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angles for this table base

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Forum topic by rjherald posted 11-17-2017 03:01 AM 502 views 0 times favorited 24 replies Add to Favorites Watch
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rjherald

14 posts in 138 days


11-17-2017 03:01 AM

I could use some assistance on figuring the angles for this table base. the length is scaled down a bit from the plans but the angle thing is giving me fits. plans call for a 20 degree angle on the long cross brace. will this angle be the same if the table is shorter that the plans? i do remember trig but this has got me baffled. the short pieces that complete the X call for a 50 degree angle on the center and 20 degree on the other end. i attached a picture of what i am working on. thanks for the assistance

rjherald

-- rjherald, indianapolis


24 replies so far

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Rick_M

10598 posts in 2212 days


#1 posted 11-17-2017 03:25 AM

If you find the vertical center it should be two isosceles triangles which will simplify things, won’t even need Pythagoras.

-- http://thewoodknack.blogspot.com/

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Lazyman

1493 posts in 1220 days


#2 posted 11-17-2017 03:50 AM

Unless you scale the height and length by the same percentage, the angles will change. Is this something you have already built or is this a picture of what you are going to build? If you already built it, then you may be able to use a rafter square to figure out one of the angles using rise/foot. Since these appear to be isosceles triangles, then you can figure out all of the other angles using that one angle because the sum of the angles of a triangle = 180 degrees.

You haven’t said why you need the angle. How exact do you need it?

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

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Rick_M

10598 posts in 2212 days


#3 posted 11-17-2017 05:04 AM

I think he’s trying to place the 2 short pieces of the X brace (blue in my sketch). You don’t actually need to measure anything. The angles will be the same, you just need to find the vertical center (horizontal dotted line) to get the length. Here is a crude sketch, you have one side of the triangle, all you need to do is find the center and mirror it.

-- http://thewoodknack.blogspot.com/

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Loren

9602 posts in 3480 days


#4 posted 11-17-2017 06:47 AM

I think drawing up a dimensioned drawing
in Sketchup would be worth the time. The
program will give you the angles.

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Rick_M

10598 posts in 2212 days


#5 posted 11-17-2017 06:56 AM

I meant to say, you don’t need to calculate anything. Naturally you need to measure length.

Honestly you could probably have cut both diagonals full length and half lapped them together in less time than it would take to fuss with short pieces.

-- http://thewoodknack.blogspot.com/

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Lazyman

1493 posts in 1220 days


#6 posted 11-17-2017 03:50 PM

I guess I am being a little dense (or my inexperience is showing). I don’t see how to get the correct angles with lengths alone. If you know the angle of the bottom right joint and make that cut first, then I could see that knowing the lengths of both sides would allow you to mark a cut line but if you don’t know either one, it seems like there would be an infinite combinations of angles (on both ends) that would match the lengths? That is what I assume is his dilemma? What am I missing?

Here is a calculator that computes angles based upon known dimensions without using trig yourself. I chose SAS (side-angle-side) in my experiment using 90 degrees as the angle where the sides are the height of the trestles and the distance between them. For example, using 36” tall and 72” apart, yields an angle where the diagonal hits the trestle of 26.565°. The problem with this is that it doesn’t take into account the thickness of the board so even this yields an inexact result and might not be any help at all. I am sure that their is a carpenters shortcut for this.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

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jdh122

959 posts in 2650 days


#7 posted 11-17-2017 05:22 PM

You have two right-angle triangles. On both of them, sides a and b are the height of the side pieces and the length of the piece along the bottom. Use Pythagoras to get the length of the hypotenuse and simple trig to get the angles at the top and bottom of the side pieces. With that determined, subtract 2x the angle at the bottom of the side pieces from 180 to get the angle at which the two pieces meet in the middle.

-- Jeremy, in the Acadian forests

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Rick_M

10598 posts in 2212 days


#8 posted 11-17-2017 05:29 PM

An isosceles triangle has two sides that are the same. He already has one cross piece done so he knows the angles where it joins the table, all he needs is the length of the shorter braces and angle of the vertex. Basically just use the table itself to find what you need to know. Probably simpler to make another long diagonal, hold it in place and strike your lines to find the angle of the vertices and length of the brace. Use a bevel gauge to transfer the angle if using electric saws. It’s one of those things that takes longer to explain than do. Just try to imagine how Roy Underhill would do it, but with less bleeding.

-- http://thewoodknack.blogspot.com/

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Loren

9602 posts in 3480 days


#9 posted 11-17-2017 05:32 PM


plans call for a 20 degree angle on the long cross brace. will this angle be the same if the table is shorter that the plans?

- rjherald

No.

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Lazyman

1493 posts in 1220 days


#10 posted 11-17-2017 06:46 PM

Yeah, that’s his problem as I interpret it. He has adjusted the width and needs to figure out what all of the angles are now. Using the link I provided above, it is easy enough to compute the angle from corner to corner but that is not the exactly angle that is needed for a 2×4 because the width of the board changes it slightly and if you want tight, exact joints, I would think that you want the angles to be pretty close to exact.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

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Hammerthumb

2792 posts in 1808 days


#11 posted 11-17-2017 06:50 PM


It s one of those things that takes longer to explain than do. Just try to imagine how Roy Underhill would do it, but with less bleeding.

- Rick_M

That’s funny Rick!

-- Paul, Las Vegas

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jerryminer

798 posts in 1274 days


#12 posted 11-17-2017 06:54 PM



I could use some assistance on figuring the angles for this table base.

- rjherald

Are you still here? Many of us would be happy to figure the angles for you if you give us the dimensions:

1. Distance between leg assemblies
2. Height of table base
3. Thickness of diagonal braces

As Loren said, SketchUp makes this easy

-- Jerry, making sawdust professionally since 1976

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Lazyman

1493 posts in 1220 days


#13 posted 11-17-2017 07:53 PM

LOL, there has to be an “easy” way to calculate the angles but sometimes modeling turns out to be the easiest way.

I just realized that the original 20° & 50° angles in the OP can’t be right to begin with. If the angle in the original design where it hits the side trestle is 20° then the middle angle would have to be 40°.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

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Rick_M

10598 posts in 2212 days


#14 posted 11-17-2017 08:46 PM


I guess I am being a little dense (or my inexperience is showing). I don t see how to get the correct angles with lengths alone. – Lazyman

It’s because you’re obsessing over knowing the angle in degrees when in that number isn’t important. It doesn’t matter if it’s 20°, 22.5°, 34.7°, or something else. Numbers and measurement systems are ways of communicating information. He doesn’t need to communicate it just reproduce it once so he can get away with marking it directly on the workpiece. All he needs to do is create a second cross brace identical to the first, hold it in place and mark the angles. If he needs to transfer those angles to a miter saw or miter gauge, he can use a bevel gauge.

To elaborate a minute, different measurement systems can exist because they are only ways of describing a set of conditions that you need to communicate or maybe just remember for your own purpose. You can measure a board in inches or centimeters and give that length to someone far away. Metric/inch is just a matter of preference. Or you can create a story stick and preserve the exact measurement without ever assigning an arbitrary number to the distance. In this case, he doesn’t need the numbers, he only needs to reproduce the angle and length of the short crossbraces once. Intuitively you know all this but we become accustomed to working with measurements and it becomes habit.

edit; I hope I don’t sound like an asshole, that’s not my intention. I used to think in terms of measuring everything but have learned there are other ways of doing things.

-- http://thewoodknack.blogspot.com/

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Rick_M

10598 posts in 2212 days


#15 posted 11-17-2017 09:20 PM

The guy replied in a different thread for some reason and his new pictures don’t include the original crossbrace so if he didn’t have that, it makes all my replies moot. Sure would be nice if someone who asked a question would interact with people trying to help so they don’t waste their time. I’m out. Good luck.

-- http://thewoodknack.blogspot.com/

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