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Compound Miter Question

2K views 27 replies 12 participants last post by  TopamaxSurvivor 
#1 · (Edited by Moderator)
I'm making another one of turtle boxes, and this time I want to get it right. In this first version, I added green felt to the underside of the shell to hide the miter gaps.

The angle between the top and sides is 45 degrees. So what's the compound miter angle where a triangular corner meets a rectangular side? The nine shell slabs have the same thickness.

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#2 ·
I can't do this in my head at the moment.

You might consider building a cradle to hold the
work at 45 degrees. Then you don't have to
set a bevel angle, just the miter angle and that's
far easier to get dialed in. In cutting mouldings
on the flat, as you've noticed no doubt, when
you change either the miter or the bevel it
throws off the other setting, for want of a
better way to describe it.

 
#4 ·
Crown molding is easy if you know the tricks.

I agree with Loren. That video was really good.

The same tricks in cutting crown molding could be adapted to your turtle pieces.
Looks like you have a lot of 22-1/2 degree cuts …
 
#6 ·
For small parts I like the cradle method Loren mentioned.

However, since we've got a Kapex which is super easy to dial in to the right bevel and we do larger scale work, I often mockup joints in Sketchup then extract the miter and bevel angles and take that info to the miter saw. Usually I'm close enough on the first cut to get a good joint.

According to my (untested) Sketchup drawing of that turtle joint, the miter is 30 degrees and the bevel is approximately 17.6 degrees.
 
#8 ·
First off John, I love that turtle box. I will have to make one of these as it is so cool. Apart from the discussion on getting the two settings for a compound angle just right or using an angled fence, there is one more option to insure perfect fits. That is to cut the parts slightly oversize at the desired angles and then use a sanding jig to perfect them. This requires a disk sander and a shop-made sanding jig. The jig I am familiar with and have in my shop is made for perfecting ring segments for segmented turning, which have to have perfect glue joints to look acceptable. A different jig would probably be necessary for your turtle segments. I don't have a photo of my jig, but I will take one to send to you as a basis for further discussion.
 
#9 ·
I think I found the answer at this website … http://jansson.us/jcompound.html

When I went to the N-sided Box calculator, and entered 8 sides with a 45-degree slope, the blade tilt came up as 15.7 degrees.

So …

For the top, I need 22.5-degree bevels around the sides.
For the rectangles, I need 0-degree bevel at the bottom, 22.5 at the top, and 15.7 on the sides.
For the triangles, I need 0-degree bevel at the bottom and 15.7 on the sides.

Does that sound about right?
 
#13 ·
The calculator from the website assumes a symmetrical miter joint but the picture of the turtle lid indicates the cuts for the side pieces are not the same as the cuts for the corners (otherwise the rectangles would taper to a narrower top just like the triangles). That means it's not a symmetrical miter and the calculator is not going to give the proper angles.

I'm still sure it's a 30 degree miter and an approximate 17.6 degree bevel for the triangle. The rectangles would be cut with the same bevel but with the miter set to zero degrees.
 
#14 ·
Thanks, Jacob. I'm going to do the same thing in AutoCAD that you did in SketchUp and look at the bevels. If I extrude solids beyond the intersection, and then slice on the miters, that should give me the bevels … I hope.

And just when I thought the mystery was solved. :(
 
#17 ·
JACOB"S RIGHT. I was using the wrong calculator on the website. I should have been using General Compound Angles 2.

I looked at a rectangular side and a triangular side as two boards meeting in space.
The inside angle between the boards, in plan, is 135 degrees.
I knew that the angle between a rectangular side and the top is 45 degrees up from horizontal.
My CAD model told me that a triangular side is 54.74 degrees up from horizontal.

When I put the values 135, 45 and 54.74 into the calculator, it told me that my blade angle for the bevel should be 17.6 DEGREES.

I doubt that I'll be able to hit an exact 17.6 degrees, but I'll hit something a lot closer than the 22.5 degree angle that I used in the past.

Thanks again, Jacob. Give yourself a raise. :)
 
#24 ·
oldnovice: I still think the only people who have benefitted from calculus are those who teach calculus. I HATE it, too. I have a personal dislike (no, HATRED) for it because it took down a number of talented student architects when some genius thought we'd use it. I don't know how I barely passed with a C, but I knew a few guys who took the class for the fifth time before they threw in the towel and dropped out of school. The upside is that they probably went on to careers where they made some real money.
 
#25 ·
John, my first semester of calculus was very instructive!. For some reason, I did not really prepare for the final and was very concerned about this because the instructor was a stickler for following directions.

On the first sheet of the test was a long list of instructions, and if you read through all of them VERY carefully, one of the instructions read; "sign the front sheet in the upper right corner followed by the date, put the test sheets in the basket by the door and leave"!

As people started to leave, you could see others erasing answers because they did not follow directions.

I got a B that semester.
 
#26 ·
My calculus instructor graded on a curve and tossed out the two worst tests out of eight for his class. I didn't score that well on about 4 of the tests but got an "A" in spite of that. Finals were optional for students that weren't already destined for an "F". Didn't learn that much though and forgot almost everything. I can remember my high school math better.

Curve grading was something I really detested. It was difficult to evaluate my performance when there were no hard benchmarks to compare my results against. A lot of students liked it but it's terrible for someone that really does want to learn. I'm guessing I had a "C" level performance in the class but I'll never know for sure.

The good news is that nobody needs to know advanced math to do advanced woodworking. I know someone that can execute perfect joints with curves and compound angles and never uses math to figure out any of the cuts.
 
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