Need help turning a base

[Joe McMahon]

I am turning my first base. It is 4" round and about 3/4" thick. I cut it "round" on a bandsaw then used a Coles chuck to flatten what will be the bottom and create a slight hollow so I can turn it around and mount it on a chuck with expanding jaws which will go inside that hollow I made. My question is, how deep should that hollow be so I can get a grip on it with expanding jaws?

[John K Jordan]

My question is, how deep should that hollow be so I can get a grip on it with expanding jaws?

What is the diameter of the jaws you will be using? Are they dovetailed (angled out a little on the outside rim)?
2" dovetailed jaws will hold fine in a shallow recess. With smaller jaws a little deeper would be good.

I've found that a recess (hollow) in the bottom held with 2" dovetail jaws will still hold well even if it's only 1/8" deep. I never measure I just cut, smooth, and use it. I use a bit of a dovetail in the recess but it doesn't seem to matter if it doesn't match the dovetail angle of the jaws exactly. Of course, if you have plenty of wood thickness to work with, a deeper recess is fine!

After forming the recess I cut the dovetail on the outer circumference with either a parting tool ground at a slight angle, a 1/4" skew chisel pushed in a small angle, or a special scraper I ground just for cutting dovetailed recesses. The scraper is good for cutting the recess and forming the dovetail angle. I don't have a picture of the scraper to post but if you decide to make one let me know and I'll get one - might be worth it if you plan on making a lot of things with a recess in the bottom. (With this scraper you can cut the recess without even holding the wood in the Cole jaws.)

If I drill a straight-sided recess with a Forstner bit (usually in the top of a bowl or platter blank) I make it deeper since it's not dovetailed and it's going to be cut away anyway, usually about 1/4" or so. Note that with a shallow dovetailed recess you don't have to baby the cuts but can still be quite aggressive, BUT, a strong catch might still pull the piece off the chuck.

I use shallow recess on shallow bowls and platters up to about 11-12" in diameter. They hold very well in good wood. (I usually don't turn wood with cracks and such.) I often decorate around the recess and just leave it in the bottom of the piece like this:

penta_platter_cedar_IMG_7434.jpg penta_olive_comp_IMG_7459.jpg

As an example of how well a shallow recess can hold, look at the one on this 19" Sapele platter. The recess is just the short inner curves of the three small raised triangular sections around the center:

bottom_PC012804_e.jpg

If you are using smaller jaws, such as pin jaws around 1" diameter, the recess might be a little deeper. BTW, there are some other options for holding such a piece that don't require turning a recess in the bottom. Just ask and I or someone can describe them.

JKJ

[Michael Costa]

If I understand what you are trying to do, I would want that base to sit flush with the Chuck. My Nova jaws are exactly 10mm which is just over 3/8" and that would leave you boring a hole more than half the depth of your base. I would glue a 3/4" waste block to the bottom to the bottom and chuck it normally. Cut a 2" square piece of plywood or furring strip and use that. Cut a 2x2" piece of 2x4 and use that.

If you think you'll have trouble centering it, mark the centers of your base and waste block, drill a hole the size of a tooth pick in the waste block and a tiny depth of the same hole in your base. Glue the toothpick (remove the point) into the base and the rest into the waste block. Now it's centered.

[Brice Rogers]

I think that the depth of the mortise that is required somewhat depends on the skill level of the wood turner. I attended some wood turning demos (by professional turners) and they make shallow mortises. Once only 1/16" deep, but often 1/8" deep. They didn't have problems. But they didn't have catches either.

Whenever I have had a work piece come loose, it was because of a catch.

So, if getting a catch is a rare occurance for the original poster, then he can make the mortise more shallow. But if catches are still a semi-frequent occurance, then he will want to go deeper.

[John Keeton]

Normally a recess is not recommended in an end grain situation. But, what you are describing would have minimal stress and you should be OK. I would suggest a decorative chuck recess approx 3/16” deep. I believe I have posted in the past images of decorative recesses I do and I think JKJ has, as well. If you can’t find one email me.

[Leo Van Der Loo]

To just turn that 4 inch disk to round and flatten the top, a ⅛” dept would be plenty, don’t make the recess too large, so there is enough wood left to clamp it, like here in these bowls.

recess.jpg to hold the bowl.jpg

[John Keeton]

Found an image on my phone.

[John K Jordan]

If I understand what you are trying to do, I would want that base to sit flush with the Chuck. My Nova jaws are exactly 10mm which is just over 3/8" and that would leave you boring a hole more than half the depth of your base....

Is that for the std 50mm jaws? If so, then yikes, a recess that deep is way too deep.

Maybe I misunderstand. Do you mean make the recess deep enough that the wood touches the flat part of the jaws, the part with the screws? If so, that can create severe problems. The recess needs to be shorter than the height of the jaws themselves so just the narrow flat face presses tightly against the flat cut on the inside of the recess towards at the outer diameter. If the recess is too deep the face of the jaws can't touch the wood and the piece may wobble and not be secure, especially with larger pieces. It may work for small things but it's not a good practice.

The Nova manual says this on page 20 about the depth of the recess for the 50mm Jaws set:
"Recess: Maximum recess depth 6mm (1/4)."
Many people use a much shallower recess.

This message has a drawing. It's written to explain something else, the angle of the dovetail in the wood, but note the depth of the recess relative to the height of the jaws:
https://sawmillcreek.org/showthread....50#post1300150
chuck-recess.jpg

This points out that the jaws should be in contact with the inside of the recess but the wood should never touch the base of the jaws, inside or out. (This is true for both recesses and tenons.)
https://sawmillcreek.org/showthread....35#post2717335

Several people have had trouble with a piece wobbling after reversing, and the cause was sometimes traced to using a tenon or recess that was too deep. This old message from Kurt Bird says it well:
https://sawmillcreek.org/showthread....95#post1128595

Note that recesses work best for dry wood, which sounds like the what Joe is using. For wet wood most people say it's usually best to use a tenon, the larger the better.

JKJ

[Michael Costa]

My response was based on the OP said he was turning his "first" base. Those at symposiums and trade shows are pros who can get away with a lit more than the average folk. 20181114_223509.jpg20181114_223539.jpg20181114_223607.jpg

My Nova chuck. A piece mock mounted a little bit. A piece mock mounted flush to the back.

Could you grab a piece of hardwood with a 1/16" recess. Sure you can. But can you get in on there square so is doesn't wobble when you turn the lathe on? Do you have the control to not pop that off the jaws. And this could be a catch, too much pressure on one side
, or your doorbell startling you as you're trying to be so careful.

Or, can you simply glue a waste block to the base (which would have been dry by now) and mount, turn, part off the waste.

Any time I have ever tried to half ass a lather mounting has ended up with the piece coming loose, banging off everythino before hitting the wall or my face.

[Joe McMahon]

Thanks Fellas. I did what you said and it worked perfectly!!!!