Tapered oval legs

[Samkilby]

Hello all -
How to make tapered oval legs, repeatably on a machine and very safely.
It’s to be done in a college environment, and the project is to design a batch of furniture, and make one. Of course if only making one I would advise hand tooling them, but the whole point of their project is to consider batch production.. and must be an achievable design.
Without use of a cnc I can’t imagine a great way to do it. For a tapered circular leg, you could use a taper jig on a router table, but in my head as an oval cutter isn’t a pure radius, or symmetrical then the oval would change as the leg would move in relation to the cutter, for example becoming more round at its thinnest part ?

Any thoughts ?

 

[TRITON]

Copy router lathe thingy. A paral... A parellelo... A copy router thingy.

You'd need a first one which you'd hand make. Then copy the rest off that.

 

[Samkilby]

Yeah I considered just a copy lathe too, but surely it would be a bit savage given the oval shape ?
I suppose if you had access to the router version, you cut take gentler hits and many passes..
but my guess is that the follower wouldn’t be able to cope with the quick radius change ? I.e. they would work well with rounds/cones or spirals but not ovals ? You could likely easily do a radius in the other direction too (flange ?)

 

[Jacob]

Some sort of eccentric centred lathe? Could be set up to work mechanically without the need for CNC or copy turning?

Eccentrically Turned Wood Figure

 

[mudman]

Oval turning is relatively easy. I had a go quite a few years ago and started a thread on it: Old thread on oval turning
There is a formula for determining the positions of the points given in that thread that you could probably make use of. I would say try turning all the legs round before remounting to predefined points and turning them too.
I did make a new handle for an OBM chisel using this technique and I'm sure that it should be possible to create a repeatable process for it.

 

[ChipsandSplinters]

It’s a really interesting problem. The first thing is - do you mean an oval or an elipse? An oval looks a bit like an egg in cross section as the two “ends” have different radius’ and so it only has one line of symmetry. An elipse is symmetrical across two lines. A quick google search will show the difference better than I can explain.

I made a console table with tapered elliptical legs and other straight elliptical sections earlier this year. I ended up making templates to rout around at the top (larger) and bottom (smaller) then planed the waste to join the cut shapes, then hand finishing with curved cabinet scrapers.

I think you could do an oval in the same way

Given they both have changing radius’, I think think the only way to machine it would be with a CNC and hand finish.

Would love to know how others have tackled the problem

 

[Tris]

There is a woodturning process called therming which may provide a solution, although it would require multiple steps and a bit of hand finishing.
The best description I've found is in Mike Darlow's book woodturning methods

 

[Terry - Somerset]

If using a router to form a simple taper - the blank is mounted between fixed centres, and the router runs on a track at an angle to the blank. As the router runs up and down the track, the blank is rotated creating a simple taper.

To produce a different (not round) profile, centres at each end of the blank need to move up and down as the blank is rotated. The router then removes different amounts from each side of the blank.

Ensuring the centres move up and down in a predictable way could be by fixing templates of the desired final profile to each end of the blank, with the blank running in vertical slots.

There may be some issues to think about:

• the precision with which the centres move up and down - in an industrial environment I assume they would be engineered in steel - could the principles be demonstrated using hardwood
• as the template at each end would be different the angle of the blank to the centres would vary - would some sort of universal joint be needed to avoid stresses
• need to decide how many passes of the router along the length are needed for an acceptable finish - more passes = less hand finishing
• arguably the height changes could be achieved with a stepper motor and screw mechanism - but needs the kit and a degree of IT and programming skill