Angled Miter Cut

[Birdhunter]

I'm building a couple of jewelry boxes and ran into a strange problem. The side pieces are two ply. The outside ply is Cocobola and the inside is Spanish cedar. The total width is 3/4". I cut a 45 degree miter on one side piece with the saw blade tilted to 45 degrees. I checked the cut and found that the cut was not square. I measured the cut with a digital angle gauge and it was off about 7/10 of a degree. This doesn't sound big, but it would leave a gap when the sides were glued together. I then cut a scrap piece of wood and got a perfect 90 degree cut. I did another cut using a jig with the blade at 90 degrees and, again, got an angled miter. I then thought the box sides might  not be straight. I lined them up against a Starrett rule and they appear to be straight and parallel. The saw is a SawStop Industrial. The bad cuts only occur when cutting the box sides.  I've done numerous test cuts on scrap and they all turn out perfect.

I'm at a loss to explain this anomaly.

 

[Birdhunter]

I set the sides away for a while then decided to look at the side pieces again.

I stood them on their edges and put a small square against the sides. Surprisingly, the sides leaned a little off plumb. I checked my jointer fence with a square and found it wasn't perfectly at 90 degrees to the jointer bed. I reset the fence and rejointed the sides. They all now set perfectly vertically plumb when resting on their edge. Being careful to butt the jointed edge against the miter gauge and with the blade set at 45 degrees, I recut the miters. All were perfect.

I'm not sure why the edges not being at 90 degrees to the side caused the miter to be angled. Anyone?

 

[ewils91]

Cool, I hate nagging issues like that. [mad]

 

[Bob D.]

I don't understand the position of the wood when cut.

The combined thickness of the two pieces is 3/4" you said, does that
affect the cut meaning are they being cut at the same time as a unit.

I am guessing that you have checked the piece and all faces are parallel
(consistent width and thickness). What are the dimensions of the piece?

What is the position of the piece when cut, is it vertical (standing on
the 3/4" thickness) and guided with your miter gage at 45° or laying
flat and the saw blade is set to 45°. I am thinking you're using the
miter gage.

Is the face of the miter gage square to the table at 90° and at 45° ?

I not seeing how the bottom of the piece not being square to the face
of the workpiece would affect the cut if you are holding the piece tight
to the miter gage fence. The bottom edge might not be in full contact
with the table I understand that but this shouldn't matter for a miter
cut on the end which is referenced off the face of the piece in contact
with the miter gage.

 

[tiralie]

I measured the cut with a digital angle gauge and it was off about 7/10 of a degree. This doesn't sound big, but it would leave a gap when the sides were glued together.

The anomaly could be caused by any number of issues, user error, calibration of saw, accuracy of saw, preparation of blank etc. Use a shooting board to fine tune the miter and you will be good to go.
Tim

 

[Birdhunter]

The boards are 4" by 12" with the outer layer being Cocobola and the inner layer being Spanish cedar. The layers have been glued, clamped, and dry. The boards were jointed on one edge and trimmed to the desired width on my SawStop. To cut the miter on the ends, I tilted the sawblade to 45 and used a miter gauge to push the wood through the cut. This same miter gauge was used to make the perfect test cuts of scrap wood. I even used another miter gauge and got the same results. After I corrected the tilt on my jointer fence, I got perfect miters on my box pieces.