Chucking up

[Lonsdale73]

According to the blurb, the 50mm standard jaws supplied with my chuck will work with spigots sized between 42mm and 61mm. Is it better to aim for a maximum sized spigot so there is more surface area for the jaws to grip, a minimum so the jaws are fully tightened, somewhere in between or does it not make a blind bit of difference? Also, in terms of spigot depth, do you make it exact same depth as the jaws, a touch shallower so it doesn't bottom out, longer so the shoulder is clear of the jaws or does this not make any difference either?

 

[CHJ]

This Quote from a previous thread on the subject may help.

.....the area surrounding the spigot must be flat and true so it can rest against the 'nose' of the jaws, the spigot length must be slightly less than the depth of the jaws for the same reason. ......

This sketch may help show spigot location as described

Note:- The Socket details are as used with my Axminster C type jaws, other brands and configuration may not be capable of the additional support as their dovetails protrude some distance from the chuck fixing flange.

Regarding the diameter; maximum grip potential with maximum surface contact and minimum bruising of the spigot or socket fibres is when the Jaws are forming a TRUE Circle. Usually with a 3-4mm gap between the jaws. (the gap being the thickness of the milling cutter that sliced the true circle into quarters)

 

[nev]

The size of spigot should be that which has the greatest surface contact with your jaws - I'd hazard a guess at around 50mm. If it's too big you will only be clamping with the points of the corners of each jaw segment, or if in expansion mode just using the top part of the jaw segment.

Depth should be a fraction short of bottoming out.
Butt the base of the workpiece up to the jaws to prevent any 'tilt'.

If it is a bowl for example, starting on a faceplate, turn the spigot (and make a tiny dimple in the centre) and the outer shape and sand and finish the outside. Then flip it round, mount it in the jaws and turn the inside. Once finished you can flip the bowl around again, using the tailstock located in your dimple, push it up against a large enough disc of wood mounted on a faceplate (with concentric circles marked on it to aid center-ing) and you can gently remove the spigot if required or finish finishing etc.

edit: Chas beat me to it.

 

[Lonsdale73]

Thanks chaps

 

[Dalboy]

This video is worth watching and explains a lot about mounting blanks in chucks.