Math question: diameter from a portion?
 

Hi,

I have a small wooden part with a curve to it and I want to replicate it on the lathe by turning a complete circle so trying to figure out how to take the measure of the slice to get the whole.

To make a simple comparison, if I had a slice from a pie and wanted to figure out the diameter of the pie from just the crust curve of one piece.

Hopefully the brain trust can steer me “straight”.

Thanks,
Rutager

I think this calculator gets you what you need:
https://www.omnicalculator.com/math/sector-area

Like this?

<iframe width="560" height="315" src="https://www.youtube.com/embed/vLf1742TWLY" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

I was about to say "I love banana cream pi"...but am afraid of where that might go.:eek:

You can do it by math or get a large enough piece or craft paper and stick a pin to act as the center and rotate your piece around it and draw a line on the outside until you get a full circle. Then measure across the circle.
John

Perfect- great explanation and easy to follow.

Thanks!

Thanks, I found that one, but my math skills aren’t good enough to figure out where to put all the numbers!

Mmmm... Cherry Pi :D

Happy Pi day ;)

Ha ok! There are links to definitions of each variable so that might get you close.

What about making a compass thats big enough to accomodate the wedge?

^^^ That's what she said.

Hi John,

In this case, my piece doesn’t go to the center, but I bet I could run lines down each side and measure from the middle to get the empirical answer. Just tried it and apparently the diameter would be huge as the lines weren’t even close at two feet out! I may have to give up on the lathe idea.

Here’s the project- I got a bunch of hand plane bodies and I wanted to make a custom wood infill for one. The originals were Rosewood or anodized aluminum, so I do have a piece to use as a pattern, just thought it would be easier to turn a section, but my lathe won’t turn a huge diameter inboard and I’ve never tried outboard turning.

http://forums.pelicanparts.com/uploa...1647306865.jpg

Apparently Byron left his camera at my place! Sorry about the blurry picture.

this is an easily googable problem.

Seems infinitely simpler to use one of the pieces to trace out a template then hand shape.

Yes it is but...

Yes, once I determined how huge the diameter was going to be, that’s the route I will take.

My initial thought was that since it had several facets and a compound curve that I could get a “perfect” shape if I turned it and could also make several at once and even out of different woods by doing segments on my round blank.

Clever.

You could rough-cut the shape, then attach it to a long pivoting arm and pass it over a router.

Rutager,

If I follow your issue - why don't you cut a jig to match that diameter and just use a router with a follower bit and/or bearing?

Edit - I didn't see Amail's comment which he posted after I had opened the tab a while ago.

Thanks Rusty,

I plugged in the cord and height from my tracing into the calculator; it wouldn’t accept .10 in the height, cord was 1.33, so I base 10 the thing and came up with a radius of 7.21 and doubled for a diameter of 14.42” does my math track, or do I need to correct anything because I multiplied by 10?

Here are a couple pictures.

http://forums.pelicanparts.com/uploa...1647380191.jpg

http://forums.pelicanparts.com/uploa...1647380231.png

A couple of you recommended running a trammel on a router, but as you can see from the picture above, it would take a custom bit to make the shape, but a chisel on a lathe could make the shape pretty easily; even with my basic skill level.

Thanks for all the suggestions, that’s why we bring all our toughest quandaries to the board!

Let’s try this again! Watching the video posted, I got out my calculator and punched in my numbers and came up with 4.5”. I set my compass to 2.25” Drew a circle and bingo, matches up nicely. Looks like the lathe is an option.

Probably fairly obvious by now, but I have always struggled with math and really need to prove it empirically to trust my answer.http://forums.pelicanparts.com/uploa...1647382159.jpg

Only thing I know about lathes is watching some wood turning stuff on youtube because it looked cool and was in my suggestion list...

How do you make a shape like that with a lathe? Start with a 5x5 block of wood and shave it away until you have a 4.5" circle? You'd still need to slice off a wafer and then cut the shape out (looks like you could get 4 per wafer) and then do the other required shaping on them... or is that the plan?

Get a desktop CNC.

Pretty close, I would bandsaw it round first, then shape the angle on the face, do the curved part on the outside, slice out the formed wedge, cut the slot and then do any final shaping.