Thread: When is a bowl too big for a mortise?

In my opinion 60mm mortice is plenty for that size bowl. I have turned up to 400mm, although I used the tailstock until it was well balanced. The problem with a mortice is that your bowls will all look bottom heavy. With a fair bit of prompting from my fellow club members I use tenons all the time now. I have a set of step jaws that close nicely down on a 85mm tenon and because of the increased diameter the tenon only needs to be 4 or 5mm high. Put a set of these on your wish list. Cheers, Steve

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I'd say, provided the tenon is shaped to a dovetail that matches that in the jaws and you keep the lathe speed to a reasonable level and adopt proper technique for cross grain hollowing, you'll be OK

Hi kuffy,
You could just get larger jaws for your chuck !

Not up to speed on this. Please tell me what is a mortise or tenon on a bowl?

I cant just rush out and buy everything. I won't learn anything if I use appropriate tools appropriatley. Gonna give it a crack tomorrow with a 60mm mortise. If I get smacked in the head, Stevo will have made my list of mortal enemies

Sorry Artful,
A bowl "recess" for mounting in the chuck is a "mortise" to a flat panel woodworker.
A bowl "spigot" for mounting in the chuck is a "tenon" to a flat panel woodworker.

Mostly I just point at the part I am talking about and call it a "thingo". But that doesn't work on the interwebs because you can't see which way I am pointing, at least I hope you can't!

Thanks Kuffy
So do you mean that a mortise is an inverted foot? Like an ingrown toenail but in woodturning terms to describe the bottom of a bowl
And a tenon is a regular foot.
Never heard them called that before.
Blimey! I must get out more.

I'm not sure if you are making fun of me or not???

Yes, the mortise would be an inverted foot.

If you call the tenon/spigot a foot. what do you call the mortise/recess used to mount the bowl to chuck in expansion mode? Or do you never use the chuck in expansion mode and therefore never need to give it a name?

Well make sure you wear your face shield as I would much prefer to be your live friend.��

Not making fun mate.

I am always wearing my faceshield. The air between my face and whatever is about to hit me is my shield. So for safety, I don't work inside a vacuum

The answer to your question is IMHO it varies. It will depend on your experience, how green the timber is, and what shape are you turning.

If dry solid but not too large (ie not just dia but actual weight) and you know what you are doing I'd say 60mm is fine. If you get a lot of catches; the wood is a burl with heaps of sap veins/holes etc; or out of round/unbalanced; or the wood green and weak; or complex shape where outer pressure cutting may be used etc - no use a larger one.

For the heavy out of balance hard as hell stuff I like I prefer 1/3 to 1/2 dia tenon

Originally Posted by Kuffy

Yes, the mortise would be an inverted foot.

If you call the tenon/spigot a foot. what do you call the mortise/recess used to mount the bowl to chuck in expansion mode? Or do you never use the chuck in expansion mode and therefore never need to give it a name?

Hi Kuffy

now I understand what you're proposing to do.

Given that
1. you'll be shaping the outside of the bowl while the blank is on the worm screw -- so when reversed the blank will be balanced.
2. you can shape the recess to a profile that properly keys into the dovetail profile of your chuck's jaws -- possibly the hardest bit.
3. you leave enough material between the recess and the edge of the bowl's base to support the expansion forces.
4. you can firmly seat the blank in the chuck's jaws, and
5. you stand where you should be standing when hollowing a bowl -- i.e. outside the throw zone

you'll be fine. If the bowl does explode or break out of the chuck, it will fly across you rather than at you.

I've seen recesses like what you want to use as shallow as 2-3 mm, with typically 12 to 15 mm of material outside the recess.

As a now long-term woodturner, I always assess the front of a bowl/platter, then the back/underside. If I see a recess or unturned spigot on the back, I go no further..... The turner was either short on time, lazy or not competent enough to disguise how the bowl/platter was held. Wood turning course A1.

What is so hard? You spend so much time on the front, then leave obvious holding marks on the back?

Earlier comments regarding spigot/recess size about safety are also relevant. I think the 1/3rd rule is appropriate, depending on the blank.

The question (aside from safety, which comes first) should be How did the turner do that?

I suggest that some should learn how to use jam chucks (or not as accurate, bowl jaws) and let the person inspecting your work to wonder how did they do that?

Not hard, just more tool work.

Jeff