Thread: Runaway bowl

Something is seriously not right.

I've had similar situations when I've spun up a heavy blank and turned the lathe off. The motor/shaft stopped but the blank kept on spinning, unwinding the chuck.

But never, ever from a catch except when I was running the lathe in reverse. I do not do that any more, not even for sanding. Reverse is much over-rated.

The only suggestions I can come up with are:

A: there is a mismatch between the chuck thread and the spindle thread. ie. it's tight enough to "feel" secure but a good sharp blow reveals that it isn't.

B: If you have a variable speed lathe, a heavy(ish) blank and the chuck isn't properly tightened on the spindle at the start, IF after bringing up to full speed you drop the speed drastically (and quickly) the chuck could possibly unwind similar to my "stop situation."

I find the latter doubtful, particularly if you're using tailstock support, but still... I imagine it's possible.

Either way, I'd assume A and double- & triple-check the thread sizes if, as you say, it's happened before. If they seem to be correct and your chuck has an adapter insert, I'd actually order another one just in case of a mfrs flaw.

I wouldn't worry about B until I'm absolutely sure it isn't A, then I'd just make sure the chuck is properly locked to the spindle every time I used it. This is a good habit to pick up anyway.

Hint: If it has a grub screw to lock the chuck thread to the spindle, try removing it and dropping a lead shot down the hole, before replacing the grub screw.

Originally Posted by Skew ChiDAMN!!

I've had similar situations when I've spun up a heavy blank and turned the lathe off. The motor/shaft stopped but the blank kept on spinning, unwinding the chuck.

Ditto. With very heavy blanks (30kg+) I have to remember to dial down the speed before hitting the stop button.

A very thin rubber washer between the chuck insert and the shoulder on the headstock spindle reduces the risk of overrun, but introduces its own issues if not removed once the weight has been taken out of the blank.

Originally Posted by Skew ChiDAMN!!

Reverse is much over-rated.

Agreed! Had it on some of my earlier lathes but didn't include it as a requirement when selecting my most recent lathe.




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Originally Posted by Skew ChiDAMN!!

Something is seriously not right.

I agree.

Time to relearn the basics. Stuart Batty offers good advice and well reasoned guidance on the how and why - https://vimeo.com/woodturning

https://vimeo.com/137484615

Originally Posted by Skew ChiDAMN!!

Something is seriously not right.

I've had similar situations when I've spun up a heavy blank and turned the lathe off. The motor/shaft stopped but the blank kept on spinning, unwinding the chuck.

But never, ever from a catch except when I was running the lathe in reverse. I do not do that any more, not even for sanding. Reverse is much over-rated.

The only suggestions I can come up with are:

A: there is a mismatch between the chuck thread and the spindle thread. ie. it's tight enough to "feel" secure but a good sharp blow reveals that it isn't.

B: If you have a variable speed lathe, a heavy(ish) blank and the chuck isn't properly tightened on the spindle at the start, IF after bringing up to full speed you drop the speed drastically (and quickly) the chuck could possibly unwind similar to my "stop situation."

I find the latter doubtful, particularly if you're using tailstock support, but still... I imagine it's possible.

Either way, I'd assume A and double- & triple-check the thread sizes if, as you say, it's happened before. If they seem to be correct and your chuck has an adapter insert, I'd actually order another one just in case of a mfrs flaw.

I wouldn't worry about B until I'm absolutely sure it isn't A, then I'd just make sure the chuck is properly locked to the spindle every time I used it. This is a good habit to pick up anyway.

Hint: If it has a grub screw to lock the chuck thread to the spindle, try removing it and dropping a lead shot down the hole, before replacing the grub screw.

Cheers Skew,
A. i'll check, as is probably the cause
B. It is a variable speed but the lathe was still in motion, and not slowing down

I wasn't using the tailstock as I was cutting the recess for the jaws. I know my face plate has a grub screw but not my insert or scroll chuck.

my only thought was the catch has somehow increased the speed of the job/chuck, and its ended up spinning faster than the lathe and therefore undone itself. or the catch has thrown the job off balance and the insert has somehow dislodged and jumped off the spindle. I will check to see i its cracked or split.

but its happened twice now, so might take the chuck/insert back to the shop and test on their lathe to see if it fits correctly.

I will keep you posted

If the insert is loose enough to jump off the spindle, it’ll probably wobble a fair bit before it registers and tightens up.

Has the project been completed?

Rather than dribble pieces of information could you supply some key basic information as it will be helpful for others to interpret what occurred and hasten the elimination process. A photo/s of the blank as mounted on the lathe, lathe set up, belt positions will also greatly assist.

1. Lathe make & model
2. Fitted with EVS / VFD - Y or N , original, aftermarket?
3. Fitted with Fwd / Rev - Y or N
4. Chuck make & model
5. How was the chuck / insert initially fitted to the spindle i.e. just firm, snugged up, spun on with force....
6. Was any lubricant used on the spindle / insert thread interface?
7. lathe speed
8. belt position (1,2,3 etc or L, M, H )
9. blank dimensions
10. tool in use when the event/s occurred
11. was the lathe motor, or has it been, "stalling" during turning, of this or similar sized projects?
12. which side of the lathe bed did the blank "fly" to - near or off side?
13. finally to eliminate "dumb" mistakes - lathe direction i.e. fwd or reverse. (we all make mistakes.)

If a lathe has EVS and is "underpowered" i.e. not enough grunt or the wrong belt setup is used, it is possible to stall the lathe motor with a "soft catch" (a progressive stalling rather than an abrupt stall/stop), reducing the spindle / blank RPM. When the turner reacts and withdraws the tool the EVS recovers and attempts to return to the selected / dialled speed and increases the spindle / blank RPM.

Most EVS setups are programmed to "ramp up" torque within limits to overcome stall conditions. So it is possible that the combined blank / chuck / insert that is loosely fitted to the lathe spindle may momentarily reach a temporary "overspeed" condition i.e. faster than the lathe spindle, and then becomes loose on the lathe spindle as the EVS recovers to the set speed. This can be a significant issue with larger bowl or platter projects that test the limits / power of mini / midi / cheap lathes and is alarmingly easy to do on any lathe running in "reverse".

The same occurs when a turner uses their hand as a brake on the hand wheel to abruptly slow the lathe spindle, the spindle slows however the inertia / momentum of the insert / chuck / blank overcomes the friction of either the spindle / insert or insert / chuck interface and "goes for a walk."

If its not fitted with EVS and "hand braking" was not involved then I'm

However if recommended practices, lathe speed etc are being observed and good techniques are employed, lathe setup checks performed before turning the lathe on, this should never occur.

It will be interesting to hear what the cause of this issue turns out to be. Simply a process of elimination.

Using large chucks on small lathes also creates a potential hazard due to the relative mass of the combined chuck and blank compared to the mass of the lathe itself. Inertia causes some "interesting situations" with light lathes sitting upon temporary benches - i.e. topple over.

Originally Posted by Mobyturns

Has the project been completed?

Rather than dribble pieces of information could you supply some key basic information as it will be helpful for others to interpret what occurred and hasten the elimination process. A photo/s of the blank as mounted on the lathe, lathe set up, belt positions will also greatly assist.

1. Lathe make & model
2. Fitted with EVS / VFD - Y or N , original, aftermarket?
3. Fitted with Fwd / Rev - Y or N
4. Chuck make & model
5. How was the chuck / insert initially fitted to the spindle i.e. just firm, snugged up, spun on with force....
6. Was any lubricant used on the spindle / insert thread interface?
7. lathe speed
8. belt position (1,2,3 etc or L, M, H )
9. blank dimensions
10. tool in use when the event/s occurred
11. was the lathe motor, or has it been, "stalling" during turning, of this or similar sized projects?
12. which side of the lathe bed did the blank "fly" to - near or off side?
13. finally to eliminate "dumb" mistakes - lathe direction i.e. fwd or reverse. (we all make mistakes.)

If a lathe has EVS and is "underpowered" i.e. not enough grunt or the wrong belt setup is used, it is possible to stall the lathe motor with a "soft catch" (a progressive stalling rather than an abrupt stall/stop), reducing the spindle / blank RPM. When the turner reacts and withdraws the tool the EVS recovers and attempts to return to the selected / dialled speed and increases the spindle / blank RPM.

Most EVS setups are programmed to "ramp up" torque within limits to overcome stall conditions. So it is possible that the combined blank / chuck / insert that is loosely fitted to the lathe spindle may momentarily reach a temporary "overspeed" condition i.e. faster than the lathe spindle, and then becomes loose on the lathe spindle as the EVS recovers to the set speed. This can be a significant issue with larger bowl or platter projects that test the limits / power of mini / midi / cheap lathes and is alarmingly easy to do on any lathe running in "reverse".

The same occurs when a turner uses their hand as a brake on the hand wheel to abruptly slow the lathe spindle, the spindle slows however the inertia / momentum of the insert / chuck / blank overcomes the friction of either the spindle / insert or insert / chuck interface and "goes for a walk."

If its not fitted with EVS and "hand braking" was not involved then I'm

However if recommended practices, lathe speed etc are being observed and good techniques are employed, lathe setup checks performed before turning the lathe on, this should never occur.

It will be interesting to hear what the cause of this issue turns out to be. Simply a process of elimination.

Using large chucks on small lathes also creates a potential hazard due to the relative mass of the combined chuck and blank compared to the mass of the lathe itself. Inertia causes some "interesting situations" with light lathes sitting upon temporary benches - i.e. topple over.

Firstly, yes the job is complete, a small 90mm x 60mm cool mint bowl, in coolabah burl.

1.Hafco WL-14
2. EVS - fitted - no hand brake was being applied.
3. Reverse - fitted
4. Hafco 100mm scroll chuck
5. Hand rotated snug fit, no play.
6. No lube
7. 1000rpm
8. 2 belts, on 2nd belt for higher end polishing.
9. An off- round reversed burl blank roughly 120mm diameter. on the worm screw until I could true the outside and face then set the jaw recess
10. Roughing gouge
11. No, it hasn't been stalling, but that was twice on the same blank several weeks apart
12. Near side and down, it hit the bed rolled off and round my feet. I was standing to the right.
13 normal anticlockwise direction.

i think the stall may be it because it shuddered and then went plop.

i will take the chuck/insert back to the shot and fit instore to see if I get the same feel.

cheers

Well the mystery deepens. That is a small blank (bowl wise) so it does not have the issues of large mass to create the situation I had envisioned. Nothing really untoward for that size blank, unless the chuck was not snugged up well enough to start with. The use of a roughing gouge is not generally recommended for "bowl blanks" however if it was genuinely a burl blank the "cross grain" effect is not really present.

As the bowl is now finished, what did you change to successfully complete the bowl?

Originally Posted by Mobyturns

Well the mystery deepens. That is a small blank (bowl wise) so it does not have the issues of large mass to create the situation I had envisioned. Nothing really untoward for that size blank, unless the chuck was not snugged up well enough to start with. The use of a roughing gouge is not generally recommended for "bowl blanks" however if it was genuinely a burl blank the "cross grain" effect is not really present.

As the bowl is now finished, what did you change to successfully complete the bowl?

I basically put the chuck back onto the spindle, made sure that it was snug and kept going, but as it had happened twice on the same block, I thought it best to true it up asap. Although it was a small bowl blank it was quite off kilter until I trued it up.

i will go through the steps you discussed and take it back to the shop and check it out.

Further to this, the thread is good, there is no play, and the insert seats snugly on the spindle. Also, there is no lug screw to tighten to lock the insert on.

IMG_5138.jpg IMG_5139.jpg

why are you using a roughing gouge on a bowl. they are designed for spindle use only. what happened was that the roughing gouge caused a catch. momentarly stopped the machine and caused the chuck to spin loose. you were lucky that you didn't do more serious damage. i suggest that you look at the video by retired that is in a sticky at the top of this page. it might save you from getting hurt.
cheers
jim

What happened there is certainly a mystery to me. You are right as turning counter clockwise should not unscrew anything even with a complete stop. Your tool caused and stopped all forward motion. If anything, it should have tightened the chuck even tighter on the spindle.

I'm surprised that the chuck doesn't have a grub screw to lock it onto the spindle or insert. If you have a drill press, drill out a hole on the insert and tap it for a grub screw. But looking at the pictures of your chuck, there doesn't seem to be enough room on it to put a grub screw, which is needed to keep it on the spindle.

I'm only familiar with Nova Chucks, and haven't seen any without a grub screw. I did buy inserts and tapped them for grub screws. It's pretty simple to do. ................. Jerry (in Tucson)USA

I took a look at the chuck you posted. You probably could take that index plate off, and locate where you could drill into the chuck and tap a hole for a grub screw without interfering with the mechanics of the chuck operation.
I did that to my Nova G3 chucks because the 1/4" screws interfered with the chuck key. It's pretty simple if you have a drill press and a vise............ Jerry

Any update o the insert threads matching the spindle thread etc?

I haven't been able to get out to H&F yet, but apparently this lathe is synonymous for this kind of activity. I went to my club and asked if anyone had experienced this issue. It was met with laughter. Then they said, yep, there, there and there, and proceeded to point 3 holes it the ceiling from just that issue. They then said, now look at the lathe and read the sign. It said "chuck permenamtly attatched, do not remove".
I instantly felt relief, and happy that this was not an isolated incident, I will however now be looking into fixing a grub screw.