Broken tenons

[Robert Feingold]

I'm having a lot of tenons breaking off of some ash bowl blanks. The ash has been sitting in a pile in my sons back yard on Long Island, NY for about 2 years in 2' logs. the wood is not spalted and somewhat softer than normal ash. I imagine 2 years in the elements has broken down some of the cell structure. the bark comes off easily. I'm turning it as I would green wood. I'm using dovetail jaws in a barracuda. The fit is probably not perfect but works well with other bowls. The tenon sheers off at the small diameter of the jaws at the sharp part. The wood is beautiful when turned.
What alternate methods should I try- Glue block? Faceplate?
Thanks,
Rob

 

[MichaelMouse]

First, don't "grip" with the chuck, make a shoulder and snug up only. Second, watch your tenon isn't entirely in the sapwood, which starts out softer and decays more rapidly than the heartwood. Third, try to span a couple of annuals. That's some fast-growing stuff if the pictures are any indication. Not the best situation. Another thing to try if you're fracturing along the line of the pores - annual rings - is to try reinforcing them with a round of water-thin CA glue. Run round and round the tenon, letting it sink in and cure. Scrape off any introduced irregularities before reversing.

A photo of the problem, not the outcome would certainly narrow the range of responses.

 

[hockenbery]

Robert,
I use faceplates a lot With machine screws mostly #12 for my hollow forms and some bowls.
There is no stronger hold on face grain than a faceplate.

I rough between centers then instead of making a chuck mount I turn a big wide tenon to cover the faceplate with a 1/2 overhang.
Trick with the faceplate surface is to make it slightly concave. The rim of the faceplate will seat firmly against the edge of the concave.

The depth of the screws hole is always greater than the length of the screw as the screw opens a space In the wood ahead of it.
I make the faceplate tenon a 1/4 inch longer than the screw length. If the wood is really dry you might drill pilot holes. I usually just drive screws into wet blanks.

Once the bowl is on the faceplate, I always true up the outside and then shear scrape it. Then hollow the bowl.

A slight modification is to use a say a 6" faceplate on a 12" bowl and make the bottom of the bowl inside the screws.
This give a super hold. You need to be able to recenter the bowl when reverse chucking as you must match the outside cut and turn the last inch or two of the outside base of the bowl.
Can do much the same thing with large jaws on a chuck. Larger diameter tenons are harder to break.


Have fun,
Al

 

[Steve Worcester]

If the wood is running cross grain, that is the nature of the beast with Ash. Use as big of a tenon as you can and be gentle. Or use a faceplate

 

[Bill Boehme]

One other thing is to keep the tailstock live center firmly in place to help hold the wood in the chuck or faceplate. There is a possibility that you are turning with dull tools and applying too much pressure on the wood. If the tool is sharp then almost no pressure is required. It is all to easy to be drawn into the trap of telling yourself that you can use the tool just a tiny bit longer by slightly increasing the pressure and delay the need to go to the grinder. When you get away with doing that, the next step is to delay sharpening just a bit longer. Before you know it, you might reach the point of applying enough pressure to push the lathe across the room.

 

[robo hippy]

Proper sized tenon, about 1/4 to 1/3 the diameter of the bowl.

Proper depth, no more than 1/4 inch.

Angles to match chuck jaw angles.

Ideal match of jaws to chuck: diameter of tenon to exceed diameter of closed jaws by 1/4 inch or slightly more if wood is wet.

Jaw to grain orientation: have jaws at about 45 degrees to the grain, rather than 2 on end grain, and 2 on side grain.

Do not over tighten. Get it snug, but don't armstrong it.

Some pieces just want to crack off no matter what you do, and more brittle, hard, and crack/split prone woods will do this more.

robo hippy

 

[Robert Feingold]

Do you think I'm better off using the straight jaws instead of the dovetail? Maybe the evenly spaced pressure will be better?

I have definitely been over-tightening the chuck. I have seen that on YouTube. Now it seems like a bad idea especially on dovetail jaws.

My gouges are sharp and i'm being careful. It takes a second to sharpen with the jig.

If I use a faceplate what is the ratio of screw length to blank diameter and depth.

Is a glue-block an option considering the wood is still wet?

Thanks for the very useful advice.

Rob

 

[hockenbery]

A 3" face plate with a full inch of screw into the wood and 6 or 8 screws will hold a 12 inch bowl if the wood is solid.
8" faceplate will hold a 20" bowl longer screws and more of them.

Could try #3 jaws

I have turned lots of natural edge bowls on a glue block using CA glue to hold wet wood on a dry 3" diameter glue block
Mostly 8" diameter but a few up to 12
When it is done a flat chisel breaks the glue. Glue block because there was not enough wood to give away for a tenon.

You have to work somewhat quickly as the glue block will fail as the wood dries.
2 hours is safe.

I turn the glue block round on a small faceplate. Drill a small hole thought the center and make a slight concave from rim to center.
The bowl I rough between centers and make a sort of tenon a bit large diameter than the glue block.
This need to be slightly concave. I knock the small nub under the revolving center off with a flat chisel.
The use a small drill bit twisting in my fingers to make a small hole in the center.

Use a straight length of coat hanger through the hole in the glue block into the holed you drilled on the matching part to line them up.
Press on the edges of the glue block. There should be no rocking. You need solid contact around the outside of the glue block.

Draw a around the glue block with a pencil.
I put two beads of thick CA inside the circle.
Put accelerator on the glue lock line up with the coat hanger

The just as the glue lock makes contact give it a twist.
The twist is essential as iT make the CA areas out into the concave giving a 1/2 to 3/4 inch wide glue joint

Mount the bowl bring up the tailstock true the outside, remove a liTtle wood from the opening.
Remove the tailstock and lightly tap the bowl. The bowl should stay in place. If you forgot to twist the glue block it will drop off as you will have cured the CA in the round shapes it was in.

Then hollow the bowl. Split off the glue lock with a flat chisel
Reverse chuck the bowl finish the bottom.
Clean the glue off the glue block with bowl gouge being careful to remove only glue. You can reuse the glue block until hit screws.
Maybe a hundred bowls.


I learned this method from Lyle Jamieson who learned it from Rudy Osolnik.

My concern would be if you are breaking tenons you may be getting catches.
This glue block method holds up to cutting it won't withstand a big catch.
I have done this method with students and they have good success.
It is almost as fast as using a chuck.

 

[MichaelMouse]

Do you think I'm better off using the straight jaws instead of the dovetail? Maybe the evenly spaced pressure will be better?

The "pressure" will be more even with a smooth wedge than with teeth. Proof in that teeth penetrate the surface of the tenon. Squeeze a wooden tube in the middle and the stuff tries to move both ways, possibly pushing the front of the jaws out of register with the shoulder of your tenon. Steals the resistance to racking that keeps the tenon from breaking. Pressure also has different effect on the wood depending on where it's applied. Wood will bear a lot on end, while face grain will squash, especially when wet.

Thus the advice to spread the load by making your tenon as near as possible to optimum size. Would be the point where maximum metal contacts wood and makes the least pressure per unit of area. With a wedge, you have both shoulder and undercut for surface. If you used a mortise instead of a tenon, you could make it the optimum size green - first point of circularity - and still have it optimum after the wood cures. Your bowls are cross grain, so fine candidates, as long as you don't keep trying to drive the wedge deeper!

Sure you can fiddle with faceplates, but if you own a chuck, means you don't have to. So why bother?

If you think it may be a tool handling problem, try keeping the piece between centers until it's almost hollowed out. You can get some good whacks without ruining the piece that way.

 

[Brian McInturff]

Robert,
Looking at you 2nd pic the shallow bowl. Looks like you got a catch about an inch fro the rim. If you get a catch out on the perimeter like that it could cause the rennin to break. Don't ask how I know this.

 

[Bill Boehme]

For most bowls there is no need to use a faceplate if you have a scroll chuck. If you were turning a from a really large heavy piece of wood, then a faceplate makes more sense.

Screw diameter and length needs to be larger if turning end grain. If turning an end grain hollow form that is really heavy -- meaning that you can barely manage to lift it, then use a steel faceplate and fill all of the screw holes. For a tall end grain vase that starts out close to 100 lbs., I would use #12 or #14 screws about 2.5 inches long. Use deep thread screws to get the best bite.

As far as types of chuck jaw types are concerned, I have not encountered a meaningful difference in holding a tenon between the serrated and dovetail designs. The most important thing in my opinion is to usde well built chucks that do not have slop or backlash in the gear train.