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issue with lead screw in chuck/Workholding

Joined
Sep 26, 2023
I did post to the cnc forum but thinking I may have been in the wrong location. Sorry about that

Now 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 pics with other bolts that have broken.
 

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Hmmm... If the materials you are machining are harder than the materials the leadscrews and jaws are made of then it may be that chips are getting caught between the screws and the jaws. Those clamps don't seem well protected from chips getting in there. Are you using the same type (make and model) of clamps and same materials on the other machines that don't have the problem? Have you tried a different style of clamp?
Alternatively, have you tried moving the operator to a different machine and does the 'problem' move with them?
 
My immediate reaction on seeing those photos was "Holy crap!" Looks like the threads are snapping right off. It seems super unlikely that a cutter is striking them. What about galling? Are there signs on the unbroken thread flanks that things are grinding or scraping?

If the threads are actually snapping off at the root, that smells of poor heat treatment. Too brittle throughout or too hard & too deep a case. Another possibility would be a bad "nut" that's causing a stress concentration. It's also possible that they are simply being overloaded by tightening (which is where that possible operator-dependency comes in), but I would expect more thread deformation than outright shredding.
 
The more I look athe photos... It appears that the problem is at the inboard end of the lead screw. Looks like that jaw may need to be replaced also. All it takes is for one nasty chip to embed itself in there and trash both the leadscrew and the jaw. As clean as the setup looks it appears there is a pile of chips under the part. Since there is a lot of engagement with the jaw would it be possible to chop off the offending part of the leadscrew? Would it be possible to have your people dismantle all the jaws and clean them out thoroughly between cycles? At 2-10 hour cycle times there must be some time available for cleaning.
 
Same operator/loader/unloader all the time on that one machine? never on any of the others?
not the same operator/loader or unloader but is the same machine. like mentioned did have it happen once on the bost 40 but was a sole incident on 1 of the jaws
 
Hmmm... If the materials you are machining are harder than the materials the leadscrews and jaws are made of then it may be that chips are getting caught between the screws and the jaws. Those clamps don't seem well protected from chips getting in there. Are you using the same type (make and model) of clamps and same materials on the other machines that don't have the problem? Have you tried a different style of clamp?
Alternatively, have you tried moving the operator to a different machine and does the 'problem' move with them?
they are all the same for the jaws on all the machines making it more perplexing to say the least. I am guessing the material is defiantly harder than the screws trying to get an answer from the manufacturer as to the material/standers its heat treated too. But were machining hardened chromium were high on the scale of harness 65C. Have not tried another style of clamp but like mentioned 7 other machines use the style clamp and only this machine has issues, one other machine did it once and it was years ago. It's not an operator issue never the same guy it happens too has even happened to our shop lead.
 
The more I look athe photos... It appears that the problem is at the inboard end of the lead screw. Looks like that jaw may need to be replaced also. All it takes is for one nasty chip to embed itself in there and trash both the leadscrew and the jaw. As clean as the setup looks it appears there is a pile of chips under the part. Since there is a lot of engagement with the jaw would it be possible to chop off the offending part of the leadscrew? Would it be possible to have your people dismantle all the jaws and clean them out thoroughly between cycles? At 2-10 hour cycle times there must be some time available for cleaning.
Was eyeballing the jaws thinking that maybe they are just worn out. The chips under the part is because this one was done machining so I snagged a photo once the pallet changer swapped parts. I'll look into that they do seem to break around the same spot on all the bolts i have been shown so far
 
Crap in the threads and over torquing to me

Have you asked the manufacturers for their opinion/
In the process of finding out from the manufacturers. But if that was the case it would be on all 8 machines not just one, more so since machinists switch machines regularly. We are a 3-shift shop and it's not 1 operator that it happens to. Seems to have no consistency at least found yet I'm only 1 month into being at this location. I feel if it was the operator, I'd see this issue spread out on all machines making it way easier to track down the cause.
 
My immediate reaction on seeing those photos was "Holy crap!" Looks like the threads are snapping right off. It seems super unlikely that a cutter is striking them. What about galling? Are there signs on the unbroken thread flanks that things are grinding or scraping?

If the threads are actually snapping off at the root, that smells of poor heat treatment. Too brittle throughout or too hard & too deep a case. Another possibility would be a bad "nut" that's causing a stress concentration. It's also possible that they are simply being overloaded by tightening (which is where that possible operator-dependency comes in), but I would expect more thread deformation than outright shredding.
Defiantly not striking them with a cutter. Will go out onto the shop floor and look into the galling and let you know what I see.
 
Check the length of the broken bolts. If the same length is the same on all, that is a clue. Another hint is , are all the holes the same depth? Does it happen to only one hole all the time? Maybe paint mark the hole.
 
My immediate reaction on seeing those photos was "Holy crap!" Looks like the threads are snapping right off. It seems super unlikely that a cutter is striking them. What about galling? Are there signs on the unbroken thread flanks that things are grinding or scraping?

If the threads are actually snapping off at the root, that smells of poor heat treatment. Too brittle throughout or too hard & too deep a case. Another possibility would be a bad "nut" that's causing a stress concentration. It's also possible that they are simply being overloaded by tightening (which is where that possible operator-dependency comes in), but I would expect more thread deformation than outright shredding.
Alright so looked at the bolts they started to save once they thought it was a problem updating those pics as well here shortly. Some snap at the root some show signs of galling or scraping right before it breaks at the root from what I saw. I think 1 may have been due to over tightening since it took out a fist's worth of threads. Did notice an oddity on the ratchet holes which I took pics of as well leads me to think it's the manufacturer.
 








 
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