My Toy Bow & Arrow Set Hot Wire Cutter

I'm very curious.. because I made some templates (using the CNC) several years ago in anticipation of putting together a hot-wire cutting bow.

Both internal and external/airfoil and hydrofoil..

I'm going to pick up some blue foam this week and start practicing cutting some simple wings I'll let Ya'll know it turns out

Been there done that

Not trying to dash the good idea... but you need a temp control ------12VDC at 6 amps MIGHT be just right for your wire diameter and length ....

BUT ----if it is too hot or too cool it will frustrate your attempts to cut nice shapes

And to frustrate it further you need good virgin foam...re manufacture foam is worthless
and even then---- new virgin foam can have various densities (weight per cubic foot) and each has a good temp setting

Do a bunch of reading here:
http://www.jacobs-online.biz/power_supply_design.htm

If you are not OCD... one method is to use a less than optimal cutter but make templates slightly larger....then block sand out all the imperfections....foam sands very easily with those dry wall screen type pads or even the coarse foam rubber spongy types

I got too frustrated, gave my cutter away, and get the foam cores I need from Phil Cartier (CoreHouse)

How do you cut shapes, just carefully carve it like it was a big hand plane?

Rusty when cutting something like an air foil using internal or external templates...if the wire is too hot or too cool the top and bottom of the core will have draggy wavy imperfect finish. When all is good the top and bottom are near finish smooth ready for covering or skins.

Very light block sanding with coarse grit (80) on down to finer grit 150~220 gets you back to the shape and finish needed

Corners / wing tip shapes, are easy to control using the sponge type coarse to medium grit versions....shoe Polish method with 150 grit long strips helps get a nice round profile

RknRusty wrote:How do you cut shapes, just carefully carve it like it was a big hand plane?

Rusty, I'm goint to use templates tack glued to the inside and outside of the foam. Fred was correct in saying I need to vary the current through my wire to get a clean cut, I used my battery charger as a test bed and it works ok , but I was getting a little of the wavy thing going on that Fred talked about

One hint for helping reduce the wavy cut.... make very good templates that have long lead-in and very smooth edges for the wire to slide along

I cut my templates a little large from 1/8th ply, use too holes in the sides to PIN them with Finishing nails into the edge of the blank.  I cover the outer edges of the template with Aluminum tape

I make the Trailing edge a bit fat and longer than needed....easy to trim back to width

I mark the center line and drill a hole in the LE front and TE back and force in a 1.5" piece of music wire as my lead in and lead out guides

I never try to hot wire cut out the spar slots...they are done on a router table or long purpose built sanding bar and guides

Sorry, guys, I'm just not compiling a picture of this in my little brain. Maybe if you could get a snapshot sometime Thanks.
Rusty

RknRusty wrote:Sorry, guys, I'm just not compiling a picture of this in my little brain. Maybe if you could get a snapshot sometime Thanks.
Rusty

Rusty, Here's an example of a hot-wire foam-cutting rig. This example uses a single internal-template and one end of the wire has a fixed-location, unlike a conventional bow. It's one way of producing a wing-panel with a varied chord. A conventional bow holds the hot-wire taught and usually there's a template on both-sides of the foam-block.

https://www.youtube.com/watch?v=R0ZCnR_g6ZU

Rusty

I got too deep into this with the combat planes once I saw how Jeff Dawson did it

A Lot of variables that are hard to control... especially for OCD folks like you and I

Bob Hunt can supply perfect wing cores...skinned or not ....for most any stunt plane you ever desired
Phil Cartier can deliver near perfect cores for many common stunt planes and a lot of combat planes

But I have to admit... part of modeling is doing it ALL your self... I had fun learning foam cutting, shaping, covering

For Rusty... cutting a full 42"~ 48" twister type wing is technically possible but in reality we make two half's just cuz the Bow is very hard to construct and feed (Volts/amps) to make a good clean long Wing Span cut

A SOLID foam wing in most stunt sizes is too heavy....so we need to Core out voids inside... the hard part... then there is good spars to consider

Balsa sheeting to make it all look pretty adds weight plus a lot of strength

To make a Ring Master Foam wing... because it is NOT a constant Chord requires two panels cut with a lager Root template and a smaller Tip template...

Since you are curious...I want you to just suck it up and Have Phil Cartier send you a Rugged Stunt Trainer

A-- easy and fast build
B- Cheap
C- on this subject the kit will give you a dozen AH Ha moments..
D- Near indestructible and flies as good as most any stunt plane there is

Building the bow and a suspension system can actually be fun. I built one from some plans that I think came from the RCM books. There was a pulley-system with rope.. and a counter-weight. The idea was to suspend the bow and its elec. leads from a swinging-arm mounted above with a pulley hanging from it. The rope would attach to the top/center of the bow and up through the pulley and back down to a counterweight. The counterweight could be anything.. as long as it counter-balances the bow. A piece of 2" PVC pipe a foot or so long, capped on the bottom and filled with sand is an easy way to do it. Drill a cross-hole through the top of the pipe for tying the rope. If your pulley-wheel has a 2" dia.. then the pipe/counter-weight will clear the rope on the "bow-side" of the pulley. This can really be helpful when cutting.. because the weight of the bow isn't working against you.. and the cutting-wire can be guided up or down effortlessly.

The swinging-arm is merely a piece of wood with a pivot or hinge mounted to the wall or an upright post to the side of your work-station. Make it long enough to mount the pulley directly over the center of your workstation. The arm only needs to swing the length of your cut.. and support the weight on the rope. The insulated power-wires can be zip-tied to the bow and also suspended so that they're out of the way.