7075 aluminum issues

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 typ> You have already described the problem as 'breakage'. Since most people

Is it possible to use a short radius in the threaded portion if they're being made with a die?

I selected 7075 simply because of its shear strength. I thought 6061 would be too weak. Perhaps I'll have him make a few 6061 testers.

The threaded > The shoulder should be a radius or taper to reduce stress risers. 6061

In your opinion, what would you suggest for material? I'd avoided steel because of corrosion issues, these parts are exposed to water, dirt, salt, etc.

Is there a particular type of sta> Aluminum has a fatigue life rather than fatigue resistance. Most

According to sunshine-x :

[ ... ]

For good reason. Attachments are a "no-no" in any usenet discussion (text only) newsgroup such as this. They are only acceptable in newsgroups which contain the word "binaries" in the newsgroup name.

Even if you find a news server which will allow it, you won't reach everyone (and you will offend many). The reason why you won't reach everyone is that some major news servers automatically drop any articles posting binaries to a discussion newsgroup.

However -- you *can* post the image on a website, and post the URL here. If you don't have access to a web site as part of your account, you can post it on the dropbox (visit:

and read up on how to use the dropbox.) Then, once it is there, post here the URL which will reach it. Don't use spaces in filenames, they will be automatically replaced with underscores.

Out of curiosity -- is there any reason that the threads must be aluminum? I would suggest that you get a good steel stud of the appropriate thread, and screw it into the bart from which the threaded portion is breaking off. (Obviously, put female threads into that part.)

Good Luck, DoN.

[ ... ]

I see that you've gotten beyond the though of posting them as attachments, but it looks as though the permissions are wrong on the files. Each one returns a message like this:

====================================================================== You don't have permission to access /images/zoom/PDR_2042.JPG on this server. ======================================================================

Good Luck, DoN.

I selected 7075 because of it's light weight, high shear strength, and corrision resistance. Others have suggested stainless as an option.

Any idea what type of additi> Out of curiosity -- is there any reason that the threads must be

================= I am not sure that you need stainless, although in the low quantities involved the additional cost is not *THAT* much of a consideration. What are your projected annual requirements for this part?

The trade-off is the more corrosion resistant the stainless, generally the harder it is to machine. I suggest that you talk with your machine shop and their material supplier(s) and see what they recommend.

One possible source of material for prototypes are full size stainless steel bolts from your local marine supplier or boat shop.

I would avoid the exotics such as titanium if at all possible. The materials are expensive, difficult to machine, and fine/thin titanium chips/dust will burn like magnesium. While this may not be a problem for you, your machine shop will factor in the safety costs.

If you look at history you'll find that no state has been so plagued by its rulers as when power has fallen into the hands of some dabbler in philosophy or literary addict.

Desiderius Erasmus (c. 1466-1536), Dutch humanist. Praise of Folly, ch. 24 (1509).

The design is poor. You have a stress concentration at the shoulder, you need to relieve that with a radius.

A better approach is to tap the end of the rod with the thread, and use a steel setscrew to effectively form the male threaded end.

Jim

According to sunshine-x :

O.K. Stainless work hardens and may or may not deal with the vibration involved.

Any aluminum is not *very* corrosion resistant -- though it helps that it develops an oxide coating which presents a barrier to some corrosion. If it gets wet with an alkali solution, it will dissolve away the protective aluminum oxide.

I would personally make a female thread in the basic aluminum drawbar, and thread in black oxide coated steel studs with rolled threads. This minimizes the amount of steel in the assembly, but puts the strength where it is needed while minimizing the weight added.

The black oxide does reduce the vulnerability to corrosion, and if it is kept coated with a good sticky oil (e.g. Vactra No2 Waylube, or even chainsaw bar oil) it will be even better.

Of course, the steel directly attached to the aluminum will open the door to electrolytic corrosion -- but since this is a ground vehicle, not a water one, I think that short splashes will not be a serious source of corrosion. With the vibration, consider that it will mostly get shaken off -- and a spray with plain tap water just before putting it away would probably remove any salt water which remains.

Your screw machine operator/machinist can certainly switch from cutting male threads to drilling and using a thread-forming tap in the aluminum tube fairly easily.

Look at the current MSC catalog, pages 1072 and 1073, which offer plain steel studs as small as 2-56x1", zinc-plated steel as small as 8-32x1/2". 316 Stainless steel as small as 10-32x3/4", 8-18 Stainless steel as small as 2-56x1".

One of these should work well for you. The zinc-plated steel could do a nice job of resisting corrosion.

Enjoy, DoN.

According to sunshine-x :

It would at least be significantly stronger in the failure mode observed with the current design. Whether it might make visible some other weakness remains to be seen.

Having now finally seen the images -- are the shallow grooves in the larger diameter part intended to receive setscrews, or to increase the bond with epoxy or something similar?

It will at least make the thread root strength greater. But as others have indicated, the vibration will lead to failure in the aluminum, sooner or later.

And the suggestion for a gentle curve from the minimum diameter of the thread up to the face of the larger part will also increase the likely life.

You might add some photos of ones which have failed. Letting us see where they fail would help analyze the problem.

Ideally -- removing some from the car before failure, but after some running time and photographing those might show a bit more, too.

Enjoy, DoN.

See responses below.

Yes, the grooves are to retain the epoxy.

I'll gather some images and post links within 18 hours.

I'll remove a functional but nicely abused set and will post photos within 18 hours.

Thanks a lot for all your advice so far, it's given me a lot to consider.

It's a poor design that concentrates stress at the transion between the threaded portion and the shank.

Changing to a different aluminum type or changing heat treatment will not solve your problem. You need to drill and tap the end of the shaft and install a steel setscrew to form the male threads.

Either that or relieve the threaded portion down to the root diameter of the threads, using radii at all shoulders. Also be sure there is a radius at the transition between threaded and unthreaded portions. A small ball-nose end mill can be used as a lathe tool to do this easily.

If you go with the steel setscrew approach, size the threads one size down.

Jim

I've added photos of broken parts. They're available at:

I built a small testing device to test what I believe mimics a collision similar to what these will experience.

Basically, I set them up so they're supported at one end and leaning on an edge on the other. Then I drop a weight on them from incremental heights until they fail.

My results are:

7075 T6 part as picutred - breaks at about 4.5" 7075 T6 part, re-hardened after machining - breaks at about 5.5" Stainless threads, internally threaded aluminum insert - bent at about 8", ruptured the tube at 9"

They all (the 7075 parts that is..) seem to fail at the same point, as photographed. The stainless just bent and then ruptured the tube.

I'm hav> You might add some photos of ones which have failed. Letting us

According to sunshine-x :

You never said that these were failing in a *collision* -- or if you did, I did not remember reading it.

I've been posting on the assumption that these were failing from vibration during running -- and I believe that the others have been posting under the same assumption.

Are you *sure* that you want to make these stronger during a collision? If you do -- the failure will simply move to the next weakest point -- which may be a more expensive part to replace.

I would consider failure during collisions to be a normal behavior.

Quite a lot more strength. Double the height for the un-treated aluminum, which suggests at least four times the strength.

Does the tube qualify as a more expensive part? :-)

Just how strong does this *need* to be? I presume that once you encounter a collision, the run is ended, and the vehicle needs repairs again.

BTW -- it is spelled "chamfer".

Good Luck, DoN.

I should be more clear - in some cases collision, which is generally par for the course in a typical race. The expectation is that the truck and all components survive minor collisions. Also, the trucks are designed to be able to survive flying through the air, for example I frequently put mine 10 - 15 feet in the air, and travel 20 - 30 feet off a waist-high ramp. It's expected that they survive the landing. So far, I have never personally broke one while driving, but a few customers have and I want to have the best possible product. Even though the failure rate is very low, I want to make it lower by enchancing the design, or possibly offering alternative metals.

I've never had one fail due to vibration yet (8 months) - just collisions, poorly landed jumps (collision with ground..).

This is a common issue in rc trucks/cars. Upgrade one part, the one beside it breaks next. I'd like to do everything possible to prevent having one of my products fail, however you make a good point. Perhaps having the end snap off is better than having an expesive wheel hub or a-arm break. Maybe this is a feature. =)

To give you an idea of what many consider to be normal, here's a link to a video (a little large, 25mb).

Yes, I was quite pleased to see this. I'm looking forward to testing a full part machined from stainless. I imagine it will not be as strong, based on what I've learnt from you and others in the thread.

The turnbuckle is a unit, and any failure requires replacement of the entire unit. We offer free replacements of the entire unit if any part of it fails (end snaps off, tube snaps, etc).

Stronger the better, with weight in mind. Perhaps an option one day may be to allow the customer to select either aluminum for ultimate weight reduction, or SS for increased strength.

Thanks, I'm new to this machinist tech talk. Explains why I didn't see much on Google.. =)

According to sunshine-x :

O.K. I see now why you have a professional machinist involved in the operation, rather than are simply making parts on your own machines, as most of us here are doing.

O.K. Serious overloads, then.

Let people make a choice -- protect the other components but have to replace them more often, or make them strong enough to survive, with the risk that more expensive components will fail. And limit the number of free replacements of the weaker unit.

Not going to work. You have a Windows Movie image, and I don't run Windows on the net, so I can't play it.

You can perhaps even have the back end drilled part way to reduce the weight somewhat, as long as you keep enough meat at the transition area.

Something which you may want to reconsider if you offer the choice of strong or sacrificial versions -- at least for the sacrificial version.

Agreed.

Do you make the other parts to which this connects, or are those made by other makers?

That is why I bothered to correct you -- since I understood what you meant. You probably simply heard the machinist *say* it, without seeing it in print.

Good Luck, DoN.