Realigning blanks

[Democritus]

Hi guys
I mostly turn bowls, so spindle turning is not a forte of mine.
I was asked recently to turn a goblet, and made pretty good progress with it, until I got a bit of a catch. This had the effect of making the blank wobble slightly. I tried to correct this by just tapping the blank, but this didn’t work. My question is ‘ Does anyone have a foolproof method for making wobbly spindle blanks run true?’
D

 

[Stigmorgan]

Unless you have a centre point to put your tailstock into its very difficult to reseat a piece back in perfectly. You could use a protractor to draw a circle the diameter of the goblet and cut it out and hold it to the goblet and align your tailstock with the centre point from the protractor.

 

[Democritus]

Cheers, Stig.

 

[SammyQ]

Four-edge centre, file one 'edge' to have a 'Vee' or notch in it. The mark it leaves is quite distinct from the other three. Just line up the 'vee' with the corresponding shallower dimple on the turned object's bottom. Simples.

 

[Robbo3]

I'll probably be reprimanded for telling you this but I use either a skew, or the back of a gouge, as a lever between the tool rest & the work piece. That's after turning the speed down & slightly loosening the holding pressure. Once running true I tighten everything back up.

Edit: I forgot to say that it will probably leave a burnishing mark so try it on a part that will be turned away.

 

[Paul Hannaby]

I'm assuming your problem was while the blank was mounted between centres? If so, using ring centres might make it easier to maintain alignment.

 

[Democritus]

Thanks, guys.
The blank wasn’t held between centres. The problem arose after I had hollowed out and shaped the goblet bowl, with the blank held by a tenon in the headstock chuck. While beginning to turn the stem I got a catch. Nothing spectacular, but sufficient to throw the blank a little bit off true.
I stopped the lathe and turning by hand, tapped the wood in an attempt to knock it into true alignment. No luck, so I gave up.
Robbo’s suggestion might well fit the bill in future.
D

 

[Paul Hannaby]

In which case, I suggest when you create a spigot on the end of your blank to hold in the chuck, you leave a shoulder that acts as a register against the top of the chuck jaws. i,e, the spigot that ends up in the chuck jaws is a smaller diameter than the rest of the blank. This way, if the work is dislodged, it is easy to replace it in exactly the same alignment in the chuck.

 

[Democritus]

Thanks, Paul.

 

[cmoops2]

Further to Paul's (very valid) comment, also consider a couple of simple pencil marks on the spigot where it contacts say jaw number 1 (using the gaps between the jaws as 'edge-markers). If the workpiece does 'move' after a catch, the spigot can then be accurately re-positioned in the chuck. I'd also recommend that you ensure that your spigot diameter accurately matches the 'perfect circle' diameter of the jaw set in use.

As an aside, you said you got the catch when turning the stem - were you using any tailstock support in the goblet 'cavity' (using say a wooden soft centre with router mat facing attached to a (lightly applied) live tailstock centre ?

 

[Ed Weber]

Cone center in tailstock

 

[Robbo3]

I've seen numerous professionals tap a piece back into concentricity but it only works sometimes for me so I rely on the lever method I posted above.
Edited the post to add a warning about burnishing.

 

[Democritus]

Thanks cmoops2 and Ed.
I thought about using some sort of a support from the tail stock, but I didn’t have anything to hand. I’d have had to turn it etc.
I like the idea of a cone centre, but I wonder about centring that on an out of line blank. Would it automatically bring everything into line, or might the cone find itself askew in the goblet bowl?
D.

 

[Richard_C]

I like the idea of a cone centre, but I wonder about centring that on an out of line blank.

It might not bring it into line but it might keep it in line if you use it from the outset.. Having hollowed the goblet, a centre of some kind might help it stay put if you do get a catch. No help now perhaps, but maybe next time.

 

[loftyhermes]

I've had success using a tennis ball or similar in the bowl, held in with the tailstock.

 

[Ed Weber]

Thanks cmoops2 and Ed.
I thought about using some sort of a support from the tail stock, but I didn’t have anything to hand. I’d have had to turn it etc.
I like the idea of a cone centre, but I wonder about centring that on an out of line blank. Would it automatically bring everything into line, or might the cone find itself askew in the goblet bowl?
D.

With goblets in particular, IMO it's important to never retrace your steps. Start with the bowl/cup, once it's complete, bring up the support and proceed down the piece.
If you loose alignment, you need to loosen the chuck a small amount. With the lathe running at it's slowest speed or turning by hand, bring up the tailstock and slowly advance the cone or sometype of support until the alignment is centered. Then stop and re-tighten the chuck.
You can achieve wry small details with this method
I made this goblet about 12 years ago, the stem is 1/8" or about 3mm

 

[Democritus]

Thanks for your valuable advice, Ed.
I love your goblet. Brilliant work.
D.

 

[Democritus]

Another effort. I used a cone centre this time,
D.