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Cutting long stock on CNC Lathe

Cole2534

Diamond
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.
What make and model is the turning center? I'm fairly certain that if there's a tailstock present it will be powered, just takes some understanding of the code to do so....and also creates a crash hazard. BTDT.

WakelessFoil

Aluminum
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).
I figured that the force would be constant but the lb / in^2 would decrease as you are increasing the inches without increasing the pounds. If I apply one pound per square inch I have 1 lb/in^2, but if I double that surface area without changing the force then I have only 1/2 lb/in^2. Again this may result in a better clamping arrangement for this application but the force per unit of SA would be less?

I am a second year EE. If I am really missing what it takes to become a good engineer then I ought to just switch to liberal arts! Ha! I understand the physics of what caused this failure but I don't have the decades of experience to know all the tricks of the trade. In other words, there is a difference between knowing how to design a part and actually knowing how to make it and use the tools (I would say the CNC turning center is a bit less intuitive than a hammer). I agree a good engineer knows how to do both, but that's why I took this job!

I don't want to stray too far off topic, but for context I was six years old when you joined this forum.

WakelessFoil

Aluminum
What make and model is the turning center? I'm fairly certain that if there's a tailstock present it will be powered, just takes some understanding of the code to do so....and also creates a crash hazard. BTDT.
Nakamura Slant JR. Fanuc 0TC.

It only extends about six inches, maybe it's just a cold day?

Is there a M code to control the length of extension?

Cole2534

Diamond
Nakamura Slant JR. Fanuc 0TC.

It only extends about six inches, maybe it's just a cold day?

Is there a M code to control the length of extension?
Check the M-Code list, it may require separate calls to move the tailstock quill (which is what I think you're seeing move ~6") and the tailstock body.

This vid is for Mazak controls but goes over the basics of use.

Garwood

Diamond
Nakamura Slant JR. Fanuc 0TC.

It only extends about six inches, maybe it's just a cold day?

Is there a M code to control the length of extension?

The entire tailstock is on ways. It will move towards the chuck. Then you use the ram.

RC Mech

Stainless
I am a second year EE. If I am really missing what it takes to become a good engineer then I ought to just switch to liberal arts! Ha!

Nah, don’t worry about that. There are hundreds of shitty “engineers” being pumped out of universities every year.

Stop with your bullshit about pressure the jaws are exerting. The name of the game here is contact area. You’re trying to use soft jaws as a substitution for hard jaws. For what you’re describing, you need hard jaws.

And keep your tie away from the spinny bits.

Cole2534

Diamond
I am a second year EE. If I am really missing what it takes to become a good engineer then I ought to just switch to liberal arts!
There are PLENTY of hack engineers in the world that make EXCELLENT money, \$250k/yr or more. I work with a bunch of them.

RC Mech

Stainless
There are PLENTY of hack engineers in the world that make EXCELLENT money, \$250k/yr or more. I work with a bunch of them.

Can you send me an application?

Milland

Diamond
I figured that the force would be constant but the lb / in^2 would decrease as you are increasing the inches without increasing the pounds. If I apply one pound per square inch I have 1 lb/in^2, but if I double that surface area without changing the force then I have only 1/2 lb/in^2. Again this may result in a better clamping arrangement for this application but the force per unit of SA would be less?
From a practical standpoint, you can think of clamping metallic solids as being rigid, non-compressible (as long as there's area contact, not point or line) objects, so more clamp pressure is better than less, especially if you don't know the condition of the chuck (worn or dirty power chucks may not "translate" the hydraulic pressure to jaw pressure as efficiently). Without knowing the dynamics of your lathe I can't say if 100psi was good, high, or low.

As for contact pressure, you may have lower areal force, but from a gripping and stability standpoint you're better off with the circumference of the material being held by the circular segmented faces of cut jaws. That matters more in this context.

I am a second year EE. If I am really missing what it takes to become a good engineer then I ought to just switch to liberal arts! Ha!
No worries, and I really wasn't trying to be a jerk. But your being a non-ME doesn't surprise me, as many ME students I've met (I instructed at MIT) did start out as grubby kids, playing with bikes and construction toys and getting their hands dirty early on. It does give an easier transition to proper ME or machining comprehension later on.

I don't want to stray too far off topic, but for context I was six years old when you joined this forum.
When I joined PM I was also six! I was a precocious kid...

OK, maybe a little older than that. And nice sneaky way to tell me to fluff off, Boomer...

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WakelessFoil

Aluminum
Nah, don’t worry about that. There are hundreds of shitty “engineers” being pumped out of universities every year.

Stop with your bullshit about pressure the jaws are exerting. The name of the game here is contact area. You’re trying to use soft jaws as a substitution for hard jaws. For what you’re describing, you need hard jaws.

And keep your tie away from the spinny bits.
My apologies if I offended you. Just trying to gain some experience here.

Milland

Diamond
My apologies if I offended you. Just trying to gain some experience here.
Don't take anything personally, we're a gruff bunch of grumps - it goes with the profession. If you're trying to learn that's what matters.

doug925

Titanium
Wakelessfoil
Watch this video (there are plenty of differing videos of this) regarding the boring of jaws.

WakelessFoil

Aluminum
Wakelessfoil
Watch this video (there are plenty of differing videos of this) regarding the boring of jaws.

Excellent video. I did not know that there where so many aspects to soft jaws.

604Pook

Cast Iron
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):

Big old Traub CNC Lathe we previously had, first owner when it was brand new, had an employee killed, doing this very thing. He heard some noise and looked around the end of the lathe to find the bar whipping, hit him in the head.

You really need to respect these machines.

Milland

Diamond
Big old Traub CNC Lathe we previously had, first owner when it was brand new, had an employee killed, doing this very thing. He heard some noise and looked around the end of the lathe to find the bar whipping, hit him in the head.

You really need to respect these machines.

The comments for that video speak of at least one other vid online that shows a death, probably not on Youtube though (unless it stuck by their monitors). I did have a much smaller bar whip on me one time when I was still green, fortunately only 1/4" rod and not that long, no injuries or machine damage. Woke me up a bit...

50BMG DUDE

Cast Iron
Nakamura Slant JR. Fanuc 0TC.

It only extends about six inches, maybe it's just a cold day?

Is there a M code to control the length of extension?

There are clamo bolts holding the tailstock in place. You loosen them and then (most likely) use a shot pin on the turret to drag the T-stock approx into position. Unlock shot pin, re-clamp T-stock. Now you use the quil on the T-stock to support your work. This is normally part of the setup for each different part as lengths will vary.

You will still need to chuck short, face and c-drill, uncluamp chuck, bar feed out to length, clamp, t-stock quil advance. run part

angelw

Diamond
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.
What Garwood said in Post #11, but in the very least, find a book, even if for manual machining and read up on the basic fundamentals of lathe operation and machining.

You stated in your other Threads dealing with programming, that you learn better by "hands on" and not so much from reading books. Well, the myriad of mistakes you made there didn't reinforce that philosophy. All that did was to result in a copious number of Posts to deal with programming issues that should have been done and dusted in one reply had you bent a little and read the book, or at least taken the advice given without it having to be repeated a number of times.

You can get away with making programming errors when leaning and taking the precaution to test your programs in Fresh Air, but when you start to machine parts and have them come out of the chuck, you, or someone close by, may only have that one opportunity to see the mistake.
The bar jumped out of the 3 jaw Chuck we are using
The bar is about an inch in diameter, 12" long. About 5" in the chuck with the rest hanging out.
With that Diameter/Length ratio sticking out of the chuck, it's never going to work, but I find it hard to believe that 5" of the material in the chuck would pull out with a light facing cut. Its bound to chatter the moment the tool makes contact and anyone with minuscule experience should have the sense to stop at that point. Me thinks the setup isn't quite how you're describing it.

Either get someone experienced to give you some instruction, or hit the books on basic machining before you go much further.

Regards,

Bill

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rklopp

Diamond
If you go back to your statics book, you'll see that the friction force is the coefficient of friction times the normal force. There is no consideration of area and hence no consideration of contact pressure. That is why the physics in post #22 is incorrect. The main thing that counts for your chucking is the squeeze force. Conforming jaws are better than pointy jaws for a lot of reasons, but friction force is not one of them.

WakelessFoil

Aluminum
What Garwood said in Post #11, but in the very least, find a book, even if for manual machining and read up on the basic fundamentals of lathe operation and machining.

You stated in your other Threads dealing with programming, that you learn better by "hands on" and not so much from reading books. Well, the myriad of mistakes you made there didn't reinforce that philosophy. All that did was to result in a copious number of Posts to deal with programming issues that should have been done and dusted in one reply had you bent a little and read the book, or at least taken the advice given without it having to be repeated a number of times.

You can get away with making programming errors when leaning and taking the precaution to test your programs in Fresh Air, but when you start to machine parts and have them come out of the chuck, you, or someone close by, may only have that one opportunity to see the mistake.

With that Diameter/Length ratio sticking out of the chuck, it's never going to work, but I find it hard to believe that 5" of the material in the chuck would pull out with a light facing cut. Its bound to chatter the moment the tool makes contact and anyone with minuscule experience should have the sense to stop at that point. Me thinks the setup isn't quite how you're describing it.

Either get someone experienced to give you some instruction, or hit the books on basic machining before you go much further.

Regards,

Bill
When I say jumped out of the chuck I don't mean the bar left the spindle, that's my error for not explaining it well. Once the cutter touched the end of the stock it chattered a bit and then the bar began to rotate eccentrically. I was taking a real light face at an extremely low depth of cut, I have performed a similar operation on larger manual machines, so I didn't think much of the weak clamping situation. I was also operating the machine manually so I was able to stop everything immediately after I noticed the issue.

Now, the larger lathe I performed this on had a massive 6-jaw chuck. So that is what I wanted to ask here. What is acceptable when it comes to chucking and supporting stock in a lathe, and what kind of factors play into how sound the setup is. How might one (if proper) turn a contour on a long part (6" long / 1" dia.) without use of the tailstock or any kind of steady/follow rest.

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