Unable to balance a blank

[JonF]

I've just spent a frustrating 30 minutes trying to 'true' a bowl blank. It measures about 30cm in diameter by 5cm in width, or at least it did to start with. It's now about 20cm in diameter and still making my small Axminster AWVSL lather move around the bench like a demented ferret. I've braced the lathe, tried speeds from 150rpm to 1000rpm (the last one before being kindly supplied by a speed guide from a member of the forum), but still it stubbornly resists all attempts to bring it into a symetrical shape even with newly sharpened tools.

My suspicion is that I've exceeded the capacity of my small lathe and I should stick to smaller work. Any ideas?

JonF

 

[Leo]

This may sound daft, but has it come a little loose in the jaws, or the screw chuck ???? happens with wet wood sometimes, it shrinks as the centrifugal force sends the sap/water flying, Or is the spindle running true ??? er has it lot of hardwood on one side, unbalancing it ?? er I dunno, sorry, try another piece maybe, see what happens???? Sorry, someone will be able to help you.!!!

 

[CHJ]

It may be that the 'Blank' is varying in density or moisture across its width,
If you have managed to get the circumference somewhere near round then try removing some of the surplus wood, inner and outer, and see if it vibrates less with the resultant weight reduction.

If you have it fixed to a faceplate then you could try adding balance weights to it while you reduce its mass.

Take the belt off the pulley so that the work can spin as freely as possible and spin by hand.
The heavy point should settle at the lowest point.
Fix something heavy like a large washer or nut opposite this point to the back of the blank/faceplate with wood screws.

If you do not want to have screw holes in the back of your blank use a large plywood disk between the faceplate and the blank, you can bolt your balance weights to this.

Alternately balance blank by removing wood by hand like this.

 

[JonF]

"It may be that the 'Blank' is varying in density or moisture across its width,"

You know, that's the only explanation I could come up with and I'll follow your suggestions tomorrow before I'm down to making a napkin ring.

 

[jumps]

how is it chucked/mounted?

 

[JonF]

how is it chucked/mounted?

I started with a screw chuck and then moved to a small faceplate (4 screw points) with no difference at all.

 

[jumps]

how is it chucked/mounted?

I started with a screw chuck and then moved to a small faceplate (4 screw points) with no difference at all.

thanks Jon

it would seem that you have significant variations in density (which may include moisture) within the blank such that truing it up for volume still has it uneven for mass

alternatively, the variation is at the 'chuck face' where one side, or area, is 'high' and the balance will only be established once it's reversed and trued.

or a bit of both!

 

[duncanh]

What I tend to do in situations like this is try to remove as much material as possible before trying to true up any round surface.
Mount on the faceplate and turn a foot for the chuck to hold (I assume you have one).
Then remove material from the outside to get an approximate profile - usually using very rough cuts as the funush doesn't matter at this point.
Then reverse the blank but leave the faceplate in place.
remove material between the faceplate and the edge going as deep as you can but leaving enough of a wall thickness to support further outside turning.
Hopefully at this point the blank has lost a lot of it's weight and speed can be turned up a little higher.
Reverse the blank again and mount the faceplate on the lathe.
By this point I'd hope to be able to get a better final shape and better surface finish.
Once outside is completed to satisfaction turn the inside as normal.

For blanks that are really unbalanced that cannot be safely turned at any speed I have a large plywood disc which is permanantly attached to a 2nd handwheel.
When this disc is in place I let the blank settle so the off balanced weight is resting at the bottom.
Then I take some lead weights and screw them to the playwood disc torwards the outside edge. With a bit of adjustment the blank should now be counterbalanced but the lead and the speed can be turned up.
The problems with this system are:
As you turn the bowl (or vase etc) you are changing the weight in it so the lead slowly becomes unbalanced and you need to regularly stop and change it's position.
There's no way to lock the handwheel in place so you need to be very careful with the speed and not get any catch which may stop the lathe - it is stops suddenly the playwood disc may continue turning at detatch itself :shock:

A safer way to work would be to have the disc mounted on the turning side of the lathe and have the work attached to it. Screws through the plywood to hold the piece in place oruse a glue joint (make sure the plywood is well laminated)

 

[Blister]

JonF

Are you using a Bowl gouge ?

And when trying to true up to a cylinder are you getting a " cut " no cut , Cut no cut , type of ticking noise when using the tool

or

are you keeping cutting contact at all times by pushing the bowl gouge into the wood and following the irregular shape

If you have a high point on the blank you should only cut this part away with each revolution of the workpiece , hence the tick / tick / tick noise as the work goes around gradually reducing the high point until you achieve a cylinder

Hope this helps :wink:

 

[Jacob]

...... making my small Axminster AWVSL lather move around the bench like a demented ferret. .......

Lathes like this aren't much good freestanding on a bench, except for little light things. You need to bolt it down solidly to something heavy such as a length of 10"x3" which is then fixed in turn to a solid bench (if you don't have a purpose built stand). Wider the better as this will stop it rocking.
Then turn your blank slowly and remove high points with something very sharp.

 

[miles_hot]

It could also be that the face (and back face) are out of true - have you tried making sure these are flat?

Aside from that I would be in the "remove as much as possible" camp - maybe turn the speed down to a point where it is tolerable and doesn't feel unsafe

HTH

Miles

 

[myturn]

I think Blister's prognosis sounds the most likely.

Don't worry if it's not true, it doesn't need to be a cylinder to make a bowl.

I rarely bother making a bowl blank a perfect cylinder as most of the outside is going to be cut away when I start shaping. If it's badly out of balance then I will use a roughing gouge to remove as much as is needed to get it to rotate smoothly.

Square up the foot and make your spigot or recess, whichever you are using.

Then start shaping the outside cutting from the inside (foot end) to the outside towards the rim using your bowl gouge. If you start from the bottom which you have made flat and keep the gouge in the wood you should be able to get it round as you will not be following an out-of-round surface on the outside.

 

[JonF]

Are you using a Bowl gouge ?

And when trying to true up to a cylinder are you getting a " cut " no cut , Cut no cut , type of ticking noise when using the tool

Yes, freshly sharpened which gives a very definite cut..no cut.

In the meantime I've secured the lathe properly to my bench and this has helped with some other work. I'll put the offending bowl back on tomorrow and try some of the ideas above.