Re: Bending dish ribs any tips or tricks?


John Lemay
 

I concur with Johns suggestion that hand bending the rib profiles by hand is possible. I did this last year to make a 2m diameter dish.

 

I didn’t worry about using any timber, each rib went straight in the vice and was bend bit by bit by hand. The first rib takes ages, and it’s a bit of an art getting used to the spring-back of the material.

 

I can’t see that a rolling machine would be helpful, because the nature of a parabolic curve is it ever changing radius.

 

Importantly, spend lots of time getting the template right, because you never know when the desire for a higher band might come along.

 

As an aside, I’ll shortly be replacing the mesh on my dish, and have just received some sheets of expanded aluminium mesh from the Expended Metal Company. They were easy to deal with and the material looks good.

 

Regards

 

John

 

 

From: UKMicrowaves@groups.io [mailto:UKMicrowaves@groups.io] On Behalf Of John Fell
Sent: 27 August 2020 10:05
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] Bending dish ribs any tips or tricks?

 

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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