Issue with width of hollow form base.

[Grey Darrah]

Man, I hope I'm not abusing this forum with all my questions. You all are really helpful!

First, let me say that in the following situation, I'm turning green wood to finished dimension (1/8 to 3/16 thickness) and letting it dry as it wants to dry. I'm currently working on an end grain cherry HF that's about 13" tall by 11 inches wide. As shown in the poorly drawn attached image, my base is currently about 4 inches wide. That's primarily because my tenon is 3 1/2 inches wide with a little bit of shoulder to rest against the chuck rim. I want the base to be more narrow (more delicate looking), maybe 2 to 2 1/2 inches. Because I haven't yet hollowed it, I feel I need to leave it thicker at the base (as it is now) for stability during hollowing. If I leave it as is and account for the final base dimension while I hollow, when I let it dry, I have two problems. One is that with the half inch tenon that exist, plus the extra bulk I'd be leaving to turn off later, I feel like there is serious risk of some cracking while it dries because of the walls thickness difference (3/16 walls and an inch plus at the base). The other problem is that it's no longer going to be round when it dries due to some warping, so re-turning the thick base off is going to cause some problems.

How do you think this is best handled?

Thanks,
Grey

 

[Richard Coers]

Start with stock about 1 1/2" longer than your final work. Then turn to the diameter you want, with the large tenon and waste stock still in the chuck. Part off the part when done hollowing. Use a steady rest to do the hollowing. Nearly impossible to hollow 13" deep by just holding the tenon. Well it's possible if you take a couple of days to turn with VERY shallow cuts.

 

[Fred Belknap]

Grey you can reduce the tenon. I don't know if you have a chuck but that would be how I would go. An endgrain tenon is very strong. Most chucks come with a 2" jaws. You could use a donut chuck to hold it to finish off the bottom.

 

[Mike Johnson]

After turning and hollowing the piece chucked up by the base you could reverse the piece and chuck it up by the opening of the vessel and bring the tailstock up to support the bottom of the vessel while you finish turning and shaping the base. This is where a steady rest with rubber wheels to support the outside of the vessel makes it easier to finish the bottom of a hollow form. You could sand the bottom flat but with a small diameter base you really want to have a recess to avoid the piece rocking on its flat base if it changes shape over the years.

 

[Bill Boehme]

Man, I hope I'm not abusing this forum with all my questions. You all are really helpful!

First, let me say that in the following situation, I'm turning green wood to finished dimension (1/8 to 3/16 thickness) and letting it dry as it wants to dry. I'm currently working on an end grain cherry HF that's about 13" tall by 11 inches wide. As shown in the poorly drawn attached image, my base is currently about 4 inches wide. That's primarily because my tenon is 3 1/2 inches wide with a little bit of shoulder to rest against the chuck rim. I want the base to be more narrow (more delicate looking), maybe 2 to 2 1/2 inches. Because I haven't yet hollowed it, I feel I need to leave it thicker at the base (as it is now) for stability during hollowing. If I leave it as is and account for the final base dimension while I hollow, when I let it dry, I have two problems. One is that with the half inch tenon that exist, plus the extra bulk I'd be leaving to turn off later, I feel like there is serious risk of some cracking while it dries because of the walls thickness difference (3/16 walls and an inch plus at the base). The other problem is that it's no longer going to be round when it dries due to some warping, so re-turning the thick base off is going to cause some problems.

How do you think this is best handled?

If it's end grain then the warping generally is uniform since the grain is all in the same direction unless cherry is different than most other wood. With a wall thickness of 3/16" the wood should be close to dry anyway by the time you finish.

I understand why you would like a smaller base. With the fat base it looks more like a bump on the table rather than an object sitting on it. I really like the method that I learned from Trent Bosch that I show below in my mod of your sketch.

Basically, the idea is to leave lots of material in the lower part for stability as you simultaneously work your way down both the outside and inside. This might take some practice as you learn to visualize the overall form, but it's really not very difficult. The part in red is the original diameter and the part shown in blue is where you will need to visualize the final shape. By doing it this way you have plenty of supporting material and you can turn the base thin and not face the problem of potential cracking as a result of thin walls and thick base.

Because the upper part will be drying and warping while you are turning, I believe that using a steady rest is out of the question because that would actually introduce vibration. As you near the bottom just slow the lathe way down.

This is quite a large diameter for a hollowform of that height so I imagine that you will need to take very light cuts.

 

[hockenbery]

like bill shows
I tend to think of it volume areas to remove
These are the steps below. Leave a wide block about 1/3 of the heighto support hollowing the upper 2/3s.
Shape the top 2/3 outside
1 drill a depth hole.
2 widen the depth hole leaving the walls 3/4 “ thick
3 hollow the wall to even thickness working rim to the bottom 1/3
On a really large form you might want to divide the 2&3 into a two parts

4 shape the outside bottom 1/3
5 hollow the bottom inside
6 reverse chuck an turn the bottom

In the hollowing #3 is the hook tool all others are the straight tool.

#5 is the most at risk of going through the side wall.
A camera system removes most of the risk
A laser is fairly ris free.
Hollowing with calipers is really tough.


r

 

[Odie]

Man, I hope I'm not abusing this forum with all my questions. You all are really helpful!

I'm not the right person to answer your question, Grey......but, just wanted to comment. Keep those questions coming, and don't you worry one bit about "abusing the forum". There are plenty of experienced turners who frequent this forum, and are ready, willing, able......AND enthusiastic about giving tips and advice for you, or anyone else who has questions about turning.

This is one of the main functions of these AAW forums.

-----odie-----

 

[Bill Boehme]

Man, I hope I'm not abusing this forum with all my questions. You all are really helpful!

This is one of the main functions of these AAW forums.

What Odie said! The forum exists to share information and answer questions whenever another turner needs help. I have actually seen some forums (usually computer forums) inhabited by snarky trolls where somebody is almost certain to say something like, "that question has already been answered back in 1986" or "use the search function". First of all, this isn't Wikipedia nor is it an encyclopedia nor Google. Woodturning is an evolving art form that is driven by interaction and enthusiastically sharing ideas and information. An answer to a question yesterday or last year is certainly not the final word. So, not until the Internet runs out of bits will there be such a thing as too many questions. And, as mentioned many times on this forum, the only dumb question is the one that isn't asked.

Here's a tip for forum members when you know of a prior thread that may be helpful in answering a question. Find the thread and post a link. That's far more friendly and helpful than telling somebody to do a search. After all, they might not have a teenage computer tech support person in their house.

 

[Grey Darrah]

In your #6 (reverse chuck), I'm guessing you're referring to some sort of jam chuck? This is always the scariest part for me, probably because I'm 99% done and the piece is now being held in a "not so secure" manor.

I did finish the piece, leaving the tenon on for removal later, and am posting an image of it in my next post here, but I still would like your thoughts on my clarification to item #6 above.

Thanks,
Grey

like bill shows
I tend to think of it volume areas to remove
These are the steps below. Leave a wide block about 1/3 of the heighto support hollowing the upper 2/3s.
Shape the top 2/3 outside
1 drill a depth hole.
2 widen the depth hole leaving the walls 3/4 “ thick
3 hollow the wall to even thickness working rim to the bottom 1/3
On a really large form you might want to divide the 2&3 into a two parts

4 shape the outside bottom 1/3
5 hollow the bottom inside
6 reverse chuck an turn the bottom

In the hollowing #3 is the hook tool all others are the straight tool.

#5 is the most at risk of going through the side wall.
A camera system removes most of the risk
A laser is fairly ris free.
Hollowing with calipers is really tough.


View attachment 25855r

 

[hockenbery]

#6 is finishing the bottom
I reverse turn
There are other options
Cutting off with a saw
Parting off with a parting tool

 

[Grey Darrah]

I finished it last night using bits and pieces form most of your suggestions. I ended up turning the base down to my desired finish size before hollowing and left about 1/8" of the original width resting on the outer rim of the chuck for some support. I'll turn the tenon and rim off when it finishes drying. I kind of like leaving it on because it's so handy when it comes time for final sanding and putting on a finish. I was using a steady rest...I'm a little uncomfortable hollowing a piece that long and wide without it.

Thanks for all your suggestions,
Grey

Here's a pic...

 

[Grey Darrah]

Forgive me for being an idiot, but when you say "reverse turn," are you meaning spinning the lathe in the opposite direction? If so, what's the advantage to that?

Thanks,
Grey

#6 is finishing the bottom
I reverse turn
There are other options
Cutting off with a saw
Parting off with a parting tool

 

[Mike Johnson]

Reversing the piece in the lathe allows access to the bottom of the piece to finish turning and sanding. Depending on the size of the opening in the top of your vessel you can use a set of jaws that will fit inside the opening and move the jaws outward to hold onto the inner circumference of the opening. This is were a jamb chuck or a soft set of jaws works to prevent damage to the inner circumference of the opening. With a small opening and thin wall you can not put very much pressure on this or you could split the wood. A jamb chuck can be made of wood or plastic or rubber that fits into the vessel opening and uses pressure from your tail stock center to allow the lathe to turn the piece. The jamb chuck can be turned inside your bowl chuck and made to any custom size needed for the vessel you are working on.

 

[hockenbery]

Forgive me for being an idiot, but when you say "reverse turn," are you meaning spinning the lathe in the opposite direction? If so, what's the advantage to that?

Thanks,
Grey

Nice looking form. You have a nice curve shown in the photo.

Do be aware that if the wood is wet leaving the large tenon on the form increases the risk of cracking.

 

[Bill Boehme]

Very nice looking hollowform, Grey.

6 reverse chuck an turn the bottom

?... This is always the scariest part for me, probably because I'm 99% done and the piece is now being held in a "not so secure" manor....

Nobody wants to see their 99.44% completed project rolling across the shop floor.

The manner of jam chucking described below is a secure way to hold a piece while finishing the base.

.... when you say "reverse turn," are you meaning spinning the lathe in the opposite direction? If so, what's the advantage to that?

Perfectly logical question. And the dictionary isn't any help since it seems to reinforce what it sounds like to you ... go in the opposite direction. "Reverse turn" is actually woodturner jargon which means "swap the head and tail ends of the turning". The reason for "reverse turning" is that we need unobstructed close-in access in order to finish the bottom of a bowl or hollowform, but trying to do that from the headstock end of the lathe is all but impossible. Working from the tailstock end of a lathe is so much easier where the only obstruction is the live center. When "reverse turning" a hollowform one commonly used way to hold the top end which is now at the headstock end is to use a "jam" chuck ... A piece of wood that is turned to closely fit the opening of the hollowform. The hollowform is very secure when it is being held between a jam chuck and the live center. Even so, it is still prudent to make light finishing cuts.

 

[Gerald Lawrence]

Jamb chuck is not a fixed design and there are many ways to do it. This article by John Lucas gives many methods but does leave out the vacum chuck.
https://www.ptwoodturners.org/Tips and Handouts/Methods and Jigs for Reverse Turning Bowls.pdf
Here is another article with vacum chucking included.
http://www.billjuhl.com/uploads/1/2/1/4/12147345/reverse_chucking_-_catlog_of_options.pdf
@Bill Boehme Maybe we need to have a listing of woodturning terms on the site

 

[John Spitters]

Here I am using a home built vacuum chuck as a jamb chuck to remove the tenon. This is a more squat form but the same works for a taller form. Turn down and profile the bottom of the form leaving all but a little nib for the tailstock, then remove from the lathe and remove the small remaining nib by hand.

 

[Grey Darrah]

Jam chucking for a flat edged bowl is pretty straight forward. Doing it for a live edge hollow form or one like I have in my drawing here seems more complicated/delicate. My opening at the top is only about 1 1/2 inches and the opening and initial curve are turned down to just a hair over 1/8. As I see it, I've got 3 options (though other suggestions are welcome):

1. Turn a flat piece of soft wood, press the hollow form opening against it and bring up the tail stock for support. Really simple, but I'm not sure how secure.

2. Turn a rounded stub about 1 3/4 to 2 inches at it's widest so that the "cone" of the stub will go a little bit into my opening. then bring up the tail stock to keep it in place. This option puts a lot of outward stress on the opening, so I really have to balance the tail stock pressure just enough to hold it, but not so much that it splits/cracks it. Again...delicate.

3. Turn a deeper, concave piece (like an oddly tall small bowl) with an opening of maybe 3 to 4 inches, 5 to 6 inches deep, with a rim about a 1/2 inch thick. Instead of being a flat rim, turned at an angle that sort of hugs the hollow form walls, 3 or 4 inches down from the HF opening. Then bring my tail stock up for support and maybe use something like kitchen drawer liner where the bowl rim meets the hollow form so it doesn't mar it up.

For this particular piece, which sounds like the best approach...or does anyone have a better way to do it?

Thanks,
Grey

Very nice looking hollowform, Grey.





Nobody wants to see their 99.44% completed project rolling across the shop floor.

The manner of jam chucking described below is a secure way to hold a piece while finishing the base.



Perfectly logical question. And the dictionary isn't any help since it seems to reinforce what it sounds like to you ... go in the opposite direction. "Reverse turn" is actually woodturner jargon which means "swap the head and tail ends of the turning". The reason for "reverse turning" is that we need unobstructed close-in access in order to finish the bottom of a bowl or hollowform, but trying to do that from the headstock end of the lathe is all but impossible. Working from the tailstock end of a lathe is so much easier where the only obstruction is the live center. When "reverse turning" a hollowform one commonly used way to hold the top end which is now at the headstock end is to use a "jam" chuck ... A piece of wood that is turned to closely fit the opening of the hollowform. The hollowform is very secure when it is being held between a jam chuck and the live center. Even so, it is still prudent to make light finishing cuts.

 

[Gerald Lawrence]

You can also just carve it off with a power carver or sand it off. You may have to get artistic with the foot but it is doable and then no head scratching.

 

[Richard Coers]

I finished it last night using bits and pieces form most of your suggestions. I ended up turning the base down to my desired finish size before hollowing and left about 1/8" of the original width resting on the outer rim of the chuck for some support. I'll turn the tenon and rim off when it finishes drying. I kind of like leaving it on because it's so handy when it comes time for final sanding and putting on a finish. I was using a steady rest...I'm a little uncomfortable hollowing a piece that long and wide without it.

Thanks for all your suggestions,
Grey

Here's a pic...

View attachment 25859

Have you ever heard of the "Golden Mean", or Fibonacci calipers? No form is bad if you like it, but there is a mathematical formula of a pleasing shape. Lots of people scoff at this, but when you have a gauge, go around the grocery store and just see how many designers of simple things like a ketchup bottle use these formula. Shapes that have the apex of the curve in a proportion to the height. From the base to the apex should be .625 times the height. So if the vessel is 10" tall, the apex of the curve should be 6 1/4" from the base, 3 3/4" from the top. In this link, they have plans called Golden Section Gauge. https://www.goldennumber.net/do-it-yourself/

 

[Bill Boehme]

Don't use a cone for something really thin ... like you said, it might cause the opening to crack. Instead, make a shape that conforms to the shape of the top of the HF, Have a short "cork" like straight plug in the center and then the outer part is curved to match the outside of the HF. On a few turnings I have made a mold using epoxy putty and then made a second mold of the first and sawed it in half to use a guide for shaping the jam chuck so that it conforms to the shape of the top of the HF. I have also used the epoxy putty directly as a jam chuck so there is no need to turn a wooden jam chuck. It's probably obvious, but I'll say it anyway -- use Saran wrap between the wood and the epoxy putty.

There are numerous other options for jam chucks ... if you don't see anything that works then use your imagination to come up with something that does.

 

[hockenbery]

I only use 2 jamb chuck methods.

With a large opening I use a post against the bottom. Least likely to damage a thin form



With a small opening A rimmed cup ( bowl shape) that spreads the load out over the shoulder

 

[Grey Darrah]

Being a portrait photographer and painter, I'm very aware of that rule. My form actually hits it almost exactly, but the pic I posted from my cell phone distorts that a bit.

Grey

Have you ever heard of the "Golden Mean", or Fibonacci calipers? No form is bad if you like it, but there is a mathematical formula of a pleasing shape. Lots of people scoff at this, but when you have a gauge, go around the grocery store and just see how many designers of simple things like a ketchup bottle use these formula. Shapes that have the apex of the curve in a proportion to the height. From the base to the apex should be .625 times the height. So if the vessel is 10" tall, the apex of the curve should be 6 1/4" from the base, 3 3/4" from the top. In this link, they have plans called Golden Section Gauge. https://www.goldennumber.net/do-it-yourself/

 

[Grey Darrah]

I LOVE and appreciate the drawings you take the time to make...so much clearer than words.

Thanks,
Grey

I only use 2 jamb chuck methods.

With a large opening I use a post against the bottom. Least likely to damage a thin form

View attachment 25871

With a small opening A rimmed cup ( bowl shape) that spreads the load out over the shoulder

View attachment 25869

 

[Grey Darrah]

Looking around the YouTube world, I came across this rig that somebody (Stephen Ogle) made and I thought you might like it. I'm going to put one together this week. He used a 1 1/4 wood dowel for the long center piece (turned true). I'm going to end up turning two cones to use on it, a convex one like in the attached image and a concave one that would rest on the outer shoulder of thin turned or live edge turned opening.

Grey

I only use 2 jamb chuck methods.

With a large opening I use a post against the bottom. Least likely to damage a thin form

View attachment 25871

With a small opening A rimmed cup ( bowl shape) that spreads the load out over the shoulder

View attachment 25869

 

[John Spitters]

I use the same jamb chuck for removing the tenon and shaping the bottoms of taller hollow forms as in your drawings as well as my bowls even my live edge bowls as long as the jamb chuck fits inside the live edge form. For the taller hollow forms they fit inside the opening (as in Hockenbery’s 2nd drawing) of the jamb chuck, my jamb chuck as pictured has the face of the chuck 7 1/2” away from the headstock, the radius of the jamb chuck is 6” and the face is rounded over from the outside as well as the inside and surfaced with a closed cell craft foam, this jamb chuck also doubles as a vacuum chuck.

 

[John Tisdale]

My pieces average 20"-dia, the base is always less than 4" and all pieces come off a 6" Oneway faceplate - the only time I'll use a chuck is on a tall piece and only then with a Okie Spin Doctor steady rest.
On the typical large hollow form
- Turn the outside shape between centers - I use a 1.5" spur drive
- Turn a 6" tenon at the live-center end and mount the faceplate - predrill all holes - I use 1.25" #14 in the center six holes and 1" #14 in the outer twelve. In ten-years and over 100 pcs weighing 200, 300 and even 400-lbs, never had a problem
- True up the outside, punch a hole with your largest forstner, and hollow to about 1.5" thick, depending on species
- I boil for about 2-hours and then let dry for 8-mos to a year with a computer fan evacuating the inside air - starting the drying in a closed box is a good idea
- When MC is 6 to 7%, I mount on the lathe with a Kelton Mandrel (there is a knock-off available), re-turn the tenon to "dead nuts flat", fasten the 6" faceplate, never using the same holes.
- True the opening, use the live-center with cone, and final shape and sand the outside
- Then re-hollow the inside to thickness. With a quality finish, a thickness around .375" to 0.5" is acceptable on a 12" dia piece, at least to my thinking.
- Second to last step in the turning is to remove the outer 12-screws and dive under the faceplate for a smaller base. I then attach a flange for 1/2" galvanized pipe, insert about a 10" pipe, and am able to go 90% through the finishing process.
- Last step is to put it back on the Kelton Mandrel and CAREFULLY remove the 3" tenon and cut a concave bottom.
Grey,
If the above seems like a lot of tedium you're probably correct - maybe I'm not smart enough to do it simpler. At the same time, I try to work only on pieces that justify the effort - judging from the picture above, you do quality work and that is a quality piece. With the right finish and in the right gallery it's easily a $2K to $4K piece.

 

[Grey Darrah]

Do be aware that if the wood is wet leaving the large tenon on the form increases the risk of cracking.

from

So if I'm turning a HF from green to finished dimensions, there are a couple of issues I have with the tenon and its potential removal that I'm not sure how best to resolve.

1. Do you sand to final grit while still a bit wet before removing the tenon? Sanding seems easiest when you can use the convenience of the lathe. I'm wanting a glass like finish, so sanding to seriously smooth is important. After the piece has finished drying (a few days to a week later) and is ready for a finish, would you lightly hit it with the higher grits again? If so, would that be by hand holding the piece or would you jam chuck it up?

2. It seems like finishes are easiest to apply while you can have the piece held in the lathe to hand spin as a lacquer or wipe on poly is applied. Do you use a jam chuck to hold it here as well? With a small, thin opening, I could imagine this being a problem, as the head of the piece would be better held by a bowl with the rim resting on the HF shoulders (as in your drawing). That bowl would then be in the way of applying your finish.

Leaving the tenon on fixes these issues, but definitely introduces the likely-hood of some cracking.

What do you do?

Thanks,
Grey

 

[hockenbery]

I do almost all my sanding off the lathe.
The exceptions are if I have a detail like a bead or groove these must be lightly sanded on the lathe.
Also some figured wood I sand wet.

I sand with the grain with every grit is use.
I generally sand to 320 using an angle drill and Velcro discs.
I put on a coat of Waterlox, wipe off, let dry over night, then sand this with 400 folded paper
Then repeated coats of Waterlox, wipe off, dry overnight, sand with scotchbrite 0000 equivalent.

3-6 coats does i for me.
I will use the Beall buffing on some pieces. This give some sheen if I want.
I prefer softer finishes.


My friends who sand wet wood use abranet. It tends to cut the wet wood while the typical sandpaper bends the fibers over instead of cutting them. Some burls and some crotch figure I do sand wet with abranet lathe at 0 rpm. I like the rippled texture it gets when dry sort of a leather feel.

What some friends do is mount their pieces on something like an old BQ rotisserie motor so they turn slowly while they spray lacquer or brush on an epoxy finish. The rotation levels the finish for them,