LumberJocks

Need some math help

  • Advertise with us

« back to Woodworking Skill Share forum

Forum topic by papadan posted 05-19-2017 04:32 AM 1107 views 0 times favorited 21 replies Add to Favorites Watch
View papadan's profile

papadan

3584 posts in 3517 days


05-19-2017 04:32 AM

Topic tags/keywords: question resource walnut miter saw tablesaw sander joining modern

Dropped out of school and don’t remember what I did learn. I need to build one of these lamps and can’t figure out all the angles I need to make it. I know a pentagon is 36* cuts but keep confusing myself on the 3D part of this. Any help will be appreciated.


21 replies so far

View Rich's profile

Rich

3668 posts in 738 days


#1 posted 05-19-2017 05:00 AM

42

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

View papadan's profile

papadan

3584 posts in 3517 days


#2 posted 05-19-2017 05:01 AM

42 for which part?

View Rich's profile

Rich

3668 posts in 738 days


#3 posted 05-19-2017 05:15 AM

I was being a jerk, papadan. 42 is the The Answer to the Ultimate Question of Life, The Universe, and Everything, from the The Hitchhiker’s Guide to the Galaxy.

Seriously though, that’s a dodecahedron. A prism with 12 sides. The one you show is a regular dodecahedron. I did a web search for “angles to cut a regular dodecahedron” and it returned a number of results that might assist you. Sorry, that’s about all my brain can handle :)

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

View papadan's profile

papadan

3584 posts in 3517 days


#4 posted 05-19-2017 05:52 AM

Ok Rich. I followed a couple links supposed to show how to cut them but they just want to sell them.

View Woodknack's profile

Woodknack

12369 posts in 2529 days


#5 posted 05-19-2017 06:01 AM

I don’t know the answer but this might put you on the right track
http://woodgears.ca/geodesic/

-- Rick M, http://thewoodknack.blogspot.com/

View MrUnix's profile

MrUnix

6949 posts in 2347 days


#6 posted 05-19-2017 06:04 AM

Or this thread: dodecahedron

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

View Rich's profile

Rich

3668 posts in 738 days


#7 posted 05-19-2017 06:04 AM

I feel your pain buddy. It’s a complicated compound angle, but once you nail it, you can cut them all day. I could do the math, but then I’d have to charge you for the answer :)

This looks helpful: https://www.youtube.com/watch?v=YxPVGe0PR5g

There are probably more, but a lot of them look like a solid surface solution, not a frame.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

View AlaskaGuy's profile (online now)

AlaskaGuy

4521 posts in 2458 days


#8 posted 05-19-2017 06:40 AM

If you don’t have all the answers don’t hang that above a boat cradle. :)

I just had to.

-- Alaskan's for Global warming!

View papadan's profile

papadan

3584 posts in 3517 days


#9 posted 05-19-2017 07:11 AM

Rich, that video gave me everything I needed. Thank you to all who replied.

View mudflap4869's profile

mudflap4869

1849 posts in 1608 days


#10 posted 05-19-2017 10:02 AM

They tried to teach me how to use a slide rule. I quit school and got a GED. I finally learned enough algebra to get my degree in Creative Writing then promptly forgot all I had learned about math.

-- Still trying to master kindling making

View JCamp's profile

JCamp

819 posts in 699 days


#11 posted 05-19-2017 10:39 AM

I think the most interesting thing is that you were awake from midnight to 3am for this thread papadan…

-- Whatsoever thy hand findeth to do, do it with all thy might

View EricTwice's profile

EricTwice

237 posts in 682 days


#12 posted 05-19-2017 11:08 AM

The interior angles of a pentagon are 108 degrees and they are mitered at 54 degrees.

A problem arises with the the angle of the faces to each other I believe this is called the dihedral angle and is 116.56 degrees. (check me on this) It is critical. You don’t have to miter it, you just have to have it in the exact middle at the top of your stock for your frames to line up properly. You will miter stock upside down. This way when you cut the miters right and left, the space for the third side automatically happens.

(this keeps you from having to figure out the compound miter.)

You should be able set up a jig and get all your pieces right and the same. Make lots of extra stock you will burn through it getting your angles correct.

once you have the angles solved, you will need to have some kind of joinery in the miter so they will stay together.

Looks like a fun project

-- nice recovery, They should pay extra for that mistake, Eric E.

View steliart's profile

steliart

2805 posts in 2837 days


#13 posted 05-19-2017 12:19 PM

I hope this could help a bit my friend :)

-- Stelios L.A. Stavrinides: - I am not so rich to buy cheap tools, but... necessity is the mother of inventions !!!

View dhazelton's profile

dhazelton

2789 posts in 2445 days


#14 posted 05-19-2017 12:22 PM

If it was me I’d just buy the freakin’ thing.

View EricTwice's profile

EricTwice

237 posts in 682 days


#15 posted 05-19-2017 01:53 PM



If it was me I d just buy the freakin thing.

- dhazelton

But what’s the fun of that?

-- nice recovery, They should pay extra for that mistake, Eric E.

showing 1 through 15 of 21 replies

Have your say...

You must be signed in to reply.

DISCLAIMER: Any posts on LJ are posted by individuals acting in their own right and do not necessarily reflect the views of LJ. LJ will not be held liable for the actions of any user.

Latest Projects | Latest Blog Entries | Latest Forum Topics

HomeRefurbers.com