RC Keith CNC Hot Wire build log

[thenated0g]

At the moment i have a selection of springs now and they are working "good enough", but this is definately in my list of upgrades for the future. Along with a more permanent table for the foam to sit on that doesn't require any weight to keep the foam in place. I plan on making a vacuum powered hold down box using peg board.

 

[TheSFReader]

Along with a more permanent table for the foam to sit on that doesn't require any weight to keep the foam in place. I plan on making a vacuum powered hold down box using peg board.

Not sure how that would work when cutting the underside of the profile : wouldn't the upper part separate/ shear ? I suppose the cut starts with the upper side, which could make the problem less problematic ;p

Thanks for the detailed log, it's really interesting even if I don't plan tu make a hot wire CNC.

 

[thenated0g]

Yes, most of the cuts so far that i have seen are top down, so most of the forces are horizontal. But you dont want anything as high as the foam cause your wire path may get stuck on stuff. So i was thinking that a totally flat area with holes under it would work well. Plus i have all the scrap material to try it out. And i have always wanted a vacuum forming station.....

 

[thenated0g]

Since i am having issues with the gcode generation and havent quite got Jedicut figured out yet i was trying to think of something i could cut and get working on. I realized that my only problem is with swept wings. So decided to take my swept wing plank setup and just do a straight wing plank instead.

Basic idea is a 60 inch wingspan slope plank with a nose out front to help with cg and a tail in back. Something like this but unpowered:


So i generated some code using the pw1211 airfoil. 265mm tip and root chord, 1524mm total wingspan. Cutup 4x blocks of foam that are 300mm deep and 15 inches wide. These just need to be glued together to get the 60in/1524mm wingspan. Grab a large tin can to use to round the leading edge wing tips. Add in spars, cover, etc.


Than i decided to try and tell linuxcnc that the foam was about 2 inches higher to try and cutout a wing saddle in the fuselage. I stuck two pieces of 1" foam together with double sided tape and place horizontal to the table. Worked out pretty good. I was able to find the offset setting, cant remember at the moment what its called, but its right next to "home".



After cutting out the wing saddle i did a manual horizontal cut right across the top of the airfoil. Now i can shape the fuselage and put the electronics internally.

 

[thenated0g]

 

[thenated0g]

First off the conversion to using the free Jedicut software is going well. Have a couple issues to figure out, but this is more related to using the minimum amount of foam, the cuts are really nice. Here is an example of just the top cut. I like to take scraps of foam on end to make sure the speed and location on the table is correct. In this picture you can see not only the top airfoil but also the spar track i programmed in. So no more router, hot melting, or razer work to get a slot for spars. Pretty amazing.

I have continued to work on the straight wing plank. One thing i realized is that the trailing edge is so fine, its pretty much useless. Im not sure if this is because the cnc is so precise or its simply this airfoil shape, but basically i lost almost 20% of the wing chord. So now i have a wing that has "failed" on two accounts. One i couldnt make it swept so i went straight wing and now it has a shorter chord. But thats ok, i will make it work, CG will just be more "fun" now

Basically in the future i am going to create my plans as normal, but than when moving to the cutting software i am going to add about 20% to all the chords to compensate and see what that does to the final output.
I used hot glue (low) on the wingbeds to get the 4 cores glued together. I added a 48" fiberglass spar about 50mm from the leading edge and top and bottom and than a 24" spark closer to the trailing edge top and bottom. Here are some 50mm balsa aileron stock in place. I removed the trailing edge of foam so that the 2 thickness's matched. Final shape is 175mm of foam and 50mm aileron stock for a root/tip chord of 225mm.


Than i wrapped the entire wing top and bottom with single direction filament tape and than all the edges got bi-weave filament tape.
Couple rattle cans later and than ironed on 5mm laminate. This thing is STIFF.


I shaped the nose cone i had made in the previous post, cutting out a hole that matched the width of the battery with room for servos and reciever. I added a couple pieces of balsa to help with nose landings and the wing being force forward and also added a carbon spar inside to help with #landed.



Finally i wrapped the fuselage in some light fiberglass and 5 minute epoxy. I was in a hurry, its "good enough" for my flying. Installed 2x 9gram servos with some carbon fiber and heat shrink tubing reinforced control rods. Cut out a TLAR vertical stablizer. Both the stabilizer and fuselage are connected to the wing with the red double sided tape. CG at the moment is about 25mm behind where it needs to be so some lead will be needed.

 

[thenated0g]

 

[iCrash]

Great to hear you are continuing to work on this. I'm looking forward to seeing more on this. Good luck with your project! Let me know if there is any part I might be able to help with. I use NCGen for my CNC. I love the plugin, but don't care for sketchup so much.

 

[thenated0g]

Previously i had stated i was having some issues getting jedicut working correctly. I had some time this past week (ie it wasnt as hot in my shop so i actually used it) to play around with jedicut and I got some responses finally in the jedicut forums. It appears my software is working fine, it was my understanding of the situation that was wrong.

HOME position of my machine is when everything is fully collapsed against the wall. This becomes 0,0,0,0. That is all axis are hitting the homing buttons. My assumption, which was reinforced by the way the free webpage software i was first using worked, was that the software would generate a cutfile that would start from home and work its way towards the full extension of my table. So it would almost never fully extend the horizontal axis of X and U to 710mm (28" drawer slides). And so when viewing the cut wizard in jedicut i assumed the trailing edge would be right up against HOME on my table. So the placement of my foam should be the straight edge of foam right against HOME, as in this picture:

But what is actually happening, by design, is that it is taking the deminsions you tell it about in setup, meaning my 711mm table depth, and placing the foam at the other side. At the full extension of 711mm. As seen in this picture:

Now this isnt really a problem. As i stated the cuts turn out perfect as designed. The issues was more that i had not placed my towers in a way to take advantage of this and i dont have any table at that end yet. Long term i had planned on making a more permanent table the full section of the cutting area. So this weekend i started working on that. I decided to make a downdraft/hold down table using peg board and a vacuum.


Before i lock it down in position i need to finish testing a new way to tension the wire. At the moment i am using various tension springs from the hardware store to keep the wire tight. The problem is that at different positions on the table the wire can have almost double the tension as the towers can get up to 10" more apart when one side is fully extended. So i am working on a pulley system that will keep the tension the same at all times. The purpose of the pulley is not to get any mechanical advantage but to keep the tension right around 2lbs and have 10" of adjustment in the line.

 

[thenated0g]

V1 of my pulley tension setup. String will be in the pulleys and than connect to a ring and than cutting wire.

 

[thenated0g]

 

[thenated0g]

 

[thenated0g]

When 3d printing you will usually print a test model, like the benchy or test cube, to make sure that the machine is working correctly. Printing this 2 part wing was my benchy. Taking two different widths of foam and cutting them in a way that the root chord of one meets up to the tip chord of the other.

One thing i did notice though, and it makes sense, is that when cutting a smaller section of foam i need to turn down the power. If you think of it like everywhere the wire is touching foam it is taking power from the wire than obviously when cutting a smaller section of wire you need less power...or in this case i didn't turn it down and melted the top a little. Oh well. Its sandable and even if i threw it away its like $2 worth of foam. That's a cheap lesson.

 

[thenated0g]

 

[thenated0g]

 

[thenated0g]

 

[sheppard]

That looks great!

 

[Scottsdale John]

Shifting gears and going to a different gcode/cutting file design software.

Over the past couple weeks i have been slowly dialing in the speed and heat i want for this xps/Homedepot foam. I have finally settled on Rene 41 28.5g wire. It is amazingly small and strong. Kerf so small its almost like doing a razer blade cut on the fast side (root). I guess because it is so small, the mass, that even putting what would normally be too much heat thru it, it just doesnt radiate as much heat out into the foam, or not as far. Pretty happy with it. I think i got about 10 feet for $6 or so dollars from Jacobs online. Shipped super quick.
At the time of posting this i am using 100mm a minute speed and around 11 watts of power, around 11.96v i think.

Here is an example of how nicely its cutting as of 6/16/2018

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But, since i have been testing on scraps, i have never had a reason to try cutting out a left and a right side. This weekend i was attempting my first project and ran into problems with the mirrored side. Long story short, there is something not quite right with the web page cutting program. One side not only cuts a different speed, but the geometry is drastically different as show below.
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Linuxcnc display window is fine, but it comes out really strange. It should be cutting out both roots on one side and both tips on the other, which it is, but as shown they are way different.

So as I wait for the web page to get fixed Keith suggested i try out Jedicut. Its also a free product, but is a windows program, not a webpage. It will do cutting files and CAM, but i will be using it to output a gdcode file since i have linuxcnc working already. While they do have an english version to download, the error messages are still in the french language, so some googling is required lol

Also as a side bonus when googling issues with it you pretty quickly get into some light saber battle results

I will post as get more into this program.

https://www.jedicut.com/en/

View attachment 108984

Did you get your problem with the left and right panels cutting at different speeds? I am having the same problem with Jedicut when there is a lot of sweep back (similar to your illustration above)

 

[thenated0g]

My different cutting speeds was not with jedicut it was with the previous free webpage i was using. I havent had any speed issues between doing left and right cuts.

 

[TheSFReader]

One wonders if the

When 3d printing you will usually print a test model, like the benchy or test cube, to make sure that the machine is working correctly. Printing this 2 part wing was my benchy. Taking two different widths of foam and cutting them in a way that the root chord of one meets up to the tip chord of the other.

One thing i did notice though, and it makes sense, is that when cutting a smaller section of foam i need to turn down the power. If you think of it like everywhere the wire is touching foam it is taking power from the wire than obviously when cutting a smaller section of wire you need less power...or in this case i didn't turn it down and melted the top a little. Oh well. Its sandable and even if i threw it away its like $2 worth of foam. That's a cheap lesson.

One wonders if there could be a way to adjust the power given a target temp and actual load on the wire (through either pre-computation, or measuring the wire's actual temp)...