I have a Nova 2000 lathe and a Supernova chuck used primarily for bowl turning. My bowls are out of round, with two thick and two thin walls. I end up sanding for hours to get the inside uniform becuz of the out of round situation. Most bowls are 10" or less and the lathe speed is around 900-1000. My bowl gouge is sharp as a razor. Is it a technique issue or possibly a machine wobble from bearings or belt alignment? I'm getting dang tired of sanding my days away! Any suggestions welcome. Thanks in advance.
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Unresolved issue
- Thread starter Richard H Buckley
- Start date
There could be a couple of long videos about this. Most of the time, it is a drying issue, and the wetter the wood is, the more the difference will be. Generally the end grain will shrink very minimally, and the side grain shrinks a lot, so a 1/2 inch thick bowl may dry out at 1/2 inch on the end grain, by 3/8 on the side grain. Fine for warped bowls.
If you are trying to go for very thin bowls, less than 1/8 inch, variable wall thickness doesn't look good. This again seems to follow the pattern of end grain thicker than side grain. If your finish cuts are with a gouge and rubbing the bevel, there is always a bit of a bounce, twice each revolution from cutting with/against the grain. If you use a shear scrape, then since it does not use the bevel rub, you end up just nibbling off the high spots. So, after reversing, you true up the outside of the bowl again, then start on the inside. You have to finish turn in stages with the shear scrape. If you ever master the bevel rub cut (the bevel should rub the wood, but the wood should not know it), this reduces it some what, and you are not causing the bowl to distort as you turn by pushing too hard, I still find the shear scrape to be the better finish cut.
Hope that helps some, or raises a few more questions...
robo hippy
If you are trying to go for very thin bowls, less than 1/8 inch, variable wall thickness doesn't look good. This again seems to follow the pattern of end grain thicker than side grain. If your finish cuts are with a gouge and rubbing the bevel, there is always a bit of a bounce, twice each revolution from cutting with/against the grain. If you use a shear scrape, then since it does not use the bevel rub, you end up just nibbling off the high spots. So, after reversing, you true up the outside of the bowl again, then start on the inside. You have to finish turn in stages with the shear scrape. If you ever master the bevel rub cut (the bevel should rub the wood, but the wood should not know it), this reduces it some what, and you are not causing the bowl to distort as you turn by pushing too hard, I still find the shear scrape to be the better finish cut.
Hope that helps some, or raises a few more questions...
robo hippy
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In addition to wet wood moving, dry wood can also move ... usually not as much as wet wood, but sometimes a surprising amount.
If the wood isn't completely dry then it needs to be twice turned as Reed said, allowing a few months drying time between rough turning and final turning. Even with dry wood, it doesn't hurt to leave the piece a little thick and give it a few days to let it stabilize before doing the final turning.
Centrifugal forces can distort a bowl that is spinning at high speed when the walls are thin. The distortion is likely to induce flutter. When the tool starts skipping the flutter can get out of hand in a hurry.
Whatever the root cause, I would suggest slowing the lathe speed down considerably, turn the interior in stages, and take very light bevel rubbing cuts.
If the wood isn't completely dry then it needs to be twice turned as Reed said, allowing a few months drying time between rough turning and final turning. Even with dry wood, it doesn't hurt to leave the piece a little thick and give it a few days to let it stabilize before doing the final turning.
Centrifugal forces can distort a bowl that is spinning at high speed when the walls are thin. The distortion is likely to induce flutter. When the tool starts skipping the flutter can get out of hand in a hurry.
Whatever the root cause, I would suggest slowing the lathe speed down considerably, turn the interior in stages, and take very light bevel rubbing cuts.
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This is where a class or a mentor will get you in the right footing.
There are lots of things that might create the uneven walls.
Wood shrinkage is one Reed mentioned. If you are not getting a clean bevel riding cut the tool can bounce and cut less deeply. Once that uneven surface is created it can get worse and the tool will tend to follow the uneven surface. Too much bevel pressure can create uneven walls too.
If you are doing a single turning as Reed explained the end grain side wall does not shrink and the sidegrain wall will. A thinner wall makes this harder to see. I turn natural edge bowls once withna wall thickness of about 1/4" or or less and the ends will be a little thicker when dry. I just sand them smooth and don't try to make the thickness match.
I try to leave some thickness in the walls near the bottom of the bowlmfor support and cut the walls to thickness from rim to bottom in couple inch increments.
In the tutorials and tips there is a thread I started on working withngreen wood.
http://www.aawforum.org/community/index.php?threads/working-with-green-wood.11626/
Links to 2 videos from a demo one roughing a green bowl, and one turning the dried bowl.
The dried bowl warped a lot so you can see how I line it up and turn it round.
The both bowls will have even walls.
Yellowstone Woodturner's meet in Billings.
There are lots of things that might create the uneven walls.
Wood shrinkage is one Reed mentioned. If you are not getting a clean bevel riding cut the tool can bounce and cut less deeply. Once that uneven surface is created it can get worse and the tool will tend to follow the uneven surface. Too much bevel pressure can create uneven walls too.
If you are doing a single turning as Reed explained the end grain side wall does not shrink and the sidegrain wall will. A thinner wall makes this harder to see. I turn natural edge bowls once withna wall thickness of about 1/4" or or less and the ends will be a little thicker when dry. I just sand them smooth and don't try to make the thickness match.
I try to leave some thickness in the walls near the bottom of the bowlmfor support and cut the walls to thickness from rim to bottom in couple inch increments.
In the tutorials and tips there is a thread I started on working withngreen wood.
http://www.aawforum.org/community/index.php?threads/working-with-green-wood.11626/
Links to 2 videos from a demo one roughing a green bowl, and one turning the dried bowl.
The dried bowl warped a lot so you can see how I line it up and turn it round.
The both bowls will have even walls.
Yellowstone Woodturner's meet in Billings.
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