# Long miter joint



## Kmfoley (May 12, 2021)

I hope this is just a noob question. I am hanging shelves on brackets with MDF to be painted. The shelves are 11.5 in wide and will be mounted inside a nook and I want to do miters on the inside corners. 
Knowing my walls would not be perfectly square, I measured the angle at 88 degrees. I bisected that angle, set it on the miter saw and cut a few 1x3 to test. They fit perfectly.

Feeling good about my test pieces, I cut the shelves but they don’t even remotely fit! What could be my (probably simple) explanation?
Thanks in advanced.


----------



## Steve Neul (Sep 2, 2011)

Not necessary to know the angle. Get someone to help you and set both square boards up to the corner letting them overlap. Where they intersect on the inside corner mark a line there and then mark a line from the corner of the boards to the inside corner that you marked and cut it there and it will fit..


----------



## woodnthings (Jan 24, 2009)

The short pieces worked, because in that length of the wall there was not a change in the angle. When you try to fit the longer piece against the wall the angle changes because the wall is not "even" along it's length. There may be a bump somewhere that changes the angle on the longer boards.
To fix it, lay a thin strip about 1/8" thick, across the intersection when the pieces are as tightly bumped together as you can make them and draw lines on both sides of that thin strip. These lines are your new cut lines. Cut to them, or hand plane to them or sand to them for the best fit. Yes, it will shorten the length of each board slightly, but you have no other choice other than filling the gap..... ugly.


----------



## B Coll (Nov 2, 2019)

There could be many reasons. The 1x3 miter looks a bit off, it will be exaggerated on a longer length. The 1x3 were only short samples. Walls all change their profile over the surface, the walls could have slight bows or cups which would only show when the board runs the entire length of the wall. Here is how I would likely approach it. On your long wall, scribe the back of the board into the wall (if you want a tight fit). Otherwise, fit the long wall the length of the wall butting square into the opposing wall. Now, do the same to the shorter wall. The two board should now be overlapped. Mark the front edge/corner where the two boards intersect. Now, cut one board with a straight edge from the back corner, that was fitted, to the intersection point. That will be your corner angle. Now, put that board back in place and trace the angle line you just cut onto the other board, and cut. That should give you a perfect miter, accounting for the walls, that meet in both the back corner and front inside corner.


----------



## Kmfoley (May 12, 2021)

Thank you, I will give this a try and agree that it could be a half a degree or so may not show up on a 1x3 but may on a larger piece. Thanks again


----------



## Tool Agnostic (Aug 13, 2017)

A half degree is a lot when you extend it out. My spouse and I were testing tilt angles on shelf supports and found that whole degree increments were too much, so we tested with half degrees. For our future garage lumber shelves, a 2-1/2 degree tilt seems just right.

I was surprised. I remember the small protractors we had in school, and the one degree increments were very tight together.


----------



## RepairmanJack (Aug 4, 2013)

some long straight boards to account for the overall angle and direction, couples with a good miter protractor would remove a lot of the guesswork.









Starrett - ProSite Miter Saw Protractor


Legendary Starrett accuracy takes error prone calculations out of the process of setting up miter cuts. Simply set your saw to the angle that you read on the protractor.




www.woodcraft.com





You don't have to use one, Steve's suggestion is a pretty typical solution for such problems, but if your marking/transfering skills are not quite there, a measuring tool is a nice thing to have.


----------

