, then I would concur and definelty undercut, the advantages here are significant, and minimising fracture points critical, but thats my view only becuase I spent many many years inspecting and carrying out material analysis on metals for cracking and wear for a living
 

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I've got a nice collection of old rod screws, gleaned from my work on period instruments.
The vast majority of them have no shoulder at all. Many of them were used on very high quality instruments, the build quality of which far exceeds that of most modern production instruments.
It suggests that the evidence points towards modern mass-production techniques being responsible for the shoulder.

Regards,
 

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Depends. It usually gets undercut by virtue of the need for a slightly smaller diameter on the thread stud - but where I've done 'quick and dirty' one-offs I've simply deburred the stub and left it at that, with no problems.

Regards,
 

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Ive carried over this qoute from the key fitting thread



Im curious who here undercuts the shank of the rod when they make new replacement rods for instruments as a standard repair practice.

Hands up,

For those that are unsure what we are referring to Ive attached a picture

I know I don't undercut the back of the thread, when Im making new rods for keys. I cut the thread as far back as I need to, and then file off any raised burr from the process.

Im interested in how prevalent this practice is amongst other repairers.
I was taught to do as Gordon indicated and undercut the end of the threads to presumably create a flat for the rod to seat against the post. However if you analyze how a thread is cut your method should work just fine as long as the dag or what ever you people from the southern hemisphere call it is removed. I think the under cut makes for a "prettier" machined fit but it may not be functionally any better.

Another reason to undercut the termination for the thread isn't so much for the functionality of the thread but for a measuring tool. I was also taught to do this first so that you would know the proper length of the threads in the post.
 

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Oh yeah, having a bolt manufacturer chime in may not be of great help, because the threads on many production made bolts are not cut, they are rolled...but you already knew that. :)
Matt
 

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Distinguished SOTW Member/Forum Contributor 2009
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I undercut because the fitting is nicer with undercut shoulders. Bot of the rod in them posts, and the hinge tubing over the rod.
 

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Discussion Starter #11
Cool, I am actually surprised by how many who have spoken up undercut the back of the thread so far,

I had made an assumption that becuase most techs that post here seem not to have a lathe (well that was my interpretation) that no one would be undercutting the thread

I may review my habits and see if it makes a difference for me

Just to clarify but, we are talking not of reducing the diameter of the shaft and then facing the shoulder to accomodate a smaller thread than the rods core dia, but deliberatly undercutting the back of the thread to at least the root diameter of the thread thereby creating an undercut at the end

Further to this, those that do not have a lathe and are repairers how are you undercutting the thread at the end if you do so.
 

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My process goes as follows: I reduce the diameter of the rod, cut the threads, and undercut/face the shoulder in the same operation. I even undercut when I make pivot screws. In my opinion it's the best for how consistently the rod stops shoulder against the post after the shoulder is faced and undercut
 

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I Face the shoulder (much better term than undercutting IMO), for all the reasons listed above. I don't use a lathe for it. I spin the rod in my bench motor (equivalent), and use a spinning diamond saw in my hand-held dental micromotor to do the undercutting, simply because for me it is quicker.

However I would not bother on the low end of instruments. As I wrote in the other thread, for certain pivot rods on high end instruments made to high tolerances, if you want to screw the rod in tight, without risk of introducing some binding of the pivot, a decent shoulder at right angles is often vital.
 

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Discussion Starter #16
Thats nicely done juan, especially when doing it by hand (not cnc). Extremely neat end finish.

I will acknowledge it certainly looks cleaner than having left the threads all the way to the end of the shoulder.
 

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thanks mate I'm a fairly run of the mill machinist (pun intended) but I know what material to use... you gotta love 12L14 for those pivots and other similar hardware.
 

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I see that a major porperty of 12L14 is easy machining. I'm interested... Is it's strength similar enough to silver steel or drill rod steel (not heat-treated)?
 

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I see that a major porperty of 12L14 is easy machining. I'm interested... Is it's strength similar enough to silver steel or drill rod steel (not heat-treated)?

Jicaino?
Simso?
 

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seeing that old semi steel pivots and rods has lasted what, 70+ years of not only active duty but substained and survived less than ideal repair approaches... I wouldn't be too concerned about 12L14 being "soft". It's certainly softer than 302, 303 or 304 stainless, silversteel, 4140 or 4340, though. The quality of some modern martensitic steels and other variatons or ellaborations on that sort of materials are light years ahead of what used to be "standard" in the 20's and 30's. And as mentioned, the french whom are supposedly the manufacturers of the "finest" saxophones entered into the screws, rods and pivots (hardware in general) refinement well entered the '60s
 
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