Mortise on bowl how deep?

[Tim Elett]

What is the least safest depth?

[Don Stephan]

Not to avoid answering, but "it depends." How strong is the wood? How heavy is the blank? Will the tailstock be tight against the bland? How clean is the edge of the mortise, and does the angle of the wall match the angle of the chuck jaws wall? How often does the turner experience catches, and how experienced is the turner?

I have Vicmarc chucks and undercut the side wall of the mortise slightly. But I have only used mortises to turn octagonal bowl blanks from 2x6 framing lumber at demonstrations, and 12" x 3" x 3" cypress blanks for weed pots. Those mortises were 1/8" or slightly deeper.

I did see once a rotating large bowl blank with a mortise break free of the chuck - immediately got everyone's attention. The mortise was s mainly in bark and cambium, very little solid wood.

With so little experience, if making a mortise in a bowl blank of any weight I might make it 1/4" deep. I measure the depth with a steel ruler, rather than trust my judgement, and make sure it is flat with a little triangular piece - a technique I learned watching Richard Raffan Youtube videos.

[Richard Coers]

Depends on the chuck and jaws. You can go less deep if the jaws are dovetail.

[Reed Gray]

Well, one video I did was Mounting things on the Lathe, and I do cover this. Like Richard said, dove tail jaws hold better, especially if you haven't taken the dove tail edge off when going through the bottoms of your bowls, and the McNaughton is probably worst for that. I seldom go more than 1/8 inch deep. If I go deeper, it is by accident. If you are turning a platter, and the wood is dry, I can get away with 1/16 inch, but that might be professional level. Most green woods, 1/8 is max depth needed. Far more important is how much shoulder you have outside your recess. I can turn and core bowls up to about 16 inch diameter with a recess that is 1/8 inch deep and a shoulder about 1 1/2 inch wide. I might be able to get away with less than 1 inch, but one good catch, and the bowl goes flying. Size the shoulder appropriately and no problems.

robo hippy

[Jim Hipp]

The fear factor usually makes the decision for me but and I have found that as I turned more the depth was less.
YMMV
Jim

[Brice Rogers]

Some highly skilled turners have said that they can work with 1/8 inch mortise. But a factor not yet mentioned was the skill of the turner. If a turner very very seldom has a catch, then he/she could use a more shallow mortise. But if the turner is somewhat of a beginner and regularly has catches, then they should probably make 1/4" or deeper mortises.

Another factor is how aggressive the turner is. If they are trying hog out deep cuts, then a deeper mortise may make sense. Also, if a turner if using carbide and doing scraping cuts, I think that there are more forces on the mortise than if the turner is doing slicing/shearing cuts.

[Edward Weber]

I agree, and dare I say, no two mortises are the same.
Wood species, size & weight of the blank, jaw profile, additional support (tailstock or steadyrest), experience, tools. Everything comes into play, there is no "least safest depth".

Don't take this the wrong way but if you're concerned about mortise depth, for whatever reason, perhaps you should use a glue block and eliminate the issue all together.

[Bill Howatt]

I agree with both Brice and Edward but would add, check what the supplier of the jaws says and then be mindful of these 2 posts.
A very shallow sweep on the bottom of a bowl can mean little supporting wood for the mortise and tightening it up to be safe can break out the edges of the mortise.

[Tim Elett]

Again thanks, was admiring a well seasoned turners bowls this week end and was amazed how nice the bottom looked with a nice shallow mortise.
I'll keep practicing.

[Bill Howatt]

I don't think it is necessary to have a shallow mortise to have a nice bowl bottom but perhaps it takes a little advance planning. I typically use an external tenon.
Regardless of the tenon, either internal (mortise) or external, the bowl bottom should be pleasant to look at and the first thought in the observer's mind should not be, "this is where it was held on the lathe". If the bowl was being adjudicated this would be flagged.
Either style of tenon can be cleaned up which may mean rounding and blending the mortise hole and the external tenon can be turned off sometimes leaving a blended foot. Just don't make it look like a tenon of either type in my opinion.

[Edward Weber]

There is no reason at all that the foot of a bowl (recess or spigot) should look like the chuck that was used to hold it.
Remount the bowl, clean up the foot and refine the mounting area.

[Reed Gray]

I did shows for 30+ years. The only ones who care about the bottoms of the bowls are other turners. I really don't see the need to remove my recess, and in some cases, since I do once turned bowls, they warp so much it would be impossible. Sanded, no tool marks, signed, wood species, and year I turned it. With a recess, the signature will last a lot longer. As far as entering any type of competition/adjudicated, I don't care, and never have done that. I only compete with myself.

When making a recess, how much shoulder you have is maybe more important than depth. You can expand to the point where you can break a small shoulder, and/or even a tiny catch can cause the recess to break. Some say a tenon is stronger than a recess. I say they are equally strong AS LONG AS THEY ARE MADE CORECTLY, and you may have to experiment to figure that out. If you do return the bottom, some say you lose more wood to a recess than a tenon. I say, 1/8 inch is 1/8 inch, and no matter which method you use, you will lose the same amount of wood.

robo hippy

[Neil Strong]

I did shows for 30+ years. The only ones who care about the bottoms of the bowls are other turners. I really don't see the need to remove my recess, and in some cases, since I do once turned bowls, they warp so much it would be impossible. Sanded, no tool marks, signed, wood species, and year I turned it. With a recess, the signature will last a lot longer. As far as entering any type of competition/adjudicated, I don't care, and never have done that. I only compete with myself.

I agree fully with Reed on this. It is only other turners who get their nickers in a twist over leaving obvious chucking recesses. The buying public doesn't notice or care. Every piece I send to the gallery retains the recess that held in on the lathe. I make them a design feature and give it the same care and attention that I give to the overall piece.

I've been doing them that way for decades and it seems that the hundreds of buyers that have been paying good money for those pieces at the gallery, where they could have chosen other pieces, mustn't have been other turners!

There is a history on how the obsession about not leaving foot recesses came about, which relates to the holding methods prior to the modern scroll chuck. Before those most of us screwed the blanks onto a faceplate, which left screw holes in the bottom of pieces that had to be disguised somehow. This somehow carried over as an aversion to obvious holding methods when scroll chucks came in.

As for judges who mark down pieces because of an obvious holding recesses, I suspect they are too caught up in their own technical obsessions to make an fair assessment of a piece on its aesthetics merits and for the want of something to say.

[Edward Weber]

I am a turner and personally hate it when I turn over a bowl and can tell you exactly how it was held and by what.
Everyone has their own way

[Bill Howatt]

I don't have a problem with a recess in the bottom of the bowl, I have a problem with it, or a tenon, being left in its raw state rather than cleaned and blended. Sure a turner is bound to be bit pickier than Joe Public who likes the shiny grain or size. Don't see most of them feeling for the bump or depression in the center of the bowl that a turner would do either. If your only goal is to peddle your work, then so be it.