Which jaws?

[Silverbirch]

I'm trying to figure out which of these chuck jaws,which I possess, should give me the most secure grip on a dovetail spigot when hollowing fairly deep, wet and heavy bowl blanks.
A) 90mm diameter x 5mm deep step jaws
B) 60mm diameter x 10 mm deep standard jaws on a Versachuck

The 90mm jaws obviously give the larger circumference to grip, whereas the 60mm jaws give the larger total surface area to grip according to my calculations.
I've used the 60mm jaws quite a bit, though not on such heavy work, and the 90mm ones scarcely at all.
Any thoughts?

Ian

 

[wcndave]

I am not sure your figures are right.

A 90mm circumference you say, that gives a diameter of about 3cm, and the other one less than 2 cm.
That does not seem right.

Assuming you mean a diameter of 90 and 60, then the 90 gives a larger volume held by the chuck, and the 60 the larger surface area. I would feel the volume held is a better measure, so would go with the 90.

However, a 5mm recess seems really small, especially for a larger juck...

 

[Silverbirch]

Well spotted,wcndave,
Now edited

Ian

 

[chipmunk]

I'd have thought that, rather than the volume of spigot gripped, it's the radius of the outside of the spigot gripped that counts - resisting the leverage which is forcing the blank out of alignment.

Same conclusion, slightly different reasoning :wink:

But for this to work the spigot needs to match the ideal shape of the inside of the jaws, in terms of ideal diameter and dovetail angle or whatever.

Jon

 

[Robbo3]

I'm not sure that I've understood the question properly but ..... each set of jaws only make a perfect circle at one point in their travel & that is the size of spigot that should be made.

Too large a spigot & only the corners of the jaws touch. Too small a spigot & only the centre of each jaw touches.

Also it is where the face of the step of the spigot butts against the jaw face that stops the wood moving out of line. Chas posted a drawing showing the relationship but I can't find it to link to.

Wet wood is a problem to overcome in it's own right & needs continual checking for tightness. Probably the ideal would be a straight spigot ie not a dovetail, held by gripper jaws.
- http://www.axminster.co.uk/axminster-ty ... prod21947/


Robbo

 

[jumps]

agree almost everything Robbo, with one v small embelishment - using wet wood I will slightly oversize the spigot from the perfect circle for the jaws, with the intention of compressing the spigot down to that size after the inevitable couple of adjustments early on.

ideally a long bore chuck such as the multistar duplex for about a 40mm spigot, but long nose gripper jaws will do the gripping best, whilst a good flat upper surface on large dovetail jaws will hold the piece square best.

 

[Silverbirch]

Thanks for the replies so far.

Robbo,

I'm not sure that I've understood the question properly

What I`m trying to decide is, basically:

Will I have a more secure hold with a shallow spigot of a large diameter, or a deeper spigot with a smaller diameter

Probably the ideal would be a straight spigot ie not a dovetail, held by gripper jaws.

I have gripper jaws which I use for end grain work, but they take an even smaller diameter spigot (48mm) which is much deeper too (33mm). I think this is too narrow to support work of this type, and too deep (I`d lose 33mm off the potential depth of a bowl)
I guess I`ll try both sets of jaws (cautiously!) and see how I get on.

Ian

 

[CHJ]

..........Also it is where the face of the step of the spigot butts against the jaw face that stops the wood moving out of line. Chas posted a drawing showing the relationship but I can't find it to link to.......

 

[Silverbirch]

Thanks Chas.

Ian

 

[John. B]

I think a little purchasing with these, - VicMarc Compatible Mounting Jaws, £21. 91

plus the Vicmark shark jaws. Shark 55mm £51.92

Then the question of what will hold better becomes academic.

John. B

 

[Silverbirch]

Hi John

The jaws you are suggesting are the next size up in diameter from the gripper/shark jaws I have but are the same depth.
Could be a purchase to consider in the future, but I`m a bit wary of becoming a jaw collector :wink:

Ian

 

[Paul Hannaby]

Personally I would go for the larger diameter jaws. Perhaps more important than both the contact area or the volume of wood being gripped is the effective width of support because his is what will counteract the leverage forces when there is a considerable overhang as in deep hollow forms.

 

[Silverbirch]

Thanks Paul.
That makes sense.

Ian