Dimensions don't seem to jibe

I’m working on a design for a small bench which has splayed legs. I have an issue with dimensions on the top stringer which fits into a cutout at the top of two legs. When I dimension the lengths at the top of the stringer, I shows 9 1/2" on each side but when I dimension the lengths at the bottom of the stringer, one side shows 10 5/64" while the other side shows 10 7/64". When I look at the stringer component itself, it doesn’t show any out-of-line planes, everything looks good.

I realize the two dimensions in question are not exact but why do they display as being different?

My file is attached.
Outdoor bench-splayed legs-2.skp (52.5 KB)

Nothing attached. Please try again.

Yeah, my bad. I forgot to hit the insert image icon.

It appears your angles aren’t totally squared up. They seem ever so slightly tilted. Not sure how that happened, and I think you’ll have a difficult time fixing it.

@wataru.kannuzuki is correct. You have somehow gotten about .8 degree of twist into the bottom of the mortise.

Edit: That’s because the bottom is drawn at 90 degrees to the sloping side of the leg (which means perpendicular to the sloped apron face), not perpendicular to the z axis. Given the combined rake and splay of the leg, this really does make the distance between the outside corners slightly greater than the distance between the inside corners. The top of the apron doesn’t have this problem because it is cut perpendicular to z, but that means it will need to be beveled with respect to the face of the apron.

Compound angles can certainly cause a lot of confusion and fussy challenges fitting things!

Edit 2: Here I drew a red axis line across both end faces of the apron. You can see that the measurement there matches the one at the other edge of the apron. So, if you beveled both edges of the apron and made the cut in the leg level, it would be easier.

1 Like

Here’s how I would do it.

Outdoor bench-splayed legs-2.skp (75.2 KB)

1 Like

Yup, you is right my friend. When I draw a reference point so that it is now aligned with the z axis, both front and back dimensions show as 10 7/64". Dem compound angles can git ya every time.

1 Like

Thanks for helping show me what I was missing. I may still stick with my approach. As it is the slight gap (i.e. 1/32") is kinda “in the sawdust” and it is buried in the internal part of the legs, so it won’t be visible anyway.

Thanks again.

May I ask, why did you choose the rake and splay angles that you did?

Good question Dave. If you drop a vertical from a corner of the top (prior to the 45 º cutout), the outside corner of each leg is the bottom of that vertical. I arbitrarily selected a 2" setback on the long side and a 1 3/8" setback on the short side that locates where each leg hits the underside of the top. That combined with the desired bench height created the odd angles. Truth be told, the bench height was supposed to be 14" (to match a bench this is replacing), but somewhere on the “way to the wood pile” I messed up and made the legs 14" high so the 1 1/2" top give me a bench that will be 15 1/2" high overall. I thought about correcting that oversight, but then, a bench that’s a little higher is okay and perhaps more importantly, it would take me more time to reset all the above stuff to maintain my arbitrary “wheelbase” and topside setbacks.

Hey, you had to ask.

1 Like

I understand what you were thinking. I wonder if you wouldn’t have an easier time if you choose an easy to set angle.

Just a note about the angles on rails between the legs. When you both rake and splay the legs the faces on the ends of the rails need to be cut at a compound angle. This is normal so you shouldn’t expect 90° angles. Here’s a stool I did plans for many years ago. Same sort of issue. As the rake and splay angles increase the angles on the ends of the rails increases.

I have run into this many times. In the “old” days they would actually give the legs a parallelogram cross section to deal with this.

Easier? Well sure, but is easy fun? No.

In my case the angles just fell out of design rather than being a driving factor, kinda doing is bass-ackward. I won’t be able to set those “off” angles directly on my machines so I’ll do what I did on another project. I’ll create triangles in SketchUp, print them out, and then cut and paste to a cardboard backing (cereal box works fine). I can use those as the setup guide.

Oh yeah, and wish me luck. If I remember, I’ll take a photo and post it back to this thread when I’m all done. But don’t hold your breath, this is a fill-in project among other pressing matters.

1 Like