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Issue with chuck/Workholding lead screws breaking threads.

Joined
Sep 26, 2023
Guys my shop is having a crazy issue I haven't seen or heard of in my 7 years of manufacturing engineering and no one in my contacts has been helpful so to the internet I turn! We have 8 machines only 1 machine has this issue.

First off, the issue at hand. We are having the chuck/Workholding jaw adjustment screws fail prematurely. We are getting charged almost 1k per lead screw! This is a pretty big issue to say the least. The Bost 20 is a vertical lathe and it uses 4 jaw boxes, and it happens on all 4 of these all 8 if we include the pallet changer side. What could be causing this issue? It's not constant when it breaks from my understanding and the lead screws don't all fail at the same time, we have now broken bolt #10.

Our parts are large enough that we use supports as well with the clamps, so I don't think were over torquing it at an angle. Only thing I can think of is that maybe its backlash coupled with temperature and vibrations. We are machining very hard materials a chromium mixture and run times go from 2 hours too 10ish hours. Also, we are in Arizona with a shop that's non-AC controlled, and bay doors open all day can easily have a 20 degree temp shift before adding in any machine operational temp changes. I don't think it is stress corrosion, cyclic fatigue failure or any form of shearing or even tensile overloading. Like mentioned we have 7 other machines this is the only one with the issue, and this machine doesn't even see our largest parts which weigh up too 24,000 lbs. We did have 1 other machine break one of those bolts, but it only happened 1 time so maybe it is something the machinists are causing with loading and unloading with the cranes? Any input could greatly help and would be appreciated.

If more information is needed let me know! Someone has had have been around long enough to have seen or talked to someone this happened to help me out guys!

updated with a photo of one of the smaller parts clmaped down with supports
 

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Looks like it must be pulling up some chips from below and jamming up?


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Jamming
it may be, but its only 1 of 8 machines that has this issue all have the same jaw types so if that were the case shouldn't I see this on all machines not 1 specific one?
 
Have you quantified anything yet, standard manufacturing engineering?
Who is running the machine, do they only run this machine or others.
Do they use a torque wrench? Is it the same or higher torque than other machines, Is the operator using the torque wrench correctly or just over tightening it. Have you swapped operators to confirm?
Are you replacing the nut when the bolt brakes, ie is it the nut.
Is the nut over or under sized for the t slot compared to spec and compared to the other machines, are the teeth in the nut over or undersized (worn) compared to the other machines.
If you haven't replaced the nut, have you swapped it into one of the other machines.
Is the T slot over or under sized, or deformed in an area compared to the other machines or to spec.
Is the part being clamped smaller or larger than other machines.
Have you tested the hardness of the new screws to the other machines screws, maybe they are too hard (brittle) even slighty?
Have you tried the new screws in a difference machine using the same torque profile, maybe they are being made from a lesser material now, different supplier, chinesium...?
..........
 
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Have you quantified anything yet, standard manufacturing engineering?
Who is running the machine, do they only run this machine or others.
Do they use a torque wrench? Is it the same or higher torque than other machines, Is the operator using the torque wrench correctly or just over tightening it. Have you swapped operators to confirm?
Are you replacing the nut when the bolt brakes, ie is it the nut.
Is the nut over or under sized for the t slot compared to spec and compared to the other machines, are the teeth in the nut over or undersized (worn) compared to the other machines.
If you haven't replaced the nut, have you swapped it into one of the other machines.
Is the T slot over or under sized, or deformed in an area compared to the other machines or to spec.
Is the part being clamped smaller or larger than other machines.
Have you tested the hardness of the new screws to the other machines screws, maybe they are too hard (brittle) even slightly?
Have you tried the new screws in a difference machine using the same torque profile, maybe they are being made from a lesser material now, different supplier, chinesium...?
..........
Seeming like to me the jaws themselves are worn causing the stress on the lead screw. All the screws but 1 have normally 2-4 threads that break and not all of them are at the root. Have noticed on a lot of these that before where the thread is broken, I am seeing galling on about every other bolt I have seen.

We are 3 shift shop and operators switch machines every month or so and it's hasn't been traced back to any operator. It has happened on all shifts with different people including the shop lead who's been here for 16 years.
Torque wrenches are kept at each machine and are calibrated often and are all the same brand/make.
The nut and lead screws are replaced when this happens.
Will check on the nut size and threads.
We did replace the nut but maybe I should swap the jaws from one machine to the other and see if the issue lies on the machine or the equipment itself.
T slots I'll look into as well haven't had a chance to have the machine down to look Friday I'll have that chance
Parts on this machine are smaller and more than half the size/weight we put on the other machines.
Have not tested the harness yet but will be.
Have not tried that, maybe I will take a screw from another machine and slap it in there and see what happens. It's the same supplier talking with a rep next week to see what I can find out for their process and if anything has changed.

Thank you though for the informative things to look into
 
What do you lubricate the jaw screw threads with?? I might suggest slathering them with never seeze or a good moly grease and see if that will allow you to use less torque when loading a part and tightening the jaws. Yup, you might attract more chips on the threads but you can clean that.
 
That image you shared has the damaged threads on the end of the shaft. Your not extending the jaws out too far and just using the last few threads of the screw are you?
I don’t like how the jaw is just a half nut. That’s only engaging half the screw and putting a lot of tangential force on the screw. It’s also not supported on the end. I don’t like that design. I think the other VTL chuck jaws I have encountered use a full thread nut like a Kurt vise.

Edit: looks like that’s a pretty common design. I hadn’t realized that before.

Try some other brand jaw assemblies to try? used even. They are expensive, but would have been cheaper than 10x screws.

Lastly, those broken threads look like cast material fractures. Ask the screw manufacturer what material the screws are made from.
 
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If I really had to guess without being there and looking at it, Assuming there isn't any draft to the surface your clamping on adding to the issue, And you would have to reference to other parts being clamped similarly,
But I would say that having the jaw only biting the part 1/3 at the top of it is causing the issue, if it isn't the problem I still would correct this if its a production part, If you can lower the part down further into the fixture, and machine off the excess jaw so the part doesn't have all that unsupported leverage.
 
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If you cant do the above post, or don't want to modify a set of hard jaws, another thing to try is shimming the play out of the nut, so it will minimize its rotation with that setup, this should minimize the torque or at least its max angle applied to the screw.
 
After looking up this type of VTL chuck, Some other things, I would say make sure to add some type of Molybdenum grease on the threads, and I always make custom covers for any of my fixturing to keep chips out of the screws, So I would design something that covers and keeps chips out of the crew area, just for the large majority anyway.
Also if this chuck has been used alot for a different size part, it would be warn unevenly, more than likely, and allowing the nut to excessively pivot, or adding a stress fulcrum for it.
I really wonder if this chuck has ran a lot of larger parts and warn out/oversized that extended area, and now your running a smaller part, that the nut is being held tighter in the inner area and loose out toward the tip, and as mentioned is a stress/fulcrum point, allowing the tip of the screw to flex. 2 Cents.
If I was a betting man this would be it.
 
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