Miters on a tapered column

[maxglitz]

I have a series of projects that revolve around this, the first being a mission (or craftsman) style lamp post in the yard. So it's a good example. It will sit on a short cement block base covered with stone.

I'd like to build a flat-sided, tapered wooden column similar to those being used on craftsman-style homes on porches. (See photo.) It doesn't necessarily have to support any weight. My question is how does one figure the miter degrees on a tapered "box?" This is the angle where the four sides join. My guess is that it wouldn't be 90 degrees because of the taper. I could be wrong. Any math wizards among you experts? Any formulas? I'll probably go though a lot of mdf trying to figure this out otherwise.

I'll be cutting this on my Ridgid table saw (shameless plug).

Mike
Atlanta, GA

[jbergstrom]

They'll still be 45° miters because you still have 4 sides...

The sides will be isosceles trapezoids (http://en.wikipedia.org/wiki/Isosceles_trapezoid).

You'll need to layout your isosceles trapezoids and use either a taper jig on the table saw (a really long one) or clamps and a straight edge but either way you can still make the cut with the blade angled to 45°...

Hope this makes sense...

[Bob D.]

Why not just use a 45 deg chamfer bit in a router after cutting the taper on each board?

[gear junkie]

On painted projects, I'll often go with angles of 35/55. The inside corners will match up and I'll hand plane/sand down the outside corner on the greater angle. The advantage is that there is a "definite" 90 so it's less noticable if the miters don't match up. It's also stronger because of the thicker corner and less likely to be dented.

[LONGHAIR]

If it is a painted project, miters are not really required anyway. Butt joints that are nailed/screwed will be stronger and if you "pinwheel" the joints around the unit, there is only one visable joint per side. Plus all of the parts are identical. You make one that becomes the "master" and pattern cut the rest with a router.

[Gofor]

The more the angle tapers in (ie slope) the greater the angle. For 4 sides, the miter is 45 degrees. It will increase referenced to the outside as you slope inward to a maximum of 90 degrees when it is flat on the ground. For the post you pictured, the slope is about 3" in 7', or about 2 degrees. The increase from 45 degrees relative to the outside surface is only about .04 degrees.

If you use one of the calculators designed for cove molding compound miters, realize cove molding settings are on the inside of the column (the cove molding slopes to the inside of the room). Therefore, for something like a lamp post, you would need to make sure that the angle of miter is referenced from the inside face (top of the board on a TS). Let me explain: If you cut a 45 degree bevel on a board on a TS the angle is 45 degrees reference from the top of the board, but is 135 degrees referenced from the face down side of the board after you cut.
So lets say I want a lamp post with a 3" slope inward on each side and it will be 6' tall. that means i have a slope of 3" in 72", or 1:24 ratio. In trigonometry this is the Sine of the angle of slope which is an angle of 2 degrees 23 minutes (2.38 degrees).
If I plug that angle into a cove molding calculator, it says my bevel should be 44.951 degrees from the inside of the column ( all coves lean to the inside of the room). You can't cut more than a 45 on most TS so you are stuck with the tearout on the outside face which will be visible, but in this case I would call it 45 degrees and put the outside face up.

Hope this helps and doesn't confuse. If you pm me, I can send you an excell spreadsheet that you can plug in the slope for cove molding and it will give you the angle which you can plug in for degrees and miter settings.

I guess what I am trying to say is that most of the online calculators for compound miters are for cove molding, which is slanted inward, so are referenced to the inside face of the board. When building a tapered column, you are usually concerned with the outside, so you have to adjust for the difference.

Go

[jbateman]

I don't know all the math involved, but if you got to this link and download, then run the Polycut.exe program, it will calculate the the angles for you. I've used it on truncated pyramid type shapes, which are really just short versions of what you wish to make.

http://www.woodcentral.com/bparticles/polyinst.exe

[jbergstrom]

I don't know all the math involved, but if you got to this link and download, then run the Polycut.exe program, it will calculate the the angles for you. I've used it on truncated pyramid type shapes, which are really just short versions of what you wish to make.

http://www.woodcentral.com/bparticles/polyinst.exe

Very cool little program :cool:

Thanx for sharing that :)

[WWPierre]

If it is a painted project, miters are not really required anyway. Butt joints that are nailed/screwed will be stronger and if you "pinwheel" the joints around the unit, there is only one visable joint per side. Plus all of the parts are identical. You make one that becomes the "master" and pattern cut the rest with a router.

I am with Longhair on this. My only refinment would be a shallow dado for the length of the joint. I generally take the time to do this as it makes assembly so much easier. As a former (and maybe future) boat builder, I generally avoid miter joints exposed to the weather. Actually, I generally try to avoid miter joints.........period. ;)