3D printed former for 3DSS coil intended for Minipulse

[aeberbach]

Hi all, I'm new here, have a minipulse kit on the way. A couple of weeks ago in Beechworth, Victoria, I met an interesting guy who showed me an ounce or so of gold found "nearby" - and the kids can't believe I'm not rushing out and buying a $10,000 detector right away, because obviously I'd make that money back in no time!

Anyway I read the Chance PI Coil http://www.geotech1.com/forums/showt...Chance-PI-coil thread as many in the Minipulse forums suggested and I think I'll try building one of those. Fortunately a big shiny new 3D printer just turned up at work so rather than cut out a coil former I made this 3D model...

All the extra holes are to save on filament, it might even improve the coil to have more air and less former. Any thoughts? I've made it 210mm diameter (close to the 8" people talked about) with slots to accommodate 20AWG teflon-insulated stranded wire, which I have plenty of. The form is 3mm thick and the slots 3mm wide, so the wires should cross at right angles. Hopefully I can print this before the end of the week if nobody points out any flaws.

[6666]

so rather than cut out a coil former I made this 3D model.

Well done , Ok so how much posted here in Oz ?

[aeberbach]

I put it on Shapeways: http://shpws.me/LYUY - AUD$36 or so. (I make nothing, there is zero markup)

I wish I could offer to print using the 3D printer I can use but producing multiples might be stretching the privilege a bit far. Interested to know if you can see the item in the Shapeways store since I just set it up. Maybe wait until I print mine, but I don't think there's anything to change.

Once I learn the 3D software a bit better it should be possible to produce any size of circle or ellipse if people want different. I will probably also do a housing, coil mount, case for the PCB & controls, arm brace, etc. as the project progresses.

[FatBob]

"20AWG teflon-insulated stranded wire"

20 AWG is a bit big. Most people use 24 or 26 AWG.

The coil will see itself if the surface area of the wire is too great.

I used the inside of a coax once. It took days to get the shield off.
In the end it didn't work too good as it could see a big target (itself) all the time...

I think it was 18 AWG solid wire.

[baum7154]

A very nice looking coil form.

I agree that the 20 awg wire will be too large if you want the coil to be as fast as possible because larger wire means more undesired capacitance. Also in the optimization of the 3DSS coil it is important to use at least 600 volt PTFE wire insulation. Thinner insulation will again mean more undesired capacitance.

I am curious how deep the slots are in your form and the width across one side of the form? I have found that the width of about 1" to 1.1" gives good performance with slot depths of .375" for 24awg wire. Is your coil form 210mm in outside diameter? If so the coil center will come out pretty close to 8".

Good luck in your build!

Dan

[baum7154]

Looking at the grid background of the coil form screenshot it looks like the slots are 10mm deep and the form is about 33mm wide or 1.3". I have not fabricated one that wide thinking it may begin to degrade the self shielding characteristic of the 3DSS. It would be interesting to see if that turns out to be a concern. It may also turn out that it improves sensitivity. In either case it might be better to make a cardboard form of about 2" width with .4" slots and wind a trial coil to see the effect of the wider form. One other effect will be that the coil will take a bit more wire to get to your inductance target value.

Regards,

Dan

[Chet]

Hi aeberbach
You have made a nice looking coil former. The holes and additional cutouts are good. They provide more air dielectric spaces between windings which reduces capacitance. I agree with Dan on the wider pattern. The coil will turn out flatter than the original dimensions. This will most likely diminish the self-shielding effectiveness. The performance may be similar to a flat Spider wound coil which is easier to wind.
Have a good day,
Chet

[aeberbach]

I'm still working on this so those suggestions are useful, thanks all. I'll pick up some lighter wire.

[aeberbach]

Here's my first 3D printed coil former.


This is the largest one-piece coil I can print at 200mm diameter. I took some advice from people here and decided to go with lighter wire, choosing PTFE-insulated 26AWG 600V wire with an outside diameter of ~1mm. Thus the slots are 6mm deep, 2mm wide and the former is 2mm thick. I did not produce one with all the cutouts as the first image showed but will do so and will be able to test one against the other with identical wire in the same detector - I have a scope so hopefully some useful information will come out of the exercise. The model is available on shapeways:

http://www.shapeways.com/product/SAM7K3CEP/200mm-coil-former-for-metal-detector?li=shop-inventory&optionId=59586209 - US$18.

I'll be working on a connector design so that I can print coils in three or four pieces, jigsawing them together to form a larger former. For me the ABS will be easy to weld with a little acetone on the join. If anyone wants a larger former ordered through shapeways it shouldn't be an issue to print a much larger shape. Of course the print is more expensive if price goes up...

If anyone wants the .SLT file just PM, I don't think I can attach a file which is not video or image here.

[aeberbach]

...and here's the cutout version. Same as the previous one but with less plastic used. Effect on coil characteristics to be measured. When my wire arrives. When RS says 5 business days delivery they mean "and not a minute sooner".


Shape ways link:
http://www.shapeways.com/product/KQT...metal-detector

[Qiaozhi]

If anyone wants the .SLT file just PM, I don't think I can attach a file which is not video or image here.

Compress the file to zip or rar.
Both these file types can be attached.

[raygdunn]

Hi aeberbach,

That is a great idea for an accurate former. I keep finding things that could be 3D printable and wondering if it's time to buy one. But the market is still moving forward and signs of much faster speeds, so waiting seems a good option.☺

Both producing the coil former and winding I found a bit of a challenge. I used the original Russian (I think) template glued on 1/16" ply as a former and a lot of fretsaw work.

I took over an hour for my last (3rd) wind. One has to be really focused to move forward the correct number of slots all the time, whilst making quite sure one doesn't drop the reel of wire ...which would just love to unravel. Admittedly I was using the thin end of the wire range, which made this worse. With thicker stiffer wire it's probably easier to take breaks, without losing wire tension. I'm thinking that your weight saving notches may make the concentration required more challenging. There's one way to find out☺

I feel the style of coil is worth the effort though, and the structure makes it easy to tune the inductance, by putting extra on and taking some off. It's a pig with the regular wire coils, if you get it too far out.

On your first wind, the thing to watch out for is the end of the first layer. The obscure point is moving forward one slot on the inside and then "winding from the other side" so to speak.

Good luck with your build.

Ray

[baum7154]

3DSS COIL FORMS

...and here's the cutout version. Same as the previous one but with less plastic used. Effect on coil characteristics to be measured. When my wire arrives. When RS says 5 business days delivery they mean "and not a minute sooner".
[ATTACH]35770[/ATTACH]

Shape ways link:
http://www.shapeways.com/product/KQT...metal-detector

__________________________________________________ ______

While this cutout version saves material and a small amount of weight I wonder if it has the necessary rigidity to allow proper winding tension during coil fabrication. It seems to me that this form will be too flexible and will consequently take more time to wind well especially with larger wire. I think your form without extra cutouts should be much better in this regard especially since the form thickness is only 2mm or .079". My original designs used .093" thick LEXAN, a much stiffer plastic than ABS, for this reason. I guess only winding the coils can reveal the truth on this.

Regards,

Dan

[Qiaozhi]

You could most likely make something similar with a tabletop CNC machine.

[baum7154]

Agreed, a CNC router would be great for this work and can machine most any reasonable thickness plastic material with great accuracy and repeatability, probably at lower cost per unit.

For development and optimization of the 3DSS coil I did my form on a manual milling machine with a rotary table but I also posted a hand tool process to do the same thing in the CHANCE PI COIL thread for those with out access to machine tools.

Dan