Killed a Mori today!!!

[WOLOG]

This morning I woke up with a weird feeling. By 4 o'clock this afternoon, I knew why. I just finished setting up and programming a part on my NL-2500SY. I ran simulation, read through the code and said to my operator "Good to go!" I walked to my office to write another program for monday morning and order tooling for next weeks jobs. My door opens and my manual machinist says to me "You may want to go to the back of the shop. It sounded like a helicopter just landed on the roof!" I got up from my desk and started for the shop. As soon as I rounded the corner, from 250 feet away, I saw my welder and 2 operators staring at something behind the 2500. When I cleared the NL-3000, I saw what used to be a straight piece of 1 1/4" 4140 bar stock now bent 45 degrees hanging out of the rear of the machine. The chip catcher for the draw tube is crunched. The wall behind the machine has a hole in it. The aluminum housing for the draw tube cylinder has been knocked out of alignment. And after evaluating the damage, I realize that the brake disc is tuning .045" out of true. I think the entire spindle is bent. And you may ask how did that happen! Because my operator with 10 plus years of experience asked an operator with 3 years if he thought 18" of material sticking out of the draw tube would run OK!!!!!!!!! I guess I am at fault because I did not verify the machine was actually "GOOD TO GO" Boy, this is gonna be expensive. I will be sure to post pictures of the "OID" (operator induced destruction) I'm really looking forward to the report I will have to write Monday.

[tobyaxis]

I had an operator do this with their 10 plus years experience. They were fired on the spot.

Another threw a 8Lb. Disk through the top of the Lathe that also went through the roof of the shop landing one parking lot over. Luckily there wasn't a car in that spot that day.

Scary when someone that is supposed to be experienced can't cut jaws or know what amount of chuck pressure they should be using.

This taught me to check everything after the operator touches a machine no matter what experience they say they have. They might be insulted, but that's tough cookies. Takes more time but at least nothing gets wrecked.

A down machine means no money and I really do not like explaining to the boss that they hired an idiot.

Sorry to hear about your misfortune WOLOG.

At least no one got hurt. That is always a plus.

[Technical Ted]

Just be glad that nobody got hurt. We had a guy do the same thing but the bar broke off & went flying about 100 feet through the shop. It certainly could have killed someone if they were in its path.

I've done some bar feeding over the years but not a lot. I would think that a 1 1/4" bar sticking out 18" would have been fine if the max RPM had been limited to under 1,000.

Hopefully nobody gets fired. I'm sure this operator won't make that mistake again. Sometimes we have to learn by our mistakes & sometimes mistakes are costly.

[CalG]

Exactly

How does one "become experienced" with how much stock can be left hanging?

No one ever showed the formula.

For this? Fired! That is small.

I do hope the damage can be restored with less than a spindle replacement. And like a car wreck, if no one is injured or killed, the rest can be replaced.



Regards all

[DSL PWR]

Well I'm not so sympathetic...

I have to wonder if you guys (TT, and CalG) have your own shop, or work for a company (even not related to cnc). If Wolog needs a new spindle it will probably be 50k+, and downtime. The total cost of this learning experience could be over 100k.

If it were me, that operator would be lucky if the door didn't hit his a$$ on the way out!

[Technical Ted]

I'm not an owner but have been a machinist for 30+ years. Even the best make mistakes from time to time. The guy who had this "crash" is a lot less liely to make the same mistake as someone hired to replace him. I have a couple questions for you (DSL PWR).

#1 If an operator made the same error without causing any damage would you still fire him?

#2 How far should he have let the bar stick out?

[DSL PWR]

Well if we are all allowed to make mistakes, (and since you think it would be a mistake to fire him) I would rather fire him and know that he won't be able to kill himself or any others in my shop ever again.

"#1 If an operator made the same error without causing any damage would you still fire him?"

Yes I would fire him, and I can't see how that couldn't cause damage. Anything past about 18 inches of length in the draw tube will generaly cause vibrations in the cut anyway, it is a whole different story if you have a liner.


"#2 How far should he have let the bar stick out?"

In free air on an 8 or 10 inch chuck? NOT AT ALL! All of the work should be contained with in the draw tube. I know in some applications (drill stem for example) the work sticks out 20+ feet, but there are generally 2 chucks on the lathe ,(these generally are not hydraulic chucks) and the work is supported very well on top and bottom rollers. They also turn the work quite slow.

On this Mori to have 18 inches sticking out the operator would have started with about a 6 foot bar. So TT if you ever have a chance take a 6 foot bar, clamp it in a 8 or 10 inch chuck with about 3 inches sticking out of the nose. Next max out the chuck pressure, and mount a dial indicator on the turret and touch it on the end of the stock. Now grab the free air end of the bar and push on it. You will produce enough force to overcome the hydraulics in the chuck. Remember this is on a stationary chuck, the rotation forces give the chuck less grip as you speed up.

So once the shaft gets just a little bit of play on the out board end, the chuck starts to loosen off, and the wobble gets bigger, and you have OID.

[ckirchen]

The formula I like to use is:
> barstock length past the spindle opening = zero

Yes, it sometimes makes things difficult, especially since it always seems to result in at least one cut with the saw before you can load the lathe, but in my opinion, it is the safest.

I also believe that clamping the spindle speed is not enough. What happens if a programmer knows that 1,000 rpm is "ok", but accidentally enters 10,000? Some controllers will let that pass and clamp to the maximum spindle speed. There is also the fact that the spindle rpm can be overridden. For example, if 1,000 rpm is "safe" and has been proven out, what happens if the operator slowly increases the spindle to 120% over time; that might be enough to push the barstock into bending. Or what about accidentally hitting spindle rpm override when he meant to hit feed rate override?

It was explained to me that, for a given piece of barstock of given length and diameter, there is a certain rpm, when enough centripetal force is generated to overcome the barstock's strength, and bending begins. And as soon as bending begins, there is no stopping it, because the further away the barstock gets from the spindle centerline, the more centripetal force there is to continue bending.

The problem is that the only way you can find that rpm is by going past it, because there are almost no differences in behaviour between 10% and 90% of that rpm. Also, if the barstock inconsistant from one piece to another, the "safe" rpm for one piece may not be safe for the next. And if the barstock has been weakened, well, we've all heard the one about the shop owner who marked the barstock with a light hacksaw cut to tell the operator when to stop making parts.

I've even seen bar feeders lose control of the barstock. Granted, that's part of what they are designed to do (contain the barstock), but they do get beat up pretty bad. Now take away all the iron and sheetmetal... That's why I will never hang stock past the spindle opening, unless there is a barfeeder right next to it.

Chris Kirchen

[CalG]

We should ALL propagate this standard. Educate and support our valued employees.

"Bar stock shall not hang out of the back end of spindle unless contained within a bar feeder ".

Simple, but I have never heard it said in any shop I've been in.

CalG

(Jeesh I've broken that rule by at least a foot in 1.25 inch 6061 bar, but no more!! ;-)

[Technical Ted]

[QUOTE=DSL PWR;604581]Well if we are all allowed to make mistakes, /QUOTE]

I'm sorry - I didn't realize we were in the company of someone who has never made a mistake. I bow to you. (Please don't take offense, I'm just trying to make a point.)

I think its a cop out to say you wouldn't let a bar stick out even a few inches. There could be plenty of times when that could raise the material cost by 20% or even more. If that is your company policy & someone violated it then they certainly should be fired. In certain shops over a couple years that policy could cost you more than the price of a new machine. (I'm not suggesting safety be compromised to get another piece or two out of a bar.)

I also don't understand why you'd be so quick to fire an operator if you don't have a policy on how far a bar can or can't stick out. Who knows a what length a certain diameter bar will bend? If the operator shouldn't have had the bar sticking out past the tube then why was that length of bar ordered?

[DSL PWR]

TT, it isn't a cop out, it's plain and simple, no unsuported work outside of the draw tube!

Not every shop orders specific lengths of material. Most of it comes in 20 foot pieces.

[Technical Ted]

I have no problem with that policy. It's very clear cut for the operator. However, I doubt many other shops work that way.