Need help with calculation - Woodworking Talk - Woodworkers Forum
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post #1 of 20 Old 01-09-2019, 05:01 PM Thread Starter
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Need help with calculation

Hey folks, long time listener first time caller. I’m more of a craftsman and mission style guy working with easy cuts and joints. But I have a range hood I’m trying to build a frame for. I’ve included a pic of what the desired outcome is, I’ve also included pics of 2 frames I’ve made with the measurements. I’m having trouble calculating the compound cut I need for the 4 corner posts. The height between the top of the bottom frame to the bottom of the top frame is to be 24”.
In the last pic attached I have red circles around the compound cut I’m looking to make. I used a “rectangular frustum” calculator at www.jansson.us but I’m having a hard time making heads or tails of it. Any help would be beneficial.Name:  60F78E16-0F5E-4249-83DD-367DB9C64859_1547071204438.jpeg
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post #2 of 20 Old 01-09-2019, 05:05 PM Thread Starter
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post #3 of 20 Old 01-09-2019, 05:19 PM
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Get a large sheet of graph paper. ( I keep a pad of desk pad sized graph paper around for just this purpose.)



Lay out your design to scale in 2d. You can then use a protractor to measure any and all angles. The angles can be calculated, but first you have to know the angles involved.


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post #4 of 20 Old 01-09-2019, 05:58 PM
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I have bent my share of sheet metal ..

I made an entire truck body from sheet metal, see My Photos. Here's my advice. Make a full size model/mockup out of cardboard, or poster board, or illustration board. A hot glue gun or tape will work to hold it together.

Make two identical pieces of the sides and the front/back.
Obviously the lengths of the sides and the front/back will be the same. The bottom dimensions will be different of course. If you know those dimensions, start there. The height will determine the lengths of the sides. If you know the dimensions of the transition piece that will be another good thing.

You can also start with a full size drawing, but 2 dimensions is difficult to deal with and won't give you the angles in all views. You would need a side view and front view and possibly a top view for orthographic projection. I'd be lost using trig to make this, but I could make a easily make a mockup.

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 20 Old 01-09-2019, 08:53 PM
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Hi Heinlein,

How computer savvy are you? Do you know you can download a free version of Sketchup (CAD program.)

I love physical modeling...There fun to do, and great to think about the project while your doing them...They are also a lot of work by comparison.

A CAD model lets you do virtually the same thing and changes are fast if you want to see something different...

I love the Mission and Craftsman style. Some of the timber frames I have designed and worked on are in these styles with the heavy copper, iron strapping with draw wedges...!!!...That project of yours is going to be awesome!!!

With this and the advise thus far you should be on your way...Unless (??) you are just looking for the "trig" formulas for doing the math...That is pretty fundamental too...

Hope some of this helps...?

j
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post #6 of 20 Old 01-09-2019, 09:06 PM
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post #7 of 20 Old 01-09-2019, 10:17 PM Thread Starter
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I Will definitely look into sketchup. I wish the other pictures would download, but only one is showing up. The other ones show the frames I made With the measurements. I’m assuming from these measurements you could make the correct calculations But like I said it’s hard to Explain what I’m asking with words when pictures can tell a bigger story
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post #8 of 20 Old 01-09-2019, 10:19 PM Thread Starter
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post #9 of 20 Old 01-09-2019, 10:42 PM
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Bit more time...???

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Originally Posted by Heinlein0311 View Post
...But like I said it’s hard to Explain what I’m asking with words when pictures can tell a bigger story...
Agreed!!!

That's why I suggested the sketchup CAD...

I can't promise the future...but if you download the program and give me to the weekend (??)...it should not take me much more than a "cup of coffee" to make a quick CAD model of this for you...Send me an email (mine is in my profile) and I can link you to a file with the model in it if nothing comes up between now and Sunday to blow my weekend away...From there you can see angles and/or "reverse engineer" the math needed to make the cuts and keep a record for the future...
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post #10 of 20 Old 01-09-2019, 11:02 PM Thread Starter
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Thanks Jay, that would be great! I will download sketchup when I get done working and I will shoot you an email with some of the pictures that wouldn’t upload on the forum. Thanks again!
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post #11 of 20 Old 01-09-2019, 11:35 PM
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you're almost there with this ...

Just mark some centerlines on both frames then elevate the center one until it's the height you want. Then simply measure from the corner on the upper frame to the corner on the lower frame and that's the length of the sides of your panel.... done. You have all the other horizontal dimensions as labeled.

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 #12 of 20 Old 01-10-2019, 09:35 AM
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My take on the need (hopefully my sketch is attached):

Look at each side (2) projections are identical, assume from plan view the centerlines are same. Both from a two-dimensional view are frustums and have equal heights of 24 inches.
First: A base of 32.5" and top of 16.5"
Second A base of 22.5" and top of 12"

Reduce to a right triangle on each side as shown ... then solve for the angle.

Does this not provide the direction you need?
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post #13 of 20 Old 01-10-2019, 02:18 PM Thread Starter
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Yes I had a guy from another forum suggest the same thing. Using that formula I come up with angles of roughly 71 and 79°. If I subtract those from 90 I get 19 and 13°. Does this mean that I tilt my blade 13° And rotate it 19°?
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post #14 of 20 Old 01-11-2019, 08:41 AM
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I think you would tilt, but from a "plan-view" you remain at 90-deg as each of the 4-sides tilt inward toward your center. Rotation, as I visualize, would be a wrong move. A mockup of cardboard saves wood, at this point.
-don
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post #15 of 20 Old 01-11-2019, 02:40 PM
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Quote:
Originally Posted by Heinlein0311 View Post
Yes I had a guy from another forum suggest the same thing. Using that formula I come up with angles of roughly 71 and 79°. If I subtract those from 90 I get 19 and 13°. Does this mean that I tilt my blade 13° And rotate it 19°?
Haven't verified the numbers but you are heading in the right direction, it has to be a compound angled cut same as making a tapered planter box.

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post #16 of 20 Old 01-11-2019, 03:48 PM
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Woeking in 3 dimensions ....

Quote:
Originally Posted by woodnthings View Post
Just mark some centerlines on both frames then elevate the center one until it's the height you want. Then simply measure from the corner on the upper frame to the corner on the lower frame and that's the length of the sides of your panel.... done. You have all the other horizontal dimensions as labeled.

You can calculate the angles in 2 dimensions, front and side view BUT the faces or planes are rotated inwards towards the smaller rectangle at the top in 3 dimensions, therefore the lengths of the faces are longer.
By raising the center frame to the height it is needed using a milk crate or whatever, it would be simple to measure those lengths. Then you can cut cardboard shapes to test it out. Then you can use a digital protractor to measure the angles to assure symmetry.

Of course you can do all this in a 3D modeling program, but for stone age woodworkers, like me this is my approach. Remember, the guys who built the pyramids didn't have computers, at least as far as we know?

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)

Last edited by woodnthings; 01-11-2019 at 05:35 PM.
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post #17 of 20 Old 01-13-2019, 09:02 PM
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These are just a few screen shots of the CAD I sent to Josh this evening and thought some here would like to see them?

They illustrate some of the angles within the design he offered up for assistance with. ...





This is a cross section of the bottom of the "hip leg" joining the bottom to the top...
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Last edited by 35015; 01-13-2019 at 09:10 PM.
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post #18 of 20 Old 01-14-2019, 07:28 AM Thread Starter
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Thanks again Jay, awesome work!
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post #19 of 20 Old 01-29-2019, 08:05 AM Thread Starter
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Thanks for all the help, I was able to get the angles with everybody’s help and input. Here is the final product
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post #20 of 20 Old 01-29-2019, 08:02 PM
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When I grow up I wanna have skills like y'all Josh and Jay! Great execution of the plan Josh!
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