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Registered Member #61926
Joined: Wed Nov 22 2017, 04:08PM
Location:
Posts: 34
DerAlbi wrote ...
I have built a 3d printed winding machine too (fully automatic) but it was way too flexible.[...] However I have found that there is no substantial loss in performance of the coil, if wound wild. It is really more a cosmetic thing. Keep going. Dont invest too much time in printing. Its too far from viable.
Wow, I can hardly imagine how long that machine took you to make. You do not have any images/videos of it, or do you? Did you construct it for mass facturing your coil gun?
I will not try to get perfect coils anymore, the wild winding style should be sufficient, but of course I still want to make some improvements. The last coil created the problem that I could not remove it from the acrylic tube without damaging the coil (see photo), even though I had several layers of paper wrapped around the tube to prevent this from happening. On the next try I will have stronger printed limitation-discs that can be slides without deformation, and I will take more care of the paperwork. Is this a common problem?
I only changed a little bit on my winding machine: The two pink big cog-wheels are directly connected to the nuts on the threaded rod (melted into it), so I can move the complete rod instead of just the central part like last time to prevent it from clogging up. Also I improved the wire routing and other minor things, but I know it is still very primitive. At least the conversion was fast and simple. Unfortunately I do not have enough wire to test it at the moment.
V2006 wrote ...
But with such an investment of time and labor, I would have wound it up manually a long time ago.
I guess you might be right, but I thought the procedure would have been too complex for a two handed person like me without some helping device. In the end my machine is just a mounting for the rod and a clamp for the wire, so I have to confess that a lot of time was wasted.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
It was during the time i was new to CAD and 3D-Printing. It took about 10 weeks i guess. But i also wrote the software for the stepper motors, the wire tension system and the PC (usb) user interface where i could program the machine. I have the machine still in my room, but i dont plan to ever use it again - its not in shape for a photo. I didnt had full automatization in mind, but just to get a high quality coil (which i didnt get from the machine). It was basically a hind-winding machine, where my hands executed stepper motor sequences. Once refined i could ususally play sequences that wound 2 layers failry quickly.
Your coils look fine. You might want to make the sides of the coil former aluminum to make the sides stiffer. Put a layer of this brown packaging tape on it and treat it with vaseline (this makes the epoxy not stick to the sides). For the inner coil former i recommend POM plastic rods (get some from ebay). Epoxy does not stick to POM plastics. Then use epoxy that does not hardens quickly. 1h minimum - better 3h to give you time to work. After every wound layer apply epoxy with a brush (you can clean the brush with acetone afterwards). Dont use too much epoxy or it will ooze out. After you are done, heat the coil up to 80 - 100°C which is easy by running current through it. You can use step-downs from ebay that can deliver 20A or more ; Heat the coil with high power, then let it sit at 10W power consumption to keep it warm. With high temperature your epoxy will cure fast after you are done winding. It should give you very good coils and disassembly of the coilf ormer wont rip apart the coil.
That said, your coil shape seems a bit weird. They are very short and fat. Got such a short projectile?
Registered Member #61926
Joined: Wed Nov 22 2017, 04:08PM
Location:
Posts: 34
DerAlbi wrote ...
Your coils look fine. You might want to make the sides of the coil former aluminum to make the sides stiffer. Put a layer of this brown packaging tape on it and treat it with vaseline (this makes the epoxy not stick to the sides). For the inner coil former i recommend POM plastic rods (get some from ebay). Epoxy does not stick to POM plastics. Then use epoxy that does not hardens quickly. 1h minimum - better 3h to give you time to work. After every wound layer apply epoxy with a brush (you can clean the brush with acetone afterwards). Dont use too much epoxy or it will ooze out. After you are done, heat the coil up to 80 - 100°C which is easy by running current through it. You can use step-downs from ebay that can deliver 20A or more ; Heat the coil with high power, then let it sit at 10W power consumption to keep it warm. With high temperature your epoxy will cure fast after you are done winding. It should give you very good coils and disassembly of the coilf ormer wont rip apart the coil.
That said, your coil shape seems a bit weird. They are very short and fat. Got such a short projectile?
Thanks for the thorough advice! As Epoxy I was using "UHU Endfest", which has a very long hardening time. On the subject of the short coils: "Tomspol" wrote that ealier (14 posts above), too, but I did not understand why the coil should be of the same length as the projectile. From my little knowledge a shorter coil should have a stronger magnetic field, but I would like to be corrected if that is wrong. My plan was to make two different coils as a short-term objective and test which one works better.
V2006 wrote ...
Zuckerstange47, I can send you my coils.
Thank you for that offering, but I would like to make my own ones ("the route is the goal"). I already wasted too much time on the machine to stop making coils now Furthermore I already got new wire today.
Registered Member #61926
Joined: Wed Nov 22 2017, 04:08PM
Location:
Posts: 34
I just realized that I might not be able to build a coil gun the way I planned to, because I did not read the fineprint on some component's manual (and as mentioned earlier I am a newbee on the subject of electronics). When looking at the datasheet I thought my MOSFET could be used for switching 209A @ 75V, but when looking at the "Maximum Safe Operating Area"-Graph it seems that the 209A (continuously) are only permitted at a voltage of about 1.4V, and that only 0.5A can be switched @75V for a time of 10ms. Is my interpretation correct?
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Well, if your mosfet is on, then there is just little voltage drop across the mosfet (since Drain sopurce is virtually shorted), so the specs make sense. Also if your mosfet is off (Drain-Source not "shorted"), which allows the 75V across drain.source, then NO current will flow, so you are below the 0.5A rating as well. The only critical thing is, switching from On to Off. There is a transition period where there is actually 200Amps AND 75V across the mosfet. the faster you switch the shorter the time. And you will switch faster than 10µs. So dont worry about the 10ms rating. However the energy from the coil must go somewhere. The current will continue to flow even if the mosfet is off. A freewheeling diode wont conduct instantaniously but it will also take time for it to "turn on". This effect (forward recovery) does not exist in Schottky diodes, but they often dont have a beefy current rating and only 2x overload safety - in contrast to silicon diodes which suffer from slow forward recovery but are much more resistant to overloading. Why am i telling you that? The time it takes the diode to take over the current is time that the mosfet spends in the transition state between on and off - this is what is (mainly) shown in the SOA diagram. Honestly, just try and see what happens.
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Yet another coil gun prototype
Furthermore I already got new wire today.