How much sticking out of the spindle is too much?

Captdave · Oct 6, 2010

I have heard stories of incidents where someone has left stock hanging out of the spindle and things going bad but never saw it happen until today.

Running a bushing job I have done several times in the past out of 3/4" brass in 4' sticks. The closest spindle liner I have is a 1" so I use a length of PVC which is real close to 3/4" ID and cap the RPM to 3500.

With 4' sticks and running real close to the chuck face to avoid waste leaves and tad over 6" outside of the spindle. Set the job up using a short piece to get it dialed in then loaded a full length bar. Boy did things get exciting quickly. Before I could hit the E-stop the whole bar came flying out of the machine :hitsthefan:

Luckily I was still at the control and no one was near the machine when that thing came flying out and hit the ceiling.

Needless to say NOTHING OUTSIDE THE SPINDLE EVER AGAIN!

Where did I put Trusty Cook's number?

Joe788 · Oct 6, 2010

It's always amazing to me how such a small amount of stick-out can cause all hell to break loose. Glad you weren't hurt or killed! I bet your heart was POUNDING though!

SteveinAZ · Oct 6, 2010

Captdave said:
.... tad over 6" outside of the spindle.

I think that answers your question. :confused:

I would not have thought that 6" on a 3/4" bar would be a problem, I may have capped the max at 2500, but either way, that' really not much sticking out.

Steve

SND · Oct 6, 2010

I'm surprised it did that with only 6" sticking out.

I had it happen once(or twice?) with hardinge HLV-H lathes, high and low range, figured ahh its just 1 quick part I'll leave it in low range which I do until the part is almost done and have now forgotten to keep it in low range, and brain just know I should be in high and then... excitement happens. But I had like 2feet sticking out, knocked the garbage bin out of the way

Not as crazy as the time I was told to run 12' SS 3/4bars and part them off in I think 2' lengths, not allowed to chop saw and face, had to part them. Had that thing steadied on the whipping end by in a C clamp on the end of the forklift fork, real safe, thankfully that machine had no high range, 300rpm all the way. If I was asked to do such thing today, my response would be different.

3t3d · Oct 6, 2010

Does that mean that you could not make a part 7" if you had to work the end and not have a tail stock holding it?
How fast could you turn that part to center drill it?

Or is the fact that there was a bit of "tolerance" from the spindle liner, the stick out would be less than the same material held firmly in a collet/chuck?

tacticalcnc · Oct 6, 2010

I run some parts outta cold roll and aluminum that hang outta my collet chuck around 5 inchs long with cuttoff probally around 5.25 inchs, i take up to .100 depth of cuts and run around 3000 rpm and feed around .006 per rev and have never had a problem on 7/8 stock and 3/4 inch, possibly slow your rpm and take lighter depth of cuts is my suggestion

Ox · Oct 6, 2010

6" of 3/4?

You sure that you had it chucked up?

Or was it sticking 6" past the end of your spindle liner? (PVC) And then the liner gave away - allowing 12" of it to float free?

Just kant believe 6" of 3/4 could doo that - even at 3500, but then I don't twist'm that hard either....

---------

Think Snow Eh!
Ox

Captdave · Oct 6, 2010

Or is the fact that there was a bit of "tolerance" from the spindle liner, the stick out would be less than the same material held firmly in a collet/chuck?

I'm sure it was from the slop in the spindle liner / PVC and the vibration that set it off.

JP Machining · Oct 6, 2010

Well, you can tell the Doc you had your heart check taken care of for awhile.

Coppa5 · Oct 6, 2010

Something went way wrong. that should not have happened and theres no way hydraulic chuck lets go of the bar(unless chucked end now looks like a speer point). looks like you didnt have a liner/bushing in the middle between the chuck and pvc pipe. the whip could start in the middle of the bar and translate to the end. once that happens your fkd.

need more info on your setup.
chuck or collet closer?
what type of jaws or collet?
spindle liner placement?
clamping pressure?
did it happen during acceleration, top speed or cutting?

CBlair · Oct 6, 2010

Well here are some lovely warning stickers they put on my Okuma along with a steel spindle liner I made for a job. I let the end stick out about 7" from the back of the machine and there was nothing of the material showing once I loaded the bar. As my lathe is up close to a wall that length is about as long as I can use anyway, approx 48".

Glad to hear all is well Dave.

Charles

cnctoolcat · Oct 6, 2010

Dave,

You get an extra tag end, but you can run (4) 3 foot bars instead of (3) 4 foot bars.

Still need a good spindle liner setup.

Glad you and your QT are OK.

Greg

Kyle Smith · Oct 6, 2010

I had some 1/4" brass go through 3/4" tongue and groove barn siding after forgetting the stock tube on a screw machine. I always thought this would be good for Myth Busters, see if it had enough to go through a person.

metaltech · Oct 6, 2010

Been running, then fixing, CNCs for 34 years. I've got so many stories about bars bending over that my fingers would wear to nubs before I could tell them all here.

Here is what is important to remember: if that stock is perfectly on-center, it's also balanced. If it's just a little off-center, it's heavier on one side. When the RPM goes up, the imbalance becomes greater. Because it's out of balance, the stock wants to bend even more, which makes it more out of balance. See what's happening here? It's amazing how quickly it can go from fine to bending at a right angle, or worse, breaking off. In short, don't stick it out, you just don't know the point when "Okay" turns to "Oh s..." :eek:

wippin' boy · Oct 7, 2010

I'm in the "that ain't really a spindle liner" group
this is a spindle liner

your device seems to be nothing but pressure fit
that will not do

i also think ox is on to something thinking there was less than ideal chucking

i hate to get shty here
but
what if this mess would have come apart 30 seconds later instead of right away
some un named operator could have been on the way back in the house to finish up dinner and been right in the way of this spinning death
the only reason this thing didn't rip someone apart was because it didn't feel like it
anything in a multi thousand rpm spindle has to be bolted in and you need to look seriously at your chucking system, that thing walked right out of that machine and that should never happen.

visionmeister · Oct 7, 2010

This happened in a machine with a spindle liner many years ago on a Hardinge Cobra CNC with 3/4 brass hex stock. The operator at the time was not very experienced, and when cutting stock, cut the bars in random lengths approx 30" +/-. The Cobra could handle 24" inside the spindle.

Some of the bars the guy cut were36-42", so now we're at 12+ inches stick out. He put one of those long bars in, pressed cycle start and as the spindle started to turn, the bar started to run out almost instantly. By the time the spindle was at about 1000-2000 rpm, the bar must have already been bent at 90 deg, it felt like the machine was going to take off.

As in your story, this all happened before the operator could react & hit the E-stop. 12+ inches of that brass hex broke off, hit the concrete floor, taking a golf ball size chunk out of it, bounced up, and stuck into a wood catwalk that was over the machine. If it had gone sideways instead of vertical, it would have easily gone through a person standing there.

The rule in my shop is, ZERO stick out is acceptable

number 2 · Oct 7, 2010

A story like that goes around our shop from a decade ago. Shift change new program. No clamp speed. Unsupported bar. 2000rpm whip n all that. Machine moved a foot. No light weight machine either must weigh a good 5 ton.

tc1999 · Oct 7, 2010

Why didn't you just make a donut for it? I've run lathes for years most not having actual spindle liners and I always just made a precision donut for the side opposite the chuck. Put it about 4-8 inches from the end of the stock and never had a problem. (as long as it is inside the spindle, you will be turning most of the bar perfectly true)

It is way more of a pain in the ass since you have to push it out once you use up the stock to the donut, but at least it is a pretty simple fix to a cheapskate boss (who won't buy a complete set of actual liners)...

And also, I can't believe you didn't ramp up the rpm's...Thank God nobody was hurt and a serious lesson was learned.

Captdave · Oct 7, 2010

Ever have that moment at 3 AM where you sit straight up in bed and say "Oh *&$#"? Well I did. As I was setting that chuck jaws up yesyerday my wife calls (you know when the boss calls she gets your full attention) and I forgot to go back and finishing tighting them up. My 25 yr old son works in the shop as well and I'm always getting on to him about that damn cell phone.

No offense taken wippin boy, I more than deserved a good bitch slappin for that mistake.

I'm using liners from Trusty Cook than are the same as in your picture, bolted in the back side of the chuck actuator except I don't have a 3/4" yet...ordered it this morning.

Yes, God was looking out for my son and me as He always is

706jim · Oct 7, 2010

The rule that I have taught for years regarding unsupported bar stock is 5 times diamater MAXIMUM extension from the chuck jaws. The same number is used for boring bar overhang. The strength of the bar (it is a cantilever) is inversely proportional to the unsupported length to the third power. So if you had a 1" bar sticking out 1" from the jaws or collet, it would be 125X stronger than the same bar sticking out 5" from the jaws. At 6" extension, it becomes 1/216 as strong. The OP mentioned 3/4" brass bar at 6" extension. This is even WORSE than my examples above.

Knowing all of this, I attempted to lightly face and center drill 7/8" SPS at about 6" unsupported length. The bar never whipped, but the tool just TOUCHED the end of the bar and the bar climbed up onto it , breaking the insert.

So these overhang limits really DO make sense!

How much sticking out of the spindle is too much?

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