Routing Ovals ?

[Dog]

I'm about to start on a project to make a mould that will be used to create a fibre glass canopy for use on a custom trike. I need to make two one-off oval templates but I'm wondering what is the most accurate way to measure and cut them out to then use with a router ?

 

[9fingers]

The easiest way to mark out an elipse is to place two pins along the major axis and then take a knotted loop of string, place this around the two pins and with a pencil, make the string taught. Draw the shape keeping the string taught at all times.

A bit of simple trig will give you the length of the string and the spacing of the pins to give the major and minor axis dimensions you want.

Bob

 

[kevinr]

A look at wikipedia may help although the math is quite heavy.

There are a few interesting looking links at the bottom of the article. Two on drawing elipses and one on woodworking...

 

[Chris Knight]

Dog,
There are many cheaper ways but this kind of jig makes it easy http://www.trendmachinery.co.uk/ellipsejigs/

 

[jasonB]

Depends on the size, I find for small ones any drawing programme with a circle/elipse tool will plat what you need, then print it out and stick to the timber, unless you have a big printer you will be limited to about A3 size if you use two half elipses.

If larger then its easy to plot using the same principal of the trend jig.

Jason

 

[Gill]

You could buy a cheap oval plastic garden table and use that as a template.

I'll get me hat an' coat...

Gill

 

[SketchUp Guru]

How about this? Make a template in MDF and spring a batten over the edge to smooth out the curve. Start with the MDF. Lay out your curve and deduct for the thickness of a thin batten, say 4mm, cut near the line and clean up to it with rasps and sandpaper make the edge fit as close as you can but don't get too crazy. Now bend the batten around the curve and check to see how well it describes the curve you have in mind. Make adjustments to the MDF as needed. Glue the batten in place.

The batten should give you a fair curve and will hide any small dips in the curve from the bearing on the router bit. If you have larger dips that might allow the batten to flex when the bearing is over that area, fill in the gap with slivers of wood and glue.

It might be a little more work than just shaping the edge of the template but I think it will go fast enough and if the fiberglass canopy will be finished with a gloss, the fairness of the curves will be important.

 

[Dog]

Many thanks for all your replies. When all is completed I'll post the finished project in the Projects forum, it might be a while though :wink:

 

[mailee]

I don't know if this is of help to you but I made my own oval jig based on the Trend one. All I did was cut a square of MDF and rout two dovetailed slots at 90 degrees to each other. Then make two runners out of the same or any plastic and screw an arm to each one using the fixing as a pivot point. Make a false base for your router and attach the arm to this at the desired distance and hey presto! A oval cutting jig! :wink:

 

[Dog]

Cheers for that, I'll give it a try