How to make a very specific large "bowl" lathe

[Josh Knechtel]

First off, let me introduce myself. My name is Josh. I recently (last few years) began woodworking, predominantly for my main hobby which is DIY audio. I like to build speakers and vacuum tube amps.

Well fast forward to today and I have a strong desire to turn horns to make horn speakers. We are not taking about little tweeter horns, I am talking 20-30" horns up to 34" deep.

I've come across one or two individuals who do this themselves. I've seen the results. They turn up to 34" round x 34" deep horns out of glued up MDF. The results weigh in near 200lbs.

I've also seen the setup used to turn it. The one I've been in communication with uses as old craftsman lathe robbed of the main body and set on a wooden stand. Then another stand is fashioned to be the tool stand. A pulley was devices to slow the revolutions way down.

I'd like to build something like this myself. Except, I am thinking about making some a bit more structurally sturdy. What do you guys suggest in terms of where to start looking for parts to scavage?

(I am not a pro wood turner and I do know that this is a pretty risk project. I will take necessary precautions. I have turned before, many years ago, 14" round oak nut bowl, 3" thick. I also have lots of dust collecting equipment (mask, filter, dust collector)).

[David Reed]

Check out this website. I have plans to reproduce something similar, but it is on a very long list.
http://www.finewoodworking.com/SkillsAndTechniques/SkillsAndTechniquesArticle.aspx?id=26160

David

[Greg Wittler]

David, thanks for that link, I think I will put that on my long list too. it reminds me of a "Stubby"

[alex carey]

what capabilities do you have for this, i.e. welding and concrete specifically.

Some picture/ideas.

http://www.vkleibrant.com/process.html

http://www.fredwilliamson.com/Pages-Main/My-Lathe.htm

http://www.virgiltreeart.com/TheProcess/tabid/400/Default.aspx

[Jim King]

This fellow has stopped in to see us a couple of times looking for new woods.

http://www.timberlinedesigns.com.au/pages/timbervases.htm

http://www.timberlinedesigns.com.au/images/Enlarged_Click/16%20the%20master%20in%20a%20bottle%202003.jpg

[Dick Strauss]

Josh,
I'm all for you building horn speakers if you choose. There have already been some great suggestions for DIY lathes!

I know MDF does a good job of dampening vibrations across lots of frequencies (thus its use in lots of speaker applications). However, MDF can split along the seam (I've had 6" waste blocks crack right off along the glue/paper joint). MDF is the equivalent of a bunch of sheets of paper glued together to get a thicker sheet. This glue to paper surface can separate. I assume you would use MDF rings/circles stacked up to get that size horn. As you turn it, it will want to crack off straight across the thickness of the individual sheets through the side of the horn to make a big funnel. I don't know how thick the other folks leave their MDF horns, but just keep this in mind if you go forward. MDF has strength in the width and height dimensions (the way you normally use it), but not across its thickness!

If you go forward, read as much as you can on safety. That type of project is not for the faint at heart. Something of that size could easily kill someone if it broke away from the lathe!

Take care,
Dick

[Josh Knechtel]

Yeah I completely understand the danger. Lots of kinetic energy in turning an object in excess of 100lbs.

I haven't considered the issue of the MDF layers wanting to sheer. I usually haven't had issue with MDF glue ups before. I prefer to build speakers with baltic birch 13 ply, but I've been told that would be difficult to turn. Plus it is pretty expensive considering how much is needed. Its lighter however.

There is another option I've considered. It doesn't involve a lathe at all. It would be devise a jig for my Bosch Colt with angle attachment like a Jasper circle jig and cut a layer at a time and change the angle as you go. This will get you an approximation to the curve (like calculus) and then sand down or turn after to finish the profile.

Either way, the layers would be built up with the center mostly removed so I wouldn't be removing tons of inner material. The turning is more of a finishing process.

Things I know so far:
1) this is dangerous.
2) you need a lot of weight on the ballast
3) you have to spin very slow, like ~50rpms or less
4) you should spins a few times by hand before turning on the power
5) you need to secure the horn to the lathe with lots of bolts to make sure it doesn't break free. I've seen examples and they use lots of large bolts to the disc that is attached to the spindle (I don't know the lingo).

I've also thought about making the horn face upwards rather than sideways. Think about how clay pots are turned. I think this might be less dangerous. The securing plate has a lot less stress on it. The gravitational vector is in the direction of the torque. However, the inner tool rest would be hard to figure out. The outside one is pretty simple.

This project isn't something I am rushing into. I plan to do a lot of planning.

Someone suggested I look into basswood instead of MDF. Is that something any of you have experience with? Can you get lots of it inexpensively?

Thanks for all the suggestions so far.

P.S. I don't have kids. My wife usually isn't in my shop when I am working, so the only one at risk of injury is myself.

[Mark Hubl]

This should handle your needs. These folks turn really large segmented bowls.

http://www.proserpinewoodturners.com/The_Big_Bowl.html

[Dennis Ford]

[QUOTE=Josh Knechtel;1265184]

Things I know so far:
1) this is dangerous.
Some practice with medium size items and a fair amout of caution should reduce the danger.
2) you need a lot of weight on the ballast
You need a very strong lathe that is also heavy. An ordinary lathe with ballast added will be less than satisfactory.
3) you have to spin very slow, like ~50rpms or less
If your lathe is strong and heavy enough, you may want to go faster than this. It will require a very steady hand to get a good surface at 50 rpm. You will likely have to start out very slow, so plan on variable speed. I highly recommend a variable frequency drive and 3 phase motor.

[Jim Underwood]

Why not use thick MDF (say 2 or 3 inch thick) and use a stave construction rather than a layered construction?
And using this construction, you can still turn it.

For that matter, use some thick wood in a staved construction. You'd be able to get a fair amount of curve, and it would be lighter and stronger than MDF....

[Dick Strauss]

Josh,
You can also cut out half rings of BB ply or some other material on the bandsaw and stack them so that you don't have as much waste material. I'm sure there are better descriptions of the stacked ring lamination process but here's some info I found...

http://gr8bear.tripod.com/Jan2002FRTDemo.pdf

http://woodtreks.com/how-to-cut-rings-on-a-lathe-using-custom-tooling/67/

[Josh Knechtel]

Why not use thick MDF (say 2 or 3 inch thick) and use a stave construction rather than a layered construction?
And using this construction, you can still turn it.

For that matter, use some thick wood in a staved construction. You'd be able to get a fair amount of curve, and it would be lighter and stronger than MDF....

Yeah that is what normally call pedal horns. But typically pedal horns aren't then turned to make round, they are just circular approximations. I hadn't thought of this. This is why I ask on other forums, not just audio forums.

All good ideas. Thanks gents.