Figure this one out

[steve swartz]

I was making some small boxes the other day and I can't figure out what happened and why.
I made some square boxes about 5"x5"x5"'. I cut the 45's on my chop saw and everything turned out good. I made some other boxes 5" wide and 5" high with a 15 degree angle on the sides, like the letter "V" with the point cut off. The finished box is a square on the top and has a smaller square on the bottom because of the 15 degree angle. The problem was that the 45's didn't fit together. I ended up shimming the stop post on the chop saw with pieces of sandpaper until I found an angle that would bring the corners together to make the boxes square on the top and bottom. It ended up being something like 42 degrees plus or minus a tad. (I would have used my Wixey angle gauge to get the real number, but a while ago I used it to check an angle on my table saw and I forget to remove it before I turned the saw on so good-bye Mr. Wixey) I have to assume that it had something to do with the compound cut, but I don't know. Can anyone explain why the 45's didn't work like they always do on square corners?

[Mark Bolton]

Google compound miter

[Rod Sheridan]

Yes, you need to make a compound mitre cut because your boxes are tapered.

here's a good link

http://www.pdxtex.com/canoe/compound.htm

Regards, Rod.

[Bradley Gray]

You can avoid the compound angle calculation if you make a shim to hold the side at the proper angle against the chop saw fence. This way the saw stays at 45 degrees.

[steve swartz]

Well guys, it wasn't that I didn't pay attention in class as much as it was that I never took trig in school and I don't know that I have ever needed it before now. The other thing is that school for me was 45 years ago and I have to tell you, I have forgotten a lot since then.
Thank you all for telling me about the compound miter calculators online. I didn't even know they existed. Thank you internet. The thing is that both of the calculators I found online told me that making a box with 4 sides with a 15 degree slope will make the miters 44 degrees. Last night I checked the angles with my Wixey digital protractor and the cuts were as I said at 42 degrees. I must be getting old.

[Yonak Hawkins]

Bradley's suggestion is my go-to solution for problems like this. It's less likely to produce inferior results .. at least for me.

[Rich Engelhardt]

You can avoid the compound angle calculation if you make a shim to hold the side at the proper angle against the chop saw fence. This way the saw stays at 45 degrees.I might be wrong on this - but - I believe you have to hold the stock upside down and backwards against the fence to make that cut also - just like crown molding.

[Bradley Gray]

Actually, this works either way as long as the shim is flipped also. Put a side and the shim against the fence, cut at 45 degrees, flip and repeat.

[Jim Finn]

When I make tapered boxes I find that if you only slope it five degrees you can ignore the variance you run into with steeper angles.