I have been using them for years at about 1000, seldom over 1200RPM, without the tailstock pulled up and have never encountered a problem. If, however, I am working on anything larger than an inch or so of left over tenon I have always pulled up the tailstock and not worried about the speed, up to about 1700RPM where the sound of them attempting to take flight becomes unnerving.
I have a large plywood additions/extensions attached to a separate set, handling anything up to roughly 29 inches and those I run slow - never more than about 800RPM as I am concerned they actually WILL take flight! Since I am working on a 16" lathe the larger ones must always be in an outboard position, precluding the use of the tailstock.
That is a spectacular failure of a Longworth Chuck. It is truly fortunate that no one was seriously hurt.
How fast was it going and what was loaded on it at the time? Was the tailstock involved? Does this tell us anything about chucks made from phenolic formaldehyde resins? This raises far more questions than the speed and load. Any chemists or structural/material engineers out there who can weigh in?
Other than as general caution does this have any bearing on a set of Cole jaws with their eight screw fasteners and metal construction? I really have no idea the answers to these questions, I just know that that is one terrifying set of photos.
By way of explanation of my previous note, I absolutely agree that using any tool beyond the recommended limits placed on it by the manufacturer is potentially more dangerous than using it within the prescribed limits. I also accept the full responsibility for my actions when I do so, as should we all.
When we are turning speed is our friend - until it is not. The better cut and reduced sanding must be weighed against the potential failure of our equipment and our technique. Be careful, be smart, and know that when it blows up it is likely due to something you did or did not do.
I don't remember the 600 rpm max on the jaws but I did read it in the manual. I'm a compulsive manual reader (and re-reader).
I've used them often above 600 but not above 1000 and always with the tailstock. And always out of the line of fire.
I'm sure that without the warning many people would just crank it way up.
I suspect the issue is the danger of the piece coming off, not the jaws coming apart. Without analysis of the forces and materials, my gut feeling is mechanical failure of the jaws or screws at 1800 rpm is unlikely. (This, of course is given the absence of defects like casting porosity/inclusions and stress cracks. I think mine are castings but I haven't looked at them in a long time.) That could be tested in the shop but it might be expensive. And noisy.
Our lab used to test small high speed motors. It was surprising how fast you can spin a thing before the materials failed. The failures were sometimes spectacular. We tested in heavy containment.
The danger in spinning Cole jaws at high RPM lies in the centrifugal force that will tend to make the metal jaws fly away. The presence or absence of tailstock support does absolutely nothing to mitigate this. Also, the size or weight of the workpiece has no bearing on it.
Take note that only two screws hold each jaw. And they are fairly small. Plus, the screws are threaded, which means that the effective diameter of the screws is even smaller than it appears.
Now add in the possibility of stress cracks, metal fatigue, and so forth which may be present at the sharp corner at the bottom of the screw thread and you wind up with a situation where the holding power of the screws may be seriously degraded. And no doubt when you install the jaws, you crank down on the screws really tight - which further stresses the screws.
If a set of Cole jaws is rated for 600 RPM max and you spin them at 1800, you are not stressing the system a mere three times more than recommended - you are stressing them NINE times as much because centrifugal force varies as the square of the RPM. And three times three is NINE. Few manufacturers build in a safety margin of nine.
For a Longworth chuck, note that the disks have slots cut in them. These slots weaken the disk, especially near the inner diameter where the slots converge. If you look, there is not a lot of material holding things together at that point.
I have two sets of Oneway Cole jaws. My small set is rated for 1200 RPM max. The large set, 1000 RPM. I seldom if ever run the small set over 800 RPM nor the larger set over 600 RPM.
Nuff said. You cannot litigate the laws of physics.
I agree with Doc. I would add though that there is no locking mechaniism in the nova chuck to keep it tight; it relies on friction within the mechanism to do that. Given a well lubricated chuck, high rpm, and a bit of vibration, I can imagine the chuck unwinding and releasing the whole kaboodle.
It scares the hell out of me to use that thing at any kind of higher rpm. three just isn't enough mass down where the screws are to hold much force. I just take my time and take small careful cuts when I have to use it.
1) to the turner, it was a classic failure of the "S" part of the age-old S.A.F.E.R. list for starting the lathe. Yes, it was loaded with a bowl. Once he popped the button, it was an Oh Sh**, but it was then too late to do anything about it. He did not have a second "off" switch on the other end of the lathe (he does now!!), and once he started it, he could not cross the plane of the chuck to get back to the switch or to the wall plug. It took several seconds for it to wind itself up into a frightfully loud aircraft propeller before it disintegrated. Several other things in his shop were damaged by the flying parts, including the bumpers. It's a lesson for us all.
2) to the manufacturer, he replaced the chuck gratis, even though it certainly wasn't his fault. This is very much in line with the recent comment here on WC about what good people woodturners are -- as we all know very well, that goes for many of our small tool manufacturers too!
The lathe speed is unknown, but was not purposeful and was clearly WAY beyond the design limit of the chuck.
I just thought it would be a good idea to share this to remind us all that we are using equipment that can kill us if we don't constantly THINK about what we're doing -- and plan ahead to hopefully reduce the probability of such occurrences.
Having power OFF switches on both ends of the lathe should be a requirement from the manufacturers, but it isn't, so it's up to us to add our own.
There are many people to whom I listen, but none more carefully than Doc Green. I will modify my behavior based on his advice. Life is, contrary to popular belief, not a race; so a bit of slowing down will do none of us any harm.
I don't know what diameter or depth of turning you were working on, but there is no reason whatsoever to crank the lathe up to 1800 rpm to remove a tenon. You could die. Stop it.
>>> Life is, contrary to popular belief, not a race
Now THAT's a great saying! I can't believe some of the rush, rush, rush, even when turning as a hobby. I'm probably the slowest woodturner on the planet.
I'm trying to remember, do the cole jaws key into the slots on the chuck jaws or slides?
OTOH, even if they do, 1800 RPM is way to fast. As Clay said, cut it out. Stop it.
Another question....What is the max recommended RPM for other chucks? I bet its lower than most of us think.
And Raffin had rules about how fast to turn bowls, based on there diameter. The limit got below a 1000 RPM pretty quick, maybe 8".
The statement "Life is too short to turn crappy wood" (forgive me I forget who said that, John Jordan Maybe) is true. Turning crappy wood too fast can shorten your life.
In general, I'm using 700/1000/1400 rpm on my lathe...Demo or otherwise. 700 is about as fast as I run my Cole jaws, because they blow crap all over the shop any faster than that. (And mine are aluminum oneway)
S A F E R is a simple set of meaningful letters that can be used to remind you of a list of very important (!) things that you should do EVERY TIME BEFORE YOU HIT THE 'ON' BUTTON/SWITCH on your lathe.
The source for this has been reported to me to be the original Keith Rowley book "Woodturning: A Foundation Course". The "New Edition", that I have, has similar material, but not the acronym, per se.
The simple version of the associated safety rules is as follows:
Before starting the lathe,
•Speed - make sure the lathe Speed is compatible with the size of the blank;
•Aside - stand Aside, out of the "firing line" of the blank;
•Fasten - Fasten blank securely and confirm all locking handles are tight;
•Eye - confirm that Eye and breathing protection are in place;
•Rotate - Rotate the wood freehand, at least one full turn, to be certain that it is unobstructed.
More details can be added, as you wish, but this is a good start.
The person to whom this happened is a very careful, thoughtful guy.
He is even more so now!
His lathe has a couple more OFF switches on it now!
That's a sound he never wants to hear again!
He left those blades in the ceiling as a reminder.
He had to patch a few holes clean through his garage door.
Some tool storage cabinets and shelves took some serious dents.
One of the buttons nicked his leg on a ricochet -- luckily, that was the extent of injury. Come on down to a meeting some time, and I'll introduce you.
Our next meeting is 3/9, and he'll be there.
This kind of thing is SO easy to do with a VS lathe.
Turn a pen and then put on the Longworth. Oops.
It has to be second nature (and third and fourth) to turn down that dial first.
