Bowls not centered once chucked?

[xraymtb]

I've finally properly finished my first bowl but had an issue with it not being centered when reversed.

I started with the blank mounted on a screw chuck held in the chuck and turned the outside along with a spigot.

Reversed the bowl and mounted it in the chuck jaws but the rim wasn't running true leaving an uneven width rim.

Is this to be expected? Everything I've read suggests finishing the outside before turning the blank around but this is the second time it's ended up this way.

 

[Bodrighy]

Several things can cause this and it is fairly common.
1: Make sure that the spigot is true and also not too long. The bowl should butt up against the base of the bowl tightly when reversed
2: Some softer woods or woods with an odd grain can squeeze up when tightening the chuck. Whilst the chuck needs to be firmly holding the spigot it can be tightened too much causing the bowl to tilt slightly.
3: If the wood is wet the same can happen if you tighten too much.
4: Aggressive cutting can shift the bowl in the chuck, having a catch can as well.

Hope this helps

Pete

 

[Spindle]

Hi

Rather than forming a spigot try cutting a dovetailed recess and re-mount using the chuck in expansion mode.

Regards Mick

 

[CHJ]

As Pete says make sure that your spigot or socket is correctly dimensioned to match your jaws at there optimum PCD, and make sure as he says that your jaws are clamping correctly to give maximum support.
On softer woods do not over tighten which will accentuate any distortion due to unbalanced side grain compressing.

 

[dj.]

I`ve had this happen before on a lathe & found that the shaft the chuck mounts on was not running true, new bearings sorted it out.



Regards.


dj.

 

[Silverbirch]

After reversing the bowl, and before fully tightening the chuck jaws,try bringing up the tailstock and locating the tailstock centre into the hole left by the screw chuck. Apply gentle pressure from the tailstock, tighten up the chuck, then withdraw the tailstock. This should help ensure the bowl remains as centred as possible and doesn`t slip slightly off centre while you`re tightening the chuck.

ian

 

[stevenw1963]

Rather than forming a spigot try cutting a dovetailed recess and re-mount using the chuck in expansion mode.

Regards Mick

As Mick says Mike, try a dovetailed recess instead of a spigot. 99% of the time that is what I do as I feel safer with a recess than a spigot.
I must say I didn't have any of what you're experiencing with the lathe so I think when you're reversing the bowl it isn't seating properly in the chuck by the sounds of it.
Are you making sure the spigot face is flat? Even the smallest discrepancy could make the bowl run out of true.

 

[paulm]

What kind of timber and how well seasoned is it Mike ?

The advice above will probably solve your problems, but worth checking the timber blanks are well seasoned and stable. If not then I generally rough turn them oversized, leave to settle for a few months, and then finish turn afterwards once all the movement and distortion has (hopefully !) taken place.

Cheers, Paul

 

[woodyturner]

What mike said about liinning the bowl up with the tailstock may be your answer I do it when I make deep vases also this can also happen if you don't remove the inside on the same day you turn the outside because most woods will have moved and done its own thing I had an old friend who had the same problem in his early days of turning and it turned out he finished the outside and didnt remove the inside till a few days latter

 

[xraymtb]

I'm fairly certain the lathe is OK as I've held a metal bar against the spindle and it appears to run true. Same for the chuck body.

The chuck jaws are harder to check as they have a bit of play until they are done up against something and don't form a perfect circle when fully closed.

Thinking about a recess, the one time I've tried a recess I didn't notice the blank being out of true so you might have cracked it. I'll finish this one then use a recess next time.

 

[stevenw1963]

Mike,

If your jaws don't close properly then that could also cause the piece to run out of true.
Put your jaws on the chuck but don't tighten the screws fully. Close the jaws on the chuck until they butt up against each other and make sure they are seated correctly in the chuck. If they don't, something is stopping them doing so, sawdust on the chuck face or on the rear of the jaws. Clean them. Once the jaws are closed properly and even on the face, tighten the screws up.

 

[CHJ]

.....The chuck jaws are harder to check as they ..... don't form a perfect circle when fully closed.
..

They are not intended to, hence why my comment about correct PCD, if you check you will find that they form a circle with something like a 3 or 4mm gap between the accessory jaws. This due to method of manufacture when they are turned as a circle then sliced into four segments.

This is the diameter, internal/external that you aim for with your spigot/socket. The gap in the accessory jaws may be greater than shown here but this is typical.

 

[EnErY]

I feel More At Ease With A dovtail rather Than A Spigot And Find its A neater Finish to The bowl after turning as well as the bowl seems to hold better
regards
Bill

 

[woodyturner]

A dovetail or a spigot created the right size for your chuck are both equal to each other and both work just as well as each other the cherry bowl I just finished was turned with a spigot which was reversed and the spigot remover and the walls are an equal thickness all the way round it is just a matter of personnel preference some of the most experienced turners prefer to use a spigot and others a dovetail mostly it depends on what you are turning'
If you turn a bowl out of very wet wood a lot of turners will turn a spigot on the inside of the bowl as well so that when it is dry it can be remounted from the inside to redefine the outside then redefine the inside both ways are very important to most experienced long term turners and both ways work

 

[shaver01]

As already said holding on reverse chucking is really down to what you are turning, my personal preference is a spigot. When you hold on a dovetail the size of the jaws dictate the size of the foot of the bowl, where as with a spigot you decide the size of the foot then remove the spigot on completion.

Reverse chucking, IME if the wall thickness is approx. 1/2" or more, and the edges are not defined too sharply you won't notice it being slightly off centre.

Thin wall is different though, my way is to hold the blank with your chosen method, shape the outside of the bowl and spigot. Rough sand and when happy reverse chuck, no need to get uptight if it runs out a bit, just loosen the chuck a bit and knock to centre as best you can with the handle of your tool. Once centred as best you can, go over the outside with your gouge in a sheer cut to take of a small skim which will centre the blank perfectly. Now sand to finish and get on with removing the inside. With this method you can get you wall thickness down to as thin as your able and all even thickness as well.

If you leave the blank for a few hours or more before removing the inside you will probably have to re-skim the outside as chances are the wood will have moved. When turning thin you must work as quickly as possible.

 

[Mark Hancock]

Mike

To avoid this problem simply turn a rough outside shape and spigot (I wouldn't recommend a recess) whilst the blank is held on the screw chuck. Then with the rough form in the chuck refine and finalise the outside before you start the hollowing and as if by magic your problem disappears

Mark

 

[Spindle]

Hi

In my opinion, the advantage of using a dovetailed recess is that the work does not need to be re-chucked again to remove the spigot. Any slight external inaccuracies post mounting on the dovetail recess can be trued up during the hollowing out phase.

Regards Mick

 

[chipmunk]

the advantage of using a dovetailed recess is that the work does not need to be re-chucked again to remove the spigot.

Hi Mick,
That's technically true since if you use a recess there is no spigot :wink:

...but it's quite a controversial statement because even if I use a recsss I will reverse turn to remove any trace of it. Admittedly some will try to "disguise" the recess but it's not for everyone.

Jon

 

[CHJ]

..
In my opinion, the advantage of using a dovetailed recess is that the work does not need to be re-chucked again to remove the spigot. ...

That all depends upon whether you consider a piece finished if it still carries the chucking details.

I personally see no reason one would not finish a piece off by removing all signs of work holding, to me it's akin to leaving tooling marks.

The only time I have seen pieces of quality being made that retained their final chuck holding spigot was at a demonstration where the turner concerned had taken great care to incorporate a very wide (150+mm dia.) narrow (1-2mm) spigot in the base design which looked when finished to be part of the decorated form.

 

[stevenw1963]

In my opinion, the advantage of using a dovetailed recess is that the work does not need to be re-chucked again to remove the spigot

I think there is no right or wrong way, just what is comfortable for each turner plus,
I would not consider a dovetail recess to be akin to leaving toolmarks, each & every individual should design & finish their creations as they (or their customers if they have them) want.
What is pleasing to the eye to one person may not be pleasing to others.