Tenon diameter axioms

[Doug Freeman]

Great thread!

I'm curious what you folks (primarily Vicmarc users) choose for jaws in order to properly hold hollow forms in a chuck?

Shark jaws of some type or the same dovetail jaws as for making bowls?

When hf’s start to get ~1.5 L/D I find a longer tenon, ~1”, helps stability. The base and tenon can be made large, with a shorter tenon, and then cut down as you approach the bottom, or a longer tenon, either works to offset the torque created by the length.

Cant speak for Vicmarc, but I use Oneway tower jaws for this. The tower jaws hold bowls just fine and the taller jaw provides easier access to the bottom, a win-win solution.

 

[hockenbery]

Great thread!

I'm curious what you folks (primarily Vicmarc users) choose for jaws in order to properly hold hollow forms in a chuck?

Shark jaws of some type or the same dovetail jaws as for making bowls?

Endgrain hollow forms to about 9” height 6” diameter will hold fine in a Chuck with #2 jaws
Leave the bottom 1/3 really thick until hollowed. Then finish the outside shape.
Longer tenon more support.

I use a faceplate for facegrain hollow-forms. I have done a few small ones in a Chuck 5” tall 5” diameter.

 

[Allen Mattsen]

So tower or shark jaws are used mainly for clearance purposes rather than extra stability, if I understand the last two posts?

I've only turned end grain HFs so far - the face grain / faceplate info is interesting and something I'll remember as I want to try those too.

There seem to be many longer type jaw sets on the market (shark, tower, etc) and I just can't find much info on how and why they are used.

 

[Bill Blasic]

Dovetail jaws for all my chucks including Vicmarc.

 

[Roger Wiegand]

Great thread!

I'm curious what you folks (primarily Vicmarc users) choose for jaws in order to properly hold hollow forms in a chuck?

Shark jaws of some type or the same dovetail jaws as for making bowls?

I use the same dovetail jaws. Because the grain is running the other direction the tenon is much stronger for hollow forms.

 

[Doug Freeman]

So tower or shark jaws are used mainly for clearance purposes rather than extra stability, if I understand the last two posts?

I've only turned end grain HFs so far - the face grain / faceplate info is interesting and something I'll remember as I want to try those too.

Read post #41 again. I have had a few taller hi L/D hf’s pull out of standard jaws (1/4-3/8” tenon length) due to the increased torque generated and the smallish tenon due to the base size. All were end grain, hi L/D typically are. Think tallish vase type form. Cant give you a hard and fast rule.

Face grain hf’s are typically below 1.0 L/D ratio and regular jaws work fine.

 

[robo hippy]

Well, I do have a pair of extended type pin jaws. I really can't see the need for tower type jaws. A properly made dove tail will hold equally well, and you don't lose that extra inch or so off the one end. Possibly for trimming up the end of a spindle project, or maybe a rolling pin to hold it a bit more steady, but at that point you are only taking very light and dainty cuts.

robo hippy

 

[hockenbery]

There seem to be many longer type jaw sets on the market (shark, tower, etc) and I just can't find much info on how and why they are used.

I don’t own tower or shark jaws.
Not sure if it is because I never needed them or because didn’t know I needed them.

I have seen a few box turners who used them to have more working room on the left side.

I think Stewart Batty used them on a long stemmed goblet demo for our club. As I recall he used the long tenon while hollowing the cup.
Then reversed it and made a short tenon with a finish turned bottom. The turned the outside of the cup and stem with the short tenon.

 

[Dennis Weiner]

Great thread!

I'm curious what you folks (primarily Vicmarc users) choose for jaws in order to properly hold hollow forms in a chuck?

Shark jaws of some type or the same dovetail jaws as for making bowls?

I own vicmarc 5, 7 and 9" dovetail jaws. I also own the stronghold along with their smooth and profiled jaws which I do not hesitate to use.

The size, shape, and grain orientation of a hollow form dictate which size dovetail jaws on my Vicmarc I will use. I generally go with the biggest I can get on a blank anyway which is their 9" dovetail jaws. I make The tenon near the same size. It is better to "over hold" than underhold. But for larger pieces or long pieces(vases), I go with faceplates and large 1/4" hex bolts. Add a steady rest, if necessary to further support your chuck or faceplate.

.

 

[Jamie Straw]

Just want everyone to know how much I appreciate your replies and reflections on this topic. I haven't had time to distill it all, but the info will really help for the next bowl class, especially. Students who have their own lathes (or prospect of) often ask me about chucks and jaws, now I have a broader base of knowledge to help them.

 

[Rachel Aberle]

One thought about newbies is that they will inevitably overshoot the mark sometimes. Some method of marking or gauging the desired diameter should be part of the method, and maybe a little acceptance of a slightly oversize tenon as they get the hang of it.

I will say, when I was new, I had a lot of tenons break off in my Nova chucks. Presumably it was due to the tenons being oversize or the edge poorly shaped. (In hindsight, I think it was a combination of making a dovetail, when they actually had the birdbeak jaws, and not getting a good flat on the bowl bottom for the jaws to press against)

I’m curious about this as it happened to me yesterday. I have a Vicmarc and made dovetail tenon around 2” diameter and when tightened on chuck, I couldn’t slip a piece of paper between wood and chuck and appeared stable. Bowl snapped right off of tenon.

 

[Brian Gustin]

I’m curious about this as it happened to me yesterday. I have a Vicmarc and made dovetail tenon around 2” diameter and when tightened on chuck, I couldn’t slip a piece of paper between wood and chuck and appeared stable. Bowl snapped right off of tenon.

Might check the grain orientation of the bowl. I have had that happen as well, if the grain of the wood runs straight and parallel through the tenon, the wood will be weaker, and the hold will be dependent on the strength of the lignin between the grain... mine was and tenon snapped right along the wood fibers... So I learned to look for that when I form my tenons, if need be, I try and adjust my tenons to be a little more cross-grain if at all possible, or make them as large as possible....

 

[hockenbery]

I

I’m curious about this as it happened to me yesterday. I have a Vicmarc and made dovetail tenon around 2” diameter and when tightened on chuck, I couldn’t slip a piece of paper between wood and chuck and appeared stable. Bowl snapped right off of tenon.

Can’t see your problem from here.
Assuming #2 jaws 48mm A 2” diameter tenon is fine.

@Brian Gustin makes a point if the grain is flat and in line with where the tenon joins the blank it can be a weak tenon in easily split wood.

The dovetail angle should match the jaws - 13 degrees for the vicmarc.
The tenon should be a little shorter than the jaw height
And flat turned where the tenon meets the blank should rest on the top of the jaws

A weak tenon results if it is too tall, wrong angle, or no flat for the tops of the jaws.

A good tenon will usually survive a minor catch. Few tenons will survive a big catch
Tenon needs to be close not perfect.

 

[Rachel Aberle]

I
Can’t see your problem from here.
Assuming #2 jaws 48mm A 2” diameter tenon is fine.

@Brian Gustin makes a point if the grain is flat and in line with where the tenon joins the blank it can be a weak tenon in easily split wood.

The dovetail angle should match the jaws - 13 degrees for the vicmarc.
The tenon should be a little shorter than the jaw height
And flat turned where the tenon meets the blank should rest on the top of the jaws

A weak tenon results if it is too tall, wrong angle, or no flat for the tops of the jaws.

A good tenon will usually survive a minor catch. Few tenons will survive a big catch
Tenon needs to be close not perfect.
View attachment 43435

thank you. This is the bottom, note tenon has been reshaped so no longer dovetail as this was finishing.

 

[robo hippy]

Well, some times it is the wood. The growth rings in that piece look huge, and if a tree grows too fast, it generally loses strength. Your tenon also may have been a bit small. Not an expert on tenons since I use a recess all the time...

robo hippy

 

[Rachel Aberle]

Well, some times it is the wood. The growth rings in that piece look huge, and if a tree grows too fast, it generally loses strength. Your tenon also may have been a bit small. Not an expert on tenons since I use a recess all the time...

robo hippy

Thank you. Yes now understanding more what was stated and the growth rings, it may have needed a much larger tenon.

 

[Brian Deakin]

Thank you. Yes now understanding more what was stated and the growth rings, it may have needed a much larger tenon.

Rachel I would suggest you watch the Stuart Batty videos on Vimeo and follow Stuarts advise on forming tenons

SB Tools

Stuart Batty combines his unique heritage with thirty-five years of experience as a world-class instructor to offer a breakthrough in both tool design and method,…

vimeo.com

 

[Leo Van Der Loo]

Well, some times it is the wood. The growth rings in that piece look huge, and if a tree grows too fast, it generally loses strength. Your tenon also may have been a bit small. Not an expert on tenons since I use a recess all the time...

robo hippy

When cut through wood at an acute angle the growth rings will seem to be very wide, while they are not very wide at all, look at this picture, if the tenon was made from the side of the bowl (the bowl would have been oriented different of course) it would look like wide growth rings, while they are not wide at all in reality.