Perfecting a Circle

karim · Post by **karim** » Mon Jun 25, 2007 1:46 pm

So I've got a shell built in Aves, circular with compound curves-- essentially the out-side surfaces of a torus.... Imagine a tire made out of Aves.

The central hole is meant to be a perfect circle... its not, quite.

Any ideas on how I can true it up? I can spare enough material to shave off some and still have it the right size... I just am not sure how to do the shaving/sanding without free-handing it, which I don't want to do.

Here are the problems:

The circle is not an even measurement in diameter... its something like 2.44 inches, and this is non-negotiable... it has to be slightly smaller than an existing part.

Since this is a shell, there is no center-point... there's a hole where the center of the circle would be... so no compass-work.

Its a thin shell of Aves in a 3-dimensional shape, so drawing on/drafting on the part is difficult with any precision.

Kylwell · Post by **Kylwell** » Mon Jun 25, 2007 2:41 pm

And a lathe won't do it? Well, got some ideas but not sure how to put them to text.

Give me a moment.

Andrew Gorman · Post by **Andrew Gorman** » Mon Jun 25, 2007 3:02 pm

If the OUTSIDE of the shape is reasonably true, you can run around the outside with a pair of dividers and scribe or mark a reasonably true inner diameter. Of course a pair of dividers won't really work, but if you made something like a beam compass with a long outer leg and a short inner leg that should do the trick.
Another possibility would be that since the shape does not have a true center, why not give it a temporary one? Fill it with plaster/foam/clay/cardboard disks white-glued in place or something like that. Mark your center, mark concentric inner and outer surfaces and you're there.
OR find a hole saw or wrap some sandpaper around a cylindrical form that's about the right diameter and hog it out to a fairly close fit.

How big is this hole anyway?

Andrew

karim · Post by **karim** » Mon Jun 25, 2007 3:08 pm

inner-hole size is about 2.44"

Can't use a lathe because... well... no lathe, but also its too fragile to chuck... its a shell of Aves about 1/16" thick.

The outer diameter isn't perfectly true... it would be nice if it was, but it doesn't have to be, where as the found part going in the hole is a perfect circle, so any irregularities of the hole will be immediately obvious.

I thought of filling with clay or somthing, but the problem is that I have nothing to construct off of, so I have no good way of finding the center, short of trial-and-error

I suppose I could start the whole part over again, but.... ugh.

futch · Post by **futch** » Mon Jun 25, 2007 3:42 pm

draw a circle on your computer scaled to 2.44". print it out and stick on the part. sand around it.

if you can drill through the part, stick the circle on one side, drill through the center, glue a second circle on the reverse side (centered on the drill hole). shape.

lastly, you don't need a lathe if you have a dremel or power drill. use the buffer bit and attach it to the part, crank the tool and sand down to your diameter.

karim · Post by **karim** » Mon Jun 25, 2007 4:34 pm

futch wrote:draw a circle on your computer scaled to 2.44". print it out and stick on the part. sand around it.

if you can drill through the part, stick the circle on one side, drill through the center, glue a second circle on the reverse side (centered on the drill hole). shape.

lastly, you don't need a lathe if you have a dremel or power drill. use the buffer bit and attach it to the part, crank the tool and sand down to your diameter.

THe part is already hollow, and the circle I need to trim is an inside circle... a hole.

Kylwell · Post by **Kylwell** » Mon Jun 25, 2007 5:02 pm

Well.... a sheet of brass, cut to the proper width and pointed, edges sharpened, slowly working in.

karim · Post by **karim** » Mon Jun 25, 2007 5:06 pm

Kylwell wrote:Well.... a sheet of brass, cut to the proper width and pointed, edges sharpened, slowly working in.

Hmmm... now that's interesting..

karim · Post by **karim** » Mon Jun 25, 2007 8:45 pm

I solved the problem with a modification of Robb's idea:

I used a cone (cut from the top of a bottle) with sandpaper glued to the outer surface of the cone... Then inserted it into the existing hole and reamed it out with the conical sanding form until it was even all the way around.

Worked perfectly!

--Karim

Kylwell · Post by **Kylwell** » Mon Jun 25, 2007 9:03 pm

That was going to be my next suggestion.

futch · Post by **futch** » Tue Jun 26, 2007 8:47 am

Kylwell wrote:That was going to be my next suggestion.

same here ;-p

Stu Pidasso · Post by **Stu Pidasso** » Sun Jul 01, 2007 10:33 pm

huh huh hhuuuhhhuh... Karim said "reaming holes..." huh huh huh...