How to calculate an angle - Woodworking Talk - Woodworkers Forum
Old 11-06-2016, 04:46 PM Thread Starter
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How to calculate an angle

I saw a video one time on how you can "lenthen" a board by cutting it diagonally and "sliding" the two pieces on that diagonal in order to lengthen it. I have a scrap piece of plywood I'm trying to use and it is 3" too wide but it is 2" too short for my use. I want to try that angle cutting but I'm not sure what the angle should be to attain the desired length. Is it 45 degrees or more or less? Can anybody give me any help? Thanks in advance,

Ray
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Old 11-06-2016, 05:02 PM
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If you scroll down to the bottom of this page there os an example, basically cut the board on a diagonal corner to corner.

It actually works better with solid wood than plywood.

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Old 11-06-2016, 05:26 PM Thread Starter
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Thanks Frank, I have been pondering the situation and I think that if it's cut on a 45, the movement both up and to the side should be equal. So, if I fiddle around with that angle, I should be able to move it 2" up and 3" to the side and solve my problem. Or just do the 45 and work it out accordingly.

Thanks again,

Ray
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Old 11-07-2016, 12:54 AM
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Um ... how strong does it need to be when you're finished?
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Old 11-07-2016, 11:50 AM
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With this type of joint, usually called a skarf joint which is used a lot in boatbuilding, 45 degrees is almost never used, not enough glue line. You want the angle that is most parallel to length of the board, within reason but at least 4 to 1. So, for 3/4"x 3/4" material the joint would be at least 3" long but more likely 4" to 5". For a 2" wide board I would go for 6", and so forth.

An easy way to make this perfectly in these smaller sizes is to use a chop saw. First, set the saw at 90 degrees, clamp and cut something to use as a guide. then rotate the blade to the correct angle which will be somewhere near 20 degrees. Put both sticks, clamped together, on the saw table against the 90 degree board you clamped and cut first such that the sticks are projecting straight out from the saw, and cut both at the same time.

For wider boards I would use a portable saw and a guide, but the principle is the same, cut both at the same time with the boards side-by-side. Then, no matter what the angle is, they will match.

There is no reduction in width at all.

Last edited by Brian(J); 11-07-2016 at 11:53 AM.
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Old 11-07-2016, 12:53 PM
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Quote:
Originally Posted by Brian(J) View Post
With this type of joint, usually called a skarf joint which is used a lot in boatbuilding, 45 degrees is almost never used, not enough glue line. You want the angle that is most parallel to length of the board, within reason but at least 4 to 1. So, for 3/4"x 3/4" material the joint would be at least 3" long but more likely 4" to 5". For a 2" wide board I would go for 6", and so forth.

An easy way to make this perfectly in these smaller sizes is to use a chop saw. First, set the saw at 90 degrees, clamp and cut something to use as a guide. then rotate the blade to the correct angle which will be somewhere near 20 degrees. Put both sticks, clamped together, on the saw table against the 90 degree board you clamped and cut first such that the sticks are projecting straight out from the saw, and cut both at the same time.

For wider boards I would use a portable saw and a guide, but the principle is the same, cut both at the same time with the boards side-by-side. Then, no matter what the angle is, they will match.

There is no reduction in width at all.
I think you missed to point of the question.

Wise men speak because they have something to say; fools because they have to say something -Plato

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Old 11-07-2016, 01:29 PM
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Yep...

Quote:
Originally Posted by FrankC View Post
I think you missed to point of the question.

He is not joining 2 boards together. He is cutting one board at a diagonal across it's face, then sliding them along the cut to extend the length.

My issue is we don't know the overall size of the "scrap". Just that it's 3" too wide and 2" too short. The overall size would help determine which approach to use, but neither involves a cut at 45 degrees. Like this, but ignore the dimensions:

You may run out of sliding length before you reach the desired length....?

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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Old 11-07-2016, 03:04 PM Thread Starter
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Thanks to all of you for taking the time to help me. I have figured it out by looking at the link below. My dilemma came about from building a shelf under my workbench to store all of my small portable power tools. I was trying to use up some of my scrap wood and I needed a piece of plywood for the shelf but the only one I had was 2 inches too short and 3 inches too wide. I needed 87 inches by 21 inches and the board was 85 inches by 24 inches. I knew there was a way to "stretch" a piece of wood but I wasn't sure of the angle but after reading the link below and then posted by one of you, it became clear in my foggy old mind. So, I will cut it long ways from one corner to the opposite corner and it will be fine. And, I will have cleared my shop of a lot of scrap wood. Thanks to all of you for helping me. I will become more active here on this forum.

Ray

Last edited by smallshopguy; 11-07-2016 at 03:31 PM. Reason: to correct mistakes.
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Old 11-08-2016, 07:23 AM
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Edge glueing plywood without a spline or many dowles will not work.

Did you not notice that the new "board is not only longer, but also wider?

George
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Old 11-08-2016, 08:59 AM
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Originally Posted by GeorgeC View Post
... new "board is not only longer, but also wider?

George
One or the other, but not both. Once the edges are squared and trimmed to make one continuous line, that dimension will be less than original.
This is an extreme example, but it's accurate.
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Old 11-08-2016, 02:56 PM Thread Starter
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Thanks GeorgeC, I am aware of what you speak. Fortunately, the two pieces will just be laying next to each other and not have to be "joined". I made a 2x2 frame underneath and after cutting the plywood and trimming the sides, I layed the pieces on the frame and shot 16 gauge finish nails through them to the frame. No need to join then. I just wanted to use up a scrap that I had laying around and this made it possible. Thanks for your input though. This is what makes these forums so valuable. Lots of different ideas.

R
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Old 11-08-2016, 02:58 PM Thread Starter
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After doing my angle cut, you are right MikeChell. I gained the length I needed but also a bit more width. All I had to do was scribe a line along my front frame rail and then cut the excess width off. It worked out great. No need for any fancy joinery since the two pieces will just be laying next to each other and nailed in place.

R
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