flyingagin

I made a foam cutting bow from some 3/4" pvc pipe, some scrap 1x2 peices, and some safty wire. The arms are the 1x2 and about 12" long. the are free to rotate in and out. I am using a bungee cord to provide tension.

For power I have an auto transformer (at least I think it is called that). I piked it up second hand back around 95. The voltage can be adjusted as much as you want by turning the knob.

I had a bunch of pink foam that I use to have up in the bedroom window to block light as I was working nights. Now that I have a day job the foam kept screaming wings to me.

I made some triangles to attach to the foam for vertical cuts to squar the foam and size it.

So I made some template out of 1/4" ply. The first set I forgot to provide lead in and out ramps? Flat surfaces before and after the foil.
So first attempt was not a usable product. Ripples, and could see lots of evedince of wire lag. I also made the cut intirely be myselve. Trying to watch both sides of the bow at the same time is not an easy task.

So I made a second set of templates. This time with about 2"of straight before and after the foil section. I but marks every 1/2" And numbered them.

So I tried again, still by myself. Still not a usable product. Boy trying to stay even on both ends by yourselve is not an easy task.

So I just tried a third time. This time I managed to enlist my wife. She is very afraid of electricity and is very much a most reluctanct helper. It does not help that you can feel the bow buzz very slightly. We still did not come up with a usable core. Rpples and eveidance of wire lag.

So here is I stand at this time. I need to crank up the temp a little bit so it cuts a little faster and their is less wire lag. Also my bow SUCKS BIG TIME. Not enough tension on the wire. The 3/4" pvc pipe is not stiff enough. The arms and the end are alkward to say the least.

Next I will use 1" pvc and use curved 90 degree pvc conduit for the arms. Maybe attach a short peice of 1" dowl to the arms to act as handles.

flyingagin

Here are my templates.
The first pic is the squars for cutting foam square. There is a peice of tri stock glued to the templates dead on square. I just lay the tri stock part on the foam and then fasten with sheet rock screws.

This actually works a lot better than I thought it would.

The top 2 templates were the first core tempaltes No lead in or out flats (no idea what that part is called).

The lower 2 are the same foil, but have about 2" of flat add to start and exit the wire. And I decided to eliminate the philips entry.

WHEN I finnally get usable cores I will remove 1/4" top to bottom from the high point and replace with a peice of 1x1/4" balsa as a spar. Also The back will get a peice of 1/4x1/4" balsa and then sand to match the foil shape. Be a good surface for hinges. The front will get a peice of squar balsa then sanded to the correct leading edge shape.

At this pont I am planning on using poster board to cover. Or some low temp film.

flyingagin

The first core

flyingagin

Second core

flyingagin

third core

flyingagin

Ok is the fuse of this project. A real simple box fuse. 3/32 sheet balsa with some 1/ 32" hard ply to glue the fire wall to and some 1/8 light ply top and bottom extending aft to stiffen things up and carry the ply landing gear plate and to carry the wing hold down dowls. !/8 sheet balsa tail feathers.
Power is ST .34
Still have to install radoi, linkages and tank.
So far pretty light.
After I finally get a wing made and the gear inside the plane I will then determine wing lacaton for balance.
The wing will have a fair amount of dihedral and have airlerons.
It is a basic trainer.

flyingagin

I have not had the chance to fly but once or twice for a few minutes in the last 2 years, and so I am not willing to risk the plane I have been working on for so many months.
http://www.rcuniverse.com/forum/m_10675544/tm.htm

flyingagin

This plane is to get my groove back.
I will make a second wing later on. Symitrical foil and ZERO dihedral. Bigger airlerons.
I have not yet made templates for the second wing. I will build up a flat area where it sits on the fuse so I do not need to cut out for the bottom foil curve.

flyingagin

In the frist post I mentioned that I bought the transformer back in 95.
At that time I had a good place to work, a garrage turned shop. I built a cutter machine that was gravity driven and would draw the bow for me. I could even make tapered or and swept wing with it. Well it went bye bye a long time ago.
Now I live in an apartment and have very little space to work.
I have been exploring Lows and Home Depot trying to figure out what to to make a cutting table from.
It needs to be dead on, AND CHEAP.
Once I can make a cuttting table I will make another hands off cutting machine.
I have some really nice pulleys for the bow pull lines.

flyingagin

I have some ideas on what to make a cutting table from. And some new ideals on a bow.
I have the pulleys to make a new cutting machine. they are aluminum and were turned on a lathe for me with ball bearings pressed in.
But all of that will take time. I want to fast track this bird and start getting some stick in. Get my groov back.
I am going to get some 1/16 balsa sheet and use one of the first templates to make a bunch of ribs.
I can make a built up wing faster than I can get some usable foam cores going.
I have a Balsa building board on order from Tower http://www3.towerhobbies.com/cgi-bin...?&I=LXPF36&P=7 so I will have a true suface to build on. And I have a magnectic building board to lay on top of it.
I will get a wing built up shortly.

Then I will revisit building a cutting machine. The one I made in 95 worked great. I have since seen similar machines. The wife absalutly does not want to help handle a cuting bow. To scared of electricty. She supports me and encourages me in my hobbie but she is not goin to be a foam cutting partner.

EscapeFlyer

My Feedback: (1)

Please be sure to show pics of every part of this project. I would love to gravity cut my own cores but know nothing about trig....

Brian

longdan

I remember my first attempt at cutting foam cores. I made a set up much like yours. I had a variable lab type power supply, and a wooden bow tensioned on the back side with a trampoline spring and the wire was stainless steel safety lock wire.
I aslo experienced wire drag which made for some awful cores. What I found was that as I turned up the current to get more heat into the wire for a cleaner cut, the heat would expand the wire causing it to drag no matter how much tension I put into it, until the heat and tension were too much and the wire would break.
Against all advice, I changed the wire to nichrome resistance wire and got much better results.

Boomerang1

Be careful with the autotransformer, some of these have full line voltage to earth regardless
of what you have the knob set at & YOU are at earth potential. - John.

flyingagin

Yeah longdan
I do need to turn up the voltage. The proplem with that is the templates are just plywood, so that sets an upper limit.

flyingagin

This is one to be carfull with as terminals 1 and 2 are power in and terminal 2 and 3 are power out. There is a direct connection to the source power at the bow wire.
I stand on a rubber matt. Gloves might not be a bad idea either.

After I turned the power off yesterday I did manage to find the wire with my bare arm. The wire was still plenty hot after maybe 10 seconds.

longdan

I think you will be surprised how much heat the plywood will take, as long as the wire is moving, its not going to burn into it. The foam will cut at a far lower temperature than what the ply will burn at.
I learnt a lesson when I tried to use some lasercut stainless foil templates. The stainless templates (1.5 mm thick I think) actually conducted so much heat away from the wire that it wouldn't cut the foam for the first few mm beside the templates.
I have gone back to built up wings, mainly from buying ARFs, and also building a few because I enjoy it more that sanding and glassing and sheeting and trying to cut foam cores.
Your wife has good reason to be scared of electricity with that transformer
An autotransformer can be dangerous because that central conection that you connect to your bow is still directly connected to the phase from the mains power. It basically means that if you touch it and your body has a good earth connection, then the current will take a path through your body to earth instead of through the bow to the neutral connection.
Its worse here where our mains voltage is 240V, not 120V.

yel914

Just a couple of .02 here. Formica, or other brands of plastic laminate make great, durable templates. Scraps can be had for free from places that do laminate countertops. Bearings, such as the ones shown are usually available from home centers in the screen door repair parts section. No need to have them made, although they're beautiful just the same.

LesUyeda

"plywood"

Masonite works quite well. My son and I used that when we built his Long EZ (full scale).

Les

Russmall

Plywood templates work great if you apply a thin strip of furnace tape around the edge. The hot wire will glide right over it and it works just as good as Formica and much easier to make. I can't take credit for this idea though because I learned about it here on RCU. RS

MTK

For constant chord, I simply fix the bow and move the panel through the hot wire. Simple, effective perfection every time. No need for a second pair of hands. If you wanted to get a little fancy, mount the panel on a surface that is installed on a pair of rails (linear bearings can also be used but might be pricy). The rails I use are 1" angle ally from Home Depot with the corner facing up

Tapered wings are also not that difficult once you understand that you are dealing with a piece of a pie section. There is a center point or fulcrum that you can use, and you may consider the bow as a radius of a circle, the cutting end of which is you actual hot wire segment of that radius. For low taper ratios (root : tip = 1.5:1 or less), the radius gets a bit long. A gravity table makes this task easier. For large taper ratios, the bow-radius is much shorter, so one may fix one end and simply move the other over templates.

Templates I've used before were made from 1/16" phenolic stock, also from AS&S or CST sales. But man are these time consuming, difficult to construct right. A much simpler technique I've come up with, uses simple, easy to precisely shape balsa templates with the edge lined with Teflon tape. McMaster Carr sells adhesive backed Teflon tape that works great for this. Try it, I think you'll like it......

BTW- an inexpensive power supply can be purchased in kit form from AS&S for around 40$. It's a supply recommended by Burt Rutan's company for smaller projects....

kenh3497

Hey Ken!!

I bought some stainless steel wire leader at a local fishing place. I think it was 120 LB test???? about .020 dia.???? it takes he heat well and you can get it tight enough. My bow is a 1 inch wood dowel with 1/4 music wire in each end. I splayed out wire ends about 4 inches wider than the dowel so when tensioned the 1/4 music wire was pulled in and provided all the tension I needed. drill angled hoels on each end of the dowel to accomplish this. An up side, as the wire heated and stretched, the 1/4 mw kept the tension more or less constant.

I loosely used this as a model https://www.tekoa.com/product.php?pr...2&cat=3&page=1

Ken

sidgates

My Feedback: (2)

http://www.cstsales.com/tekoa_hot_wire.html
http://www.tekoa.com/home.php

See above addresses. I purchased the Tekoa manual system I thought is was a waste till I tried it. Works great, why reinvent the machine?

Sid

flyingagin

Beleive me I have always understood the autotransformer to be dangerous. I stand on rubber matts.
Whenever I can find a 1:1 or even a 2:1 transformer of suffeicient wattage with a true isolated secondary I will put that in the circuit.

I have been surprised at how weel the ply templates have worked. Easy as heck to make, cheap. and they have not had any burn damage or cruves made by the wire. so next time I will turn the juice up some more.

My very first bow power supply was a light dimmer switch. Just as dangerous. Worked fine till the magic smoke puffed out in the middle of a cut. My buddy had a real good laugh at that or maybe it was me? Garage stank like heck for a little while.

flyingagin

Thanks Ken
That is a good idea on a bow.
The one I made a long time ago and lost during many moves, was wood with 1/4 piano wire arms, but needed a line (heavey fishing line) on top with a turnbulckle to tune the tension.
I think you have the right idea.

Ken

Avaiojet

I need a few foam cores cut.

Know anyone who's doing it professionally? Good quality at a reasonable price.

Charles