Long jaws vs. spigot jaws

[Wayne Spence]

At the local woodturners group last week the demonstrator turned a hollow form vase about 12 inches long. Poor guy, he had an unfortunate catch and ended up with an open form. He used Nova 3" long jaws and turned a tenon nearly the 2 inch length the jaws will accommodate.

I am about to go the sixty or so bucks for a set of these but have also considered the 45mm spigot jaws. They seem robust with serrations on the inside but not nearly as long as the long jaws.

I am looking for some advice from those who have used both jaws. Any takers?

 

[Jim Carroll]

The spigot jaws are better as they give more purchase than the long nose jaws.

The other option for hollow forms is the 100mm power grip jaws. They are like the 45mm spigot jaws only 100mm in diameter.

With these jaws open them about 8-10mm and make the tenon nearly as deep as the recess and make sure there is a square edge for the jaws to seat onto.

If you do have a catch it will not go very far and realignment is easy.

Bigger is better.

 

[MichaelMouse]

I have a set of spigot jaws, but haven't used them in years. They chew up wood, don't shoulder properly, and you couldn't rechuck a piece on a bet. I don't use the big power grip that much, but they wedge tightly and hold well when I do. For tenons I generally go to my "75mm" jaws which have a much smaller minimum diameter.

Shoulder is more important than serrations in maintaining the alignment of the piece, and the big dovetail is much better at holding against pullout.

 

[Bernie Weishapl]

On large hollow forms and like large canister sets I use the Oneway #3 smooth jaws. They will hold from 3-3/8'' to 4-1/2'' tenon. Never have had one break or tear out. As MM said your shoulders and tenon shape is most important.

 

[Wayne Spence]

Jaws

I have been using the 50mm bowl jaws and last week turned 6 bowls of 2 inch laminated pine. On one bowl I had catches and broke the bowl off at the tenon 3 times. The tenon held but there was not enough meat there to keep the bowl from shearing off. Number one, I realize that having catches is the number one problem but until that time comes along I will do the best I can with what I can get or buy. Number two, that laminated pine had grain changing direction frequently which was a real challenge. Number three, the 75mm bowl jaws that MM uses may be the best solution. I have the 100mm bowl jaws but that minimum diameter is too much for much of my work.

 

[Katherine Kowalski]

Wayne,
I'd highly recommend using larger jaws for your bowls -- the general rule of thumb is that the tenon (and jaw size) should be no less that 40% of the diameter of your finished piece. So a 6 inch bowl would need a 2.5 inch diameter tenon.

Another tip -- always chamfer the lower edge of the tenon -- it will help prevent the tenon from shearing off in a case when you knock the bowl out of the chuck.

Katherine

 

[Mark Warden]

Where does the 40% rule come from? Oneways #6 jaws Max 7-3/8 " has the capacity to hold a 26" bowl

 

[Matt Hutchinson]

I have to agree with MM that the deep jaws really eat up wood, i.e. you end up with a lot of waste blocks that were once tenons. I have to say, however, that for very long forms where you don't use s steady rest or tailstock, the spigot tenons are going to hold much better.

For example: When I started working on a back hollowing technique (when you hollow endgrain on a spindle cutting on the back/far side using a spindle gouge) I tried holding it with just a dovetail tenon. I got a nasty catch, I mean really nasty, and knocked the piece so far out of the chuck I can't believe it didn't launch. After this, I started using deep spigot jaws, and no matter if I got a catch or not, the piece didn't budge but a hair.

I think a large part of your decision depends on what work you intend to do with the jaws.

Hutch

 

[MichaelMouse]

Where does the 40% rule come from? Oneways #6 jaws Max 7-3/8 " has the capacity to hold a 26" bowl

The legal team, mostly. They try to protect their company from the preventable errors of others with recommended limits on bowl diameter when the mass, if the lesser in the MVsquared, is still a big player in safety. They also recommend a particular "no greater than" rpm, not mentioning a diameter of work, or even the jaws used.

How much this really means can be estimated by the change in the 600 rpm limit on my ancient nova chuck to the newer, with the same jaws, of 680. Coincidentally, the rpm resulting from use of the third pulley on my 3000. Or maybe it's not really coincidence? 😉

Rules of thumb translated to canons of faith too often become rules of dumb. Anecdotes and hearsay, anyone?

 

[MichaelMouse]

I have to agree with MM that the deep jaws really eat up wood, i.e. you end up with a lot of waste blocks that were once tenons. I have to say, however, that for very long forms where you don't use s steady rest or tailstock, the spigot tenons are going to hold much better.

Doesn't the tenon normally fail by shearing at the base of the piece on cross-grain or chewing through on long-grain? Makes all tenons, regardless of length, equal in my mind. Difference is in breadth bearing on the shoulder. The standard 50mm and long jaws don't have much, nor a lot of dovetail. The bigger do. That's why the flat side puts a shoulder on the tenon - to keep it from racking. While the tenon may deform under stress with no shoulder, the end grain bearing against the opposite member takes that condition away.

 

[Matt Hutchinson]

Oh, I was assuming spindle orientation since the OP wrote of the incident involving a hollow vase form. I haven't had a long spigot tenon in spindle orientation do much more that get knocked a little off center. By long I mean 1.125". For face plate/cross grain orientation I agree, not as much gained with a deeper tenon.

Hutch