7075 aluminum issues

I'm making small parts with M4 (close to 8-32) threaded male ends. The metal ends are occasionally snapping off, usually near the shoulder.
Can suggest a way of increasing the strength of the part? They're made on a screw machine using a die, and are made from 7075 aluminum, T6. I can't change/increase the thread size.
I'm testing out re-artificially ageing them, at 250 deg. F for 24 hours in a small oven. I'm not sure how I'll test if they're stronger than before, but I'll figure out some creative way.
Is re-ageing them even worthwile? My thoughts are that the screw-machine is stressing them, and that this will strengthen them.
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I think you should send the parts out to be thread rolled. Or see if you can get a die to roll the threads on your machine. Rolled threads are stronger. Jim

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At this time thread rolling is cost-prohibative, and we're not having any problems with the threads stripping, they just snap. Would rolling make a difference for shear strength?
Jim Sehr wrote:

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I agree - but in a F.A. mode - are the threads full - or does the die slam into the head and then reverse out - stressing / stretching the head ....
Might be an adjustment needed.
Martin
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member http://lufkinced.com /
Jim Sehr wrote:

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That's a great question - I'll have to ask our machinist. From looking at the part, I'd think it's not, however I'll ask the expert tomorrow and update the thread.
Martin H. Eastburn wrote:

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What grade fasteners are you buying?
Gunner
"I think this is because of your belief in biological Marxism. As a genetic communist you feel that noticing behavioural patterns relating to race would cause a conflict with your belief in biological Marxism." Big Pete, famous Usenet Racist
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They're actually attaching to nylon plastic ball-ends, they're part of a tie-rod.
Gunner wrote:

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Is the material Aluminum, or steel? If aluminum..they are going to fail. Period. End program. Full stop. This application calls for something other than aluminim. Something steel and properly heat treated.
As Ive read the follow up comments about this being for RC car front ends...you are using the wrong material or the wrong heat treat if steel. You may have somewhat better luck if they were made of titanium than aluminum, assuming the proper grade and heat treat. And rolled threads.
Sometimes..there simply is no free lunch
Gunner

"I think this is because of your belief in biological Marxism. As a genetic communist you feel that noticing behavioural patterns relating to race would cause a conflict with your belief in biological Marxism." Big Pete, famous Usenet Racist
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They're currently made from 7075 Aluminum T6 rod, and are machined using a screw machine with a die for the threads.
I'm not certain what I should be doing post-machining for heat treating. I've tried re-ageing some and will be testing them shortly to see if there's a difference between them. My procedure was very simple, 250 - 270 deg. F in the toaster oven for 24 hours.
I'm thinking I may have to investigate alternative materials, such as grade 5 titanium or stainless. Do you think they'd be more suitable?
Gunner wrote:

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What is the radius of the shoulder?
Richard
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This part is about 0.2" at the shoulder, is round, and inserted and epoxied into a tube. The threaded end extends out the tube, and is snapping at the base of the threads.
Richard Ferguson wrote:

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Maybe I did not understand. I take it the threaded part is the same overall diameter as the unthreaded part. I think that I assumed that the threaded part was noticeably smaller in diameter than the main body.
If the threads are the same diameter, then there is no shoulder, and my question/comment did not apply.
Richard
sunshine-x wrote:

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The diameter of the shoulder is 0.2", while the threaded portion is M4 thread with a diameter of about 0.152".
Richard Ferguson wrote:

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sunshine-x wrote:

Geeze. Everyone is just asking you questions instead of answering yours.
Are these parts subjected to much vibration? If so, is all due attention being paid to avoiding stress risers -- at least as much as is possible in a threaded part?
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wrote:

Your two questions....<G>
Gunner
"I think this is because of your belief in biological Marxism. As a genetic communist you feel that noticing behavioural patterns relating to race would cause a conflict with your belief in biological Marxism." Big Pete, famous Usenet Racist
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Yes, there's a LOT of vibration, they're part of a turnbuckle assembly on RC cars/trucks (nitro/gas powered).
They're threaded into nylon rod ends. I'm not sure how to answer your question.
Tim Wescott wrote:

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sunshine-x wrote:

> Yes, there's a LOT of vibration, they're part of a turnbuckle assembly > on RC cars/trucks (nitro/gas powered). > > They're threaded into nylon rod ends. I'm not sure how to answer your > question. > You certainly answered my first question, and if you don't know the answer to the second -- well, I think you answered it, too.
I don't know how 7075 is for fatigue resistance, but aluminum is generally always worse than steel, and you're in an environment where I wouldn't be surprised seeing steel fail in fatigue.
Do some web searches on "fatigue" and "stress risers". You should find some food for thought. There was an interesting thread on this group recently about a model boat gearbox shaft that a guy was having problems with, with a good response detailing the design of fatigue-resistant threads.
Basically you want to avoid sharp edges, particularly ones that go into the material. With time and vibration such edges will crack, and once they start the crack will propagate pretty quickly.
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Tim Wescott
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You have already described the problem as 'breakage'. Since most people running screw machines seem to know their stuff, please don't be offended by the simplistic questions/remarks.
Have you examined the thread using an optical comparitor to see if the proper thread form is indeed being generated?
Have you specifically looked at the ending of the threaded portion to see if there is any abrupt ending which would create an abnormal stress point? From my limited experience with 7075, it tends to come off in chunks.
Perhaps a simple change in type of lubricant, or method of application could make a significant difference.
Are the chips loading up in the threading die?
Is there a 'neck' at the end of the thread to minimize stress?
Another possibility is the application itself, and the parts are not installed 'square' and the resulting stress in use is causing the breakage.
Good luck, Ace

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I honestly know very little about machining, I'm having them manufactured by a screw machinist, using some sort of giant CNC screw machine.
I don't have access to and wouldn't know what do with an otical comparator. I could post high-resolution images of the part in question, would that help?
The threads are very smooth and not chipped up which leads me to believe they're being adequately lubricated during the process, is that perhaps incorrect?
There isn't much of a neck at all, the threads go almost to the shoulder. They're being threaded into nylon ends, which are often not threaded on all the way (these are adjustable turnbuckles, so it's expected there will be partially exposed threads).
While typing this I've decided to also post a high res image of the part. I'll have to do this from another usenet account, google doesn't seem to allow attachments.
Ace wrote:

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High-resolution images are available here:
http://jdcarbon.com/images/zoom /
sunshine-x wrote:

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