Scribe Tool for Octagon Cuts

[Wmcullen]

I'm not sure if this process is right or wrong; unique or common. But I want to share in case it's interesting.​

29: Scribe Tool for Octagon Cuts
last post- 28: Cutting Corners with Bandsaw
(3d *.obj file attached)

Thanks for the GREAT discussion on the utility and methods of turning pen blanks into octagons.
I have one more simple tool to contribute if you want to go a step beyond "eyeballing" the necessary cuts.​

The Problem
While my last jig focused on making 45 degree cuts in blanks, it failed to indicate where to make the cuts if you want an octagon.

I may have accidentally cut one squared blank into a smaller square. (Doh!!)

A better octagon should have all sides (S) equal, but I wasn't sure where to cut.

My Solution
(Not rocket surgery... but a bit of geometry.)
I started by building a simple "center finding" scribe tool... and modified it by offsetting the scribing hole to 29.29% of the length and replacing the more common rounded ends with triangles. I made the scribing hole just big enough for a mechanical pencil.

Why 29.29% across?

This school website provided the main equations.

Summary
Because we typically deal with smaller blocks of wood in penturning, this tool is designed to handle blanks with side measurements from 5/8" to 1" wide.
Cutting at 45 degrees along this line should yield a perfect octagon.

I've attached a 3d *.obj file called octFinder3.obj. I always need to scale it up by 10x before printing. Model offered as-is.

Thanks for looking
- Cullen

 

[egnald]

That is way cool!!!! - Dave
PS My cheat has been to go to MS Publisher, draw two squares the size of my stock and rotate one of them 45-degrees, then cut the template out and paste it to the end of my blank. I like your approach much better!

 

[Wmcullen]

That is way cool!!!! - Dave

Thanks Dave. That's a great way to use some very practical geometry! While playing with this topic I came across some other methods you might find useful. Or at the very least, interesting.

Calipers

1. Draw an X to find the blank's center.​
2. Using calipers or a compass, measure the distance from the center to a corner.​
3. This measurement will mark 29.29% from the other direction along a side.​
4. Cut at 45 degree for a perfect octagon
   (Geometry is incredible, isn't it!??!)​

Tablesaw
Watch this great demonstration of a Fine Woodworking tip:

Close enough
Precision is a fickle goal.
If you're just looking to remove material before turning you might eyeball "a tad less than 1/3 of the way across" and cut. Done.

- Cullen

PS- After writing this post yesterday, I found another excellent article that covers a lot of this and more at:
http://www.timberframe-tools.com/techniques/making-square-stock-octagonal/

 

[Artie1015]

hey Cullen,

Very ingenious. I admire your analytical mind.

Best,

Artie

 

[Joebobber]

With the right tool in the right persons hands, anything is possible. The 3d printer and you were made for each other. You make some awesome jigs man.

 

[KMCloonan]

Very cool.