Cutting foam wing cores. What is max span?

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stuart knowles 1611
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Joined: December 27th, 2008, 11:46 am
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Cutting foam wing cores. What is max span?

Postby stuart knowles 1611 » November 10th, 2020, 1:46 pm

Hi to All,

This years building season has arrived by Government imposition so perhaps best to make a start.

I want to build a three piece wing where the plug in outer wings have a span of 60 inches.

I have cut cores in the past with a span of 40 inch each core. As I recall, they were cut very slowly with minimum heat in the wire and that the finished core came out quite well.

My question is, is it possible / practicable to cut a 60 inch core in one go?? I can make an 'H' section bow of the appropriate span but incurring failures at this size would soon start to get expensive.

Can a five foot core be hot wire cut without excessive lag? Has anyone out there done this and made it work. Any tips to offer??

Thanks . Good luck with your own builds

regards

stu knowles

DAVE JOHNSON
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Re: Cutting foam wing cores. What is max span?

Postby DAVE JOHNSON » November 10th, 2020, 7:07 pm

Hi Stuart--we have cut several 8ft wing panels here for the B17s---i use an 8ft 6"" bow and a mains variac---if you are stuck you are welcome to borrow it ---we shant need it for a few months----DAVE

stuart knowles 1611
Posts: 241
Joined: December 27th, 2008, 11:46 am
Contact:

Re: Cutting foam wing cores. What is max span?

Postby stuart knowles 1611 » November 10th, 2020, 10:55 pm

Hi Dave, That's encouraging to know. I sort of thought it should be OK if I'm careful but didn't want to waste a load of foam without checking. 8 1/2 foot!! what am I worried about!

Thanks for the advice, it very helpful. I'll try cutting the blanks with my gear and see how it goes. I may be starting to run out of amps with a bigger bow but I won't know until I try.
regards

stu k

stuart knowles 1611
Posts: 241
Joined: December 27th, 2008, 11:46 am
Contact:

Re: Cutting foam wing cores. What is max span?

Postby stuart knowles 1611 » December 21st, 2020, 6:47 pm

Just to add a little more to this which may be of use to someone else, I have made a 6ft bow but I then hit a wall in finding something with enough volts/ amps to bring the wire to a suitable temperature.

On the smaller bows I have an old MFA power unit, 20 volts ac current, No detectable gain in the temperature of the wire. Then I tried two 12 volt car batteries wired in series to give 24 v dc, result - just barely warm. I took advice from someone who knows which suggested that something in the region of 50 volts would be required. Ebay, YouTube and Google diligently interrogated but I couldn't find anything remotely suitable at a sensible price.

Then someone suggested that I look at an electric 'stick' welder. Sure enough there are outputs at 50 and 80 volts and adjustable amps from 20 to 120. What is more, there's one sitting in my garage!

Today I rigged up the new bow and connected the electric welder. Set at 50v 20 amp and it powers the bow beautifully, Not too hot, just right. Nothing went bang and the lights stayed on. Result!

When the bench is cleared in a few weeks I shall have a go at cutting the full wing cores but, based on todays experiment, I am confident that it will work.


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