I need to trim around a stovepipe that penetrates a vaulted ceiling. How can I duplicate the oval (or is it an elipse?) that a round object creates when it penetrates a plane on an angle?
It is an insulated stove pipe in a sauna. I plan to end the finished ceiling 4″ shy of the pipe, I’d like to fashion metal trim to close the gap and prevent heat loss. How can I draw the cutline?
Replies
I'd use trial and error to make a template out of manilla folders, felt paper, or thin cardboard - not the most cerebral approach maybe, but it works.
I'm with Jim on this one, you can spend 45 minutes doing the math only to find you still have to make a template to make sure your whole project conforms to the classic textbook case. Best to spend 15 minutes making a paper template via trial and error and be done with it.
The only problem with making a pattern of the existing irregularities without discovering what they are, is that your pattern and it's errors will only compound the existing errors. If you lay it out properly you lessen some of the existing errors and force the work back in line.
You surely wouldn't lay out an addition on a structure by assuming the existing structure was square, plumb and otherwise correct. Proper layout technique is the key to proper construction. I admire technicians who can and do master good layout techniques.
Edited 10/4/2003 8:08:59 PM ET by Slater
Gee, the math shouldn't take 45 minutes. The minor axis is the stove pipe diameter, and the major axis is (pipe diameter/12) * sqrt(rise2 +run2) . Plugging three numbers into a one line equation is fast. I timed myself at 20 seconds punching in numbers. Then, knowing the axes, using a compass and string like Slater described would only be a few more minutes.
To me that would be faster than trying to cut through a scrap of duct without mangling it or working overhead with a level in one hand, duct in the other, and a pencil in the third.
Probably by now, though, the work is done and installed! J
If they don't have time to do it right the first time.......
Wayne, your right it shouldn't take 45 minutes, but there are those who simply look at the task at hand and go for it, then there are others who agonize everything to the n th. degree, burn up a day and have nothing to show for it. My point was, is the pipe really round and is the vaulted ceiling really at the anticipated pitch? With a paper template it doesn't matter, it will conform to the shape needed. Generally I use geometry to get the expected size, layout on paper, cut hole out undersize, cut slits around the parameter, slide paper around pipe and crease the edges with my fingernail, perfect fit every time.
You might want to try posting in a math teacher's forum. There's more likelihood of finding someone there who knows the answer. Or, if you have kids, they can ask their math teacher.
If I come across anything, I'll post it.
I found something. Use the technique on the attached image. To do this you first need to know distances AB and CD used in the description.
CD = diameter of the stovepipe
To find AB,
r = rise of roof pitch
R = run of roof pitch
then AB = (CD/12) * sqrt(r2 +R2)
So, for example, if your stove pipe is 8" diameter, and your roof pitch is 6 in 12, then
CD = 8, and
AB = (8/12) * square root of (62 + 122)
= 8/12 * square root of (36 + 144)
= 0.67 * 13.42
AB = 8.94
Edited 10/2/2003 11:29:09 AM ET by WAYNEL5
Edited 10/2/2003 11:30:18 AM ET by WAYNEL5
Here's the attachment. There's no way to revise an attachment once it's posted. It has to be deleted and posted in a subsequent message. Dumb.
That looks like the CBS television way to do an ellipse.... ;-)
-- J.S.
try joe fusco .com or e-mail.. he's the man...bear
"expectations are premeditated resentments"
If you want to try the "direct" method, you could hold (with aid of helper) or better yet, prop a piece of the same pipe up to the ceiling. Dead plumb, of course. Then use say a 12" metal rule held flat against the side of the pipe as you proceed around the perimeter making frequent punch marks with a corner of the rule on a piece of paper or light cardboard you taped up there first. Connect the dots, cut it out, then cut in half and offer it up to the pipe. Adjust if necessary.
I think this is what Jim Blodgett was suggesting, too.
Wayner,
You can view a Stove Pipe as if it were a Gable Stud. It's running Vertically up to the bottom of a Rafter and then through it.
Now all you need to do is find the pitch of the roof. In my attached drawing the pitch is a 6/12 or 26.57°.
If you were to cut a Gable stud, you would set your circular saw at a 26.57°.
If you want to you can just take a piece of the same exact size stove pipe and cut it on a Miter Box with it set at 26.57° then you can scribe that for your molding it will fit exact. It will have the Elliptical shape already there.
This way you don't have to draw out an Ellipse or try and figure out the Major Axis and Minor Axis like I have in my drawing.
The key thing is knowing the pitch of the roof and how you get the pitch of the roof.
For Example with a 6/12 pitch and 7" Stove Pipe:
6 [Inch] [Pitch]
7 [Inch] [Run]
[Diagonal} = 7.8262 or 7-13/16" (This is the Major Axis)
The Minor Axis would be 7" the width of the stove pipe.
Another way you can figure the Major Axis:
6/12 Pitch Unit Legth x Pipe Width(Diameter) / Unit Run = Major Axis.
13.4164 x 7 = 93.9148 / 12 = 7.8262 or 7-13/16" = (Major Axis)
http://forums.taunton.com/n/docs/docDownload.aspx?guid=3A98D629-8068-465C-879D-CD3579F56360&webtag=tp-breaktime
Joe Carola
Edited 10/3/2003 9:48:12 PM ET by Framer
Edited 10/6/2004 11:20 pm ET by Framer
You are trying to draw an ellipse. So first you need to know your short axis and long axis sizes. Your short axis is the diameter of the stove pipe hole you wish to cut. The long axis is the hypotenuse of this pipe hole as it relates to the pitch of your roof. To solve for this hypotenuse, use the run (pipe hole size), and the related rise of your roof and the Pythagorean theorem and this will give you your long axis.
Here's a quick sample, if your pipe has a 12 inch diameter and your roof is a 6/12 pitch. The Pythagorean theorem says:
Run Squared + Rise Squared = Hypotenuse Squared
Therefore in my sample:
(12 x 12) + (6 x 6) = the square root of 180 or 13.416 inches
12 is your short axis and 13.416 is your long axis
Now to draw an ellipse. You'll need a piece of string and three nails. Draw a square x on the floor, center your long axis and short axis on the x . For our sample measure from the center out 6 inches up and 6 inches down on the x, this is your short axis length of 12"s now measure 6.7"s left and 6.7"s right, this is your long axis length of 13.416"s. From your top short axis point swing a compass 6.7"s and intersect your long axis on both sides of the x. Drive a nail at both of the intersection points, drive another nail at the top short axis point. Now wrap a non stretch string around all three nails, and tie it tight. Now remove the the nail from the top short axis point and replace it, inside the string, with a pencil. If you swing the pencil inside the string in an arc it will intersect all the exterior axis points and describe your ellipse.
I'd be glad to solve for your long axis, if you'll supply me with your pipe diameter and your roof pitch. Then it's just a matter of clarifying how to draw the ellipse. I hope this helps, and no, I'm not a math wiz just a slate roofer, who happens to do this a lot.
wayner,
Profile gauge would work,if long enough,cheers,Phil.If it is to be.... 'twil be done by me.
joe... great visual..
i was gonna say make a mock stove pipe out of cardboard and set it plumb to the ceiling, scribe it and use the finished truncated tube as the templateMike Smith Rhode Island : Design / Build / Repair / Restore
That is very slick and really rather apparent......if I'd stopped to think about it. Cutting these holes is a once-in-a-while task and so I've never taken the time to think it thru well mathematically. Instead, I"ve aklways just offered up the pipe and "scribed" the result.
Once I saw your demonstration it made me wonder why I never realized it myself. Makes perfect sense.
Thank you for the insight of a simple to understand method yielding exacting results........(providing the pipe is truly round and is standing plumb, I think).
Knowledge is power, but only if applied in a timely fashion.
take a pipe the size you want use curve cutting snips and cut pipe at same pitch as ceiling than trace shape on ceiling
The Alaskan approach would, of course, involve chainsaws and duct tape.
Take a 6" diameter piece of firewood. Chainsaw it at the angle of the ceiling. Tape it in place. Trace its outline. Free-hand through the ceiling with the chainsaw (or cheat and use a jigsaw).
But really, a 6-inch diameter piece of wood, cut on a mitre saw will give you the perfect (and totally sturdy) template).
In "Tips and Techniques for Builders", published by Taunton, there is a tip that illustrates the inversion of your quandery.
Ian R. Walker of Princeton, New Jersey need to scribe sonotubes to the "irregular bedrock surfaces" when constructing forms for deck piers.
He had a made tambour like cylinder out of movable wooden slats, which could wrap around the sonotube, and the frame that holds the slats allows you to tap the slats into the contour required. A line is traced around the ends of the slats.
rg
Since 1997 I’ve answered this question no less then 6 times on this site alone. From technical drawings like the one Joe Carola did to formula like the ones posted.
The really answer to this question is knowing what you CAN do and what you SHOULD do in your situation. How practical is it to “miter” a 6,8 or 12” flue pipe? How practical is it to build some type of apparatus for the one off time you need to do this?
I guess the point is not to make anything more complicated then it needs to be. View Image
Amen!!!!!
This is probably one of the most interesting and informative posts I've read here since I've come on board. This kind of stuff fascinates me.
But if you call the supplier or manufacturer of the stove pipe, he should have templates or measurements for each diameter stove pipe he sells, for all standard roof pitches, already worked out.
Or maybe just extrude the base of the stove pipe (plus your four inches) through the roof slope on your ACAD. Plot it, then have your copy shop blow it up to full size.
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
Boy is this getting heavy....... try using a bright flashlight shinning it inside the vertical pipe onto the ceiling, just trace it out. You will have some extra time to read all the previous posts. [email protected]