How to determine radius

fohukulele · Mar 30, 2019

Hi guys, just wanted to ask if anyone could help me with a quick method to determine a radius. I have a wooden seat from a bar stool with a nice curve, I have tested it with spruce bracing and sandpaper and it works well, would just like to know the approximate radius but searches online only seem to refer to starting with half circles to determine the radius. My maths knowledge doesnt stretch this far unfortunately.
Any info appreciated thanks.

Timbuck · Mar 30, 2019

Radius is half the diameter...or is it a dished seat and you want to know the radius of the dish.?? a photo would help.

fohukulele · Mar 30, 2019

Timbuck said:
Radius is half the diameter...or is it a dished seat and you want to know the radius of the dish.?? a photo would help.

Thanks for your reply, yes it's a dished seat and I'd like to know the radius, I'll add a picture shortly. Cheers

fohukulele · Mar 30, 2019

Heres a picture hopefully you'll see what I mean. Thanks

Graham Greenbag · Mar 30, 2019

fohukulele said:
Heres a picture hopefully you'll see what I mean. Thanks

I’m not volunteering but don’t think that that will be too hard for someone here to work out for you. If I were to attempt it - an approximation is the best that could be offered - I’d want you to lay a long straight edge within the radius and measure both its length and (accurately for this) the maximum gap under it. IIRC a bit of trigonometry should give a radius from those figures ........ well many decades back it would.

fohukulele · Mar 30, 2019

Approx 450mm length and gap under around 17mm if that's of any use

SpaceForRent · Mar 30, 2019

fohukulele said:
Approx 450mm length and gap under around 17mm if that's of any use

According to Google, its ((L/2)^2 + H^2) / 2H
Where L is the length across the dish (450mm), H is the depth (17mm). Hope that helps.

BuzzBD · Mar 30, 2019

I recently made two radius dishes and found the formulas on line. Do a search on how to make a radius dish. My dishes are 15’ and 25’. They are both 16” in diameter. The sag on the 15’ radius is .178” and on the 25’ is .107”.

Timbuck · Mar 30, 2019

I'll work it out for you ..but not tonight co's i'm off out

lauburu · Mar 30, 2019

You're looking for the radius of an arc. Try this

https://www.mathopenref.com/arcradius.html

Miguel

fohukulele · Mar 30, 2019

BuzzBD said:
I recently made two radius dishes and found the formulas on line. Do a search on how to make a radius dish. My dishes are 15’ and 25’. They are both 16” in diameter. The sag on the 15’ radius is .178” and on the 25’ is .107”.

Thanks for your reply. According to this website

http://randypeterman.com/radius-dish-depth-calculator/

I think I have a 5ft radius, not 100% sure but on this calculator if I put in 5ft, 1.5ft width, the depth comes up at around 17mm so hopefully that's it

Graham Greenbag · Mar 30, 2019

I cheated and used the web too ...... getting old.

You gave use the chord length and depth. An on line calculator did the rest for me and came out with a radius of about 1.5 meters.
https://planetcalc.com/1421/

Hope that that helps. I love UU, so many people willing and happy to help

.

fohukulele · Mar 30, 2019

Graham Greenbag said:
I cheated and used the web too ...... getting old.

You gave use the chord length and depth. An on line calculator did the rest for me and came out with a radius of about 1.5 meters.
https://planetcalc.com/1421/

Hope that that helps. I love UU, so many people willing and happy to help .

Agreed

thank you so much everyone for your input

sequoia · Mar 30, 2019

1497.47059 mm or 58 61/64th of an inch. More or less.

mikeyb2 · Mar 30, 2019

Search for "calculating the sagitta of an arc" by Lutaio Mottola. this gives a formula where you can enter parameters and it will the calculate for you. For some reason UU won't let me post the link.
https://www.liutaiomottola.com/formulae/sag.htm Finally , it let me post the link.

fohukulele · Mar 30, 2019

mikeyb2 said:
Search for "calculating the sagitta of an arc" by Lutaio Mottola. this gives a formula where you can enter parameters and it will the calculate for you. For some reason UU won't let me post the link.
https://www.liutaiomottola.com/formulae/sag.htm Finally , it let me post the link.

Another great link thank you, bookmarked them all they'll certainly be useful. Cheers

Titchtheclown · Mar 31, 2019

I hate to be a grumpy old git in the ointment but are you sure you have a spherical section and not an elipsoid or some other shape freehanded out by a skilled bloke withan angle grinder?

BuzzBD · Mar 31, 2019

The above link to the formulas allow you to calculate the sag at any interval from the center. For my dishes I did them at 0, 2”, 4”, and 6”.

Titchtheclown · Mar 31, 2019

BuzzBD said:
The above link to the formulas allow you to calculate the sag at any interval from the center. For my dishes I did them at 0, 2”, 4”, and 6”.

Sounds like a way to check if it is an actual spherical section, either that or use CAD (Cardboard Aided Design) and knock up a cardboard circle section of the diameter you calculate and give it a spin to see how close you are.

fohukulele · Mar 31, 2019

Titchtheclown said:
I hate to be a grumpy old git in the ointment but are you sure you have a spherical section and not an elipsoid or some other shape freehanded out by a skilled bloke withan angle grinder?

All I'm using it for is adding a curve to the backs of some mini ukes I'm making and after testing on scrap it seems to do the trick. Only wanted to know the approximate radius out of curiosity really

How to determine radius

Member

Well-known member

Member

Member

Well-known member

Member

Well-known member

Well-known member

Well-known member

Well-known member

Member

Well-known member

Member

Well-known member

Well-known member

Member

Well-known member

Well-known member

Well-known member

Member

Similar threads