Instructables
Picture of Easy Cure for Miter Joint
This miter joint is close to fitting, but it has a slight gap on the inside of the corner. Few things diminish a good wood project like miters with visible gaps. It is a great feeling to make a picture frame or install molding and have tight, clean miters.
 
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Step 1: A quick and easy solution to miter joint gaps

Picture of A quick and easy solution to miter joint gaps
Firmly clamp the members of the joint at the angle needed, in this case a 90 degree corner. Gently run a fine tooth saw down the joint between the two pieces. A back saw or a dovetail saw works perfectly. When finished, slide the members of the joint together and fasten in place with glue or nails.

Step 2: The final fit

Picture of The final fit
This is the same joint after running a dovetail saw through it by hand. The fit is tight. The look is perfect and very professional.

If your gap is large, bring the two members of the joint together and run the saw between them again. In most cases, you will not create a fit problem for the other joints by removing a tiny bit more material at this corner.

If you are working at installing molding in a house, be aware that the corners on many walls will often be a bit greater or lesser than a perfect 90 degrees. Take a couple of pieces of 1 x 2 two or three feet long that are very straight. Fasten them together at one of their ends with a bolt and wing nut. Use this as an angle finder to check the actual angles of your corners. Measure the opening in the angle finder with a protractor. Adjust your miter saw accordingly. If there are still inaccuracies in your cuts, you will have less material to remove with a dovetail saw in order to get rid of any gapping.

This idea originally came from a high school woodworking textbook published in 1910. I believe it was called "Woodworking for Secondary Schools."
ardnon2 months ago

will use this to beautify the gap on my porch railing. thanks

Phil B (author)  ardnon2 months ago
Thank you. I wish the idea had been original with me!
Great tip! I've often ran into problems installing base boards because of the irregular corners that were over filled with drywall mud, or the factory taper used to bed the tape. I would just cut the miter less than a 45 degree so that the outside edge of the cut is "tight" and the inside edge only needs a little caulk at the top corner.

Another problem is that the drywall board doesn't go all the way to the floor, and the base board sometimes "rocks" into the gap at the bottom, making your miter really hard to stay tight. If you find the bottom of the miter "opening" up, drive a drywall screw into the bottom plate, just under the drywall, to give your base board a backing in witch to rest.
vincent75202 years ago
Holly mackerel !…

Why didn't I think of it ???…

The simplest ideas are always the more brilliant !…
Phil B (author)  vincent75202 years ago
I sure wish it were an original idea with me. I shamelessly stole it from a book. Since I have run into other people who learned it other places.
The let's you brilliantly brought it back to the surface ! …  ;)
The let's you brilliantly brought it back to the surface ! …  ;)
cb922 years ago
I need a perfect miter joint for my custom screens. I plan to get a table disk/belt sander with a nice big disk. This tool has a miter guide for the disk. Once set up perfectly at 45 degrees, it will make a perfect miter joint, dead square to the lumber. My miter saw gets close, but when I'm cutting two inch thick lumber for corner braces, they can get crooked. This will make them perfectly square and flat.
pfred22 years ago
If all of your angles add up to 360 then all of your miters will be tight, even if none of them are exactly 45 degrees. It is kind of a case of 4 wrongs sort of make a right? So there is a trick to cutting them in pairs that is a little hard to explain but when you do it they always work out.

I think I got it out of a woodworking magazine. You set the miter gauge for your 45ish cut, make two corners, put them together, then trim your ends without moving the miter gauge adjustment. Getting all of your angles correct involves flipping pieces over somewhere in the process. This is the big drawback of this technique if you are cutting moldings that aren't flat on both faces.

Another trick is to err on the side of the outside being the gap, then run a metal rod on the outside closing up the gap. Wood is somewhat flexible in this regard :)

I bought a gadget at a yard sale that sort of relies on your method Phil. I got it so cheap I just bought it for the saw it came with:

http://www.magicmitre.co.uk/
Phil B (author)  pfred22 years ago
The magic miter link is interesting. The nearest thing I have seen is a Craftsman electric miter saw with a device to push into the corner so the angle can be set on it. Then it is transferred to the two movable fences so they can be locked down. I think i understand the concept of flipping over complimentary pieces so the amount of error is reversed to make a 90 degree corner, still. (In the picture of the Craftsman miter saw the angle measuring device is stored on top of the electric saw.) Thanks.
dwildemann2 years ago
Where can I get this clamp in the US, would like a cheap source if possible.

Donw
Phil B (author)  dwildemann2 years ago
The clamp shown is adjustable. I bought it many years ago at a local hardware store. If you do not need the adjustable feature, you can get a fixed clamp for a 90 degree corner inexpensively here.
Dagless2 years ago
This is a fantastic, simple tip that I never would have thought of myself. Thanks for sharing!

From this day forth, all of my mitre joints shall be nanometre-perfect.
Phil B (author)  Dagless2 years ago
I went a long time before I learned about this way of making good miters. After I learned about it I discovered most carpenters know and use it, as well as many others who work with wood. I wish I had been the one who first came to this idea and I could say it is mine, but it comes from someone else, as I mentioned. Post some photos of your projects sometime, if you have not already.
crazyg4 years ago
what about the other 3 corners?,BEHOLD THE MAGIC EVERSHRINKING PICTURE FRAME,dont mind me i just stretch canvas over mine after sticking em together with a nail gun then wolloping with hamer to straghten up,a good quality miter chop saw would fix but im sticking with the £40 special with +/-5degree every way , a warped platter,rusty coarse blade and a really loud gearbox.
Phil B (author)  crazyg4 years ago
My statement that three corners always fit, but it is the fourth that does not is tongue in cheek humor, not a serious justification for making three corners inaccurate. But, any inaccuracy does not usually show itself until you are fitting the fourth corner. If a job had to be really precise, I would recommend fitting the two members of the corner into a jig known to be an accurate 90 degrees and remove any difference in the way I described above. The picture frame actually does not shrink appreciably. You are talking about less than the width of a pencil line in most cases.
crazyg Phil B4 years ago
ahh everybody has there inaccuracies,now if i cant fit a pencil in the gap its ok,on the subject of jigs and clampy things both of which i never had much luck with in the past,there is however an acception mhich i cannot recomend more(see photo),recent ish exhibition i did involved a machine that could paint streched canvases 9 at a time in about a minute,which ment a lot of frames
jet-angle-joining-WMH-9055.jpg
This is really handy !

Can you point me to a place i can get one of those clamps ??
I might have to make my own if i cant find one
Phil B (author)  stephenniall4 years ago
Stephen,

I checked your profile before answering and see you are in the UK.  I just checked and you can get similar clamps (or cramps) here if you cannot find them where you buy tools locally.  They cost a little over four pounds in the UK.  The clamp in the photos is adjustable for almost any angle.  I bought it at a local hardware store in the USA back in the 1960's.

Yes, this is really a handy way to make miters fit.  Several serious woodworkers I know have long been aware of it, too.

Thank you for your comment.  I hope you are able to post the Instructables you have in mind and to do it soon.  All the best to you.
Ahh thank you ! have only just seen your comment and I have the exact clamp i bought from a Pound shop for the big ammount of £1 !

Hopefully it should work but if not i shall invest in a better one
Phil B (author)  stephenniall4 years ago
I have seen clamps like these with a saw guide to fit around the saw's blade from both sides and guide it, but have not seen these for many years.  Such a clamp could serve as a miter box or guide.  I am glad you already have a clamp.  If you use a standard miter box or guide to make the initial cuts, trimming them with your clamp should give you excellent results. 

Your pound stores must be based on the same idea as our dollar stores, although you do not get much there for a dollar these days. 
Yeah same idea just different currency!
PhahQ4 years ago
I just made a couple frames and wish I knew this trick.  I found the book downloadable on google.  Here's the link:
http://books.google.com/books?id=hZkBAAAAYAAJ&dq=woodwork+for+secondary+schools&printsec=frontcover&source=bl&ots=FBVQr4N_Gk&sig=TqadpMrsipNuSYI7YxioyAOrQlM&hl=en&ei=xNdAS_WHCI30McSwqKcF&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBUQ6AEwAA#v=onepage&q=&f=false
Phil B (author)  PhahQ4 years ago
Thank you for your comment and for the link.  I need to reduce the number of books I have on shelves and this helps.  You will have opportunity to use this technique again sometime, I am sure.
seandogue4 years ago
(removed by author or community request)
Phil B (author)  seandogue4 years ago
Thank you for your comment. I wish I could say this was my idea, but as you saw in the last step, it came from a book published almost 100 years ago. Your comment reminds me of something I often heard when young: "Every carpenter makes mistakes. A good one covers them up."
hedgiehog5 years ago
thats a great idea, of course the smart thing would be to get it right the first time, but this is better than mixing saw dust and glue together lol
Phil B (author)  hedgiehog5 years ago
I agree on both counts. Thank you.