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Oval Turning

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mudman

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Another thread in a forum far far away, prompted me to have a go at oval turning last night. It was really fun and I'd recommend everyone with a lathe has a go.

Only using some old scrap pine as I was fully expecting to produce scrap but very instructive as an exercise.

Started with a piece that I turned to round and then off-centred one end only by about half the radius. Ran the tool along the piece as if I was rounding it off and then moved the centre to the other side of the original centre (does that make sense) and repeated the cut. This resulted in what would make a good lump hammer shaft with an oval end that becomes progressively 'less' oval as you go to the other end.

Next I tried a screwdriver handle shape. Turned the shape in the round and then moved both centres as before. This time I only removed wood from the 'handle' bit of the wood leaving the part to receive the tang round. Re-centred to the third point and finished the shaping. This resulted in a very nice shape that fitted the hand quite nicely.

Although I did only produce firewood, I was very impressed with the method and I think that with some practise that it is not a difficult technique to learn.

In the other thread:
Alf":1hjhvxt3 said:
I don't have the required mathematical mind to work out what off-set to use given the length of the blank, so I guessed. Badly.
Anyone? Is there a solution, or just a matter of practice and experience...
I don't doubt that there is some sort of mathematical method for determining the sort of offset to use but I think that it is probably a case for trial and error and experience. You probably noticed that the degree of 'ovalness' is related to how far apart the two outer centres are, the further apart they are, the more oval it becomes.

Great fun though creating something that definitely not round off of the lathe. :)
 

Alf

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mudman":1gj5g2ed said:
In the other thread:
Alf":1gj5g2ed said:
I don't have the required mathematical mind to work out what off-set to use given the length of the blank, so I guessed. Badly.
Anyone? Is there a solution, or just a matter of practice and experience...
I don't doubt that there is some sort of mathematical method for determining the sort of offset to use but I think that it is probably a case for trial and error and experience. You probably noticed that the degree of 'ovalness' is related to how far apart the two outer centres are, the further apart they are, the more oval it becomes.
Barry,

Excellent. You can fill Pete's screwdriver requirements then...


It seems to me, both in practice and just visualising it, that the distance between centres makes a difference too. I had what seemed like a likely off-set this last time, based on my first attempt, but forgot to allow for the longer blank I had. D'oh, as they say. I fear quite a few more goes are needed as far as I'm concerned; but it's really going to have to wait until after Christmas now.


Anyway, I was in the supermarket this morning and bought a new digital camera, as you do. So here's the handle I made a couple of days ago. I need to work at my skills with this new camera; the rather interesting figure in the sycamore looks more like damage.
It's not very oval, but it's not round either. It's progress, of a sort. It's good fun though, isn't it?

Cheers, Alf

 
A

Anonymous

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Well done Mudaman and Alf :shock:

As far as sorting out the positions for mounting the wood when turning an oval, how about the old string and two nails method of drawing an elipse?
Seems to me that this is exactly the same problem and so you could determine the positions very quickly with an offcut and a loop of string (+a pencil)

I did a quick search through a couple of books and the points for the foci (nail positions) can be found fairly easily.

the distance from the centre of the ellipse to each focus (nail) is found from

a^2/(sqrt(a^2-b^2))

Where a^2 means 'a squared', and sqrt means the squareroot of stuff in brackets

a is the distance from the centre to the furthest point on the ellipse and b is the distance from the centre to the nearest point. So, if the ellipse is taken as lying across the page (wide rather than tall), then 'a' is the horizontal radius and 'b' is the vertical radius


Double this distance gives the nail positions as it is the distance from the centre to each focus


I haven't tried this yet but book says it works
 

mudman

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waterhead37":31672aqb said:
I think these ovals are a good reason to invest in a couple of nice rasps!
But that'd be cheating! :shock:

Alf,

Your one looks very nice, I can't see you needing much more practice, looks like you've got it sussed already.

Tony,

Thanks for the formula, I was thinking of doing a bit of algebra to see what is going on.
I've been trying to visualise what happens (and doodling circles when I should have been working) and from an empirical point of view I think that the distance from the original centre should define the finished oval. This should mean that it could be as simple as find the centre and the centreline then measure an equal fixed distance up this line from the centre. However there are other variables involved such as depth of cut and probably some others. I'm going to have to do some more thinking on this and I can see me getting obsessed with this for a bit. :? I think a bit of experimentation is called for, but like Alf, it's going to have to wait, I wasn't using my lathe last night and I still can't get to mine. :(
 
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Anonymous

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Hi Barry

hope the formula works. I got interested and looked a bit further as I wanted to find the equation that determines the points on the perimeter.

AQnyway, I found the following (which is common snesne but perhaps not obvious)

If c is the distance from centre to focus (nail position or turning centre)

c^2 = a^2 - b^2

Clearly a/b must be more than 0 and less than 1.
If a/b = 0, then it is a circle and if a/b is more than 1, then the axes have swapped
 

Hans

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mudman":3tu539fq said:
I don't have the required mathematical mind to work out what to use given the length of the blank
I think drawing can help a lot with off-center work.
I am just a beginner in the use of (Turbo)CAD, but I can get a clear picture of the result when changing a radius or moving a center. It takes (me) some time though.

Hans
 

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