Templating an Arch/Ellipse to cut using the shopbot?

[bjones]

Hi.. I need to make 4 arched molding templates for some 8 foot windows, wanted to know if anyone had a good technique to determine the mathematics of the arches in order to plot them into part wizard to cut out? I thought it would be as simple as slicing a circle, but when the arch is elliptical in shape it gets more complicated.. anyone run across this before??

[paco]

To start, an oval shape is not necessarily an elliptical shape... keep that in mind. It's not because it's expensive (the window) that it's what its supposed to be... that window can still look very good.

You could try with a vertical and horizontal measurements, cut a very cheap sheet (1/8-3/16 Masonite) and see if it fit... if not where or how it doesn't.

You could try to get a very nice FRONT picture of the window and trace it as accurately as feasible.

You could try to pencil trace it on a cheap Masonite (part of it with some good references), maybe cut it with the hacksaw and then digitized it.

My prior concern would be how much is that customer is willing to lay for to get is template/part...!

[sam_harbold]

Hi Brian
If you know the manufacture of the window a lot of times they can provide you with a cad drawing of the window. You can then import this into part wizard and go from there. What we have found is most window manufactures don't use a true ellipse when they produce a window. Most are made from combining two or three radius to form the shape. If this is the case with out a cad drawing becomes more difficult. If we can't get a cad drawing from the manufacture then we will draw the inside edge of the window jamb onto a piece of cardboard. When back at the shop take the drawing and input the data into cad then produce our template. If you want more detail on producing the template and inputing it into cad drop me an e-mail and I will be happy to explain in detail.

Sam

[kfitz]

I did this several years ago. A carpenter made tracings on kraft paper. I laid the paper on the floor and created a grid, plotted points, re-created in the curves TurboCad and/or Rhino, and exported to Part Wizard.

Also, somebody (Bill Young???) created a Circle Top window molding application that worked well.

[cnc_works]

For other applications, I often take a photograph and digitize (trace) it. It occurs to me that this may work for you if you were to take a photo direct on and from enough distance to avoid wide-angle distortion, trace it, then with one known dimension, scale the tracing.

You could then cut a 1/4" plywood mock up to test the arc if you wished.

For the profile of a molding...if I can get a piece of it...I will put it cross section down on my scanner, scan it, then trace the resulting scan and again scale to a known dimension. I've cut many knife templates for molders in this manner.

Donn

[waynelocke]

I did this once for a stair landing. I made a template at the jobsite out of 1/4" ply with a pilot bit and then used the probe to get the template into the computer and onto the Shopbot.

You could probably do good enough by making a tracing and using it to make a plywood template which you could probe.

Wayne

[paco]

I like Sam's suggestion. It couldn't cost too much to ask and the solution could be a phone call away,

Still, if you're to machine a sophisticated molding, I would recommend to cut the basic elliptical shape from the CAD file just to make sure it fit the actual window.

Let us know which route you take and how the project turns out.