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View Full Version : When is a bowl too big for a mortise?



Kuffy
21st September 2016, 04:22 PM
I have a couple of side grain Walnut blanks here. ~285mm diameter x 70mm thick. I have been using the wormscrew to turn the bottoms and then a ~60mm mortise to reverse it in the chuck and turn the inside. the biggest bowl I have done so far like this is 190mm diameter, also in Walnut

Is a 60mm mortise too small for a 285mm blank? It seems to me that there might have too much leverage against the mortise when I am cutting the rim, especially when my gouge or machine does something it shouldn't (yes, I do blame my tools and machinery :D )

I can do a tenon, but I only have standard chuck jaws for my 100mm chuck and the tenon needs to be ~44mm diameter which I reckon will snap off, or compress enough that the bowl start flying.

StevoWoodi
21st September 2016, 04:44 PM
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

Sent from my C6903 using Tapatalk

ian
21st September 2016, 04:49 PM
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

smiife
21st September 2016, 08:51 PM
Hi kuffy,
You could just get larger jaws for your chuck !

artful bodger
21st September 2016, 08:57 PM
Not up to speed on this. Please tell me what is a mortise or tenon on a bowl?

Kuffy
21st September 2016, 09:01 PM
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 :D

Kuffy
21st September 2016, 09:14 PM
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!

artful bodger
21st September 2016, 09:46 PM
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.

Kuffy
21st September 2016, 10:01 PM
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?

StevoWoodi
21st September 2016, 10:07 PM
Well make sure you wear your face shield as I would much prefer to be your live friend.��

artful bodger
21st September 2016, 10:09 PM
Not making fun mate.

Kuffy
21st September 2016, 10:50 PM
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 :D

dai sensei
21st September 2016, 11:10 PM
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

ian
22nd September 2016, 12:42 AM
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.

Jeff Leslie
22nd September 2016, 01:21 AM
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

Kuffy
22nd September 2016, 03:30 AM
I dunno Jeff. It sounds like bowl snobbery, much the same when flat panel jerks turn their noses up to dovetails which have tails wider than 1/8". But when the jerks are asked why skinny tails are better, they will most likely say that they look more appealing which may be so, but it isn't the answer to the "why".

If the finished bowl will sell for ~80bux, what does it matter if there is an "acceptable looking" mounting recess on the unseen bottom of a bowl. If the bowl was targetted to the upmarket snobs, then sure, spend the extra 10mins messing about to remove it and get paid for it.

Function can be right or wrong. Design is what it is, like it or not.

Luke Maddux
22nd September 2016, 04:54 AM
I've turned up to a ~15.5" bowl using a standard chuck sized mortise in expansion mode.

No problems.

By the time you've used screws to turn the bottom, and then removed a bunch of material from the inside while stabilizing it with the live center, you've likely lost enough weight in waste material that it's not going to me much of a concern anyway.

It should be noted that it's important to get the diameter of the mortise and the dovetail angle correct so that surface contact is maximized inside the mortise.

In all honesty I've never had a bowl slip off of an expansion mode chuck. I did have one explode because I overexpanded... but that's a different animal.

Good luck,
Luke

Dalboy
22nd September 2016, 05:42 AM
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

I agree with dai sensei. I quite often turn bowls the diameter you are looking to do on a standard jaw set. And even when using a larger jaw set I use a very narrow tenon as with the walnut bowl I posted the other day, the tenon on that is only 1/8" deep and the jaws used in that case are 3 1/2".

pommyphil
22nd September 2016, 08:19 AM
I've turned a few bowls ands mostly use a shallow mortise, as long as it's the right size for the jaws and plenty of wood outside. Use the tailstock until it's balanced and lighter.
I have had bowls back to refinish, mostly from rotten fruit, and always leave a few mm mortice to rechuck. For a practical bowl the base needs to be close to half the diam. for stability.
Lots of " Gallery Pieces " just fall over when you drop a banana in. :) Phil.

Bob Wemm
22nd September 2016, 01:22 PM
I agree, but it is really good when someone asks "How the hell did you finish the bottom like that"??????

Bob

Kuffy
22nd September 2016, 05:52 PM
Cheers fellas. It all worked out well. My little 3/4hp lathe didn't argue much with a blank ~285mm diameter running on the second slowest speed. It had just enough torque to allow me to tear through the material. I didn't use the tailstock to steady the blank at all and the recess held nicely. Even when I shove my bodyweight into the bowl to apply the shellawax with epic force the recess still held. The bowl/platter ended up as one of those classical ogee shape bowls. Nothing special.

Jeff Leslie
22nd September 2016, 07:10 PM
Kuffy, sorry to cause you any grief on the merits or otherwise of recesses on bowls. I didn't realise that you were a production turner and that time and $ were so important. Do whatever you need to make it work financially. I guess that most of us turn wood for non-financial reasons. For me (and probably a good many others), having a nice foot or feet lifts the piece and makes it unique.

I add that I don't hand-cut my dovetails - I have a Leigh dovetail jig plus a Gifkins for that purpose. Ultimately, I would like to do it by hand but skill levels ATM are not as required.

I have no idea how long you have been turning, Kuffy. Or who is teaching you on design and more importantly, on safety.

My personal preference - as you noted earlier - is mostly a question of taste. And safety.

I can turn inboard to 750mm on my Stubby and would not recommend your recess approach. Sorry.

Also, I have in my turning collection a bowl turned by Stephen Hughes out of red malley burl that is maybe 4mm thick in the walls. I just can't see how you would turn such a bowl using a spigot.

I believe that the aspirations of most of us on the board are all about excellence/perfection and time is not the main issue. I also add that re-turning a dovetailed foot is not an onerous task. The method of holding disappears quickly and gives an insight into the talent of the turner. Starting with jam chucks.

Kuffy, we will disagree on many things, starting with this one. After you've had some instruction from say Vic Wood or Ken Wraight (or Retired), you may change your stance. I want to be the best turner I can, and as of 2012, I have not used a recess to show all and sundry how I turned a piece. It's up to you entirely. All I know is that a dovetail recess on a piece for the Australian Woodturning Exhibition will come last. And that is not my objective.

Again, if you are turning out bowls as quickly as you can, go for it. I can only assume, given your remarks about the "show side" that you only sand and finish the front of your work that is visible. My preference is to give the same effort to both the front and back, but if $ are the killer, go for it. But please, let all know on the board that you are doing a quick fix because you can't afford the time to do it properly.

I suspect that negative feedback is coming. All I know is that my tutors are better than yours. And that is a fact, not an opinion.

Jeff

Colin62
22nd September 2016, 07:43 PM
I suspect that we're deviating a bit from the topic here, but then again, probably not too far - we're still discussing choices for holding pieces.

My first consideration when choosing how to hold a piece is safety - I don't like pain, and I hate losing a bowl. Assuming that it's a fairly standard shaped piece of wood I'm using, and I have a choice of methods, my next consideration is how I'm going to get rid of the evidence when I'm done.

I usually manage to make a plan with Cole jaws and I have a friend who has a vacuum chuck who I can visit as a last resort, but the little bowl I made last night has me stumped - so I'm going to have to make a jam chuck. It's the top end of an old fence post, and the outside is natural (well weathered) so there no way too grip it with cole jaws, there are splits that run deep enough into the bowl to make a vacuum chuck useless, so I can't see any other way to do it. The good thing is that it's many years old, and won't be moving much, so I can take my time.

Kuffy
22nd September 2016, 08:37 PM
Oh Jeff, my "lazy" and "incompetent" way of a "quick fix".


https://youtu.be/uctKBl_n7nQ

I am a production wood machinist by trade. An acceptable product done in a reasonable amount of time is the aim of the game.

And my tutors, which freely offer there time and assistance are bloody brilliant and I have nothing but good things to say about them and I would never say that your tutors suck so bad that they couldn't possibly be as good as mine.

Woodturnerjosh
22nd September 2016, 09:32 PM
I'm not really a fan of using a recess, but I do use a vacuum chucking system for finishing the foot and it's just quick and easy to remove a spigot.
If you haven't seen them here is a great video by Stuart Batty about using recesses (he has hours of great free videos on here that you can legally download for free in high definition!!!).
https://vimeo.com/68649135