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Old Dec 20, 2013, 07:24 PM
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United States, AZ, Tucson
Joined Nov 2013
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Cutting Foam with a Hot Wire

A question on how to cut foam with a hot wire. I am a bona fide newbie, just starting. I will be building a trainer and anticipate making multiple sets of wings for replacing those destroyed in the process of learning to fly. (Probably multiple aircraft is more like it.) Somewhere between 40" and 48" WS, simple rectangular shape, simple flat bottomed Clark Y airfoil. In shaping the airfoil of the half spans, what is the preferred practice (and why)? Bringing the hot wire cutter to a securely mounted work piece? OR bringing the work piece to the wire which is set out from the edge of the work bench by several inches, the frame being clamped to the bench.

I have seen at least one video of someone using the second procedure, supporting the work piece with a piece flat material. This looks it might be easier to do.

If bringing the wire to the work piece is preferred, how, with what, does a body secure/mount the work piece to something to keep it from moving around.
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Old Dec 20, 2013, 07:36 PM
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Ward Hagaman's Avatar
San Diego
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Not sure if you are asking about "blocking" or cutting the actual airfoil?

I assume you have a bow and templates already?
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Old Dec 20, 2013, 07:58 PM
Red Merle ALES
Curtis Suter's Avatar
United States, Mt, Helena
Joined Apr 2002
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hoss4659,

Welcome. You can start by reading the article I wrote for RCSD called "Build an Automated Foam Wing Core Cutter". The link to it is here: http://tailwindgliders.com/Files.html#Articles

To answer your question the foam is heavily weighted down and the wire is moved from leading edge to trailing edge cutting the bottom airfoil first then the top airfoil.

Happy Holidays
Curtis
Montana
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Old Dec 20, 2013, 10:01 PM
Little Red Plane
United States, TN, Knoxville
Joined Dec 2012
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Buy Curtis' videos, they will give you the help you need! Well worth every penny that yo will spend.
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Old Dec 21, 2013, 03:30 AM
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Australia, NSW, Bellingen
Joined Aug 2008
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Gravity hot wire cutter.

I've not seen Curtis's video, but I have seen Phil Barnes which is good too.
I have built a simple gravity driven hot wire cutter that has the bow suspended under a cantilevered bench. I copied Joe Cormer's example which can be seen here: http://www.rcgroups.com/forums/showp...&postcount=163

And my copy using similar principles can be seen here @ report #15:

http://www.rcgroups.com/forums/showt...r#post16646458

Template shaping and cutting is a subject for an entirely separate discussion.

Jim.
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Old Dec 21, 2013, 10:42 AM
Canada
Joined Jun 2009
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I read and built Curtis's rig and have been using a calculator that runs in excel that gives you the set-up positions for tapered wings, Thanks Curtis!

Also recommend sketching out the wing shape on the bench and test the gravity drop hot wire cutter set-up to fine tune before cutting and place the leading edge as close to the front of the foam block as you can.

Good luck
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Old Dec 21, 2013, 10:48 AM
Red Merle ALES
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United States, Mt, Helena
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Quote:
Originally Posted by Paul Harvey View Post
I read and built Curtis's rig and have been using a calculator that runs in excel that gives you the set-up positions for tapered wings, Thanks Curtis!

Also recommend sketching out the wing shape on the bench and test the gravity drop hot wire cutter set-up to fine tune before cutting and place the leading edge as close to the front of the foam block as you can.

Good luck
Thanks Paul and Garry for the nice comments.

I test the timing of the cut by running the wire along the top of the foam core while holding the wire down to the foam with one hand and the other slowly lowering the swing arm with the other.

What we are talking about here Hoss is that on a tapered core (wing) the wire should enter the Leading Edge at the same time at the root and tip and exit the trailing edge at the same time at the root and tip of the core. So one wire must travel faster than the other.

The DVD's show all of this and much, much more. It's like going to a clinic but one that you can view over and over. I know I refer to them myself from time to time to refresh my memory on procedures I have not used in awhile.

Curtis
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Old Dec 22, 2013, 09:58 AM
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United States, VT, Castleton
Joined Dec 2007
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Hoss, there is a very simple method of cutting airfoils described here. The wire is tied to a pivot point at one end. No bow, and only a single template to make. It works well. The drawbacks are that you can only cut a tapered wing (not the rectangular wing you describe) and you can't cut with tip washout (not easily or accurately anyway). But if a slightly tapered wing would work for you, and you haven't invested the time in building a bow yet, it might be a good way to get started quickly. The longer the wire, the less taper you will have. I've built a few gliders over the last 2 years using this method and they fly great. I'm just now starting to think about building a gravity bow cutter. I like the design that Jim T. describes above.
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Old Dec 22, 2013, 10:13 AM
Red Merle ALES
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United States, Mt, Helena
Joined Apr 2002
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Hey JimZinVT thanks as that reminds me that I offer up a FREE video on how I do a Pivot Cut. Scroll down to Building Tips and download the "Pivot Cut" video.

http://tailwindgliders.com/Tips.html

Curtis
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Old Dec 30, 2013, 09:12 AM
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United States, AZ, Tucson
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Thanks

Gentlemen-----

Returned from a Christmas break holiday to find all the very useful responses to my question. Thanks for your help!!
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Old Dec 30, 2013, 06:14 PM
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Screw Eyes instead of sheaves

Quote:
Originally Posted by JimZinVT View Post
................. I'm just now starting to think about building a gravity bow cutter. I like the design that Jim T. describes above.
If you choose to do so Jim, may I suggest a further simplification. The sheaves (sometimes wrongly called: "pulleys") can be substituted for using common brass or screw eyes. If synthetic string is used, it will slide through these at the required 90 deg angle with little friction.
Which means that a suitable aluminium extrusion to allow adjustment of the sheave position is simply not required. This can be very hard to find.
So, all that is required to replace this is a length of soft wood into which the screw eyes can be positioned as determined by the taper ratio.

Just by the way, I have found no need for a spreadsheet to arrange the two pull strings pivots or sheaves to obtain the correct distance travelled for a good cut.
(Curtis and I have disagreed on this before - I'm not trying to be controversial for it's own sake, just suggesting streamlining the whole operation).
After all, the ratio is all that is required, not the actual travel.
More about this if it is not immediately obvious what I mean.

Life just gets simpler.......

Jim
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Old Jan 01, 2014, 07:37 PM
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United States, VT, Castleton
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Hi Jim. I haven't really studied the geometry of these gravity driven cutters yet, but I'll get in touch when I get started building one. Not opposed to using a spreadsheet program to work it out, but if I can do without, that's cool.

Speaking of spreadsheets, I've been using Curtis' "Sailplane Calc" spreadsheets to design all of my scratchbuilt gliders and they've all flown beautifully from the first toss. Thanks Curtis!
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Old Jan 01, 2014, 09:14 PM
Red Merle ALES
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United States, Mt, Helena
Joined Apr 2002
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Jim,
You are most welcome I glad you find them useful. You don't need the Calc for foam cutting either with the drop arm method or Pivot but it sure makes my life easier! Then again I can go for a year or so without cutting any foam! That's where my videos are a nice resource for me too. Heck I just referenced them earlier today to get a boom layup for someone.

Curtis
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Old Feb 12, 2014, 03:54 PM
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goodfruit's Avatar
New Caledonia, South Province, Noumea
Joined Feb 2014
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Hi everyone,

I'm new to foam cutting.
What's the difference between Curtis and Jims' machines, especially concerning the swing arm ?
Thanks.
Yann
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Old Feb 12, 2014, 04:20 PM
Red Merle ALES
Curtis Suter's Avatar
United States, Mt, Helena
Joined Apr 2002
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Yann,

Please feel free to read the article I wrote for RCSD called "Build an Automated Foam Wing Cutter" here: http://tailwindgliders.com/Files.html#Articles

Curtis
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