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Author Topic: Pulse Induction detector build  (Read 28055 times)
probono
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« Reply #15 on: September 06, 2015, 04:35:15 PM »

Talk to Woodbob - he's made a few detectors recently and is also on the Geotech forum
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Cymro
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« Reply #16 on: September 06, 2015, 06:11:41 PM »

Thanks - will do!
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woodbob123
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« Reply #17 on: September 06, 2015, 06:13:46 PM »

Would be happy to help and exchange ideas on PIs I'm in Newport not sure where you are.
 Also built the Magnum but didn't use it much and came back to the hobby a few years ago and as has been mentioned PIs don't miss a thing as many are designed to find very small pieces of gold in OZ and USA they sure do seek out every piece of iron and rust particle.
 I would suggest building one of silverdogs kits, before getting too advanced there is another the MPP which is an improved version of one of Eric Fosters design, not for the inexperienced
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Cymro
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« Reply #18 on: September 06, 2015, 06:58:01 PM »

Ta for the reply, woodbob123 - PM'd you.
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Cymro
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« Reply #19 on: September 21, 2015, 07:37:28 PM »

Right then - it's time for the next instalment of the PI build.

I wanted to make a solid job of the hardware, and another potential weak spot was the connection between the control box and the shaft. It would have been easy enough to chuck a couple of self-tapping screws through the box and into the shaft but the box which houses the PCB also houses the sealed lead-acid battery which is fairly heavy.

What I wanted was a connecting piece which would saddle the tube of the shaft and have a substantial flat base which I could drill and tap into - machine screws are less prone to working loose than self-tappers. I do have reasonable workshop facilities but I lack the one machine tool which would have made the piece in half an hour - a milling machine.

What I do have is a small foundry setup, so I set to and made a wooden pattern for the saddle piece - see 1st and 2nd pics. One reason it's taken me so long to get to this stage is because I needed to paint the pattern so the sand didn't stick to it - and paint takes its own sweet time to dry. Couple of coats of emulsion, sanded between and after coats, four or five coats of acrylic aerosol car paint, couple of days for it to harden . . . Soon mounts up. Then there's the weather - don't want to go pouring molten aluminium in the rain or you might just get a face-full as the water flashes to steam and expands at a rate of knots . . .

So one way and another the first day I had that was predicted to be dry all day was last Saturday. It took me most of the morning to prepare everything, then I rammed the pattern up in the sand and lit the furnace. Any odd bits of aluminium went into the crucible - an old mitre saw frame, bits of alternators, lawn mower engine - anything . . .  Took about 20 minutes for it to melt, then I poured it. Pic 3 shows the flask just after the pour, then Pic 4 shows it after I'd shaken most of the sand off it - I like to leave it about as long as it takes to make and drink a cup of coffee before I open the mould . . .

When it was cool enough to handle I sawed the sprue (the big mushroom-shaped bit) off the casting(Pic 5) and tried it against the tube. The saddle part was a bit too tight so I had to hand fit it with a half-round bearing scraper until it was just so, then trued up the remaining five sides  on the shaper (Pic 6.) Couple of hours hand work with the scraper and half an hour on the shaper got the job done.

All that was left to do then was to braze the saddle piece to the shaft (Pic 7) so I can attach the PCB box - just finished brazing it an hour ago and I'm calling that it for today. Pic 8 shows how it will fit on the box.

It looks like this part took a lot of effort for not much reward - it might even be said it's over-engineered - but I'm trying to eliminate any weak spots in the design. If I ever want to make another piece like it I'll have the pattern on the shelf ready - and making the pattern is the biggest part of making the casting.

Next up is deciding how I'm going to make the control box which sits on top of the handle - I'll update this thread as I make progress.


* Pattern1.jpg (118.68 KB, 960x949 - viewed 205 times.)

* Pattern2.jpg (136.23 KB, 872x893 - viewed 194 times.)

* Flask.jpg (212.63 KB, 1187x1084 - viewed 190 times.)

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* Brazed.jpg (179.22 KB, 1095x1080 - viewed 287 times.)

* Finished.jpg (152.36 KB, 1559x917 - viewed 295 times.)
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Cymro
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« Reply #20 on: September 30, 2015, 08:19:13 PM »

I know I said I was going to make the control pod next but I'm stalled on that while I consider whether to include a meter or not, so I made the arm-cup instead. It all has to be done and it doesn't really make much difference which order it's done in.

I'm sure you all (both . . . ?) remember the casting I made for the control box mounting? OK - since the biggest job in any casting is making the pattern, I cast another for the mounting block I intend to hold the arm cup. Just a case of ramming it up in the sand, melting the metal and pouring now that I've got the pattern.

When it was cool enough to handle I sawed it in half, machined it all over in the shaper then clamped the two halves together and mounted the whole issue in a 4-jaw chuck in the lathe. Bored the inside to fit over the shaft tube and faced both ends - 10 minutes work (or 30 if you include centering it up with a clock gauge . . . ) 1st pic.

I then needed to make the actual arm cup. I used some 1mm aluminium sheet I had lying around - cut 2 the same but drilled 1/2" holes in one of them - pic2.

Brazed through the holes so they became one piece, chain-drilled and filed the slots for the strap and then formed it into the shape I wanted on the folding jig I made at the beginning of this whole epic story. I say 'the shape I wanted,' - what I actually mean is the shape I could make with the tools I have. Whatever. Pic 3.

I drilled the arm cup, then drilled and tapped the upper mounting block for M5 pan-head socket screws to hold the arm cup on. Not much to see there so no pic. I also drilled and tapped the same part for an M6 thumbscrew (yet to be made) to hold the whole thing to the shaft. Again, no pic.

Waste not, want not. My mother dinned that into me as a child and it's stuck. I had the sprue  from the casting I'd made for the mounting block lying around. I'd normally put it in the crucible to re--melt next time I fired the furnace up, but it was a roughly cylindrical shape if you squinted at it in a certain way . . . Pic 4.

Sawed both ends off and mounted the middle bit in the lathe - a few minutes later it looked like Pic 5.

Knurled it for a finger-grip, drilled and bored it to accept the head of an M6 bolt and started parting it off - Pic 6.

When it was off the lathe I put the head of the M6 bolt in the recess, clamped it in place and aluminium brazed it in. The filler rod I'm using won't adhere to steel but it will to zinc - and the bolt is zinc plated . . . Pic 7.

When it was assembled to the arm cup mount it was as you can see it in the 8th pic, and I placed the shaft, with the arm cup onto the control box to show how it will all look in the 9th pic.

I've been thinking about how I'm going to mount the PCB and the battery, so I think I might make that my next mission before I make the control pod. After all that's done, *all* I have to do is to hook everything up and finish it all off - paint etc.











* arm cup mounting.jpg (73.9 KB, 851x645 - viewed 188 times.)

* arm cup sheets.jpg (197.25 KB, 1347x1148 - viewed 193 times.)

* arm cup.jpg (74.97 KB, 886x645 - viewed 180 times.)

* thumbscrew mat'l.jpg (75.39 KB, 476x635 - viewed 192 times.)

* thumbscrew.jpg (107.93 KB, 848x538 - viewed 428 times.)

* knurled thumbscrew.jpg (92.86 KB, 648x559 - viewed 424 times.)

* brazing screw.jpg (51.33 KB, 625x520 - viewed 622 times.)

* armcup fitted.jpg (54.99 KB, 767x602 - viewed 1202 times.)

* final pos.jpg (69.15 KB, 842x636 - viewed 1241 times.)
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Chef Geoff
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« Reply #21 on: September 30, 2015, 08:52:37 PM »

It's certainly getting there have you thought how the metal upper section will effect the coil field?
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nobby
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« Reply #22 on: September 30, 2015, 09:08:56 PM »

Massive respect for your fabrication skills...... I was also wondering how the metal would effect the function of the detector?
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« Reply #23 on: September 30, 2015, 09:16:00 PM »

Me thinks you must have been an alchemist in a previous life!  Smiley
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Cymro
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« Reply #24 on: September 30, 2015, 09:57:20 PM »

Hi Geoff - I'm hoping that all the metalwork will be far enough away not to affect the coil - it would do if the shaft was vertically above the coil but as it will be at an angle it should be out of range since the field of the coil *should* be at right angles to the plane of the coil.

Looking at the PI machines out there commercially they all seem to use the same arrangement, and a friend actually did an experiment with his machine which seems to bear this out.

Fingers crossed . . .  Smiley

Ta to Nobby and Resurgam for the heads-up - nice to have some feedback on my efforts! I've been wondering if anybody was actually following the thread . . .
« Last Edit: September 30, 2015, 10:01:29 PM by Cymro » Logged
Dryland
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« Reply #25 on: October 01, 2015, 01:52:44 AM »

"Following this thread" ? I can't wait to see he finished article lol.. As Nobby said it must be wonderful to have the skills and the know how to be able to turn your hand to something like this. If the Whites XLT etc is anything to go by, I think you should be ok as far as the upper metal section affecting the coil, but I speak as a complete novice in these things.
                   Just a thought but  Whites use a carbon fibre lower shaft to lighten the detector, Have you given any more thought as to the final weight of yours ?
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Cymro
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« Reply #26 on: October 01, 2015, 06:17:47 AM »

Hi Dryland - thanks very much for taking such an interest in my project, and for the complimentary remarks.

Have to confess - I cheated . . . I did this sort of thing for a living for 25 years so I'm not a complete tyro at it. Got made redundant in my 40s cos no-one wanted those sort of skills (it was cheaper in Taiwan/India/China or somewhere . . . ) but it gets in your blood. I'm lucky that it's also my hobby nowadays - I set up a decent workshop years ago and I'm always on the lookout for opportunities/excuses to use it.

As you'll know, the design of the machine is a straight steal from my old Whites Eagle Spectrum which was a heavy machine to begin with (even more so after a previous owner changed the 'C' cell rechargeables for 'D' cells . . .) but the balance is good.

The heaviest part of this machine is going to be the battery - it's a sealed lead-acid one because PIs are power-hungry. I deliberately haven't fixed the position of the control box yet so I can experiment with it to get the best balance I can. If you can't do anything about the weight at least you can make sure it works with you rather than against you . . . The rest of the metalwork isn't that heavy.

Hey - who knows; if this works out maybe I'll get a queue of people wanting bespoke hand-built metal detectors . . .  Cool
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Resurgam
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« Reply #27 on: October 01, 2015, 06:49:53 AM »

                            Following you all the way and looking forward to when you run the beast over a wet salty beach.  Smiley

                           As an aside, I noticed Geoff's comments about the possibility of the metal structure interfering with the coils magnetic field from above and I set to wondering if the magnetic field from a coil can be induced to be directional like some short wave radio aerials. ie.......big magnetic field below the coil and minimal magnetic field above the coil; or the magnetic field above the coil deflected back down to strengthen the lower one?  Undecided 
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Cymro
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« Reply #28 on: October 01, 2015, 09:08:14 AM »

Interesting idea about a directional coil . . . I passed the Radio Amateurs' Exam 30 years ago (never got licenced though . . . ) and what you're describing is a beam antenna. It uses a driven element (which is connected to the radio) with parasitic elements - a reflector behind it - which acts just like a mirror - and multiple directors in front. They're arranged on a logarithmic scale IIRC and the calculations would make your eyes water. Together they produce forward gain by preventing the radio waves from going in all directions as they naturally would from a single vertical (or horizontal) wire and focusing them in the direction the antenna is pointing. That's what a TV aerial is and does.

The size and spacing of all the elements is dependent upon the frequency you want to operate on - the higher the frequency the smaller the elements - that's why you see huge arrays outside Hams' houses if they're working (say) the 11-metre band (that's 25 - 26 MHz) and much smaller ones if they're on 2 metres (144 MHz-ish. . . )

You can make the antenna smaller by using a fractional wavelength - say 1/2 or 1/4 wave but at cost of efficiency. I'm not an antenna designer so there are probably (make that definitely) some tricks I've missed . . .

When you consider that the PI I'm building runs at 100Hz you can see that the coil would be . . . ummmm . . . cumbersome. Even for your CS4 running at 17KHz it would need to be massive. Interestingly, the Sniffer I'm building is designed to use big coils - 1 metre square or even 2m x 1m - the 280mm plain loop I intend using is inferior to their depth but you have to balance that against usability.

Thanks for following - it's good to know folks are interested. If it eventually works I'll let you have a play with it . . .

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woodbob123
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« Reply #29 on: October 01, 2015, 03:37:45 PM »

Fantastic job with the casting what I use to make up shafts is much simpler not having a furnace etc (wife wont let me) using standard Ali tube turn down a plug of plastic 1-2" long to a "tap in fit" tap it down to where the strength is needed then either fix it with a couple of self tappers or make up a "swaging tool" by removing the cutter from a pipe cutter and replacing it with a rounded blade and running that around to force Ali into plastic. I use this method a lot when making telescopes and stuff. Hope that makes sense. Once fixed it can be drilled through with no danger of crushing the tube however much you tighten the bolts.
If you use plastic pipe for stem it can be made much stronger by wrapping with the epoxy bandage they use to make lightweight casts for broken arms available from e-bay in black looks a little like carbon fibre from a distance and can be painted.
Cymro I've sent you a pm.      Bob
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