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# Cutting long stock on CNC Lathe

#### WakelessFoil

##### Aluminum
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
I figured this was gonna be the best solution. Complicates the manufacturing procedure a bit but it seems like the most stable and doable option.

#### David Ferguson

##### Aluminum
And just because you have 10" of stock behind the chuck, remember it can still bend if you run it too fast. That is why CNC lathes often use spindle liners with an ID that is very close to the stock size.

#### WakelessFoil

##### Aluminum
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.
I assume this is why a collet chuck is ideal? Considering that it conforms to the bar and "hugs" it better than 3, spaced out individual jaws?

I understand now what you are saying, but apart from the effects on the clamping security, I do not see what is wrong with the math in post #22. It intuitively makes since for me that if you have a force applied to a surface, the force per square unit decreases as the surface area increases (or at least up to a certain point).

#### angelw

##### Diamond
I assume this is why a collet chuck is ideal? Considering that it conforms to the bar and "hugs" it better than 3, spaced out individual jaws?
The Clamping force from the collet chuck is no better than that of the three-jaw hydraulic chuck. Its greatest claim to fame is that it's gripping force is not affected by centrifugal (centripetal) force. You can get serrated collets but with smooth bore collets, the bore size should match that of the bar being machined. Soft Jaws, machined to the diameter of the workpiece, "hugs", as you put it, the workpiece quite well. Your problem starts and finishes with the Diameter/Length ratio of the material stick out from the chuck jaws.

Using a Tailstock may complicate the manufacture in your mind, but how you're trying to go about it at the moment is never going to work. Just suck it up and set up the job using a Tail Stock. If the job can tolerate a relatively small diameter of the face not being machined, you will be able to:

1. get a Centre Drill in between the retracted tailstock and the end of the workpiece to centre drill the end.

2. advance the tailstock to support the part.

3. face the part, using a 55deg, or 35deg included angle tool, down to a diameter just clear of the tool touching the tailstock centre and then continue to profile the part

Regatds,

Bill

#### EPAIII

##### Diamond
Well, yes if the force is distributed over a larger area then there will be less force per square inch or mm. That is basic physics. But look at it this way, if the jaws have flat faces, then they only contact the stock on a narrow line. We are talking about sideways forces so there is a lot less surface area in contact with the surface and a sideways force is resisted by frictional forces which are proportional to the area in contact (yes I know that has been discussed but more pressure usually means more actual area as the surfaces deform).

Now when the faces of the jaws are curved to wrap around the stock, then there is a lot more area in contact and if the stock tries to rotate sideways, it will be up against the sides of those trenches. So it will be a lot harder to twist it.

As for the soft jaws being only good at one diameter, most soft jaws have a lot of meat on them and can be turned down multiple times. Or they can have different areas turned to different diameters for work that is not as difficult as yours.

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.

#### angelw

##### Diamond
I figured this was gonna be the best solution. Complicates the manufacturing procedure a bit but it seems like the most stable and doable option.
Following are pictures of the type of Centre Drill Tool Holder and the Turning Tool that can be used if you have to centre drill to use a Tail Stock in the program. The stroke of the Tail Stock Quill is greater than the length of the Centre Drill, therefore, you can fit the Centre Dill between the end of the part and the Tail Stock Centre when the Quill is retracted. The Centre Drill Tool assembly could be made even shorter than that shown in the picture, if the centre drill were to be cut in half. However, the length as is works very well.

The Turning Tool shown is a 35deg included angle tool. I wouldn't necessarily use this type of tool as a roughing tool, its shown for example only. A 55deg tool will work close to the Tail Stock Centre just as well and would be a better choice for roughing.

Regards,

Bill

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#### Overland

##### Stainless
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.
I just watched this video again.
I guess the guy hit a stop button, but it sure didn't stop very quickly !
I wonder if he hit the emergency stop - surely that would have stopped it almost instantly, for all the good reasons.
Bob

#### cngbrick

##### Aluminum
I assume this is why a collet chuck is ideal? Considering that it conforms to the bar and "hugs" it better than 3, spaced out individual jaws?

I understand now what you are saying, but apart from the effects on the clamping security, I do not see what is wrong with the math in post #22. It intuitively makes since for me that if you have a force applied to a surface, the force per square unit decreases as the surface area increases (or at least up to a certain point).
With higher force per unit area you are much closer to subjecting the material to local yielding when applying cutting forces. Yielding at the jaw contact line will result in a reduction in clamping force. Having a larger contact area (machined soft jaw) will give you a much larger safety factor before yielding and you will have a better chance of operating within the elastic range of material deformation.

#### 50BMG DUDE

##### Cast Iron
With higher force per unit area you are much closer to subjecting the material to local yielding when applying cutting forces. Yielding at the jaw contact line will result in a reduction in clamping force. Having a larger contact area (machined soft jaw) will give you a much larger safety factor before yielding and you will have a better chance of operating within the elastic range of material deformation.

I watched a bar of 1" 4140 sticking out the left end of an Okuma LB3000 bend and destroy the headstock end of the lathe. Bar was only sticking out 12-13" and was supported in a trusty cook liner. Machine went CSS to face and, BANG CRASH - Happened almost instantly. That bar had a nearly perfect 80 deg bend in it. The setup guy at the time had to go change his shorts, nobody got hurt.

#### rklopp

##### Diamond
I watched a bar of 1" 4140 sticking out the left end of an Okuma LB3000 bend and destroy the headstock end of the lathe. Bar was only sticking out 12-13" and was supported in a trusty cook liner. Machine went CSS to face and, BANG CRASH - Happened almost instantly. That bar had a nearly perfect 80 deg bend in it. The setup guy at the time had to go change his shorts, nobody got hurt.
I investigated a very similar accident that resulted in a fatality. The bar was sticking out a little further, bent, and whipped. There was no support, however. The decedent was found with his arm torn off, bled to death. Nobody knows exactly what happened because the fellow was working alone on a holiday eve. The other oddity was that the bar was not the normal material for the parts the lathe usually turned.

#### Cole2534

##### Diamond
I investigated a very similar accident that resulted in a fatality. The bar was sticking out a little further, bent, and whipped. There was no support, however. The decedent was found with his arm torn off, bled to death. Nobody knows exactly what happened because the fellow was working alone on a holiday eve. The other oddity was that the bar was not the normal material for the parts the lathe usually turned.
I betcha it went something like.....

DEAD GUY: No one else is here, if I just hurry up and slide my personal/moonlight/govt work through this machine while no one is looking then no harm no foul.

Few other places on Earth does haste make waste quite like a machine shop.

#### Milland

##### Diamond
I investigated a very similar accident that resulted in a fatality. The bar was sticking out a little further, bent, and whipped. There was no support, however. The decedent was found with his arm torn off, bled to death.

Ugh - while it beats (?) being pulled into a lathe spindle, that's not a good way to go. I'll admit to prefering milling work, as I generally think it's "safer" than lathe work. Maybe just a dumb bias...

#### RC Mech

##### Stainless
I just watched this video again.
I guess the guy hit a stop button, but it sure didn't stop very quickly !
I wonder if he hit the emergency stop - surely that would have stopped it almost instantly, for all the good reasons.
Bob

Any older lathe, some newer, will not activate the braking circuit of the spindle on E-stop. It’ll just cut power and coast down. Ditto for mill spindles.

#### rklopp

##### Diamond
Ugh - while it beats (?) being pulled into a lathe spindle, that's not a good way to go. I'll admit to prefering milling work, as I generally think it's "safer" than lathe work. Maybe just a dumb bias...
Yeah, then there was the vacuum-chucked plate that frisbeed through a VMC enclosure and hit some poor guy in the jaw. A long 3/4" endmill at 12,000 RPM can put a lot of energy into a plate the size of a sheet of paper and 1/4" thick.

#### nissan300ztt

##### Hot Rolled
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
You will need a center in your part for this. I tell engineers all the time that long parts like this if not being cut on a swiss turn machine(the part moves forward and backward and tools only move up and down) you will almost always need a center. The longest parts I make are 1/2" Diameter 304SS rod that needs to be turned down to .460" over 11". The only way other than using a Centerless grinding machine is by use a #2 center drill about .150" deep and face then use a live center.

#### Mtndew

##### Diamond
I figured this was gonna be the best solution. Complicates the manufacturing procedure a bit but it seems like the most stable and doable option.
If a tailstock hinders the manufacturing side of things, consider getting a spindle liner so you would only then need an inch or so hanging out of the chuck to do the end work.

https://www.trustycook.com/products/spindle-liners/

But first, I'd say you need someone to educate you on how to run a cnc lathe before you end up hurting or even worse, killing someone.
Not knowing that you need to bore the soft jaws is a major red flag and your boss should be fired for letting you anywhere near that machine.
Not really your fault because you don't know what you don't know.

##### Diamond
If a tailstock hinders the manufacturing side of things, consider getting a spindle liner so you would only then need an inch or so hanging out of the chuck to do the end work.

https://www.trustycook.com/products/spindle-liners/

But first, I'd say you need someone to educate you on how to run a cnc lathe before you end up hurting or even worse, killing someone.
Not knowing that you need to bore the soft jaws is a major red flag and your boss should be fired for letting you anywhere near that machine.
Not really your fault because you don't know what you don't know.

Based on some of his questions, and his phrasing of "we are" in some of his older posts, I kinda don't think his boss knows any better, either.

#### WakelessFoil

##### Aluminum
Based on some of his questions, and his phrasing of "we are" in some of his older posts, I kinda don't think his boss knows any better, either.
Allow me to clarify a bit. This is as much of a hobby and personal challenge for me as it is a job. I like trying to figure out how things work. I accept that there is inherent danger in running any piece of machinery with limited or no experience.

That being said, I come here to get that experience so I can excel, I know you all possess much more than I do. Also, the bar never left the chuck. It rattled a bit but was held in place.

#### cngbrick

##### Aluminum
Do you have access to a community college that offers night courses in machining? Hands-on instruction from a qualified instructor would go a long way in furthering your understanding the mechanics of work holding and forces involved in cutting metal. A course would also address safety in the machine shop.

Most of the advice given on this site assumes a basic level of competence in machining. The questions you ask and your interpretation of the advice given are frankly very worrying. You are at risk of doing damage or harm to your equipment, to yourself or to a bystander.

If we flipped this situation around: as an EE would you feel ok about giving advice to a machinist on where to put his voltmeter probes to measure the voltage on a 3kV motor driving a mineshaft elevator? Simply asking this question would betray a significant lack of understanding of the dangers surrounding this situation.

#### WakelessFoil

##### Aluminum
Do you have access to a community college that offers night courses in machining? Hands-on instruction from a qualified instructor would go a long way in furthering your understanding the mechanics of work holding and forces involved in cutting metal. A course would also address safety in the machine shop.

Most of the advice given on this site assumes a basic level of competence in machining. The questions you ask and your interpretation of the advice given are frankly very worrying. You are at risk of doing damage or harm to your equipment, to yourself or to a bystander.

If we flipped this situation around: as an EE would you feel ok about giving advice to a machinist on where to put his voltmeter probes to measure the voltage on a 3kV motor driving a mineshaft elevator? Simply asking this question would betray a significant lack of understanding of the dangers surrounding this situation.
The university I am attending has only one machinist, and the engineering department is poorly outfitted IMO. I guess the bureaucrats up top think the arts are more important.

I have worked in a machine shop (manual and CNC) for over three years now. I never had a professional machinist as an instructor but I am surrounded by guys who have worked in related technical trades via a metalworking society here in Virginia. I have worked on dozens of projects almost all of which were completed without loss of quality or limb (knock on wood). I would argue that my knowledge of soft jaws is not a perfect metric for measuring my experience with metalworking in general. Regardless, I still consider myself a beginner seeking knowledge in a hobby I wish to pursue.

And I would tell the machinist where to put his voltmeter, on the basis that if I do not, he would do it anyway without guidance because he still has a job to do. I would share my experience and knowledge, rather then engaging in an internet ego battle.

-Justin

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