scary mishap today ...

[wquiles]

First of all, I am not hurt or anything, well, maybe just my pride as an apprentice machinist. And as embarrassing as it was, I rather share it in the open so that hopefully others don't make the same mistake I did ...

I am working on a project that required me to make some new 10mm screws, about 60mm long, with the very rare 1.0mm pitch. Since the screw is not going to get stress or any kind, I decided to use 12L leaded steel.

I had a long piece, and since I know I was going to have about 2 feet of overhang past the end of the spindle, I used my spider to center the piece as best as I could:

My initial operation was going to be a simple facing, so that I could drill a tiny centered, pilot hole, to use a live end while threading, as I wanted to thread the 2-3 screws all in one operation:

I used the lathe's momentary button, and it seems to be in OK balance, so I setup my tool, the AccuRite lubber, the lamps, the safety shield, etc. - nothing new. I then made the one mistake that caused all this problem. I started the lathe, at the high speed I had before, which was 1200 RPM.

Everything seemed fine for a about 10 seconds, while I moved the cutter to the piece of steel, then all of the sudden, all hell broke loose. The machine started vibrating a LOT, and right in front of my eyes the 1200 pound lathe started MOVING away from me!. It took me a second or two to realize oh crap (well, I mentally used a stronger word!), and I calmly told myself: stop the machine - turn it off.

It all seem to happen in slow motion. It took me (what seem at the time) like an eternity to think what I am supposed to do to turn the lathe off. As soon as I realized my hand was near the ON/OFF lever, I turn the lathe off. And everything stopped quickly and nicely.

As I stepped back from the lathe, this is the first thing I saw:

and then, even more horrifying, I notice this:

Yup, the lathe vibrated about 4 inches away from me in the quick time it started vibrating and the time it took me to realize the problem and turn it off.

Here is the offending piece of steel (about 4 foot long):

I was very calm, which surprised me a lot. I was also not hurt, at least not physically. And by virtue of standing in the right place, the safe place to be in a lathe, to the right of the chuck, I was never in danger of getting hit by the now-bent-piece of steel, which just a few seconds ago was flipping around at 1200 RPMs. Yup, I got lucky today.

Of course, after reflecting on what went wrong (and mental pictures of how bad could it been), I noted the three very important lessons for future work:
- don't have that much over hang - EVER!
- start at much, much slower speeds!
- next time keep the foot close to the lathe brake - just in case!

Once I reflected on those lessons, I reposition the lathe:

Re aligned the lathe with my Starrett level:

Checked everything on the lathe, once over, making sure everything was tight. I then cut the piece of leaded steel so that the whole piece was inside the spindle, and proceeded to continue on the job, of course, at a more reasonable 500 rpm's:

That is all. Please remember my mishap today, and hopefully you will learn from it, just as I did.

 

[Illum]

I'm not a machinist but I cringed when I read "and right in front of my eyes the 1200 pound lathe started MOVING away from me!"

 

[ninemm]

That's pretty nuts. Glad you didn't get hurt! Thanks for sharing, hopefully others won't make the same mistake.

 

[wquiles]

I'm not a machinist but I cringed when I read "and right in front of my eyes the 1200 pound lathe started MOVING away from me!"

Yup. It was definitely quite a sight. I did not expected that the lathe had travel that much, until I looked down and saw the old marks against the final resting position.

That's pretty nuts. Glad you didn't get hurt! Thanks for sharing, hopefully others won't make the same mistake.

Thank you. I know I got lucky today.

If my mistake and me sharing it helps prevent this to at least one person out there, it was worth the embarrassing feeling I had in sharing this stupid mistake openly.

 

[saabluster]

As soon as I saw that first shot I knew what had happened. Funny how the brain just skips town sometimes. I am soo glad you didn't get hurt. It blows my mind to think that bar could move the lathe. Hey maybe that would be an easy way to move it around from now on. Just stick that bar back in, crank the ol lathe up and push it where you want it.

 

[Norm]

Lucky it wasn't a bit longer it would have been beating your BMW on the bonnet
Norm

 

[Chauncey Gardner]

Very scary. I'd check the lathe out very carefully also.

That kind of force with the out of balance weight at that rpm.....well, you saw how far it moved the 1,200 lb lathe.

Your description of the slow mo the brain does under extreme stress (much like what happens before serious physical trauma) was enough to make me squirm around a bit.

Glad you are ok, but that was very nearly something very bad.

Just a tiny bit of sag from your bar stock would cause this to happen given the length of overhang & rotational speed.

Don't even want to think what your leg (or arm) would have looked like if you had moved, slipped or gotten too curious...

Good post. It's amazing sometimes in retrospect how we avoid getting wrecked (or do anyway & see it coming in super-slo-mo) & look at what we did & say, "what was I thinking?".

 

[Th232]

As soon as I saw that first shot I knew what had happened.

+1. Also glad the car wasn't damaged. Bit hard to tell with that 3rd pic, how far was the end of the bar from the car?

 

[Per-Sev]

I was a machinist for 16 years and I have never seen anything like that before. You said you used 12L steel I take it that is a very soft steel. I am glad you are still here to tell us about this I guess it had more run out then you thought. I have had it bang around inside the spindle but I ran a 24" dia swing lathe and could turn up to 26'' dia using a boring bar in the front of the tool rest.

 

[gadget_lover]

Before I read the first line I saw that picture and knew the majority of the story. Knowing the size of your lathe I knew that long, thick stick-o-steel there was just waiting to become a wrecking bar.

Boy, are you lucky!

Thanks for sharing.

Dan

 

[precisionworks]

As soon as I saw that first shot I knew what had happened.

+1

That's pretty common, I did the same thing some years back. If your lathe has a "jog" button, that can help avoid disaster - I always jog the spindle first before starting any new op to make sure all the settings are correct, that the part is centered, etc. It adds a few seconds to the process but quickly indicates an incorrect setup.

check the lathe out very carefully also

+1

There's a pretty good chance that you'll have to reset the Set-Tru screws, especially if the vibration caused any of them to back out.

Not a bad time to check the bearing preload. I believe your machine has a 1.5" spindle bore, if so you'll need a 24" length of 1.5" solid round stock, although heavy walled tube like Schedule 80 will also work. Remove the chuck & insert 12" of the bar into the headstock. Bring a dial test indicator into contact with the spindle nose & pull hard on the end of the bar. The indicator should show at least .0005" movement, but no more than .0010". Repeat the test by pushing hard, readings should be the same. Adjust preload if needed following the well written Chinglish instructions

 

[TorchBoy]

I'm not a machinist either, and when I saw that first photo my only thought was "I hope he's not going to spin it quickly." I guess quick is relative. I've done similar things with a masonry drill bit (or two). Thanks for a good write-up and the lesson we can learn from it.

 

[Tom Anderson]

Scary stuff!

Thankfully, you weren't injured!

 

[wquiles]

If your lathe has a "jog" button, that can help avoid disaster - I always jog the spindle first before starting any new op to make sure all the settings are correct, that the part is centered, etc. It adds a few seconds to the process but quickly indicates an incorrect setup.

My lathe has a jog button, and I do use it a lot. I used it on this job as well, and for the 2-3 seconds of the jog, everything was fine. It was not after the 10+ seconds of actual running that all hell broke loose. I guess that once I started to move the carriage, cutter, etc., everything aligned "just right" for the harmonic frequency to start making the shake. Once it started, it only got worst - again, it all happened very quickly. Of course, if I would had started much slower ...

Definitely a mistake I will never do again :thumbsup:

There's a pretty good chance that you'll have to reset the Set-Tru screws, especially if the vibration caused any of them to back out.

Not a bad time to check the bearing preload. I believe your machine has a 1.5" spindle bore, if so you'll need a 24" length of 1.5" solid round stock, although heavy walled tube like Schedule 80 will also work. Remove the chuck & insert 12" of the bar into the headstock. Bring a dial test indicator into contact with the spindle nose & pull hard on the end of the bar. The indicator should show at least .0005" movement, but no more than .0010". Repeat the test by pushing hard, readings should be the same. Adjust preload if needed following the well written Chinglish instructions

Ever since I saw how you adjusted your bearing pre-load I have been wanting to check it, and since I also own one of those infrared temp sensors, this is the perfect "excuse" to do it.

The other thing I must do is to find a way to mount the lathe in a more stable/solid position. I have never liked how top heavy my setup is, and I have always found the base/feet too narrow for my taste. Myself and others with these 12x36 size lathes have commented in some of the newsgroups that these lathes could really benefit from a much heavier/wider base, so this "incident" just re-affirms this fact. There is just too much flex on those mounting cabinets - I definitely need something more sturdy and bottom heavy to get a more solid setup.

 

[F250XLT]

Scary experience indeed, glad you are okay. I also applaud you decision to post it here, I am sure it will help educate aspiring machinists.

 

[Illum]

Scary experience indeed, glad you are okay. I also applaud you decision to post it here, I am sure it will help educate aspiring machinists.

makes me wonder how well you could vent a garage if you mount a propeller in the chuck....::green:

being that close to the car if you had been hesitant for a couple seconds longer there could have been a hefty scratch near the tire well
could the motor assembly be damaged through such an unbalanced load?

 

[KC2IXE]

As soon as I saw that first shot I knew what had happened....snip...

Yep - saw the picture, first thought was - Oh, he didn't

BTDT, most of us probably have. No harm, no foul in this case. Now you need to get some outside supports

 

[gadget_lover]

Yep - saw the picture, first thought was - Oh, he didn't

BTDT, most of us probably have. No harm, no foul in this case. Now you need to get some outside supports

I was going to mention that, but figured it might encourage bad behavior.

Here's my understanding;

The idea behind outboard support is that it lessens the oscillations to the point where the rod never gets far enough off center to allow centrifugal force to overcome the stiffness. The further off center, the faster it is moving. Moving faster = more force. More force = more deflection = moving faster.

If you constrain it in any two planes it will not be able to continue that cycle.

I hope that is accurate enough for general understanding.

Daniel

 

[HarryN]

One way to improve the hold down is to bolt it to the floor. The normal method is to drill some holes into the concrete through a convenient location in the base, and epoxy some all thread into the concrete. A few nuts and lock washers and you are finished.

This is used all of the time in earthquake prone areas.

 

[KC2IXE]

actually, I find that for a stock stand, a PIPE the same ID as you spindle thru on a stand - actually a couple of them works fine - slide the stock through