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When I bought my Incra miter gauge on the recommendation of a forum member - one of my better recent acquisitions by the way - I read that the way to check the accuracy was to cut the parts of a hexagon and check the fit of the joints - what would be an insignificant error when checking a single part becomes more noticeable when it's an accumulation of errors. As I cut a fair number of octagons, I thought that I would use this idea to check the accuracy of my miter saw which was set and checked using an adjustable bevel and protractor. I had some lengths of 3/4" red oak and used that, with my new stop system, to cut out the parts of an octagon. When clamped, one of the joints had a very minor gap at the outside of the joint - but I can juggle the parts and get the same gap at an inside joint - so I'm assuming that this gap is the accumulated error produced by the combination of the saw setting, any loosenes in the saw bearings/mandrel and operator technique. The adjustments on the saw consist of loosening clamp bolts, moving the parts and retightening the bolts - which normally requires adjusting the parts slightly past the required setting to compensate for movement as the bolts are torqued - so it's kind of trial and error. As the error in each cut, theoretically anyway, is 1/8 of the gap seen, I think that this is as close as I'm going to get and, for what I'm doing anyway, that this is close enough.

In the past, for larger parts, I've glued octagons into half assemblies, checked the straightness across the outer edge (toe-in or toe-out) and trimmed it straight by either clamping the assembly on a panel with the short corners against the edge of the panel and running the assembly through the TS to get a straight edge or doing the same thing by clamping a straight edge to the part and trimming with a router and pilot bit. Any minor differences at the outer points of the octagon after trimming can be blended afterwards - this works for assemblies where the initial miter angle is close and the error is not extreme, basically where you have your fixture about a close as you can get it.

To finish off the story, my wife walked out into the shop after I'd finished the clamp-up, saw the assembly sitting there and decided that it would make a nice picture frame for her - one more item on the "Honey-Do" list.
 

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well done Tom..
 

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I finally decided that the particular error I had must have been a result of slight thin kerf blade deflection. I was simply unable to eliminate the error entirely, never was able to get nearly as close as you have to perfect. I finally gave up and got the Grizzly miter trimmer (based on the old Lion design). My 45s and even 90 cuts are a perfect fit using the trimmer.

I haven't tried it yet, but I ordered a thick kerf (1/8th inch) blade to do splines, but also to see if I can zero in on specific angles using the Rockler sled. If that doesn't make a perfectly flat miter cut, then I'm not certain what to do next. I know you can set odd angles on the miter trimmer, but I doubt they will be perfectly accurate given the setup method.

My table saw is relatively new and has only had light use, so I don't think the issue is runout. One other item to consider is a blade stiffener, which I think can also be used on a miter saw. I once had a 12 inch DeWalt miter saw and the blade deflection was serious and noticable unless you used proper technique and let the blade do the cut without forcing it. I sold it and bought a 10 inch Bosch, which is much more accurate.

In researching making picture frames, I found out that a big part of perfecting a frame is judicious use of filler.

Looks to me like you've worked out your situation. Pretty nice fit.
 

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Very nice Tom.
 
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Tom I bought a so called purpose built miter blade a number of years ago. It was a Delta and I spent $60 on it thinking that would be enough. I learned after that it wasn't. I tried it on some red oak at 45* and it did a horrible job. The cut was wavy. I realized that the blade was deflecting back and forth on the grain so I decided to add a set of stabilizers to it and the next cuts were near perfect.
 
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Discussion Starter #10
Tom I bought a so called purpose built miter blade a number of years ago. It was a Delta and I spent $60 on it thinking that would be enough. I learned after that it wasn't. I tried it on some red oak at 45* and it did a horrible job. The cut was wavy. I realized that the blade was deflecting back and forth on the grain so I decided to add a set of stabilizers to it and the next cuts were near perfect.
The Ryobi saw came with a 60T Ryobi blade which has given great results over the years, but needs sharpening again. As I was cutting treated wood, I decided to use a cheaper blade and am using one that I bought at, I think, Home Depot in a pack of 3 for a ridiculous price. I wasn't expecting much, but figured that it would be good enough for the planters. I have been very pleased with the results that I get with this blade, smooth cuts on the 2x4 material, both regular and PT, but amazed at the finish on the cuts on the oak test pieces - it's almost as good as the finish I get using the high-end Freud blade I use in the TS. I have a set of stabilizers that I bought years ago - and don't remember ever using, but I've never had the need to use them on the miter saw with the Ryobi blade.
 

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On the few times that I have actually used my blade stiffeners - from Lee Valley - I did notice an improvement.

I think as much as anything it's how you use the blade. Cutting carefully and slowly with a sharp blade usually works.
 
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