I posted this on a facebook group they were foolish to let me join. I thought I would repost here just as asecond set of eyes just in case I am smoking mother nature on this topic . Thr OP question was "what went wrong" , referring to the blowout of the mortise on his bowl shown in attachment . Anyway if you absolutely nothing elas to do ..............................

I have been woodworking for a little over 50 years but I finally bought my first lathe about 5 years ago to try some bowls and vases. One of the topics I find surface a bunch relative to bowls / vases is the debate of a mortise or tenon on the bottom of the piece. Personally I like to use a mortise whenever the piece allows but I find the majority of turners prefer a tenon and recommend against a mortise. Both are completely functional and come with some advantages and disadvantages. On tenons most likely you are applying a waste piece or a thicker bottom of primary wood to include the tenon. This offers a solid grip by the chuck but will require turning the work around to cut off and properly finish the bottom of the bowl, vase. This often involves something like Cole jaws or Longworth chucks which can be a challenge to secure the work piece.

A mortise on the other hand can be added to the blank and sanded and basically starts out as a finished bottom from the start requiring only application of a finish. The disadvantage is the grip may not be as solid as offered by a tenon and if not properly turned specifically to match the chuck jaws can bring issues. The three points using a mortise are depth, taper angle of the rim and in my mind the correct diameter to align with the chuck jaws. All chucks have a range from open to closed but within that range there is an ideal setting that offers a near perfect circle to apply to the mortise this far more critical using a mortise than a tenon. The image here shows a bowl posted by someone where the mortise blew out in the final stages of turning and the question was why did this happen? You will see from the image here how a mismatched mortise diameter and a chuck opened past the "perfect circle" diameter most likely caused this failure. Think about the energy that can be applied as the chuck is cranked open. If the mortise diameter matches closely the ideal chuck jaws "perfect circle" the load is pretty much distributed evenly to the mortise full circle. If however the mortise diameter requires the chuck to open beyond the "perfect circle" diameter of the jaws the contact patch of the jaws to the mortise is reduced . This creates a situation where the same energy cranked into the chuck is applied to much smaller contact areas of the mortise creating a very high load over a smaller area. You will see in the picture the bowl bottom failed in a line parallel to the grain at that point. The grain simply could not hold against the high shear force from most likely 4 contact points between mortise and chuck jaws. mike flite AKA calabrese55

IMHO, you are exactly spot on, Mike.

My preferred bowl mounting is the mortice, although I do reverse chuck the bowl and "feather out" the mortice so it looks more delicate. It's not really too difficult unless it's a "banana" bowl or other odd shape. I copied an idea for a jig to measure the bottom thickness thus avoid making funnels. Also, when roughing a blank, for drying, the the overall wall/bottom thicknesses are more consistent.

Mortices are just the way I learned, but, I have used tenons. Sometimes, as you pointed out, with a waste block- especially on a piece that I don't waste precious wood.

21 hours ago, lew said:

IMHO, you are exactly spot on, Mike.

My preferred bowl mounting is the mortice, although I do reverse chuck the bowl and "feather out" the mortice so it looks more delicate. It's not really too difficult unless it's a "banana" bowl or other odd shape. I copied an idea for a jig to measure the bottom thickness thus avoid making funnels. Also, when roughing a blank, for drying, the the overall wall/bottom thicknesses are more consistent.

Mortices are just the way I learned, but, I have used tenons. Sometimes, as you pointed out, with a waste block- especially on a piece that I don't waste precious wood.

+1 agree with everything said except I use tenons more, even though I have had them shear off from a catch or ???, just me I guess.

I use mortises for plates and platters and some large bowls. One thing not mentioned is the orientation of the jaws for expansion can cause the surrounding material to fail. As best I can put this into words the expansion should be diagonal to the grain and not straight across it. In addition the depth is not critical as I have used 1/8 inch but the amount of material outside the mortise should be about 1/2 inch before the curvature begins. You could get by with less but that is a large risk. Oh and one more thing is you cannot crank down when expanding the jaws in a mortise the way you would a tenon.