Proper chuck use

[Edwin]

I'm still a beginner at this lathe business (have built a couple of bowls and a handle for one of my tools so far) but seem to be having a recurring problem - when turning a bowl on the chuck, I keep breaking the "key" that the chuck grabs onto (not sure what it's called) off the main workpiece. It's really annoying to be about one or two passes away from rough finishing a bowl, then SKIP, the bowl flies across the room and needs to be remounted onto MDF and cecentered and all of that. Grr. Should I be using the outward-pressure method of chucking and all that? A bigger key? This is where experience is needed, me thinks.

-Edwin

[Skew ChiDAMN!!]

It's possible that you're making the spigot too large or too small to suit your jaws. If too large, then only the very corners of your jaw will have a secure grip, if too small then most of the grip's in the "middle" of each jaw. I doubt it's too small though...

Try this: without any wood, adjust your chuck so that the inside edge of the jaws form as perfect a circle as possible. Measure that inside diameter; that's the "ideal" size to give maximum grip along the entire length of each jaw... so make your spigots a mm or two larger than that, to allow for compression of the timber. (You'll probably find that this "ideal" position is when the chuck is nearly, but not quite, fully closed. Which is why I said I doubt that too small is your problem... but I do have a chuck where 'tis in the half-way open position, so I mentioned it just in case. )

Attachment 41910

Also, take a look at the inside of your jaws and see if there's a bit of a bevel... better brands of "general" jaws will have one. (By general, I mean your normal, everyday jaw that's typically included with the chuck at purchase, as differentiated from pin-jaws, power-grips and other "specialist" jaws.)

If it does, you should angle the side of your spigot to match... still making sure that the smallest diameter of the spigot matches the "ideal" diameter for your jaws. Also, make sure the shoulder sits firmly on the egde of the jaws... when all is done properly, you don't need a long spigot for a good grip.

Attachment 41911

Hope that helps and doesn't just confuse the issue?

[Cliff Rogers]

This is where you need to get together with somebody with experience.

Lessons &/or a woodturning club will help.

[Captain Chaos]

G'day Edwin,
I'm just wonderin, what is the make of your chuck? if it is a Nova or Vicmarc chuck, there will be an Operators Manual with it. There will be a full description on mounting blanks in the manual that should shed some light on your problem. I'm not trying to be a smart alec, just pointing you in the right direction - I've been down the same road myself, back in the dim, dark days of the mid 90's when I first started turning with my Nova chuck.
I would be pleased to give you some help in person if that would benefit you. Or, depending on your location, contact either "The Hills Woodturners" or "The Northern Turners" for guidence with your issue. Please PM me if I can be of help to you.
Regards,
Barry.

[joe greiner]

If the "key" (aka tenon or spigot) is actually breaking, I'd guess it's too small for the mass of the bowl. Like Skew says, match the diameter of the spigot as much as possible to the jaws for best purchase. Also, make sure the depth of the spigot is only a hair less than the length of the jaws, with the flat spot on the bowl squared up to keep it running true.

I'm not too keen on using "outward pressure." Too easy to split the workpiece. OK with a plywood (not MDF) waste block where you have material in enough directions to prevent splitting.

Joe

[Jim Carroll]

Edwin I agree with all that is said above.

The spigot you have to make is critical in shape and size.

The diameter of the bowl also dictates the size of the jaws you should be using. As a rule of thumb 50mm jaws for a bowl up to 250mm, 100mm jaws for a bowl up to 350mm and 130mm jaws for up to 500mm. These are for a guide and not gospel but may help as you may be trying to put too large a bowl on too small a spigot.

When clamping down on the spigot you can tighten as much as you can do not be afraid to try and overtighten.

Skew your description as always is excellent. Can I have your permission to reproduce the pictures you have made.

This will help in explaining to potential customers what is the easiest and correct way to grip a tenon for a bowl. You explain to them what to do but sometimes the light is on but no one is home. A picture is worth a thousand words.

We had a gentleman over the weekend try to explain how he held his bowls and wondered why the bottom was always heavy, it turned out that he created a double hold. By this I mean he created a recess for the jaws to go into and a tenon in the centre of the recess to hold the bowl. It took a fair bit of convincing that the recess was not required. Just make a tenon and all will be well.

[Old Arn]

Great advice given so far. If I am understanding you correctly your tenon is breaking at the base of the bowl and not coming out of the chuck. If thats the case then try turning the bowl with the added support of the tail stock. If the bowl is wide or long you are putting a lot of force on that chuck tenon when you first start roughing it out. Its a pain to hollow with the tail stock in place but leave it there as long as ya can.

If its a hollow form then ya can't use the tail stock when hollowing, just take it slow and easy, make sure your tools are sharp and don't get aggressive. A bowl steady is also handy for additional support.

If the tenon is coming out of the chuck try super glueing the tenon to the chuck. I do this on most of my turnings and never had one come out. When you are done just give the chuck a good twist with the chuck key and it will pop right out.

I also disagree that ya can't over tighten a chuck. They put a lot of force on the wood when tightening them down and if ya crank down to hard you can crush or crack the wood, especially if its soft wood.

Good luck,
Shane

[Skew ChiDAMN!!]

Skew your description as always is excellent. Can I have your permission to reproduce the pictures you have made.

Why, Thank you Jim!

They aren't the best quality pix, just something I knocked together in a rush to illustrate the point. Still, as far as I'm concerned anything that hits these pages is automatically public domain... so feel free to do as you wish with 'em.

[rowie]

I agree with all of the above, this problem sounds like the spiggot needs to be longer in length, dig-ins wont help(keep the toolrest close), and dont tighten the chuck too much because as the base area becomes thinner, the spiggot will be squeezed out by the pressure of the jaws.

[BernieP]

G'Day Edwin

The good thing about people asking questions here is the answers we all get to read. Certainly a wealth of information in these replies and intend to print off and file away. By the way welcome aboard.

Cheers
Bernie

[rsser]

If it's coming adrift at the last cuts I'm guessing you're cutting at the rim where the leverage is going to be greatest.

So apart from all the sage advice so far you should consider lighter cuts. As one master said, let the wood come to the tool.

[Edwin]

Cheers Skew and everyone else who has replied, what an interesting thread this has turned out to be! I'm looking forward to carrying out your advice on my next bowl (the one in question is currently mounted on some sacrificial MDF ready to be finished. If I find my digi camera I will post some pics as I am quite proud of the design if not so much the chiseling skill!) Also can't wait to set up my dust extraction system - it sure does hurl the stuff everywhere.