# Failure Thanks to Miter Joints



## Sirnanigans (Apr 3, 2014)

So I figured my miter saw was good enough alone... 

Either it was user error, or the miter saw I used to cut the sides of a 22" x 29" frame wasn't consistent. Either way, three of the joints were mediocre but acceptable for the project, and one had a noticeable gap. Furthermore, the whole assembly is about 2 degrees off square, although this may be due to a panel fitted into grooves of each side being too tight or off square.

With more careful setup, is it possible to cut precise miters with a miter saw, or should I leave room for adjustments always?

Looks like it's back to square one (hopefully it's actually 'square' one this time, get it?), because this thing is glued up solid and it need to be made exactly square.


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## Fred Hargis (Apr 28, 2012)

I've seen and know picture framers that use nothing but a miter saw, but their saw is very carefully tuned, and one guy has 2....one that does not move from the precise 45º angle he has it set at. That aside, I still see the miter saw as a carpentry tool and will cut miters on several other tools before I use it (other than for crown molding).


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## jschaben (Apr 1, 2010)

Sirnanigans said:


> So I figured my miter saw was good enough alone...
> 
> Either it was user error, or the miter saw I used to cut the sides of a 22" x 29" frame wasn't consistent. Either way, three of the joints were mediocre but acceptable for the project, and one had a noticeable gap. Furthermore, the whole assembly is about 2 degrees off square, although this may be due to a panel fitted into grooves of each side being too tight or off square.
> 
> ...


 Actually miter joints have TWO critical parameters. One is getting the 45° angle dead on. The other is the opposite sides MUST be of equal length. Either will give you marginal fitting miter joints.:smile:


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## Fred Hargis (Apr 28, 2012)

Very good point...check that by putting the opposing sides back to back and feel the points, you can easily detect the difference.


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## mdntrdr (Dec 22, 2009)

The saw must also be set to a true 90* vertically. :smile:


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## Sirnanigans (Apr 3, 2014)

All good points. I shouldn't kid myself, I know what I did wrong.

I tested the blade angle by the laser sight, cut a 3in end off of a test piece and flipped it around to see if the resulting corner was square. It appeared to be square, but it clearly wasn't long enough to give a good respresentation. Also, I should have held my square physically up to the blade, in retrospect.

I learned my lesson: if it only has to be done once, might as well spend the time on it. This regarding the miter saw setup, of course.

also the boards were a touch warped, which I figured the plywood would correct by being straight and square, then pushing the groove onto it and gluing it. This did not work, and was lazy. Next time I will straighten the boards, too.


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## pweller (Mar 10, 2014)

I think you have to be very careful if you're going to get good results from a miter saw. One thing I noticed is how much flex there is in the saw, as a whole - this is especially true of sliders. Put the saw in the down/cut position, and then push the end of the blade from side-to-side (about where it would be cutting the workpiece). You'll see there is a lot of play, you've got flex in the hinge joint, some play in the motor bearing, some flex in the blade, etc. You can also get some inaccuracy just on how you push down on the saw handle, any subtle twist will screw up the cut. You may want to adjust/lighten the force of the return spring.

I bought a digital protractor from HD for about $20 that has been a big help. I have found it is important to check both the alignment of the saw and the actual accuracy of the cut. I thought I had my saw set up perfectly, only to find out that the cuts weren't as accurate as I thought the saw was.


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## Sirnanigans (Apr 3, 2014)

pweller said:


> I think you have to be very careful if you're going to get good results from a miter saw. One thing I noticed is how much flex there is in the saw, as a whole - this is especially true of sliders. Put the saw in the down/cut position, and then push the end of the blade from side-to-side (about where it would be cutting the workpiece). You'll see there is a lot of play, you've got flex in the hinge joint, some play in the motor bearing, some flex in the blade, etc. You can also get some inaccuracy just on how you push down on the saw handle, any subtle twist will screw up the cut. You may want to adjust/lighten the force of the return spring.
> 
> I bought a digital protractor from HD for about $20 that has been a big help. I have found it is important to check both the alignment of the saw and the actual accuracy of the cut. I thought I had my saw set up perfectly, only to find out that the cuts weren't as accurate as I thought the saw was.


That's surprising. In an effort to keep the cut smooth, I naturally cut it very slow, which might help. When cutting a 2" x 1/2" board stood up tallways (must be a better way to describe this), it's easy to take it nice and sloooow, there's not much to cut anyway. 

This time around I will be spending more time testing and setting the cut. I would like to cut it a hair long and straighten/flatten it out afterwards, but the miter saw is my only power tool besides a router table, so good luck to me for making it perfect with anything else.


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## Bonka (Mar 24, 2011)

*45d Miters*

Here is how I cut 45's. I cut the top & bottom boards at the same time for length as I do the sides. this gives me equal length boards. I make sure the boards are flat. I check the miter saw for 45d with a drafmen's triangle. I have a stop to butt the boards against and cut the top and bottom. Then I change the stop to the desired length and then cut the remaining miters.
The I place them one my bench and see how they fit.
If they are off I have a 45d shooting board. I take off a small amount counting the repetitions. This keeps all the boards the same length at top, bottom and sides.
This has worked for me time and again. I use a low angle jack plane as one will be cutting what amounts to end grain.
Seldom have I had perfect miters from a miter saw and I suffer less grief doing the forgoing rather than fuss with a miter saw.


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## Midlandbob (Sep 5, 2011)

A accurately set up saw, an excellent blade and often a clamp to hold the board are valuable for a good mitre. 
Even then, as mentioned, a really precise cut often needs to be finished with a shooting board.
If the board being cut is more than a few inches wide, it is important that it be dried to the final moisture content of the project. A cut at 45 degrees might end up at 44.5 degrees if it dries a few % .. Wood movement can open up either the inside or outside of a mitre cut in a wide flat cut board.


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## Manuka Jock (Jun 27, 2011)

Sirnanigans said:


> ....a 2" x 1/2" board stood up tallways (must be a better way to describe this)


= On Edge 

:thumbsup:


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## Johnny5 (Mar 7, 2014)

To reduce the cutting pressure required, and thus reduce the amount of flex and twist introduced into the equation, I like to make the initial cuts long, then trim off about 1/4 blade to the final size. Slicing off 1/4 blade passes produces very little resistance compared to a full blade pass.

Also, if using a slider, check the rail bearing tension. Mine has an adjuster screw that should be set right up to the edge of binding the rail, then backed off just enough to permit smooth motion. This will minimize rail movement during the cut.


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## mdntrdr (Dec 22, 2009)

Johnny5 said:


> I like to make the initial cuts long, then trim off about 1/4 blade to the final size. Slicing off 1/4 blade passes produces very little resistance compared to a full blade pass.



Full kerf cuts will provide a much more consistent angle than slicing off the end which puts uneven resistance on the blade causing it to flex. :smile:


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## Johnny5 (Mar 7, 2014)

mdntrdr said:


> Full kerf cuts will provide a much more consistent angle than slicing off the end which puts uneven resistance on the blade causing it to flex. :smile:


Hmmm, interesting point, didn't think it would be near enough offset pressure to cause the blade to flex any appreciable amount. Guess that would depend on the blade and material being cut. I have never had any issues with this practice though, I always sneak up on a cut and test fit as I go. A lot easier to take a little more off then it is to glue that sawdust back on the end....:smile:


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## Manuka Jock (Jun 27, 2011)

Sirnanigans said:


> Furthermore, the whole assembly is about 2 degrees off square, although this may be due to a panel fitted into grooves of each side being too tight or off square.


Dry fit everything , as you go along , and then all 4 sides together , and lightly clamp them for a test fit .
Check that the panel is perfectly accurate to the required dimensions , and angles , square in this case , and that it does not jamb in the groves of the frame pieces 
Then dry fit the whole thing and ensure that there is a fraction of a gap on all 4 sides within the groves so that it does not distort the frame out of square.
Lightly clamp again to check .
Glue up , but do not over glue . 





Clamps do not make woodwork fit , they merely hold the good , and the bad , together


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## mmwood_1 (Oct 24, 2007)

mdntrdr said:


> Full kerf cuts will provide a much more consistent angle than slicing off the end which puts uneven resistance on the blade causing it to flex. :smile:


This may happen with a thin kerf blade, but you should not be using a thin kerf in a miter saw anyway, IMO. A full 1/8" kerf blade is not going to flex when trimming 1/32" off the end of a board. Not unless it's as dull as a butter knife, or you've got it mounted backwards.


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## mdntrdr (Dec 22, 2009)

mmwood_1 said:


> This may happen with a thin kerf blade, but you should not be using a thin kerf in a miter saw anyway, IMO. A full 1/8" kerf blade is not going to flex when trimming 1/32" off the end of a board. Not unless it's as dull as a butter knife, or you've got it mounted backwards.


I once mounted a full kerf saw blade upside down. When I re mounted it right side up, I still found it to flex when trimming Hard Maple and other Hardwoods. Especially with LARGE/THICK mouldings. :smile:


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