Help Turning Bowl

[john lawson]

I need to turn a bowl which will be used to make a bronze casting. An image of the bowl with dimensions is attached. As you can see the bowl is 16" in diameter.

I am fairly new to turning and this one needs to be done right. I have a Jet 1642 EVS lathe and a General purpose chuck that could hold the 2" bottom part of the bowl. Assuming I turn the 2" part first, will that be strong enough to hold the larger part of the bowl while turning? Turn everything outboard?

By the way, it will be mahogony, and I have wide boards around 2" thick which I can glue up.

Any and all suggestions are welcome

[Al Wasser]

Yes your chuck will hold the bowl if the foot is done well. The problem you have is that your lathe will not turn a 16" bowl. Yes it says it will, but if you start exactly with a 16"blank, by the time you round it out and sand it, you will be lucky to have a 15 1/2" bowl. And yes this belongs under the turning section for the most suggestions

[Jim King]

John: You are using a very soft and low grade wood into which you are going to pour a casting ? Or are you going to make the form from another material using the mahogany original ?

[Dean Thomas]

Al's giving you good words there.

I'm not sure about your chuck. Is this a true wood chuck or a "general purpose" chuck that was really designed for a metal lathe?? Are there some teeth or other grippers on the inside of the jaws or are they smooth?

Make your tenon as wide as you can without causing the scrolling mechanisms to be exposed. Some call these "knuckle busters" and I'm sure you can figure out why. :rolleyes: After you've made your tenon as wide as possible, be sure not to make it too long! If you measure inside the jaws, from the shoulder of the top of the jaw to the metal body where the screws are, your tenon should be SHORTER than that.

My Oneway Talon chuck has jaws that have rows of grippers along the business edge of the jaw. Some others have jaws that have only one little ledge at the top so that it looks like half of a T. If your jaws are smoooooth along the sides that'll grip the tenon, you have issues.

A profile of your woodworking jaws look something like an L. From the top of my jaw to the top of the _ part of the L is 1/2". My tenon can be up to 7/16" long. That creates a shoulder at the bowl end of the tenon against which the bowl rests. That ensures maximum stability and the best possible shot at keeping the bowl in the correct plane should you have to rechuck.

What kind of "general purpose" chuck do you own? Brand and model if you have them.

[Dean Thomas]

Me again.

I'm presuming that you're going to use the mahogany bowl as a model, and that a casting will be made that will be the mold itself or will be an intermediate step.

Getting mahogany smooth smooth is not very easy. Furniture makers like mahogany because it is machinable and holds shape pretty well. Not terribly heavy, not terribly dense. Doesn't normally kill tools, but holds up to wear and tear pretty well. Getting the sucker smooth enough to serve as a model might be tricky. One can always doctor the plaster casting, of course. Or one can fill the pores with something and then spray it with a release chemical. I'm thinking that you could actually use plaster and then sand it out to flat and perfect. Pretty is not the deal for a model. Precise and sure-to-release are the biggies! Plaster goes away pretty quickly with a common scraper or with sandpaper. Taking it down so that you just begin to see wood ensures that you've re-attained your desired shape, yes?

I'm curious. Is your aim of a 16" bowl going to burst someone's bubble? Will they (or you?) be satisfied with a 15.5" bowl??

[john lawson]

The bowl I am to turn is for the model of the casting. They will use the bowl to make an impression in a sand mold and then pour the bronze in the cavity made by the mahogony model.

The bowl does have to finish to the dimensions shown in the drawing; hence my concern about the 16 lathe I own. The only way I could figure on doing this was to screw the block of wood to a faceplate, turn the foot, and then chuck the foot up and turn the bowl itself. All of this I would have to do outboard, sliding the head of the machine all the way down to the end of the lathe. I would also have to come up with a stand to hold the tool rest.

Since I am somewhat new to this I was concerned about several issues; making a stand to hold the rest, chucking up the foot, and then turning the bowl using the 2" foot as the only thing holding the bowl while it is being turned. Below is a picture of the chuck I have.

Thanks for the replies, looking for anything else you can add.

[Dean Thomas]

Or, find yourself a friend who has a Powermatic 3520b or similar. :)

Yes, you can do all that you're thinking by sliding the head to the end, screw chucking the blank, turning a foot (and forming your outside shape) and then flip the bowl onto the chuck. It's a good plan except for not having a tool rest already.

On most of the Jets, this one included, you can swivel the head either 45 or 90º and it'll lock into place. In that configuration, you can use your own toolrest for at least some of the work. Once you chuck it, however, it might be too far out to play. Remember that you can move the head into the middle of the bed and then put the tool rest on the left side so that you have access to the same business side of the cutting tools that you had before. Does that make sense to you?

And I THINK that there was or is a tool rest extension that fits into your banjo and then swivels out from there giving you a fair amount of additional length. It seems to me that I've a fair amount of whining that it really is not as hefty as most turners would prefer, but I know that I'm usually pushing something that I own to the utter limits of its abilities. Or pushing myself past those limits. :o

If you can manipulate your headstock so that it's at the right angle, you might be able to pull this off! The hard part will be making the banjo & tool rest work, I think. Possible, but it all depends on how the machine is really set up for "sideboard" turning.

[Reed Gray]

I wouldn't consider a 2 inch tenon enough to safely turn that size of bowl. You may get away with it, but a 4 inch or more tenon would be safer. After turning the inside, then reverse mount the bowl and trim the tenon down to size.
robo hippy

[David Epperson]

If it were me doing it I would look at cutting the lay up boards into rings first before glue up and "turn" the rough shape using the chuck as a simple pivot point while cutting using a die grinder motor and a round nose router bit. More of a metalworking approach I know, but much less stress on the tenon.
I to am wondering about your choice of wood to do this with. I would think a closer grain wood would finish out smoother. Or is this a material you have on hand.

[john lawson]

David:

Yes, it is a material I have on hand, and it turns easily. I talked with the customer about it and mahogony is ok with them. Is there some other wood I should consider? I thought mahogony would be easy to turn and fairly consistent in grain.

john

[David Epperson]

The mahogany I've seen is fairly open pored, which won't matter much if you seal it well before using it as a pattern. Otherwise a fine grained green sand mix might get stuck in the pores and transfer that "defect" to the finished casting. I haven't worked any in years, so I can't speak to the issue of how stable it will remain. Though if you alternate the grain direction between "layers" of your flatwood build up, it will probably stay more stable.

[Paul Engle]

I'd go with the segment like Dave suggested , If you have to hold the "R" to plus or minus to .030" you and a whole bunch of us are gonna cry trying to do that on one piece of wood and I don't care who it is or how good they are.At least with one ring at a time ( and as narrow as you can stand , you might end up with 30 rings ) you can guage your R and adjust and if you go too far + one ring to - on the next ring you'll be out of R by nearly a 1/16'' and that you can see and if you do that for the whole depth the bowl , it will look warped and the casting will follow it to the letter.With the rings if you mess one up , part it off , glue on another.It may be better to try it on a metal lathe to do the rings where you can control the DOC / R one layer at a time and grind a bit to R and reshape when R changes from side to bottom. It'd be nice if you had a pattern lathe available as they were made for doing things like this , maybe not bowls but with the powered feed you got good chance of making it work, and for sure the biggest tennon you can manage as the outer rim inertia is gonna want to send it in to next friday at the first mis-que or sooner.I think I'd get OCC's CNC and do it....but by hand ... well practice first for sure. maybe three or six times...Dont get me wrong, I just hope you realise how exact this may have to be , there are a lot of good turners in spinny land but I bet most could not turn a bowl to R spec with out a ton of practice much less one with two R's . now if they are gonna turn the casting after it is done then you got it made for sure , let the machinist cry ... no he'll have what he needs to make it " nice " .You will have to be better than him/her. Make 2, keep one for bragging rights cause when you pull this off you'll " BE DA MAN " for sure.:D ;)

[Richard Madison]

John,
The dimensions in your picture are surely for the finished metal piece, which, after casting has been finish turned on a metalworking lathe. As you can tell from the previous replies, someone, somewhere in the process has unrealistic expectations about a "perfect" pattern making "perfect" castings.

Mahogany was indeed a wood of choice to make patterns for sand castings, at least some years ago. We used to use it for aluminum sand castings. Seem to recall that the pattern was coated with a sealer/release material that made any porosity of the wood irrelevant.

If your Jet is like mine, the headstock does not pivot, but you can turn off the end of the bed with aforementioned accessories. Oops, you already got that part. Never mind.

Segmented rings from your 2" thick stock should work well until you get to the last ring. Forgot the base dia. of the bowl, but build it on a scrap block about that dia. on a faceplate about that dia., maybe 6" faceplate. Make the scrap block thick enough that you can turn the required tenon on it (without hitting the screws) as the last step before parting it off. Glue on a ring or two and turn them. Repeat. Last ring will have to be turned off end of lathe to get 16". Don't need expensive outboard turning stand if this is a one-off project. Cut the last ring very close to finished size and glue in place off end of lathe. Power sand with 60 and 80 grit gouges to near finished dimensions. And some more like that.

Just some thoughts, John. Been there and done some of that, but it was a long time ago, and am surely no "expert".

[Dean Thomas]

Great idea about not putting the last "ring" on until absolutely needed!! Thanks, John, we all needed that little bit of wake-up. Same with the release material preventing pore problems. DOH. Overthinking a problem and its solution, especially in committee, can really bring some interesting things to the surface, so to speak.

And one other silly thought while we're thinking this through: Saw a thing on Discovery channel recently about bronze castings, that they had to be made a certain percentage oversize to allow for the shrinkage that happens as the metal cools. Is that a problem, John or Richard? Just curious...

[john lawson]

All:

Yes, the dimension shown on the drawing is the finished dimension for the model, not the casting, they have the shrink figured in the oversized model, I believe it is 3/16", so that won't be an issue.

You are right, my Jet headstock does not rotate so I will have to slide it to the end of the lathe ways. That is a great idea about building up the rings and leaving off the last one!! Thanks.

I have a somewhat checkered past regarding castings, I used to be a part of a company that cast aluminum and iron. While I do not claim any technical expertise, as someone mentioned earlier, mahogony is the wood of choice for making models (the old fashioned way, for one offs). Now they may use something else. Anyway, I am sure they will be able to successfully cast the part if I can deliver it to the right dimension. They may have to seal it but other than that I believe they will be able to deal with the mahogony. They were unsure when I asked them about dimensional variation allowed on the model. I suggested that I could hold plus or minus 1/16". I am fairly certain that will not give them any problems in pouring the molten bronze, and it is not critical to the finished part. In other words, if they can pour it, it will be ok as long as I make it to withing 1/16".

Thanks again to everyone who has responded. I believe I have enough info to quote this now and will do so today. Assuming they give me the go ahead I will probably come back to you again for more advice.

john

[Dean Thomas]

Grrrrrrrrrrrrrrrrrrr. :mad: I really hate it when the manufacturer's website lies! I'm guessing that maybe they just scabbed the 1442 or even the 1236 page and did a couple of tweaks on it, but somehow missed the headstock swivel capability issue.

Sorry. Did not mean to mislead. It's that darned "assumption" theorum at work again...;)

[Richard Madison]

John,
With +- 1/16" tolerance on your mahogany pattern you should do ok. Probably (+) is better than (-), depending upon which dimension, of course. Usually easier to make a thing smaller than to make it bigger. Good luck.

Dean,
Whereabouts in Missouri you is? For ref. I spent some malformative years in Kirkwood (suburb in St. Louis County), and some other disfunctional years in Springfield and Rolla. Just curious.

[William Bachtel]

If I only knew where you are located maybe you could tie up with one of us, closer with a bigger lathe

[Dean Thomas]

Richard, I'm on the west coast of Misery, smack in the center of KC. I know more than a few folks from the two souther cities you mention that I'd consider to be disphyunctional. :D

William, John is in Birmingham, AL. Don't know if he added that to his profile since you asked or not.

All, this is why it's a really good thing to add at least a state to your profile, so's we might could just drop a PM or email to say, "Hey, I'm close by; want some hands-on help??" I know that some really value their privacy, but this is a pretty cool group.

[john lawson]

All:

Yes, I edited my profile, don't know why I did not do it earlier. If anyone is close by and can give some hands on help I would appreciate it very much.

I have cut out the rings for this bowl and will start gluing up the first tomorrow.

thanks Everyone

john

[Richard Madison]

Thanks, Dean. Visited a good friend in KC about a hundred years ago, but have long since lost touch, and haven't been back there.

Good luck John! You can do it.

[john lawson]

Thought I would give an update on my progress so far in turning "the bowl". I am gluing the bowl up using 2" thick segments. I have 3 segments glued up, and I will need a total of four. Today I shaped the outside of the bowl using a bowl gouge.

Before I started I made templates for the outside and inside of the bowl and the picture below shows the outside template. T then reversed the bowl and mounted it on my chuck, then bored the center hole, allowing an extra 1/2" of wood for safety. Next I have to hollow out the inside of the bowl.

What do you recommend for shaping the inside, a bowl gouge or a scraper?

After I get the inside of the bowl shaped I will glue on the last "ring" and have to start turning outboard, which means I will have to rig up some kind of stand. Any ideas for cobbling up something simple and safe?

thanks

john