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Cutting long stock on CNC Lathe

WakelessFoil

Aluminum
Joined
Aug 18, 2020
We are trying to cut a long part in our CNC turning center. I tried to start the operation with a real light facing. The bar jumped out of the 3 jaw Chuck we are using and won’t turn straight now. Maybe an issue with the soft Chuck jaws? I hate dealing with these automatic hydraulic chucks because it’s harder to get the stock perfectly centered in the jaws. Would a collet Chuck be a better option for this? Or maybe a manual self centering 6 jaw?

Thanks,
Justin
 

David Ferguson

Aluminum
Joined
Aug 13, 2008
Location
Paso Robles, CA
How about some details. Specifically, what is the diameter of the stock? What is the length of stock that you are clamped on? What is the length of the stock sticking out of the chuck? What is the chuck hydraulic pressure? What kind of jaws (material, smooth/serrated, total grip circumference)? What is the stock material?
 

Milland

Diamond
Joined
Jul 6, 2006
Location
Hillsboro, New Hampshire
You've got a bent piece of stock now, best to replace it with a fresh piece. Watch this video to see what happened (in a bigger scale):

You need to either clamp max RPM really low, or figure out a SAFE way to stabilize the bar. The guy in the video didn't...

If you were holding the bar entirely in the lathe and had the bar slip out of the jaws during facing, you were asking too much from the "system", and need to figure out a different way way to stabilize the part, either with a steady rest or something else. Perhaps face close to the chuck, drill a center hole, then extend and use a live center in the tailstock for additional processing.
 

Garwood

Diamond
Joined
Oct 10, 2009
Location
Oregon
Maybe an issue with the soft Chuck jaws? I hate dealing with these automatic hydraulic chucks because it’s harder to get the stock perfectly centered in the jaws.

You understand soft jaws are used by cutting them true right?

You really need to get someone with a clue to show you the ropes before you get hurt or destroy a machine.
 

RC Mech

Stainless
Joined
Jul 21, 2014
Location
Ontario, Canada
You understand soft jaws are used by cutting them true right?

You really need to get someone with a clue to show you the ropes before you get hurt or destroy a machine.

Yeah really. The entire point of power chucks with soft jaws are that the part is easily repeatable and consistent.
 

FredC

Titanium
Joined
Oct 29, 2010
Location
Dewees Texas
As mentioned we do not have enough information. To answer your question a collet possibly could hold a long part better. If the part popped out of the soft jaws maybe you were not chucking on much of the length? Anyway to chuck on more? Expanding stop further back in the spindle?
When facing parts with a big ration of length to diameter I put a small dimple in the end with a ball nose endmill to prevent the bar from lifting over the facing tool near the center of the cut.
 

WakelessFoil

Aluminum
Joined
Aug 18, 2020
How about some details. Specifically, what is the diameter of the stock? What is the length of stock that you are clamped on? What is the length of the stock sticking out of the chuck? What is the chuck hydraulic pressure? What kind of jaws (material, smooth/serrated, total grip circumference)? What is the stock material?
The bar is about an inch in diameter, 12" long. About 5" in the chuck with the rest hanging out. Fairly hard steel. The clamping surfaces of the jaws are flat and not serrated. I will have a peak at the pressure gauge when I get the machine warmed up later today.
 

WakelessFoil

Aluminum
Joined
Aug 18, 2020
You understand soft jaws are used by cutting them true right?

You really need to get someone with a clue to show you the ropes before you get hurt or destroy a machine.
Yes I understand that. Maybe it wasn't clear in my initial post but the bar was spinning true when I chucked it up, implying that the jaws where straight.

That would be nice but it is not in the best interest of the well-seasoned machinist to dedicate his work time to teaching a young engineering student the ropes of his trade. So I come here to learn.
 

Garwood

Diamond
Joined
Oct 10, 2009
Location
Oregon
Yes I understand that. Maybe it wasn't clear in my initial post but the bar was spinning true when I chucked it up, implying that the jaws where straight.

That would be nice but it is not in the best interest of the well-seasoned machinist to dedicate his work time to teaching a young engineering student the ropes of his trade. So I come here to learn.

Come on now. How many turning shops have you asked if there's a skilled lathe guy you could pay a few bucks to show you the ropes?

When I started out I offered a retired programmer $100/hr to come over and get me running. He was a great guy and would only accept $20/hr because he knew I was desperate and couldn't really afford him.

If you showed up at my shop, had half a brain, some manners and asked I would happily help.

I'm 40 and there aren't many people younger than me who are doing this shit. Most of us appreciate anyone younger taking an interest, but you gotta reach out. This forum won't help much if you don't know what questions to ask. The stuff you're trying to do is pretty indicative of someone who is missing alot of the basics and needs an hour of one on one time from someone who knows their stuff.
 

rklopp

Diamond
Joined
Feb 27, 2001
Location
Redwood City, CA USA
Please stop and make sure you know what you're doing. It sounds like your trying to face the end of a 1" dia. bar with 7" hanging out of the chuck. That's not going to be successful (to say the least) without a steady rest or first facing with, say, <1.5" sticking out, adding a center hole, and then extend out to 7" and bringing in tailstock support.
 

Milland

Diamond
Joined
Jul 6, 2006
Location
Hillsboro, New Hampshire
From the sound of it, the soft jaws were new, uncut with flat ends. So three lines of contact of a soft aluminum trying to hold onto a harder round steel with a lot sticking out.

So perhaps the facing cut put enough deflective force onto the lever (the sticking out bar) to push into a jaw, plastically deforming it and sending everything to heck, even if the bar itself wasn't bent. OP, the jaws are meant to be cut before use so that not only concentricity is good, but surface contact is high, preventing deformation.

As others have said, you really need a more experianced person to show you some of the "real shit yo" that machinists do before you hurt yourself. And think a bit using your engineering knowlege about the event chain that led to failure - that's what that education is for...
 

WakelessFoil

Aluminum
Joined
Aug 18, 2020
Please stop and make sure you know what you're doing. It sounds like your trying to face the end of a 1" dia. bar with 7" hanging out of the chuck. That's not going to be successful (to say the least) without a steady rest or first facing with, say, <1.5" sticking out, adding a center hole, and then extend out to 7" and bringing in tailstock support.
This is the reason I started this thread. I have done a similar operation on a much larger machine but with a six jaw manual chuck and manual lathe. The issue is that I need to face the part before turning a 6" long contour, so either I need 6" hanging out with support, or I need to feed the bar for the turning.

I cannot use a steady rest for two reasons, the turning procedure would interfere with the turning and would need to be removed and replaced each time, and this turning center does not accommodate any kind of bar support other than the chuck and a dead center some 20" away from the chuck.

So I suspect the proper and professional way to do this is to face the part close to the chuck and then use the bar feeder to bring it out to be turned. I just need to design my tool paths to limit the force applied on the end of the bar as not to lead to this same problem.
How about some details. Specifically, what is the diameter of the stock? What is the length of stock that you are clamped on? What is the length of the stock sticking out of the chuck? What is the chuck hydraulic pressure? What kind of jaws (material, smooth/serrated, total grip circumference)? What is the stock material?
Chuck pressure is just over 100 lb/in.
 
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Milland

Diamond
Joined
Jul 6, 2006
Location
Hillsboro, New Hampshire
Also be aware that most power chucks have very limited travel, so you have to adjust the jaws such that after mounting and boring true, they have enough range left to properly apply clamp loads to the part. So double-check this before finishing the jaw bore op.

And speaking of, use a spider or some other method to "preload" the jaws so they're not "free" during the bore cut - that's it's own bit of fiddling such that the jaws can't wobble or slide back and forth, ruining the boring operation. And speaking of spiders, don't use anything less than a tarantula, the smaller guys squish too easily...
 

WakelessFoil

Aluminum
Joined
Aug 18, 2020
From the sound of it, the soft jaws were new, uncut with flat ends. So three lines of contact of a soft aluminum trying to hold onto a harder round steel with a lot sticking out.

So perhaps the facing cut put enough deflective force onto the lever (the sticking out bar) to push into a jaw, plastically deforming it and sending everything to heck, even if the bar itself wasn't bent. OP, the jaws are meant to be cut before use so that not only concentricity is good, but surface contact is high, preventing deformation.

As others have said, you really need a more experianced person to show you some of the "real shit yo" that machinists do before you hurt yourself. And think a bit using your engineering knowlege about the event chain that led to failure - that's what that education is for...
So each jaw is supposed to be cut into a concave shape to hug the OD of the part? Wouldn't that decrease the pressure on the part as you are increasing the surface area? I am actually curious about this because doing this would also result in jaws that are specifically cut for one diameter.
 

WakelessFoil

Aluminum
Joined
Aug 18, 2020
Also be aware that most power chucks have very limited travel, so you have to adjust the jaws such that after mounting and boring true, they have enough range left to properly apply clamp loads to the part. So double-check this before finishing the jaw bore op.

And speaking of, use a spider or some other method to "preload" the jaws so they're not "free" during the bore cut - that's it's own bit of fiddling such that the jaws can't wobble or slide back and forth, ruining the boring operation. And speaking of spiders, don't use anything less than a tarantula, the smaller guys squish too easily...
Yes I am all too familiar with having to adjust these jaws since each one only moves about 1/4" linearly. I will look into spiders and boring the jaws.
 

Mechanola

Stainless
Joined
Mar 21, 2011
Location
Äsch
There are many ways to skin a cat. What I mean to say is that you need not imperatively turn a long bar’s face. You can grind it, skim it flat on a shaper, wipe over it with a milling cutter, and more.

What surprises me is that the tailstock with center should stand 20 inches away and not be free to be brought in closer.

A twelve-to-one ratio round bar to be turned is the typical work between centres. Fun fact is that, within limits, centre bores can be done on bar faces not perfectly perpendicular to the axis. Think things over, the chuck solution is obviously not your best choice. And still before chucks you want to employ collets.
 

rklopp

Diamond
Joined
Feb 27, 2001
Location
Redwood City, CA USA
So each jaw is supposed to be cut into a concave shape to hug the OD of the part? Wouldn't that decrease the pressure on the part as you are increasing the surface area? I am actually curious about this because doing this would also result in jaws that are specifically cut for one diameter.
Yes, it decreases the contact pressure on the part, but the area is much bigger and the force is the same. The jaws are sized to fit the part, but can be bored again to a new size or even re-used every time you have to grip 1" stock. It's best to number the jaws and serrations they fit if you're doing the latter, to get the most repeatability. Otherwise, you'll have to bore again with at least a skim cut.
 

Milland

Diamond
Joined
Jul 6, 2006
Location
Hillsboro, New Hampshire
So each jaw is supposed to be cut into a concave shape to hug the OD of the part? Wouldn't that decrease the pressure on the part as you are increasing the surface area? I am actually curious about this because doing this would also result in jaws that are specifically cut for one diameter.
Yes on jaws being cut for the actual bar, best practice is each time they're mounted. No on pressure loss - if anything, pressure is better distributed, no (or much less) chance of the plastic deformation I mentioned coming into play and lowering overall clamping force.

Would you let us know what your major is? If it's Mechanical, are you first year or second? Not trying to be a jerk (that comes naturally), but you're missing some of the "gut" insight that a good engineer (even in training) needs. You can develop it, but you have to really think about everything that's going on.

I've described machinists as "blue-collar physicists" because we have to understand everthing - forces, vectors, material properties, inertia, gravity - it all comes into play when you're applying loads (cutting) and looking for a positive outcome (a good part, no blood, and a healthy machine).
 








 
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