Re: Bending dish ribs any tips or tricks?


John Fell
 

David,
You can hand form the box section .
Find a 1m length of good solid hardwood - something like 35-50mm thick and cut a square hole through the thickness such that the section to be formed will just pass .
Clamp wood into a solid bench vice or one mounted on a solid pillar drill stand , with the square hole set to have its sides vertical and horizontal .

Pass the section through the hole and apply a gentle downwards force at around 1 metre from the hole .You will be able to "feel" the required effort needed to bend the section .
Remove section and offer up to your rib template to determine if bend is conforming to the shape - reverse thrust if required or try bending more if needed.
It is better to underbend at this stage as work hardening will occur with each applied effort stroke .

When happy push section approx 100mm further through hole than the first bend point and repeat the bending  stage - remove and test each time you perform a bend and progress along full rib length ,testing against template as you go .

Ensure the hole does not move from its initial set point and that the applied force is vertically downwards , to avoid any tendency to spiral the formed section.

Although a slow process you should finish up with the full rib following the template and it should be plenty good enough into the mid microwave area .

73
John
G0API

On Wed, 26 Aug 2020 at 16:22, David Law <law1274@...> wrote:
Hi, I'm on Mk2 of my 4m 0.38f/d dish project this time using traditional square tube and stiffening ribs.
The material will be 18mm x 1.6mm square tube.
I've made a template from wood sheet.
I wondered if anyone has any tips or tricks in how to form a nice smooth accurate reproduceable parabola.
Or indeed what not to do! Roll bending machines are not really in the budget for a one off project.
It will be attached to the stiffening rib via trusses and riveted plates which will avoid using a welder. 
It's for 23cm at this time and likely to stay that way so points of mm accuracy isn't too important.
I touched base with Howard, g4cch a while ago who gave me some tips, but the more advice based on experience the better.
David
PS Mk 1 used 1/2" round tube on a hub but it was too floppy (probably only good up to a 2m dish)
If anyone wants any bent 1/2" round tube let me know!

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