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4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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Another Coilgun

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Author Post
Wed Dec 14 2016, 01:53PM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 504
Its about 1kg of plastic equal to 50€ worth of filament.
Yes i am using copper for the prototype because aluminium is extremely expensive because it would be custom made with minimum 10kg per wire diameter which is quite a lot of wire considering its aluminium. For the prototype its not an issue. It will be uncomfortable heavy but it will work.
Square aluminium is also only available with >=0.5mm "diameter". My first coils would need 0.25mm so this prototype now also explores how i can adjust to thicker wire through PWM switching in the first few stages and what it does to the projectile and efficiency.

The prototype in the beginning exploded at stage 16 due to mechanical failure of a diode in the half bridge. Look at the beginning of the thread for photos of exploded parts, It was the end of the big prototype. Since then i did never shoot again.

Building the Coil-Winder now adds a bit delay to the project but being able to wind the coils is important and it also has the benefit that i am developing software for the microcontroller architecture that i am going to use in the end.

Meanwhile i have implemented the stepper motor drive and acceleration profiles and i am confident that this coil winding machine can do up to 3 turns per second. Its quite fast compared to hand winding however its quite slow considering its a machine.
The problem is the stepper motors loosing torque as they become faster. The fact that i added a 5:1 gear is therefore a problem, however i need it to ensure i have sufficient torque. As consequence i would need a way bigger stepper motor for direct drive of the winder, which means more cost and also much more difficulties driving the motor since there is no payable of-the-shelf solution anymore.
Anyway. first experience with a coil winder. prototype. good enough.
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Mon Jan 09 2017, 07:27PM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 504
First very primitive test inductor.

Its just a single layer of 0.2mm wire to see how precise the steppers and the wire tension algorithm handle the job.
Its denser than hand wound for sure. Not by much however.
Wound with 2 turns per second. 90% of the turn is completed until the horizontal correction is done. i will have to learn about the different behaviors. i guess with such fine wire this is ok, with thicker wire maybe not. But its in no way a limitation of the machine, its just the way i programmed it to do the job. i can also go gradually side-wards of course.
The start and the end of the inductor is not messed up. i programmed it that way to start and finish with a few turns of larger wire distance.
Now i need a desktop program to make more complex movements.
The white between some turns is not bad wire distance.. its resin that comes through.. light does the rest. (i wanted to do a finished coil just for fun.. therefore i glued it in place directly - taunting few minutes. )
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Sat Feb 18 2017, 09:20AM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 504
So its time for another update again. There was not much progress made to be honest because i am catching up on tooling here. In a sense this much is progress but in terms of the actual gun coming to life.. hmmh. As i posted above i am still building the coil winding machine. The PC-Control program is basically done and ready to use and what is still missing is a good coil former and actual knowledge how to wind a coil. Layer-changes are a bitch.
For the coil former i actually bought a CNC milling machine off Ebay (CNC3020T with 800W Spindle) which was a lot of work to get operational and of course its quite time consuming to get into all the software associated with it - tool path generation and so on.
However with this now i am quite capable of doing a hole lot of stuff in combination with the 3D-Printer. I honestly feel amazing right now. I feel the difference in quality i am able to produce if i can make the proper tools for the job. 3D-Printing the coil former was not an option btw, because the layered (surface-)structure of the print disturbs the coil turns. The only solution is to sand the surface which in turn will produce dimensional errors. So milling is indeed the right way to do it.
Here is a side wall of a milled coil former:

I wont go deeper in to its features because this part is not finished yet an the next iteration of the design is milling right now... i mill in POM plastic which is good for not adhering to epoxy.. the surface is really smooth but the material is a bitch to work with. Its too ductile and therefore hard to cut cleanly. The tool marks you see are purely optical.

As for the whole coil winder side project i want to share my doubts with you. I catch my self loosing my focus to such side project which now results in the whole CNC stuff and so on. I get side tracked a lot which does not help the project at all. Its sometime really depressing how much work is ahead of me when i realize how much of the work i dont even realize. Its always like a factor of pi more.
My problem with the whole project is that i really want to achieve something new and like Barry said - "next gen". Its impossible to invent stuff in this field because everything thats viable has been done.. its only possible to improve the current state of the art by just doing everything the absolutely right way or at least as good as i can do.
This results for example in a more complex coil shape which in turn takes time to perfect. I try to keep telling myself that all for a greater good, so when my prototype is finished i have something to show - and not only that.. i also extent my skill set so if the future ever brings me to a point where i will beg for money to make the gun bigger people can trust me. Yet i realize that conveying this trust is impossible and cheap marketing bullshit would be way more effective.
I also do not know of any alternative to how to make my desired coil shape except having everything custom made.

Anyways. I am not far from making my first coil fully automatically and in a repeatable way. I will share the results with you. Time will tell if the machine could improve the L/R-time constant of my coils which is currently 1.7ms. Actually i think it wont change much.
On the one side my coil geometry becomes worse because the inner layers have less turns to accommodate an air cavity inside the coil (this is required by my projectile sensor system) which reduces power density. On the other hand i really optimized the inner shape of the coil to reduce the air gap and to reduce overall wire length by a few percent.
Additionally the overall design (i hope) will experience an energy density increase of >10% because the coils are only separated by 1.0mm thick PCBs instead of 4mm wide spacers which hosted those unreliable light traps.

Edit: i have HUGE problems with my router:

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