a strange failure of an unalloyed Tantalum bolt 1

[elinah34]

I have a 8-33 Tantalum bolt which we manufactured according to ANSI Standard.
When tightening the bolt we got a strage and unfamiliar (at least to me) failure of the bolt's thread (I attached photos).
I thought that when designing according to the standard the shank should fail before the threads.
In addition, even more strange is the thread didn't really tear off the shank but it looke like it plastically deformed.

What can you say about it ?

 

[mfgenggear]

elinah34

so I don't have a copy of the ASME b18.3, so I pulled out my old 18Th edition machinery handbook and took the dust off it.
there is nothing special about this thread. it appears to be a standard thread. I observed the the manufacture is required to verify with thread gages.
Purchase your own they are relatively inexpensive and reverify the threads. pull a sample and reinspect them.
as a manufacture will only do a sample inspection procedure unless otherwise stated on the contract. if it is a small lot it
should have been inspected 100%. simple to inspect the major diameter with blade mic's. on a pinch a measurement over 3 wires to inspect size.
but this does not inspect the other geometry as thread gages. correct P.D size, lead, included angle of thread and so on.

so first very the thread geometry is to the standard,
second look for burrs, interference will exacerbate seizing problem
third oil the parts well to prevent corrosion,
I personally feel the pitch had errors, thus causing the last threads to fail.
as pointed out buy others,
if a small volume, a buffing operation of the threads very lightly with red wheel(scotch bright)
will help immensely. and a cleaning operation to remove debris.
edit: one other suggestion, have the threads ground, it will produce a smoother better surface finish, and much more precise on geometry and
size, generally a thread will require 7 passes, this part may require more on the last cuts to improve finish.

 

[elinah34]

Thank you very much for your detailed answer

 

[Tmoose]

The drawing notes 40.1 say the threads must be checked with go and n0-go gages.

What are you using for cutting oil when turning the threads?
Some exotic inserts supposedly need to run hot to perform at their best.
Are the threads created with Interpolated CNC, more of a manual thread turning operation?
Changes in tool relief and rake can make big differences in surface finish.
Is the tool the full thread profile?
With such a small diameter getting the cutting edge "on center" and providing sufficient rake and clearance can be a problem.
I'd paint the sides of the insert and look for "dragging" which can make a real messy surface finish.
So can "gummy materials, for which the best defense has to be found by trial an error. Tool sharpness, top rake, relief etc and .

Here is a link that talks about the various kinds of tool infeed with advantages and disadvantages.

https://www.mmsonline.com/articles/threading-on-a-lathe

Here is link to a guide for that includes screw thread surface finish/roughness.

https://www.engineersedge.com/manufacturing/maximum_surface_roughness_15411.htm

 

[mfgenggear]

I will do a machinability harden when possible and it is with in the permissible thermal treatment such as AMS2759/2.
being this material is gummy there are limitations on machining properties , where as the geometry of the tool, feeds and speeds,
as well as the coolant used, most high volume shops in America, do not use oil any more unless it is grand fathered in.
most use a soluble based coolant. it gets. tricky to dispose of chips, and lubricating oils or coolant.
EPA issues. no more dumping coolant down the drain. and has to be disposed of properly.

in softer materials it is important on the rake, but for threading there is a limit on available inserts, and I may be out of touch there may be more these days, unless
one fabricates or modifies the carbide insert with a diamond wheel. on a tool and cutter grinder. then obtaining the correct geometry becomes important.
a machinability harden steel get , better finish and the heat is removed with the chip, as with soft gummy material it tears, and has a poor finish.
and it is stringy, and it is difficult to break the chip, like harden steel.
also depth of cut as well as higher RPM is an important factor, harder, tougher material lower rpm and deeper depth of cut. except for the final pass.
softer the material higher rpm and shallower depth of cut to obtain better finish, on gummy material there has to be enough depth of cut
that the insert can cut correctly and not rub or tear. it's actually a science to it.
it is what is.

 

[GregLocock]

Since it's a standard thread I'd suggest running the turned thread through a die just to knock any burrs off and act as a quick check on dimensions.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[berkshire]

The release date on that drawing 1601 is a bit suspect, unless they are using some calendar I am not aware of.
B.E.

You are judged not by what you know, but by what you can do.

 

[elinah34]

I have to admit that I wasn't really involved in the manufacturing process of this part. It was produced by a manufacturer who claims to be familiar with working with this exotic material, and as a designer who isn't really familiar with all the tips you mentioned regarding manufacturing, I let him do his work.

 

[mfgenggear]

Yes it does not appear fun material to work with.it was probably given to a young machinist and was told to handle it.
I don't believe I would try to machine
This material.cutting threads is probably easier said than done.probably the thread rolling guys probably quote high because they did not really want it.
An other suggestions would be to thread roll on the lathe.

 

[drawoh]

elinah34,

Your initial post describes the thread as 8[‑]33. Is this a typo?

--
JHG

 

[elinah34]

drawoh, you are right.
It was a mistake.
The thread is 8-32

 

[drawoh]

elinah34,

That is good, although it eliminates a good explanation.[ ]

--
JHG