Circle Cutting Trick

Got some pics of what you’re working on?....what material is it?....also show us exactly what you intend to stick through the roof, then we’ll be better able to visualize it, and come up with the solution......NMI ( need more intel) .....😉

Pat

Hope your ready for a wild-*&^ suggestion.  Measure to determine where your blast furnace intersects the roof, and then cut your blast furnace at the same angle as the roof pitch.  Use one side of your blast furnace to mark whatever you are using to make the corrugated roof (preferably before the corrugations are "modeled"), and remove the material to make your furnace hole.  Then add your corrugations to the roofing material, slip the two pieces together (through the hole) to be sure they fit, and then glue together.

I would use material for the corrugated roof that is a good bit bigger than the intended roof size so that you can trim the roof to fit after you get the blast furnace (already glued into its hole) positioned properly.

Just a thought.

Chuck

No problem!

The attached picture is of the cast house. I have roof trusses now but no updated picture. The peak runs parallel with the length of the building. I just gotta figure out how to get the corrugated roof panels from Plastruct cut to fit around the blast furnace. I think the Plastruct corrugated panels are just ABS. I expect to be able to cut them with a razor blade. They're not extra sturdy by any means.

If the blast furnace was a straight cylinder instead of a cone shape (I think truncated is the technical term), this would be easier. But I'm just hoping there's a neat trick somebody has so I can do it faster.

@PRR1950 posted:

Hope your ready for a wild-*&^ suggestion.

Always! I like creative suggestions! Yours made me realize that I completely failed to look at all my options. This was probably the way to go from the beginning. I just made a few cone shaped cylinders and never considered what you're suggestions. I'm going to go take some measurements and see what I can do.

you’ll have to project where the stack protrudes through the roof,....then figure out the diameter that will actually poke through. Use that diameter to form a template from card stock, or similar material that can hold a circle shape. Trim the card stock to SIT on the roof as if it were going through the roof. Trace that shape onto the roof. Cut way well inside the tracing, then sneak up on the final trimming till it’s at your desired affect ,......

Pat

Using Chuck's logic but the other way around....  If your diameter is approximately 4 inches, take a piece of scrap 4 inch PVC pipe, and cut it at the angle which corresponds to the slope, or pitch of the roof.  What ever correct angle you go through the pipe with will create the elipse shape that your seeking.  You can transfer that onto to your roof stock, preferably some practice pieces to check the fit.   If the furnace diameter is changing due to "tapering", you'll have to wing it a little, slightly larger than the pipe diameter near the bottom and stay inside the line near the top.   Assuming the peak of the roof is about dead-center in the furnace stack, the taper shouldn't create nearly as much of an issue.   This would keep from cutting through the blast furnace.   

@chris a I bet I could use a piece of cardboard to create the shape and then transfer it. I'll just wrap it around the shell that I have so that I'm slightly bigger than the shell itself. That should work!

Chris’ suggestion is excellent. Also look around for mailing tubes, wrapping paper, toilet paper or paper towel tubes, etc. depending on the size you need. Wrapping paper cores these days are usually heavy Kraft paper not really fastened, so you could wrap it around for size.

Bill,

Get yourself a contour gauge! These are used for duplicating irregular edges in molding, etc. Look in a woodworking store or on line. I've seen them in sizes from 4 to 10". Today they are plastic, but years ago they were metal. Either will work.

Decide where the top edge and bottom edge of the roof will be where it is intersecting the furnace. Lay the gauge from one point to the other, and you will have your contour. Reverse it for the other side. For a trial, you can transfer the resulting edge on cardstock, cut it out, and test fit it. It should fit perfectly.

Chris

LVHR

What Pat said. Make a template out of thin card. A cereal box would be good.  The width will be the same width as the chimney, and the length to to bottom an ellipse depending on the pitch. Cut it small and nibble away and if you have to, add back masking tape until it's right. Then cut your corrugated from that. This is the kind of thing that you could send more time trying to come up with the perfect solution than it would to actually just cut it. You have so many variables- pitch, corrugation, tapered stack that just making a template is easier than doing all the math.

If you don't get it perfect, you can add flashing which the prototype would have.

Bill, that's some impressive modeling work you're doing on your layout.  Maybe this solution is more complicated than you want to try, but if a really tight fit is what you're after, here is a way to accomplish that.

I often encounter this type of puzzle when doing trim work on various home renovation projects.  Here's the way I would approach it for a close fit, with the assumption that you're planning a gable roof:

1) Mark the center of the Blast Furnace roof penetration and drill a small hole in the roof peak slightly larger than the pin you'll place in it in step 2.  Draw a line perpendicular to the roof peak, from the reference hole down the roof, past the extent of the final hole size.

2) Measure the blast furnace diameter = UBFD (Upper Blast Furnace Diameter) at the height where it will intersect the roof peak and mark half this distance on the ridge on both sides of the reference hole.  Temporarily hot glue a pin in the reference hole sticking out about 1/4".

3) Measure the blast furnace radius at the lowest roof intersection point (LBFR).

4) Next on a scrap of paper you'll need to contrive two ellipses.  The narrow axes will be UBFD on the first and 2xLBFR on the second ellipse.  The long axes will be the same on both ellipses, but will depend on the roof angle.

If it's going to be a 12:12 (45 degree) roof slope then the math is easier.  The long ellipse axes = 2[SQroot(2(LBFR^2)].  If you plan to use a roof with a different slope, the you can either use the Pathagorean Theorem or a trig function to calculate half of the ellipses' long axes and then multiply the result by 2.

5) Once you've marked these ellipses and each of their 2 axes on paper, carefully cut them out.  Starting with the smaller ellipse first, line up its center on the pin and  the long axis to the perpendicular reference line already marked on the roof in step 1.  Trace the outline of each ellipse on the roof.  Remove the papers and the pin.

6) Cut out the smaller ellipsoidal tracing in the roof using what ever tool you're comfortable with.  Personally, using the reference hole in the peak as a pilot,  I'd use a sharp fine tooth hole saw with a slightly smaller diameter than the finished small hole, going very slowly with the lightest acceptable clutch setting on my drill.  With the roof in two sections, a coping saw would also work with less chance of damage.  I'm sure you'll decide which method is best for you.

7) Now comes the fun part.  Using a course sanding drum, rounded rasp, file and/or sandpaper wrapped around a dowel, each with a diameter smaller than the hole, I would sneak up on the final shape using the larger ellipsoidal tracing as a uniform guide from which to stay back.  I'd frequently check the fitment of the roof around the Blast Furnace as I go, until I'm satisfied with the result.

Hope this makes sense (it's late) and is helpful to you.

Or a completely different approach.

Since it not really a working blast furnace, you could decide not to cut a hole in the roof.  You might want to just cut the furnace to fit the roof slope on a band saw or scroll saw.  Then you'd probably want to fill in the gaps around the roof corrugations.

How about making a roof panel out of light card stock and putting a round o in it.  Then you can trim the hole to fit and use that to transfer it onto your corrugated roofing.

It sure would tax your math skills a lot less.  (Assuming you have any math skills.  I don't!)

Ooph! This really blew up. There are a lot of great suggestions here! And I learned a contour gauge exists. I will post some pics in my layout thread when I get it figured out! Thank you all for the suggestions. I find experience usually knows the best method and I'm glad I asked. I needed a different perspective!