How do I make this angled joint? (sketch inside) - Woodworking Talk - Woodworkers Forum
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post #1 of 15 Old 03-23-2018, 06:09 PM Thread Starter
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Question How do I make this angled joint? (sketch inside)

Hi! I'm trying to figure out how to join two planks together at an angle. Basically it's a piece of a jig. It's somewhat hard to describe the end result so I've drawn a sketch. The joined part will bear stress from a moderate load, let's say about 5-7 lbs of weight. So what kind of joint should it be? I guess either a butt joint of some sort or a finger joint. Thanks for help!
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post #2 of 15 Old 03-23-2018, 06:17 PM
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No more weight than that if you just doweled the joint it should work fine. More than likely the chairs in your house are joined together with dowels and they hold well.
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post #3 of 15 Old 03-23-2018, 09:10 PM
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It's a scarf joint, same as used on a guitar neck. 7 lbs should be no problem.
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post #4 of 15 Old 03-23-2018, 09:17 PM
where's my table saw?
 
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don't use a butt joint

Gluing end grain to end grain is the weakest application for glue. Long grain to long grain is the strongest. You finger joint would have more long grain surface area, and therefore be the strongest. Granted they are a pain to make, but even 4 fingers in corresponding slot would be quite strong.

Murphy's Law says... If it can break (from an overload) it will break. Plan ahead to prevent joint failure. No one likes a joint that fails.
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The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #5 of 15 Old 03-23-2018, 09:39 PM
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In making a jig like shown, it would be best to make it out of plywood and cut the knotch or angle from the whole piece. This would be stronger and much faster than trying to piece together solid wood.
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If you don't have time to do it right the first time, when will you have time to do it over?
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post #6 of 15 Old 03-24-2018, 05:33 AM
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A butt joint will fail in this application. From the sketches you've got drawn out, the finger joint would be my top pick. I would also consider dowels as Steve mentioned or mortise and tenon.
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post #7 of 15 Old 03-24-2018, 01:35 PM
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A scarf joint will not fail.
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post #8 of 15 Old 03-24-2018, 03:46 PM
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look at this diagram and the load directions

It's not as simple as you might think because the loads are lateral vs vertical, putting the joint in a shear condition. I have enough guitars to know how strong the neck joints are, but they also use a tension rod to reduce the forces on the joint.

To me, it's still about grain direction and long grain is strongest. Admittedly, 7 lbs isn't much load so it may just be a trial and error, then test to see what does work. Mathias Wandel has tested many types of glue joints:


The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

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post #9 of 15 Old 03-24-2018, 05:37 PM
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Quote:
Originally Posted by ivan.lt View Post
Hi! I'm trying to figure out how to join two planks together at an angle. Basically it's a piece of a jig. It's somewhat hard to describe the end result so I've drawn a sketch. The joined part will bear stress from a moderate load, let's say about 5-7 lbs of weight. So what kind of joint should it be? I guess either a butt joint of some sort or a finger joint. Thanks for help!
IMO...Solid wood is most defiantly the way to go.

I would stay away from plywood for this joint union. The cross graining of ply is too weak for "scarf joints" which this would be...since a "butt joint" is the most simplistic "scarf joint" form. If you go with something thicker than 1", like even 1.5" it would become much stronger, as you will have more "meat" to work with giving you a much stronger joint.

There are a number of scarf joints that would work here well. The "Finger splice" (already offered) would render a very strong joint, yet can be a bit of a challenge to cut in such a 150 angle configuration. Some of the simpler "Splayed Scarf" joint would also work as well, and a "Pegged Scarf" with doweling (~3/8" dowel?) may well be more than you need if well glued to meet your 7 lb strength range of load capacity.

This is a really easy scarf to cut actually, and I have used it in everything from "thread grafting" in Penzai, to "down and dirty" fast splicing for just such applications as yours.

First, just create the geometry you want. Glue it up well and let set overnight!! Then simply drill by eye so that you get into the foundation piece at least 2" in. The angle of this drilled hole is not critical...straight "by eye" is level enough. Then simply glue and drive in your dowel/trunnel let set again overnight and trim off the dowel.

Good Luck,

j



P.S. The next simplest is a "scissor" or "strokes bill" scarf...If the dowel method is not appealing...We use these all the time as well for full structural unions in rafters and major beams...

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post #10 of 15 Old 03-24-2018, 06:26 PM
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Quote:
Originally Posted by woodnthings View Post
It's not as simple as you might think because the loads are lateral vs vertical, putting the joint in a shear condition. I have enough guitars to know how strong the neck joints are, but they also use a tension rod to reduce the forces on the joint.

To me, it's still about grain direction and long grain is strongest. Admittedly, 7 lbs isn't much load so it may just be a trial and error, then test to see what does work. Mathias Wandel has tested many types of glue joints:

Can you squeeze all the glue out of a joint? - YouTube
If I post an answer it's based on experience, not on a blog or watching in a video. Wandel makes money posting videos and glue joint videos get tons of views but don't mistake that as experience. A scarf joint will be fine for a 5 to 7 lb load.

Also, tension rods help prevent and correct any bowing from string tension. They are unnecessary for scarf joints.
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post #11 of 15 Old 03-25-2018, 03:55 PM Thread Starter
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Wow, thanks for such a discussion! I really appreciate all the advice. I'm not sure how to employ the "scissors scarf joint" here (isn't it for straight joins?) so I'll go with the simple scarf. Thanks again.
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post #12 of 15 Old 03-25-2018, 07:59 PM
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Originally Posted by ivan.lt View Post
Wow, thanks for such a discussion! I really appreciate all the advice. I'm not sure how to employ the "scissors scarf joint" here (isn't it for straight joins?) so I'll go with the simple scarf. Thanks again.
Hi Ivan,

I think with a good glue joint just the simple lap could fulfill your very small load requirements. If not, add the dowels and the strength will go up considerably...plus...there easy to do. If you do want to explore the "scissor scarf" I think you could pull it off. Its not much different than the "finger splice" suggested...just less complicated to layout and cut...

Good Luck,

j
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post #13 of 15 Old 03-26-2018, 06:17 AM
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Quote:
Originally Posted by Toolman50 View Post
In making a jig like shown, it would be best to make it out of plywood and cut the knotch or angle from the whole piece. This would be stronger and much faster than trying to piece together solid wood.
Agree.

George
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post #14 of 15 Old 03-26-2018, 08:54 AM
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Originally Posted by GeorgeC View Post
Agree.

George
Hello George,

I agree that this could be made of plywood...and...with the low loads it may even endure.

Quote:
Originally Posted by Toolman50 View Post
...it would be best to make it out of plywood...This would be stronger and much faster than trying to piece together solid wood.
I respectfully disagree...Plywood would not be best, or easier, nor is it every going to be stronger in this application...

That's not an opinion...that's empirical fact, and engineering tested reality for virtually any wood execution of this type.

Plywood is a convenience to manufacturing and facilitation as a sheet good material.

Plywood does not meet most of the structural load parameters of solid wood, natural or engineer timber alike...when employed beyond "sheet good" or "diaphragm" applications in loading...

j

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post #15 of 15 Old 03-26-2018, 05:49 PM
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Solid wood is much stronger than plywood. Think of how a tree grows and the forces it resists: roughly half the plies in plywood are going the opposite direction; which makes it dimensionally stable but weaker than solid wood.
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