Sheared Drawbar

[The Merry MillerParticipant @themerrymiller]

Michael’s technique is exactly the one I used many many years ago in Stepney Power Station.

I also used to run a small diamond point chisel down the inside of the thread to create a very small channel and then spend many happy hours picking out the threads.

 

I remember doing this at one time lying on my back at the bottom of a steam receiver with only 18″ of headroom and operating a large ratchet drill by hand only to get the core of the stud out. The studs in this instance were about 1.5″ diameter and in about 2.5″

Happy days.

 

[Steve Taylor 2Participant @stevetaylor2]

Well its finally out, thanks to everyone that gave advice.

 

Just so you know I ground the broken abor until I could just see where the thread was then soaked it in penetrating fluid for 48 hours and used an easy out, it came out fairly easily in the end.

The odd thing is; its a 3/8 BSW thread, (I can post a photo if needed) I suspect theres a contributing reason the old one was stuck.

[mgjParticipant @mgj]

Steve, its no surprise it is BSW – an awful lot of 2 morse drawbars are.

 

Probably its broken just above the thread where the thread runs out it runs out, which is the stress raiser. That will be true whatever steel was used.

 

When you make your next one (!) could I suggest a groove to run the die into. One of these cases where removing metal strengthens the assembly. You see a lot of these grooves cut square with an ordinary parting tool – a nasty unpleasant and barbarian habit. (nearly as as bad as just stopping the die) They need to be rounded (ish). Anything is better than a sharp corner. (With due apologies to grannies if appropriate)

Edited By mgj on 17/02/2011 22:21:38

[Steve Taylor 2Participant @stevetaylor2]

I will need to make a new drawbar, I will have 5 drawbars for one machine, 2 horizontal and 3 vertical.

 

Can you clarify your idea please, are you saying a round shear groove could be cut above the point of thread entry so if it should break it will allow something with which the stub could be removed, or a long groove along the length of the thread.

[mgjParticipant @mgj]

Sorry the writing came out a bit small. The idea is to run the thread into a rounded annular groove – this one happens to be in a bit of 3/8 with a 5/16 thread. Doesn’t matter – as long as the groove is rounded and goes to the bottom of the thread depth.

[Steve Taylor 2Participant @stevetaylor2]

What does that do (sorry if its obvious)

[mgjParticipant @mgj]

Three things – the least important first.

 

It will screw neatly up to a shoulder and is much more likely to locate square. – which is pretty handy in a drawbar

 

It means that the load form tightening up the nut or drawbar is carried in the threads and that any stress from that stays in the threads. The longitudinal load is carried in the minor diameter. Because you have a gradual transition in section (rounded groove), there is no stress raiser to concentrate the stress artificially. thus stress levles are lower and the item is stronger and less likely to shear than if you just stop the thread. nad hte hting is much stronger, than if you just stop the thread.

 

As I recall – because I haven’t done the sums for a very long time – by a factor of about x3.

 

The fatigue life is increased significantly.

 

In engineering you never want a violent change in section and all sharp corners should be radiused. Not doing so is a very nasty dirty habit which weakens things unecessarily, makes them more prone to fatigue fracture, and is akin to picking ones nose (or worse) in church. Lots of corners don’t get radiused because machinists are to damn idle to radius tool tips properly, or to put grooves in where grooves should be put in, to the considerable detriment of the item being made.

 

It should be done as a matter of course with cut threads – screwcut or die.

 

 

 

Edited By mgj on 18/02/2011 20:13:40

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