Thread: Delicate engineering problem

Originally Posted by pippin88

Able to lie down and hold in vice from the side?

A picture of the stuck piece in situ would make it much easier to suggest best removal.

I am banned from posting pics for some reason...been trying to work it out for ages.
Don't get any errors so no clues....really annoying....

Dunno if there is a limit as I have uploaded quite a few over the years...?

I uploaded a pic to Google Pics. Hopefully that works.

https://photos.app.goo.gl/Vraa7W8euN2HDesr6

Steve

Pic of the Parts.

https://photos.app.goo.gl/DimZqgB81nSFz86p8

'Distal Locking Screw' is the offending part.

Hi Steve,
Oh my god! I see what they have done... They must have used a huge amount of force to shear ALL the corners off the internal hex to get it to round like this....
What a bugger! You have my sympathy!
I don't believe a splitter is going to be the answer due to the deformation forces. Given that the ring is likely to be stainless I doubt very much that you would be able to split it anyway. If it is titanium, it may even be worse....
I really suspect that some kind of milling is required to split the ring at the point of the slot on the side with the "ring spanner torque pin hole", then wedging it apart. Maybe chain drilling very small holes...
But what about this: the head of the distal locking screw looks quite solid. What if you drilled two holes 180 degrees apart axially into the head, then made a pin spanner that fits tightly into the two holes, with a pilot that goes into the "ex" hex round hole?
Have you already made a copy of the "ring spanner" or "die holder" looking holding tool with the torque pin? You would certainly need that to remove the distal screw.
I did read that you can't tolerate vibrations. So drilling has to be done at very low speed with a very sharp cobalt or carbide drill. You will feel the cutting action (a good thing really) but it may well be tolerable at something like 1 turn per second (60rpm) or less. I doubt that you could hold a hand drill accurately enough square to the bolt face - especially in the size (maybe 2mm dia) - without breaking it off. We would all have to put our design-thinking cap on to come up with a fixture that could be rigidly fixed to the "die holder" part...
In the meantime, can you try to drill something clamped to the prothesis adaptor to experiment with cutting tools and their different transmitted vibrations/cutting or grinding sensations? Anything you clamp to the metal will transmit straight into your bone, so you can find out easily what is and what isn't tolerable.
By the way, the glue idea sounds encouraging. The heat a small amount required - of even a very exothermic reacting adhesive - will not heat up the amount of metal in you implant by any more than fractions of a degree. Certainly less than walking from an air-conditioned room into a 40 degree day. So don't be too worried about that.
Now, combining some kind of mechanical torque transfer like two pins into the head of the screw AND the adhesive proposed is in my humble opinion MUCH more likely to succeed.
One more question: how much of the dual cone and it's locking screw is outside your body? Might it be possible to loosen that somehow? So the mechanical removal of the distal screw can be done disconnected from your leg? I'm guessing that can't happen until the distal screw and taper sleeve is removed....
Lastly, does your distal screw have the flats milled into the threaded part? And if so, what are they for? Hopefully not for some kind of thread locking system.....

Keep this thread going and your information and thoughts flowing. Combining the ideas of the many people here will definitely come up with a solution or trigger new ideas for solutions!
All the best!

Originally Posted by Sterob

I am banned from posting pics for some reason...been trying to work it out for ages.
Don't get any errors so no clues....really annoying....

Dunno if there is a limit as I have uploaded quite a few over the years...?

I uploaded a pic to Google Pics. Hopefully that works.

https://photos.app.goo.gl/Vraa7W8euN2HDesr6

Steve

Hi Steve,

Have you tried deleting your cookies and browser history ?

I use Firefox and a while back discovered several hundred meg of data stored ! That stuff I've now deleted. I've since managed to stop that happening, but you would be surprised at the number of places stuff is stored. It could be part of the reason for your inability to post pictures.

"ssl,google-analytics.com" stops me on this site, I have to turn off blocking for them. They are used to track you and log what you do !

Originally Posted by Sterob

Pic of the Parts.

https://photos.app.goo.gl/DimZqgB81nSFz86p8

'Distal Locking Screw' is the offending part.

Looking at those pictures, there seems to be some kind of pad on the threads of that screw, I've seen that kind of locking before. I'm suspicious that its a chemical compound that is intended to smear into the thread and prevent rotation.

From the picture of the screw head, it takes a considerable amount of force to destroy a hex socket like that. I fear that it may be a case of drilling the screw head off and possibly even into the screw threads to break it up so that it can picked out.

Steve I'm sorry if I sound pessimistic about this, but I do wonder whether that screw was ever normally intended to be removed.

Is the head of the screw proud of the surface at all? Bolt and nut removers might work if the chamfer is ground off to a square edge, they sometimes screw themselves onto the bolt with little push needed. Just depends on if They can get a bite started.

A drilling jig could be made to slide over the outer diameter, put a slit along it and a suitable hole in the end to guide the drill.
Lay on your back comfotably and securely on a work bench, slide the drilling jig on with the slit pointing up and then tighten the vice onto the sides of the drilling jig.
The bolt head will drill off pretty easily, maybe use trefolex, when the head is off the shaft of the bolt will be loose and should come out easily, it might even come out with the drill if it is actualy a left hand thread
Lastly....i would get someone skilled in using tools to drill it...that would NOT be a doctor.

Wow they really did a number on that bolt. I'm a little surprised that they don't have a "max undoing torque" i.e. if you need more than this something is wrong, Stop. Damn the glow plugs on my truck have that!

Is it possible that someone forgot to use anti seize?

I assume there is a damn good reason they cant just remove the "dual cone".(perhaps you have an early version?)
It doesn't look like that bolt goes all the way through it.

Hi Steve,
just another thought - not engineering related.
You mentioned that you had to get a general anaesthetic for every procedure. I don't know how 'squeamy' you are, but I've had a fair few procedures under spinal anaesthetic (intrathecal anaesthetic). Not painful, but 'weird' sensation. It means your bottom half is paralysed and anaesthetised, but you don't have to recover from a general. And you can supervise what they do! They normally still want you to fast, in case they have to 'knock you out', if you panic or something requires other interventions.
I have a medical background and like to be 'involved' in the process - not in the 'control' sense, but in the 'observer' sense. You learn a lot. In your case, you may have been able to prevent them doing that damage....

Originally Posted by jhovel

Hi Steve,I don't believe a splitter is going to be the answer due to the deformation forces. Given that the ring is likely to be stainless I doubt very much that you would be able to split it anyway.

Yes, I was wondering about deformation, as others have said. Main concernn would be stressing the femal thread in the Abutment and rendering it unusable.

Originally Posted by jhovel

But what about this: the head of the distal locking screw looks quite solid. What if you drilled two holes 180 degrees apart axially into the head, then made a pin spanner that fits tightly into the two holes, with a pilot that goes into the "ex" hex round hole?
Have you already made a copy of the "ring spanner" or "die holder" looking holding tool with the torque pin? You would certainly need that to remove the distal screw.

Yes, I had the machinist at work make one for me a few years ago. I needed it as the nuts used to always come loose on me!!! ( I wish that was still my problem....lol)

I had a 'pin spanner' thought last night in bed! THAT might work, if can find one small enough. I checked all my power tool pin spanners and they are all too large. Don't want to make one unless I have to.

Originally Posted by jhovel

I did read that you can't tolerate vibrations. So drilling has to be done at very low speed with a very sharp cobalt or carbide drill. You will feel the cutting action (a good thing really) but it may well be tolerable at something like 1 turn per second (60rpm) or less. I doubt that you could hold a hand drill accurately enough square to the bolt face - especially in the size (maybe 2mm dia) - without breaking it off. We would all have to put our design-thinking cap on to come up with a fixture that could be rigidly fixed to the "die holder" part...
In the meantime, can you try to drill something clamped to the prothesis adaptor to experiment with cutting tools and their different transmitted vibrations/cutting or grinding sensations? Anything you clamp to the metal will transmit straight into your bone, so you can find out easily what is and what isn't tolerable.

I can't do the drilling myself. Prosthethist won't let me AND it would be too hard. Can't see properly and can't reach. I'm GUESSING drilling 2mm holes, 3mm deep, would be ok......

Originally Posted by jhovel

By the way, the glue idea sounds encouraging. The heat a small amount required - of even a very exothermic reacting adhesive - will not heat up the amount of metal in you implant by any more than fractions of a degree. Certainly less than walking from an air-conditioned room into a 40 degree day. So don't be too worried about that.

Good to know....A big concern and no fixing if it goes wrong...lol

Originally Posted by jhovel

Now, combining some kind of mechanical torque transfer like two pins into the head of the screw AND the adhesive proposed is in my humble opinion MUCH more likely to succeed.

Yes, I agree. Lots of prep time but would have to work.

Originally Posted by jhovel

One more question: how much of the dual cone and it's locking screw is outside your body? Might it be possible to loosen that somehow? So the mechanical removal of the distal screw can be done disconnected from your leg? I'm guessing that can't happen until the distal screw and taper sleeve is removed....

Dual cone is about 80mm long and about 40mm is outside my body. A cap screw holds the Dual Cone in, but the Bushing, Abutment AND the locked screw have to be removed before the Dual Cone can be 'pulled out'( This is the ORIGINAL problem..Dual Cone is too short, so I want a longer one fitted....)

Originally Posted by jhovel

Lastly, does your distal screw have the flats milled into the threaded part? And if so, what are they for? Hopefully not for some kind of thread locking system.....

Yes, small nylon inserts to stop it coming loose. Not a problem normally.....Did I say 'normally?....lol

Originally Posted by jhovel

Keep this thread going and your information and thoughts flowing. Combining the ideas of the many people here will definitely come up with a solution or trigger new ideas for solutions!
All the best!

You guys are doing great.....We are really getting to it now. Thank you all.

The 'SD200' and the 'Pin Spanner' Ideas are winners.

Originally Posted by BaronJ

Looking at those pictures, there seems to be some kind of pad on the threads of that screw, I've seen that kind of locking before. I'm suspicious that its a chemical compound that is intended to smear into the thread and prevent rotation.

No just nylon inserts. Nothing as fancy as that.

Originally Posted by BaronJ

From the picture of the screw head, it takes a considerable amount of force to destroy a hex socket like that. I fear that it may be a case of drilling the screw head off and possibly even into the screw threads to break it up so that it can picked out.

I agree...they have done an excellent job of borking a simple job.
Drilling the head off was a thought. Vibrations may excude that...we will see.

Originally Posted by BaronJ

Steve I'm sorry if I sound pessimistic about this, but I do wonder whether that screw was ever normally intended to be removed.

It is, but not very often. If the 'shear pins' fail, the screw needs to come of so the Bushing can be replaced. A 30 sec job.....lol

Originally Posted by mcostello

Is the head of the screw proud of the surface at all? Bolt and nut removers might work if the chamfer is ground off to a square edge, they sometimes screw themselves onto the bolt with little push needed. Just depends on if They can get a bite started.

No, they are quite flush. Nothing of any significant projection to grab on to.

Originally Posted by jhovel

Hi Steve,
just another thought - not engineering related.
You mentioned that you had to get a general anaesthetic for every procedure. I don't know how 'squeamy' you are, but I've had a fair few procedures under spinal anaesthetic (intrathecal anaesthetic). Not painful, but 'weird' sensation. It means your bottom half is paralysed and anaesthetised, but you don't have to recover from a general. And you can supervise what they do! They normally still want you to fast, in case they have to 'knock you out', if you panic or something requires other interventions.
I have a medical background and like to be 'involved' in the process - not in the 'control' sense, but in the 'observer' sense. You learn a lot. In your case, you may have been able to prevent them doing that damage....

I've had them before. Sometimes they work, sometimes they don't.
I've had 1 fail and had to have a 'pain buster, i think it was called? a plastic ball that pumps pain killers into an area?...not sure.
No-ones mentioned it for this. Maybe they don't WANT me to witness them work. I can understand that. lol

I cleared my cashe and tried again: still fail....worth a shot, thanks Baron.

I've got no new ideas on the actual removal unfortunately, but I'm pondering the possible vibration/resonance issue. Given you have a bit sticking out, I wonder if it would be possible to do something to damp or alter vibrations should they prove to be a problem?

Not by any means an area of expertise for me, but a couple of thoughts I had:

One was a pair of narrow panelbeaters type leather bags filled with lead shot - one under, and one on top. Probably won't do a whole lot, but may take the edge off?

Or possibly something a bit like the things hanging off the bottom of my diesel Land Rover that are supposed to arrest certain vibrations - they appear to be nothing more than a steel plate bracket attached to driveline parts, with big lumps of cast iron attached to said bracket by means of vulcanised rubber. Maybe it's possible to clamp something designed along that idea to the protruding part (maybe via something as simple as a hoseclamp) to at least try and pick off some of the harmonics of it?

Finding the right weights/positions etc to target the specific frequency is of course the big fault here, and it may well be completely unviable, but thought I'd throw it out there in case it sparks an idea with someone else.