Making threads lock by machining different... how to do?

[USAeng]

I was wondering if there is a best way to go about machining threads different if you want to screw them together and have them deform and kind of lock in place. Same idea of destroying threads to make things stay together just machine them different on purpose so there will be interference fit and the parts will be more difficult to take apart... what parameters of the thread would be the best to change to make this work best.

The threads are #4-40

Was wondering the best way to do this and how to instruct the machinist to do it.

Thanks for any help.

 

[hydtools]

Maybe consider Spiralok thread form rather than custom deformed thread.

http://www.spiralock.com/

Ted

 

[USAeng]

those threads are cool... thanks for the link, but we dont care about taking back apart again... and want to just machine the parts in house without spending any extra $

 

[MikeHalloran]

Spiralok taps are not terribly expensive, and the technology is mature.
I.e., the experimenting has been done already.

Rolling your own could be expensive, e.g. if you overdo it and induce a delayed failure, or overdo it more and can't assemble the parts without breaking them.





Mike Halloran
Pembroke Pines, FL, USA

 

[byrdj]

loctite

 

[rb1957]

if a nut &bolt set-up ... nuts with a nylon lock feature.

if a stud (ie bolt and tapped hole) set-up ... helicoils have a locking feature.

if you want to be sure it never comes apart ... weld the nut onto the end of the bolt.

 

[USAeng]

sorry... I should have mentioned this is about 700F and has to pass electric current... so loctite is out... and there is no surface for lockwasher... I was going to destroy some threads but figured machining a sort of defect would be nicer

the closest thing to this setup I would say would be an acorn nut on the end of a 4-40 threaded rod...

I have used ceramic and metallic high temp threadlocker but its impractical because they take a long time to setup and require additional heating to cure right... also the one is a 2 part which is another step.

I would just like to redesign the threads better... I will start looking at the threads myself... to figure what I want to change... I figured this would be something people do often so would be easy answer

 

[USAeng]

there is no room to weld either... otherwise that would be good

 

[USAeng]

also... I talked to the spiralok guy I found see they need a solid surface to mate the nut to to make it lock

this does not have a solid mating surface it just threads on the end of a little rod

 

[drawoh]

USAeng,

I believe that it is M5x0.8 and 10-32UNF threads that are close enough that they almost assemble. This would be an off-the-shelf solution.

JHG

 

[MikeHalloran]

You certainly can 'bottom' a threaded rod into an acorn nut, so Spiralok could work.

OR, if you really want a permanent assembly, just have the parts made as one piece, from headed wire with rolled threads.
Custom screws in that sort of size in any common material are surprisingly inexpensive, and the typical minimum order of 25,000 or so won't even fill a bucket, so storage space is not an issue.



Mike Halloran
Pembroke Pines, FL, USA

 

[TVP]

USAeng,

There is an ANSI/ASME standard for interference fit threads, which is what it is called when you intentionally alter the thread form (pitch, etc.). Here is a link for the official standard ($40 via download) as well as a Google search that includes some free information:

ANSI/ASME B1.12-1987 Class 5 Interference-Fit Thread

http://webstore.ansi.org/RecordDetail.aspx?sku=ANSI/ASME+B1.12-1987

http://www.google.com/search?hl=en&..._sm=e&gs_upl=1171l6357l0l38l13l0l0l0l0l0l0ll0

 

[Bigkrank]

Don't tap them at all, use these for screws:

http://www.taptite.net/taptite/taptite2000.asp

 

[Tmoose]

Are you having problems with loosening now?

What materials are you using? Are they at a stress level when creep is possible at 700 F ? Could be to initial tightening preload, or stress from differing CTEs.

If the problem originates with loosening, and that comes from fastener yield, a longer grip length or different material may be needed.

If current must pass thru surfaces clamped by the fastener, the clamping load must be maintained. Any feature that simply prevents nut rotation will not really guaranteed clamping. Using the fastener as a conductor is generally considered a bad idea.

 

[DLite30]

Once tightened you could peen the end of the rod to flare it out, against the inner surface of the nut.

 

[USAeng]

all very good replies I will be able to come up with a good solution using all this info.... thanks a lot

 

[schradea]

TVP,
I'm in a similar situation. I like the idea of creating an interference fit with the threads. However, our app is a small thread (10-32), and it looks like that standard only applies to threads .25-1.5" in size. Are you aware of a similar method for smaller threads?

 

[1gibson]

Intentional defect perpendicular to the threads? Quick tap with a hammer and small screwdriver for starters, refine the process (axial location of defect, and consistency of size) if it works.

Initial cost of 5 minutes and 1 set of components.

 

[boslab]

how about a tapered thread, like BSPT, it will reach interfence as the effective diameters converge
mark

 

[racookpe1978]

I was going to say mix a NF male and NC female thread set with "thin" nut. But I didn't check if a NF #4 is available.