Perfect Miters?

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I'm gluing up some door casings prior to installation and while some miters are perfect, some show small gaps on the visible side of the thicker portion of the profile. I'm curious as to the cause; seems like my miter saw should be cutting them perfectly. It's almost as if the ends being joined have edges slightly rounded over. Any thoughts?
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Hi Greg,
I'm not sure that I can picture exactly what you are describing. But, I still might be able to help.
Why do you think that your miter saw should be cutting miters perfectly? Have you gone through some extensive alignment and precision adjustment procedure or are you just trusting the scales on the machine?
Here's an article that you may find helpful. It talks about the accuracy needed to produce tight miter joints. It also talks about different methods for achieving that accuracy.
http://www.ts-aligner.com/accuratemiters.htm
Even when machines are perfectly aligned and adjusted, it's still no guarantee that the results will be accurate. Skill is required, even when using machines. Proper technique and fixturing can make all the difference. Here is another article that you might find helpful. It talks about the sort of problems that you can encounter on a table saw but it shouldn't be a problem to relate the same sort of issues to the miter saw:
http://www.ts-aligner.com/external.htm
Let me know if you have any questions or need more help.
Thanks, Ed Bennett snipped-for-privacy@ts-aligner.com
http://www.ts-aligner.com Home of the TS-Aligner
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Thanks Ed.
<<Why do you think that your miter saw should be cutting miters perfectly?

Well, I guess "consistently" is a better word. Some of them are perfect; why aren't they all?
<<Have you gone through some extensive alignment and precision adjustment procedure or are you just trusting the scales on the machine?>>
I have measured the angles and they seem dead on; however, I'm not concerned about that. My doors jambs probably aren't perfectly square, so the miter angles will take some adjusting anyway.
It just seems to me that if you slice two pieces of wood with the same blade, the cuts ought to join with no gap. Why not?
<<Here's an article that you may find helpful. It talks about the accuracy needed to produce tight miter joints. It also talks about different methods for achieving that accuracy.>>
I'll take a look, thanks.
<<Skill is required, even when using machines. >>
Dang!
<<Here is another article that you might find helpful. It talks about the sort of problems that you can encounter on a table saw but it shouldn't be a problem to relate the same sort of issues to the miter saw:>>
I've review that, too.
Thank you sir!
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Ed Bennett wrote:
<<Proper technique>>
Ed, I think your website gave me the clue I needed. It mentioned possible blade warping on thin cuts. I noticed the edge of my cut that was rounded was where the blade withdrew *last* from the wood, where the damping of the blade would have been least with the thin cut.
When I stopped the blade after the plunge and withdrew it while stopped, my cuts were clean again.
Thanks!
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Assuming that the saw is adjusted properly it could be that the wood is slipping slightly as you cut... some sandpaper glued to the fence and/or using the clamp that goes with the saw may solve the problem.
John
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John Grossbohlin wrote:
<<the wood is slipping slightly as you cut... some sandpaper glued to the fence and/or using the clamp that goes with the saw may solve the problem.>>
I've had a hard time figuring out how to clamp stuff with this saw (DeWalt). It didn't come with a clamp and the fence isn't the ideal clamping surface. And these flat pieces of trim don't offer much to grip either. Still, I don't think it's moving. Looks like I had poor technique.
Thank you
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don't forget the most over looked thing....the pieces most be the exact same length ie upper/lower..right side/left side

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Lee wrote:
<<don't forget the most over looked thing....the pieces most be the exact same length ie upper/lower..right side/left side >>
You mean the diagonals? Yes, I noticed that because I had to adjust the miter angle of the leg, the diagonal is slightly less than the header, so the corner juts out a bit. I figure I can shave that off once the glue dries.
Thanks
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No I mean the length of the pieces you are joining together. Think of a pic frame if the right vertical is slightly longer(or shorter) it throws off all the angles. If all is equal and your saw is tuned properly there shouldn't be a gap in any of the miter joints.....but then again it's not always a perfect world that's why they make wood filler lol

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Lee wrote:
<< No I mean the length of the pieces you are joining together. Think of a pic frame if the right vertical is slightly longer(or shorter) it throws off all the angles. >>
Gotcha. Not an issue yet. I can only do one miter at a time, so there's just a leg and header.
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RE: Subject
The easiiest way to get perfect 45 miters is to build a sled using a factory corner from a sheet of 3/4 ply.
See Fred Bingham's book, "Boat Joinery & Cabinet making", for details.
Lew
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I'd gage the corner angle before I'd trust the plywood.
--
NuWave Dave in Houston



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People put way too much stock in books and magazine articles. Just because it's in print doesn't mean it's right.
You really shouldn't trust that the unskilled minimum wage sawdust jocky at the plywood factory managed to put an accurate corner on one of the 10,000 pieces of plywood he cut that day. If the corners of plywood have any spec at all, it's probably on the order of +/- 1/8".
I'll have to add this one to the "Myths and Ledgends" section of the web site.
Ed Bennett snipped-for-privacy@ts-aligner.com
http://www.ts-aligner.com Home of the TS-Aligner
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"Ed Bennett" wrote:

MIne works quite well.

one
I don't; however, I do trust the automation used to trim 4x8 sheets to size since it is not a manual labor job these days.
Lew
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As many subfloors as I've run, I can't say I've ever seen gaps at end joints where the plywood was off square.
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I haven't done so many subfloors (two or three) but I've noticed the gaps in each one. Not that it matters, it's just a subfloor. I thought we were talking about woodworking with slightly higher standards.
Ed Bennett snipped-for-privacy@ts-aligner.com
http://www.ts-aligner.com Home of the TS-Aligner
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Your sled? I'm sure it suits your needs.

That would be another topic for the "Myths and Folklore" page. Just because its automated doesn't mean its perfect or accurate. I run a shop full of automated machines and I assure you that accurate results still depend on the skill and knowledge of the people using them. I've hired people to run the exact same programs on the exact same machines with the exact same fixturing and they still manage to screw up the parts. Heck, I've even screwed up a bunch of parts over the years. The machines used to trim plywood to size may be automated but "perfect" accuracy is not automatic.
Ed Bennett snipped-for-privacy@ts-aligner.com
http://www.ts-aligner.com Home of the TS-Aligner
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says...

Hmmm. We reclad our shearing shed with construction grade ply a couple of years back. Some of those sheets were *considerably* off square. Nearly 1/4", some of them. Didn't matter because we put battens over the joins, but .... ;-)
As an aside: I laid parquet flooring in my old folks' house a few years ago, and found that even a lot of the 12" sq. prefabricated tiles were out of square! Had to recut many on the RAS to eliminate gaps. Yeesh. Not sure to this day how I managed to eliminate pattern creep between the tiles over several runs, I guess it must've evened out.
-P.
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Ed Bennett wrote:

The factory edges are not cut by some guy with a circular saw.
The whole process is automated using something called a "DD Saw". Among other reasons, plywood right out of the press is still pretty hot.
Chris
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Yes, yes, yes, I know. I appreciate all of the comments about guys with skill saws vs the "perfection" of automation, etc.
I've seen how plywood is made. The sheets are gang ripped to width and then cross cut while in motion. The feed rate of the cross cut saw (traveling at an angle) in conjunction with the feed rate of the board determines the squareness of the cut. The factory is staffed with unskilled workers. The person overseeing the crosscutting may never have even touched a circular saw in their entire life. I'm quite sure that they care more about length and width than they do about squareness. If they track these measurements as process parameters (a big IF), then these are likely to be checked on a random audit basis with simple instruments (i.e. tape measure) by more of the same unskilled minimum wage workers.
A long time ago I heard this same folklore and tried to apply it in my woodworking. "Use a factory edge as a straight edge, use a factory corner as a square, etc." I probably read it in a magazine article written by some journalist who (while pretending to be an expert) had never touched a circular saw in his life. That's when I learned not to trust the factory edges (even on furniture grade plywood). They're good enough for construction purposes (sheathing, flooring, roofing, etc.). And, there are probably a lot of people who will never notice a problem in their own work. But, they are far from "perfect" (what Lew originally said) and don't suit my needs. I also learned not to take technical adivce from journalists - a lesson that has been reinforced countless times over the years.
The original poster has related that his problem was being caused by deflection of the blade when making thin cuts. I could be wrong, but I suspect that the demands of his work exceed the accuracy of factory cut edges on plywood. I'm glad he was able to find his solution in one of my articles.
Ed Bennett snipped-for-privacy@ts-aligner.com
http://www.ts-aligner.com Home of the TS-Aligner
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