Flying wood again!

[Bruce White]

Thought, because it is the most recommended way, to try using contraction mode on a spigot when turning a 5KG lump of dry burl (described as rib fruited gum - pretty hard wood) today. I habitually turn the outside of the bowl to finish using a faceplate, make a recess and reverse it using expansion mode. Rarely have one come off.

Gave the spigot method a go after finishing the outside and carefully matching the depth and angle of the chuck jaws and spigot. Tightened it as much as I could, ran it for a bit, tightened it again (didn't move) and still had the damn thing come off when I started to level the face. And I was not going hard at it. The same thing happened when I tried spigots on the green turnings I have stashed away for finishing - reverted to expansion mode there too.

I have a Vicmarc 120 and the old black Nova chuck. Both have straight sides in the contraction mode and dovetail on expansion. Could this be the problem?

I mentioned in an earlier post on this subject my theory on inertia on the chuck. If you turn the lathe on with the chuck empty, it naturally flies open, therefore, to my simple mind, expansion mode has to be safer as inertia is helping the chuck do its job. I finished turning a really rough, hard piece if what I think is York Gum burl using the expansion mode on the same Nova chuck with no problems immediately prior to trying the contraction mode on this one.

What am I doing wrong?

[mpax356]

I think we need a little more info here. You did not indicate which chuck or jaw set you were using nor how big the turning was. Some potential problems: having the spigot bottom out in the chuck. Another is not having a clean shoulder. You probably know this but can't tell from your post. Was the piece resting against the jaw faces? If you did not have a shoulder to provide a mechanical hold against both the face and spigot, you don't get a strong hold. I have no experience with the VM chuck but have the G3, SN2 and Titan chuck. The manufacturer does not recommend any groove at the base of the spigot for the 50mm jaws. They say the lip will bite into the wood. Some folks think they should cut a dovetail on a spigot and by not cutting it correctly it actually weakens the hold.

[Bruce White]

The piece is about 24cm in diameter and about 16cm deep. I was using my Nova 50mm jaws, had it hard up against them (used the tailstock ) and I could see on the tenon where the jaws had bitten in to it evenly all round. I had cut the tenon so the chuck was pretty close to fully closed (following advice on another thread).

[Pat]

Bruce a pic or 2 of the offending UFO and the spigot might help.

[Bruce White]

Sorry Pat, already removed the spigot, cut a recess and am on the way to finishing it the way I usually do. Probably won't be today - it is 9.13 AM and currently 41.2 degees celsius outside and and only about 3 degrees cooler in the shed.

[orraloon]

I usually go for expansion mode if the base is wide enough. I like at least 25mm of wood around the hole for heavyish lumps. I have had more problems gripping than with expansion mode. I use a SN2 for heavy bowls but for most stuff I use this Leady expansion chuck. The cutter beside it bores out the recess in seconds.
Regards
John

[ian thorn]

You say you had the tailstock up so im thining the piece didnt come out of the jaws but the spigot brock of if this is the case may be the wood is not strong enough for a 50 mm jaw if useing a large block i fit the 80 or 100mm jaws you could try drilling a small hole for the live centre to locate in untill you remove some of the centre to reduce the weight.As you said dont hog into it

[NCPaladin]

I try to go with a recess also but occassionally use a spigot.
With the 50 mm jaws and the area of a circle (pi X r square)
With a 2" spigot there are 3.14 sq inches of wood holding it attached.
With a recess and 1" of wood surrounding there are 9.42 inches of wood holding it attached (this is after deducting the size of the spigot which is not holding anything).

If design/size/wood allow I would rather use a 35mm or 40mm recess than a 50mm spigot.