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Old Dec 06, 2005, 10:01 PM
Firecracker!
BillO's Avatar
San Francisco & Santa Cruz
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Tips for cutting high-taper core?

OK, so now I'm trying to cut the fin for my Jart plug and it isn't easy. The fin has a 7 3/8" root and a 2" tip and it's only 6" high so it is highly tapered. The hot wire goes so much slower at the tip that the foam is getting burned. I'm using the "pivot block" described in the FeatherCut instructions.

One suggestion made in the manual is to cut the tip template slightly oversize to account for the excessive kerf. Does anyone know how I can print out a slightly oversize template using Profili?

There are two options I can see -- skin thickness and drawing thickness, but I wonder if either of them makes the actual outline any bigger.

Thanks for any suggestions!

Bill
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Old Dec 06, 2005, 10:22 PM
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East Stroudsburg Pocono, Pennsylvania, United States
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cutting

Quote:
Originally Posted by BillO
OK, so now I'm trying to cut the fin for my Jart plug and it isn't easy. The fin has a 7 3/8" root and a 2" tip and it's only 6" high so it is highly tapered. The hot wire goes so much slower at the tip that the foam is getting burned. I'm using the "pivot block" described in the FeatherCut instructions.

One suggestion made in the manual is to cut the tip template slightly oversize to account for the excessive kerf. Does anyone know how I can print out a slightly oversize template using Profili?

There are two options I can see -- skin thickness and drawing thickness, but I wonder if either of them makes the actual outline any bigger.

Thanks for any suggestions!

Bill
Hello Bill.
Increase the tip to 3", than cut it. After cut, glue the 2" template to the root
than send to conture.
Les
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Old Dec 07, 2005, 08:35 AM
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Quote:
Originally Posted by BillO
Does anyone know how I can print out a slightly oversize template using Profili?
  • Click on "airfoils' and then "airfoils management" from the drop down menu.
  • Select an airfoil from the list and click on it.
  • Click on "process" from the menu at the top of the window.
  • Click on "process selected airfoil" from the drop down menu.

In the airfoils processing window you will see the option for changing the thickness of the selectes airfoil.
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Old Dec 07, 2005, 10:24 AM
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Hi Phil -

Thanks, this is just what I was looking for!

Bill
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Old Dec 07, 2005, 01:00 PM
Nucking Futs
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Can you lower the temp any? To reduce the kerf, oftentimes I'll dial the temp way way down and tension the bow higher. Takes a bit longer but the foam loss is much lower.

HTH, IRJ
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Old Dec 07, 2005, 02:38 PM
Firecracker!
BillO's Avatar
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I'll give that a try too. My power supply is just a dimmer connected to a transformer and it isn't all that accurate or linear. Maybe I'll have to upgrade. Thanks.

Bill
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Old Dec 07, 2005, 11:47 PM
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That's really an extreme taper. Why not just sand the core shape from a rectangle, the way Dr Drela does for hlg tails. Really simple and takes next to no time.

Cut the airfoil profile as a rectangle, with the proper thickness taper. Then sand some tangents and fair.

A little looking on the hlg forum should find the info, or go to the Charles River RC sit and look under their plans section, in Apogee, for info on cutting airfoils in balsa. Works the same in foam.

Gary
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Old Dec 08, 2005, 12:15 AM
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Gary - yep, that's a good idea. I was thinking I may use balsa as it will be stronger than foam, but it's basically the same thing. I'll give the Feathercut a couple more tries at it and if it doesn't work, then it will be on to sanding...

Bill
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Old Dec 08, 2005, 02:59 AM
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United States, MI, Monroe
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First draw 2 lines, 1 down the LE past the tip so when you draw line 2 Fron The TE they come together. to a point. This is now the pivot point for 1 end of your Bow. Add a piece of MW to make a leg for this end and pivot off this.
On the tip end use a small fan to cool the wire it will help alot. I do this on a short run of cores.
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Old Dec 08, 2005, 07:48 AM
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Yep, I use Kevin's technique. I drilled a small hole at the pivot point and stick the end of the bow into it. This way, I only have to freehand the other side of the bow.

One thing about this method, though, is that for long span panels, you will need a very long bow in order to reach the pivot point and still have enough cutting wire to reach past the opposite side of the core.

-joedy
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Old Dec 08, 2005, 12:49 PM
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Hi Kevin -

The setup you describe is exactly the "pivot block" method I'm using. The idea about the fan is interesting though. One more thing to try.

Bill
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Old Dec 10, 2005, 05:47 AM
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Italy
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Quote:
Originally Posted by Phil Barnes
  • Click on "airfoils' and then "airfoils management" from the drop down menu.
  • Select an airfoil from the list and click on it.
  • Click on "process" from the menu at the top of the window.
  • Click on "process selected airfoil" from the drop down menu.

In the airfoils processing window you will see the option for changing the thickness of the selectes airfoil.
This is not a precise way to oversize an airfoil, as you need an offset function and not a simple thickness change.
Tru to add a negative skin thickness in Profili....
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Old Dec 10, 2005, 01:10 PM
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Pivot Cutting Photos...

The photos below show a two different pivot cutting examples and resulting cores...
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Old Dec 10, 2005, 07:11 PM
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Durone -

I tried using a negative skin thickness and it seems to do the right thing. Thanks. Fortunately in this case, accuracy is not essential. It's just a symmetrical foil for the fin.

Sled Driver -

So the two methods are 1) use a bow and fix one end to pivot around and 2) no bow, just stretch out a wire?

Bill
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Old Dec 11, 2005, 11:11 AM
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Quote:
Originally Posted by BillO
So the two methods are 1) use a bow and fix one end to pivot around and 2) no bow, just stretch out a wire?
The first set up shows using slotted pivot, which works fine for longer pivot length cuts where the heigth of the wire at the pivot does not matter. The second set up shows a notched pivot that keeps the wire at a heigth the chord height for short pivot length cuts.
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