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Old Aug 08, 2005, 11:45 PM
Registered User
Congress, AZ
Joined Sep 2001
4,783 Posts
foam cutting tapered tips

I've noticed in cutting tips for the SuperGee, that I get a lot of cores that have a thinner airfoil than the corresponding airfoil in the main core. IE, when I join the wing, the AG46 airfoil, which is on both the main core and the tip core, are not the same thickness.

I think perhaps when cutting the tip, the wire is cutting a thicker kerf than it cuts on the main core, because it's moving at different speeds. If anyone has any ideas, I'd like to hear them.

Options seem to be:

a: Tell the airfoil program that the kerf is a different thickness for the tip (but I have it set at zero now).

b: Use a thinner wire on the tip

c: Vary the heat or cutting speed on the tip

Comments?

Oh, I'm currently using .018 stainless as the wire.

Gary
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Old Aug 09, 2005, 04:13 AM
York Electronics
Gary Warner's Avatar
Dallas Tx USA
Joined Apr 1999
2,505 Posts
Quote:
Originally Posted by GaryO
I've noticed in cutting tips for the SuperGee, that I get a lot of cores that have a thinner airfoil than the corresponding airfoil in the main core. IE, when I join the wing, the AG46 airfoil, which is on both the main core and the tip core, are not the same thickness.

I think perhaps when cutting the tip, the wire is cutting a thicker kerf than it cuts on the main core, because it's moving at different speeds. If anyone has any ideas, I'd like to hear them.

Options seem to be:

a: Tell the airfoil program that the kerf is a different thickness for the tip (but I have it set at zero now).

b: Use a thinner wire on the tip

c: Vary the heat or cutting speed on the tip

Comments?

Oh, I'm currently using .018 stainless as the wire.

Gary
I've never considered this before, but I've seen what you are talking about. Since the wire speed is slower for the lesser cord, its kerf is larger. Makes sense to me.

Maybe using a direct measurement of the kerf for the affected tip will show the amount of kerf needed to be considered in an airfoil program like Compufoil. Then again, it sounds wasteful to make two cores and templates just to get one wing correct.

I'd be interested to see what you come up with. Good luck with this problem.

Gary
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Old Aug 09, 2005, 07:38 PM
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Australia, NSW, Sydney
Joined Feb 2003
1,114 Posts
How are you cutting the cores? What template system are you using?

I use the two template system (either one for the bottom and one for the top surface, or one for the bottom and one the exact airfoil shape which then rests on the first one). I make the templates exactly to the intended airfoil shape, and cut the lower surface first. After this first cut the core drops down into the bottom saddle to end up with it's bottom surface one kerf below the template outline. This exactly compensates for the kerf you get when the top surface is cut, resulting in the core being the exact size regardless of the wire thickness or kerf size.

As long as you use the same cutting speed and current for the top and bottom surfaces of each core you will end up with exact airfoils. It doesn't matter if the cutting speed and wire current vary between the inner and outer wing panels, as long as these conditions are the same for the two surfaces of each individual panel.

I've just cut 6 complete sets of HiLoad-60 Supergee 2 cores this way and the inter-panel thicknesses match up perfectly. Why six sets? Because as I got further into the build of this great model, everyone in my club got interested, and so there are now six of us building planes.

One problem I did encouter when cutting the high taper ratio tail cores, was that of the kerf varying along the length of the wire. The wire would cut with minimum kerf right next to the templates, but cut with a progressively deeper kerf going in about 15mm from the template. The problem was much worse at the smaller chord end of the panel. I solved this issue by using a higher current and a correspondingly higher cutting speed (2.5A on 0.20" stainless fishing leader). Apparently, at my original 2A current, the wire was hanging around too long at the "slow" end allowing a bigger kerf to develop. Somewhat counterintuitively, the higher current results in a lower and more consistant kerf.

I hope this helps? I learnt so much cutting these cores over the last couple of weeks, and had to solve many problems. I'm happy to pass any of this on if you have any more questions.

Graham.
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Old Aug 09, 2005, 09:11 PM
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Congress, AZ
Joined Sep 2001
4,783 Posts
Graham,

The problem only shows up when cutting a lot of taper. I think you may be right, the wire speed problem is cutting a bigger kerf, particularly at the tip.

I'll try using more current and see what happens.

The mismatch at the two panel joint is really sort of minimal. But the tip airfoil looks thin. I too use two templates and let the core drop down into the bottom. If the increase in current doesn't solve the problem, I'll just cut the tip template and allow for some kerf.
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Old Aug 10, 2005, 09:14 AM
DLD
David Layne
Tracy, Ca
Joined Jun 2004
665 Posts
You didn't mention what you used for templates. If your template material is more than 1/16", it could be caused by that. The more taper, especially in thickness, there is, the more difference there will be. I use aluminum or phenolic at about .060, and it works fine. Once I tried 1/8", and experienced exactly what you are describing. Hope this helps.

David Layne
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Old Aug 10, 2005, 11:24 PM
Registered User
Congress, AZ
Joined Sep 2001
4,783 Posts
I'm using formica, about 1/16".
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