# best mitre joint for a 15ft long piece of wood?



## iron (Nov 29, 2015)

hi, 
first post. 

i'm going to be making a 3-sided box (U-shaped) out of doug fir to wrap our existing ceiling beam. i'd like to have no visible joints, of course, so i'm trying to figure out the best way to make that happen. 

the best joints i've seen that look really nice would be the lock mitre, splined mitre, lapped mitre, or just straight up mitre using this guy's glue up technique: https://www.youtube.com/watch?v=wvxLCfnw4YQ

any thoughts? i have a crappy old contractor's table saw and no real router table (though i've been thinking of making something simple).

to me, the challenge seems to lie in being able to keep the wood tight and square to the cutting surface on such a long piece of wood. alternatively, i could hand route using a guide, and maybe that would open up jointing possibilities.

thanks in advance for your help.


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## woodnthings (Jan 24, 2009)

*your choices are .....*

1.table saw set to 45 bevel, with in and outfeed supports. You will need one straight edge for this to work accurately.

2. Hand held circular saw with straight edge guide OR if the piece already has at least one straight edge, use a circ saw rip guide attached to the saw and make your 45 degree bevel.

I don't really see the use of a hand held router or the jointer for making a bevel this long. You will need 2 bevels on the bottom piece and that won't be easy to get accurately. 

I am voting for the table saw with an extended fence and a helper who knows what to do on a long rip. You may also need some instruction on a rip this log ... I donno? With no helper or no experience I would then use the circular saw as explained above. This issue will come on the second rip with only a 45 degree bevel to register your saw guide against. If it were me, I would make a "add" on guide with a 45 degree "pocket" to register along the bevel already created. The second bevel will only be as good as the first one. duh.

This won't be easy, even for an experienced carpenter or woodworker. Do you know any? :blink:


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## iron (Nov 29, 2015)

i know a few okay carpenters, but not one that i would trust more than myself. while carpentry/woodworking are not my day job, i can handle most tasks even with crap tools. 

i'm thinking the wood will already have some pretty straight edges or else i'm not buying it!

that's why i think a router with a fence would work well enough. but, i hear you...


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## Toolman50 (Mar 22, 2015)

On a long piece like you need, a plain mitre will be your best choice. To simplify glue-up, put a board in the open end to hold everything square. Finish before installing.


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## MT Stringer (Jul 21, 2009)

Are you going to paint it, stain it or what?


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## iron (Nov 29, 2015)

i'm planning to stain it and then lift into place and use PL adhesive to secure it to the existing beam.


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## woodnthings (Jan 24, 2009)

*here's the difference....*



iron said:


> i know a few okay carpenters, but not one that i would trust more than myself. while carpentry/woodworking are not my day job, i can handle most tasks even with crap tools.
> 
> i'm thinking the wood will already have some pretty straight edges or else i'm not buying it!
> 
> that's why i think a router with a fence would work well enough. but, i hear you...



A router and bit is a "vertical line" source removal operation... kinda. A circular saw blade is a "plane" rather than a line. Any variations in the line will show much more than in a plane. You can have slight variations in the blade to guide registration without affecting the result too much. The other issue is you will need a fairly large 45 degree miter bit to miter a 45 angle on 3/4" thick stock. Your straight edge and your control had better be perfect or you will have small humps and bumps.

:yes:


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## philr (Mar 20, 2017)

*Going a step further*

If I understand your question correctly, I have had a similar experience on covering a long beam -- 32 feet.

I used western red cedar for this covering because of relatively straight grain.

Regular 45 degree miter cuts were used, matching grain as best as possible.

Additionally I use splines to connect the boards at the miter joints. 

If you absolutely want to keep the boards together and don't trust biscuits or splines in the miter joints, then use a router to make bowties (at least 2 per joint).


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## gj13us (Apr 19, 2016)

I'd cut a looong bevel as accurately as possible and then rely on the old boat builders' proverb: Putty and paint make a boat what she ain't.


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## Steve Neul (Sep 2, 2011)

If you leave a little air space between the new and old wood a simple 45 should be fine. Just glue and nail it and it should stay forever. The reason for the air space is so if the new wood shrinks it won't be putting stress on the joint.


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## philr (Mar 20, 2017)

Steve Neul said:


> If you leave a little air space between the new and old wood a simple 45 should be fine. Just glue and nail it and it should stay forever. The reason for the air space is so if the new wood shrinks it won't be putting stress on the joint.


You are absolutely right on the air space. If you go this route use a non hardening putty such as Minwax putty. That way when the board shrinks (usually in winter) the space should not show.

Here is a thought you may want to try. Instead of the intended board, use plywood and veneer. You can leave space between the plywood joints and veneer over them. With the veneer, you can use several thinner joints and let the joints fall between the plywood joints. If you don't want to use plywood, a better choice would be MDF where you typically will not have to worry about shrinkage or expansion. Then you don't need any air spacing.

Many ways to skin this cow but in the end, you need to make a decision as to which best suits your skills and project involvement.


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## Toolman50 (Mar 22, 2015)

Iron, 
Izzy was working on a piece only 2’ long. I think it would be quite different trying to work with a long piece to stretch across a room. Instead of plywood I would use solid wood. Cut the 45 degree bevel on the table saw and use glue and finish nails to put it all together. I would use an 18 gauge brad gun with a 1 1/2” brad.


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## woodnthings (Jan 24, 2009)

*Just so you guys know ......*

This thread is 2 years old. Not that good advice is not exactly that, but anything directed to the OP is going to fall on deaf ears. :|


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