Ripples ... help please?

[TobyB]

Got a new lathe - for 18 months I learned a lot using an inherited Arundel Junior which I added a Supernova chuck too - but very limited in size of wood that could be turned, and so I bought a much larger and very impressive Wivamac.

I can now try to turn the larger bowls and platters I fancied making ... but ... find new problems, maybe related to the much larger diameters I'm working with ...

I keep getting ripples, especially on the outer face (so I find there is a chattering sensation at the gouge tip as the wood is no longer quite circular), which I can only get rid of by cutting away a lot of wood, typically using the tip of the gouge - which then leaves furrows in my attempt to get a nice smooth progressive curve to the edge when using the bevel (downhill, as the books describe).

I never noticed this much on small bowls and boxes ...

Any idea what I'm doing wrong, or could do better?

Cheers

Toby

 

[CHJ]

Welcome to the forum Toby,

What you are experiencing is a common phenomenon, I believe the ripple starts because of the differing hardness levels of the wood, mainly due to the change from side to end grain as the wood rotates which allows the gouge to cut slightly easier/deeper on the softer bits.

Once this has started unless you have a very rigid control of the tool on the rest, the tool 'bounces' radially on the next revolution making the furrow even deeper each time as your natural tendency is to push the tool inwards against the bounce.

One way of helping prevent this is to turn at the highest speed you can so that there is less reaction time for the tool/your grip to respond before the next revolution. (A bit like driving fast over a rough surface, the suspension does not have time to respond to the holes.)

Once there though the best way to remove the ripples before trying again is to use a scraper to round up the blank again so that you have a smooth surface for gouge bevel contact.

 

[jasonB]

And make sure your tools are sharp, I tend to get something similar if the tool starts to loose its edge.

Jason

 

[Mark Hancock]

Toby

I agree with what Chas says but also make sure you are pressing the tool firmly down on the tool rest and not on the work. Pressing hard on the work to cut the ripple out only increases the ripple effect for the reason Chas gives. Back off and reduce the cut to just take the high spots off and as Jason says use a sharp tool.

 

[TobyB]

Thanks for that everyone. Tried the higher speed thing, and definitely helps ... and will concentrate on how firmly I hold the tool on the rest too ... managed to cut smoother profiles on a couple of bowls so at least they look OK ... not quite so happy with the feel ... but clearly need more practice. Discovered the scrapper thing myself - rounds it off OK but surface rubbish, so another few mm needs shaving. Backing off makes sense as to the cuts ... but without the bevel rubbing, how do I make a smooth progressive curve?

Cheers

Toby

 

[CHJ]

...... Backing off makes sense as to the cuts ... but without the bevel rubbing, how do I make a smooth progressive curve?
.....

By "Backing off" with the gouge you are achieving similar results to using the scraper by only cutting the high spots, BUT the gouge must be presented such that whilst cutting the bevel is rubbing during the cut, of course it will be in free air during the hollows.

As the piece becomes nearer a true cylinder then the bevel will be in contact a bigger percentage of the time.

This is just part of the knack of holding the tool at the correct angle, biggest problem is making yourself relax your grip whilst still maintaining firm control.

Having started a cut with the tool feeling good, try watching the ghost profile image of the spinning piece at the top rather than the tip of the tool, sometimes it helps to develope a feel for what the tool is doing.

Remember the bevel in contact with the wood must be in line with the cutting profile you wish to achieve, easy to say but not always so easy to achieve when you are manipulating an oddball piece of wood.

Follow Graham's (oldsoke) often quoted advise, take small cuts, everyone is a learning cut and they are free until you reach your final shape.

 

[Blister]

Also make sure the tool rest is close to the work , but not touching , this will give maximum support to the chisel and reduce the amount of leverage / movement in the tool tip while cutting :wink: