oval cutting jig

[smokeyone]

I am hoping someone on the forum can tell me where I am going wrong please. I do have an oval cutting jig which I have been playing around with - just pencil lines - but which ever way I set it I cannot seem to get a large difference between the long side and the short side.
The jig arm is made of perspex and is set on a base with cross slides which are adjustable. As an example I can mark out an oval 48" long by 43" wide plus various other sizes but all with the same sort of inch/percentage difference.
I hope to cut an oval with more like 12" difference rather than 5".
I am missing something here because I am running out of wood to pencil on.
Thanks

 

[skyechem]

Could you perhaps post a picture of the jig? An oval has two focal points. (If you use the technique of two pins with a loop of string around them to draw an oval, the pins are the focal points.) The closer the focal points, the more like a circle it is: the further apart they are, the "longer" the oval is. I hope that helps.

 

[smokeyone]

Thanks for the tip. When you said "the further apart they are" it struck me
what the problem is - the tracks are not long enough so in effect the two pins cannot move far enough apart - thus I cannot make a longer oval.
Back to the drawing board.....

 

[Chris Knight]

I take it that you are referring to a device similar to the one in this link. I have used a number of these before, and whilst the difference between major and minor axes is restricted to a degree, it is nothing like so restrictive as the figures you quote.

As you might have found, a restriction arises owing to collisions between and base of the jig housing the grooves. For this reason, you might need one or two jigs to cover a range of oval sizes. You do therefore need to trim the base in the 45° between the major and minor axes as much as possible.

 

[smokeyone]

Sorry for taking a few days to reply - pc troubles as usual..
That's exactly what I have in mind although my one is the same principle but has a lot less travel.