Reverse turning bowls, how accurate is the new tenon?

[Dean Center]

The recent thread on twice turned bowls made me wonder how reliable/accurate the returned tenon is for folks.

In other words, if you true the tenon and turn the outside of the bowl in one step, when you reverse and put the tenon in your chuck, is the outside adequately true?

Please input to our survey:
1. What percentage of the time can you accept the outside turning and what percentage of the time do you have to re-true the outside?

(for full data collection, 2. what do you use to drive the inside of the bowl? and 3. what chuck do you use?)

Thanks.

 

[hockenbery]

For returning a dried bowl I jamb chuck the bowl over an open chuck either a vicmarc 120 or a ONEWAY stronghold. For me the tenon will run almost true just about every time 95+%. If it doesn’t have a crisp look or feel smooth when running in the chuck, i loosen the chuck rotate the bowl a tiny bit and push it against the chuck as I retighten it. If this doesn’t work i tap the rim until it runs true then tighten the chuck more. If this doesn’t work I will return the outside but I haven’t done that in a long time.
If there is a tiny flutter to the touch I don’t worry about it because A bowl can be off center a 1/64 of an inch and look and feel good when the fished rim is 3/8” or thicker.

When I return a jamb chucked bowl I take the time to line up the high and low rim parts, then true rim( the most out of balanced part), true the outside, the true the tenon last. So when I true the tenon the bowl has almost no vibration.
I also cut my tenons with a spindle gouge across the grain to get a clean tenon. Scraping the tenon works ok most of the time but
scraping into endgrain & side grain can create a slightly oval tenon
Scraping may leave a little fuzz on the tenon causing it not to center well.

if my finished wall is 1/4 or less whether it is a once or twice turned bowl I always do a finish cut after it is in the chuck just to get the truest possible outside wall. It often isn’t necessary but I do it this way as my process.

 

[Emiliano Achaval]

Most new turners do not cut a tenon for a proper fit, crucial to have the bowl run true. My 3/8 40/40 lets me cut a perfect tenon. I do not like the chuck jaws to cover all of the edge of the tenon, I like about half, that's all you need. I also take the sharp edge of the tenon to prevent the tenon from shearing off. A bowl gouge with a 60* nose won't let you cut a proper tenon... I'm with AL, about 95%, and a little playing around usually gets it right. I just gave a class to a new turner, I was amazed at the tenon he had, an accident waiting to happen, no contact with the jaws at all... I have several Vicmarc 120's.

 

[Bill Blasic]

When you do the outside before turning around you take a finish cut on the tenon. I rough out the outside and the tenon. I take a finish cut on the outside. I finish sand the outside and put my finish on the outside. I then take a finish cut on the tenon and turn it around. This gives me the best chance of the outside running true to start. Remember when you are removing stock (wood) you are relieving stresses in the wood. I do the same with twice turned bowls.

 

[Doug Freeman]

if you true the tenon and turn the outside of the bowl in one step, when you reverse and put the tenon in your chuck, is the outside adequately true?

Its close, but many times removing inside material causes the OD to move. For this reason I true the tenon and the lower 1/3 of the outside. If the rim is out much top to bottom I will flatten some then turn around and finish out all surfaces with it mounted on the tenon.

I use extended jamb chucks about 6” long when between centers to give clearance for deep bowls.

 

[john lucas]

There are a lot of variables. First of course is the shape and size of the tenon. Stewart Batty has a great video in shaping the tenon properly. Then ideally it needs to be sized so that the jaws are almost closed when tight. Wood density varies. End grain does not compress as easily as side grain. I try to orient the tenon so that an equal amount of side grain and end grain are in each jaw. Wood moves due to internal stress so if you rough out the outside by the time you reverse it and turn your tenon it might ha e become slightly oval. Maybe only a 64th of an inch but just enough it wont run totally true. So.we try the best we can but sometimes you still have to true up the outside depending on how picky we are.

 

[Ed Sandle]

For the most part, I'm with Al. Almost always, after returning a tenon, my turnings will run true, especially with my Vicmark chucks. At times I do have to twist them a bit and retighten. I mostly turn thin walls, and ocaionaly have to return after mounting, when the walls are 1/8" or less, to get an even edge. Where I differ from most is, I have an 1/2" scraper ground to match the angle of my dovetail jaws, and use this to make my dovetails. I have a angle mark on my grinding platform to give me the perfect angle when sharpning, and it gives me perfect results every time. It is so simple to use, you just look down on the front face of the scraper when making a dovetail, keep the front edge at 90 deg, and push it in. Actual cutting of the dovetail takes less than 2 seconds, and leaves a perfectly sharp inner edge for the chuck jaws to lock into, even on wet wood, as long as your scraper is sharp. It is so simple, and fool proof, as long as the angle is accuratly ground to match your jaws. So to better understand the grind, the front face of the 1/2" scraper needs to be ground back to match the jaw angle, I think it's about 7 deg, and the right edge needs to be undercut to where the top edge is sharp. When using, the gouge handle is basically in line with the lathe bed, but slightly skewed to the dovetail angle. This is just the opposite of some of the commercial dovetail tools I have seen being sold, in that they need to come in from the side, and are very awkward to use. You can check your grind angle by putting your scraper in one of your jaws, and sliding it up to it. You might want to add about a 1/2 degree of angle to your grind so that when tightning up your jaws it pulls them up to the back face.

 

[Tom Albrecht]

For the most part, I'm with Al. Almost always, after returning a tenon, my turnings will run true, especially with my Vicmark chucks. At times I do have to twist them a bit and retighten. I mostly turn thin walls, and ocaionaly have to return after mounting, when the walls are 1/8" or less, to get an even edge. Where I differ from most is, I have an 1/2" scraper ground to match the angle of my dovetail jaws, and use this to make my dovetails. I have a angle mark on my grinding platform to give me the perfect angle when sharpning, and it gives me perfect results every time. It is so simple to use, you just look down on the front face of the scraper when making a dovetail, keep the front edge at 90 deg, and push it in. Actual cutting of the dovetail takes less than 2 seconds, and leaves a perfectly sharp inner edge for the chuck jaws to lock into, even on wet wood, as long as your scraper is sharp. It is so simple, and fool proof, as long as the angle is accuratly ground to match your jaws. So to better understand the grind, the front face of the 1/2" scraper needs to be ground back to match the jaw angle, I think it's about 7 deg, and the right edge needs to be undercut to where the top edge is sharp. When using, the gouge handle is basically in line with the lathe bed, but slightly skewed to the dovetail angle. This is just the opposite of some of the commercial dovetail tools I have seen being sold, in that they need to come in from the side, and are very awkward to use. You can check your grind angle by putting your scraper in one of your jaws, and sliding it up to it. You might want to add about a 1/2 degree of angle to your grind so that when tightning up your jaws it pulls them up to the back face.

Why not share a photo of this scraper, please?

 

[john lucas]

I have have Vicmarc dovetail jaws. I cut the tenon with my parting tool. I cut from the side.to finish it I rotate it clockwise and angle the tool by swinging the handle out. I want a V that is slightly narrower than the V jaws. This guarantees the jaws sit flat on the top of the tenon and the jaws squeeze on the sides.

 

[Ed Sandle]

Didn't have any pics of it,but I went out and took a few with my cell phone. Not the greatest, but I think you get the idea. Also I see I mistated on the scraper size, as it looks tobe a 3/4" not 1/2". Look close at the picture and you should be able to see that the cut at the inner point is very clean, even though I havent sharpened it in a while, and took less than 2 seconds to make this cut. Just eyeballed it for alignment, and shoved it in. Also, make sure that your cutting edge is slightly below center, or it will want to push the scraper out when cutting if you dont have enough relief angle ground in. The back face of the cut should be 90 deg to the lathe bed.

 

[John Jordan]

I always make finish cuts AFTER the piece is mounted in the chuck. That way its 100% true, I hate seeing things even slightly out of round even though a little doesn't really hurt. It doesn't matter if its the original tenon that's re-turned after drying, or a new tenon on a green piece, I make the finish cuts after its in the chuck. FWIW.

John

 

[Ed Sandle]

John, if I gave a confusing description as to running true after making my cut, weather it be wet wood, or returning, yes I also always will make a finish cut when returning after drying as I want my final rim to be of an even thickness. I almost never have turned a piece of wet wood from start to finish at one time, although I really would like to experiment with that sometime. Maybe that would be a good project for me this winter.

 

[Mike Johnson]

Re-mounting a wood work piece in a woodturning chuck is almost impossible to get mounted the same as it originally was mounted the first time. A precision metal turning chuck requires the use of a dial indicator to get a round piece of metal centered in a chuck for turning. A wood-turning chuck is not machined to the same tolerance as a metal-turning chuck and your wood work piece is constantly changing its shape based on humidity and temperature in the room. As you remove wood from the work piece it is relieving internal stress in the wood which is constantly changing the shape of the work piece. When you add up all of the variables in tolerances of the mechanical equipment and movement of the wood from moisture gain and loss, temperature changes, and internal wood stress changes, it is just another part of the wood turning art form that we all deal with every day.

 

[Randy Heinemann]

Generally speaking, with my limited experience, I find that I can re-mount small bowls after drying with the existing (in my case) recess. Sometimes it takes a little adjustment on the mounting the second time, but the bowl isn't usually out of balance enough to prevent remounting. The jaws on my Nova are numbered to I try to remember to mark the spot where the #1 jaw is on the bowl. That helps get it remounted without to much adjustment. I have found the larger the bowl, the more difficulty involved in remounting, but I haven't tried any really large bowls.

 

[Mike Johnson]

Small items are usually not a problem remounting, as you increase the diameter of the work piece the out of round or warpage of the item increases greatly and you can run into issues when turning the second time. Another option for turning a second time are the ring mounts that you screw onto the work piece.

 

[JeffSmith]

Most of what I turn are over 12 x 14” diameter, usually in the 14 - 18” range up to 24”. Local timbers are BL Maple, locust, alder and madrone - all prone to warping when twice turned. I tend to leave the tenon well larger than the final dia. After drying I use the original tenon to chuck up the blank (with the full expectation that its going to be ovalled), turn a recess inside, then mount in recess to true the tenon and finish turn and sand the outside to within about 10% of the bottom/tenon. Turn again to mount by the tenon and hollow the inside to final thickness and sand.
The bottom/tenon is dealt with with the bowl in the vac chuck. May sound like a lot of rechucking, but it goes fast. Haven’t lost or thrown a blank in the last decade and a half. When first chucking, centering can be adjusted slightly by using the oval and turning in the chuck before tightening down.

 

[Fred Belknap]

I make sure I leave the center mark on my tenons and remount using the center point and a jam chuck. I find the most important step is to get the bottom of the bowl flat so all four jaws set firmly on the bottom of the bow. I also round up the tenon. It is easy and works well.