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Old 4th Oct 2017, 8:45 am   #21
Ed_Dinning
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Default Re: Large inductor lathe wound to a spec?

Hi Al, yes there is lots of obscure theory about inductances and the shape, material and diameter they are wound on. I suppose air is the standard for low loss, but difficult to make a self supporting coil with 0.4mm wire, and if currents are high there will be mechanical forces to take into account as well.

I do have a lathe, but it is only a small Myford ML7 and I don't have a full set of change gears for screwcutting.

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Old 4th Oct 2017, 3:18 pm   #22
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Default Re: Large inductor lathe wound to a spec?

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Originally Posted by Ed_Dinning View Post
This is easily done if you cut a 0.8mm pitch thread.
Hey Ed,
A few questions, please...

Q1. what depth of substrate/material would I need to make that pitch viable, do you think. 5mm thick? 3mm?

I'm thinking of using polycarbonate if it needs to be reasonably thick.

Q2. But just to check , as it's much cheaper,does it make sense that PVC would be somewhat lossy at RF, or do you think it's ok? I'd have to check the thickness depending on your answer to Q1!

I know there's a huge voltage gradient from the earthy end to the antinode but I seem to remember that it becomes somewhat soft at the 'hot' end, proving it to be lossy?

Thank you!

Best wishes,

Al
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Old 5th Oct 2017, 8:50 am   #23
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Default Re: Large inductor lathe wound to a spec?

Hi Al, ideally the "thread " depth should be about 1/2 wire dia, no need to make it much deeper.
The thinner the substrate the better (within reason) and provided it is stiff enough as the material loss is proportional to the mass of material.
If it softens it is too lossy!

Have a look on line for books on material constants. I have a good one Reference Data for Radio Engineers by STC from the 50's. This will act as a guide only as there are many more modern materials about now. Problem is that they are mechanical materials so electrical properties are not always listed/known and constituents may vary, affecting electrical properties.
You need a bridge with a good loss balance circuit

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Old 5th Oct 2017, 9:19 am   #24
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Default Re: Large inductor lathe wound to a spec?

If the former were long, thin or flexible I can foresee problems with screwcutting it, in as much as it would deflect away from the tool. This effect would be greatest nearest the centre of the tube and could result in a shallower groove there when compared with the ends of the tube.

It's possible this effect could be minimised by using a travelling steady, but the only way to find out would be to try it and see.

Winding coil the wouldn't present too many problems, but some means of maintaining constant tension in the wire would be needed.

Checking my own lathe shows that the maximum former diameter for a typical amateur's lathe would be about 5".
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Old 7th Oct 2017, 7:26 pm   #25
David G4EBT
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Default Re: Large inductor lathe wound to a spec?

Unless I've missed something, I'm not clear on what diameter and length the former would need to be. As Graham says, there's little prospect of threading something like PVC because as soon as the cutting tool is more than perhaps 3 or four inches from the end of the former, the pipe will flex. The smaller the diameter, the worse will be the flexing. The only way I can think of avoiding that would be to make an expanding wooden mandrel to insert into the former to keep it rigid. That sort of time-consuming project is strictly DIY ('Do it for Yourself') - not 'GSI' ('Get Someone In').

The Mandrel would need to be turned from 4 square section blanks, glued together using hide glue and newspaper, then the square blank turned to round with a roughing gouge, then splitting the four sections. A length of threaded bar say 12mm diameter would need to be fitted with a cone at each end, and would need to be passed through the centre of the quartered mandrel, tightened onto each end to expand the four sections onto the inside of the former to hold it rigid. See rough sketch attached. Has to be held absolutely dead centre in the lathe - just 0.5mm out of centre is 1mm difference from side to side of the former. If 0.5mm out of true, to cut a 1mm deep groove, the tool would penetrate on one side of the former, but would not penetrate the other side at all - hopeless of course.

Clearly a rigid former made from something such as acrylic or glass-fibre with at least 5mm wall thickness would be a much better bet, but at say 100mm diameter would be eye-wateringly expensive.

An alternative worth considering would be rigid plastic draining pipe, which comes in 110mm (4.25") and 160mm (6.5") diameter. It's very rigid - it has to be, considering that it's usually buried underground. I used a 6ft length for a 'steam box' to steam lengths of ash to bend around formers to make a Windsor chair. Not affect by steam, and totally rigid. As to its RF capabilities, the only way I know if testing that it to put a sample in a microwave over, alongside a bowl of water (not in the bowl of course!), and rung it at 1kW for a few minutes to see if it heats up. If it heats only slightly, then if it's good at microwave frequencies, it should be fine at lower frequencies.

This sort of stuff:

https://www.drainagesuperstore.co.uk...ipe-110mm.html

For turning between centres, I guess a shouldered wooden plug would been to be turned to fit in each end of the former - the tailstock end running on a live centre, the headstock end held rigidly in the chuck.

Just thinking out loud really - as I say, I haven't really got a clue what diameter and length the former needs to be, so all of this might be total tosh.

Good luck with the project Al.
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Old 8th Oct 2017, 1:41 pm   #26
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Default Re: Large inductor lathe wound to a spec?

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Originally Posted by David G4EBT View Post
Unless I've missed something, I'm not clear on what diameter and length the former would need to be.
Hello David, and thanks for taking the time and effort to write such a helpful post. I have little familiarity with machining anything - I last did so at my boys' grammar school as a teenager. Metalwork and woodwork were compulsory until the third year. Without this blessing, I'd be even more clueless!

Ideally the inductor needs to have an aspect ratio (length to diameter) of at least 4:1. I had just finished doing some caculations based on acrylic tubing with external diameter of 80mm and internal diameter of 75mm -so 5mm wall thickness. This is both available and not prohibitively expensive, but only comes in lengths up to 50cm.

I sussed that 1,000 turns of 30 gauge wire takes up 31.5 cm of length. Add to that 15.7 cm with a 0.157 spacing between each turn and the whole thing fits into a 50cm length of acrylic tubing, with an outside diameter of 80mm. However, the aspect ratio (length to diameter) is too high - 6.25, too long and thin, and the inductance of this winding is only around 12mH, way too low!

So....

Thank you for your brilliant suggestion to use underground-grade pipe. It is way cheaper than acrylic and as you suggest, bomb-proof. Not only that, the 11cm diameter gives me plenty of headroom to get the inductance I need (28mH)within the correct aspect ratio. Further bonus, I can use thicker wire!

Just to be clear, and anyone suitably experienced can answer...

1) do you think that can be lathed - rock hard and 3.2mm thick?

2) Does anyone have a lathe that can handle that diameter? (11cm)

2) what is the minimum spacing between windings? I know they can be close wound to be contiguous (exactly side by side) but I don't know how tiny any deliberate space can be designed to be

Once I know these things, I can go ahead an order a sample, test it for RF leakiness and then design the inductor and see who is willing to take on the winding! Alternatively, if anyone knows what kind of plastic it is, we can just search to see if it has sufficiently good dialectric properties!

Thank you so much again!
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Old 8th Oct 2017, 7:01 pm   #27
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Default Re: Large inductor lathe wound to a spec?

Quote:
Just to be clear, and anyone suitably experienced can answer...

1) do you think that can be lathed - rock hard and 3.2mm thick?
Don't know as I'm not familiar with the material. Carbide tools will cut most things, but there's no telling what the finish will be like. It's a case of try it and see.

Quote:
2) Does anyone have a lathe that can handle that diameter? (11cm)
I do and I expect other members do too. The important thing is that the former clears the lathe's saddle, as it will need to pass under the whole length of the former during groove cutting and winding operations.

Quote:
2) what is the minimum spacing between windings? I know they can be close wound to be contiguous (exactly side by side) but I don't know how tiny any deliberate space can be designed to be
Given a lathe with suitable change gears or better still a screw cutting gear box there's no limit on how small the space can be.
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Old 8th Oct 2017, 8:10 pm   #28
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Default Re: Large inductor lathe wound to a spec?

Hi Al, the grey (above ground) and Brown (below ground) types should have suppliers data sheets on the web that detail the materials. Avoid any black tubes as they may contain carbon black as a colourant.

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Old 8th Oct 2017, 8:53 pm   #29
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Default Re: Large inductor lathe wound to a spec?

Quote:
Originally Posted by astral highway View Post
external diameter of 80mm and internal diameter of 75mm -so 5mm wall thickness.
Surely 2.5mm wall?

Hi

A rotating cutter may do this job better, far less load against the tube.
Something like a Dremel mounted on the head stock.

Richard
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Old 9th Oct 2017, 7:06 am   #30
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Default Re: Large inductor lathe wound to a spec?

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Surely 2.5mm wall?
Hi Richard, to avoid confusing anyone it's a diameter, not a radius. So there is a circle of 75mm diameter inside a circle of 80mm diameter, with an 80 minus 75 mm, i.e. a 5mm wall between them.
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Old 9th Oct 2017, 7:13 am   #31
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Default Re: Large inductor lathe wound to a spec?

Thank you Graham , that's great news. There will be something like 0.15mm between turns.
I will provide a full spec for the job in the next few days.

What can we do in terms of forum rules to agree some form of payment for anyone who has the capability to do the machining and winding the approx 1200 (tbc) turns with some kind of jig?

Ed, good call. I will get to a builders merchant and have a look around. Postage on a 3m pipe is 35, so I'll be collecting it in person and cutting down to around a 70cm section.

Please could I have some indication of who might be willing to do this ko for suitable reward? Many thanks!
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Old 9th Oct 2017, 8:31 am   #32
M0FYA Andy
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Default Re: Large inductor lathe wound to a spec?

Quote:
Originally Posted by astral highway View Post
Hi Richard, to avoid confusing anyone it's a diameter, not a radius. So there is a circle of 75mm diameter inside a circle of 80mm diameter, with an 80 minus 75 mm, i.e. a 5mm wall between them.
That makes no sense! 80mm minus 75mm leaves 5mm which is shared between both 'ends' of the diameter - so 2.5mm wall thickness in my book.

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Old 9th Oct 2017, 1:39 pm   #33
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Default Re: Large inductor lathe wound to a spec?

Hopefully the first pic below might dissipate any doubts and speculation.

As can be seen, this length of 110mm diam underground drain is 3.5mm wall thickness, which I think is standard, whatever the make. The extrusion is perfectly round and though it cuts cleanly and easily, it's very rigid and doesn't deform easily. For the want of something better to do, I've just placed a 2M length on bricks and have stood in the middle. my 100kG didn't flex the pipe at all. It's mostly made from PVCu and from sometimes HDPE, (high Density Polyurethane, which is what the yellow gas main pipes are made from).

As to its RF capabilities, I know it's not very scientific, but I've just placed a sample in the microwave oven alongside a large bowl of water and run it at 1KW for five minutes. The water was boiling furiously - the plastic was still at room temperature. I think if it will stand a KW of microwaves, it should be suitable for the intended purpose.

As regards the ease of cutting a groove in it on a screw-cutting lathe, I've turned a range of plastics including nylon, acetal, HDPE etc for various applications over the years - pulleys, bushes, valve-tester valve-base adaptors, and it turns very easily and cleanly. So much so that as often as not, I use my woodturning lathe and woodturning gouges as it's quicker and less messy than using a metalworking lathe. Not to wander too far off topic, as an example, I've just turned two adaptor bushes in acetal to enable me to connect a workshop vac and my bandsaw to a dust extractor cylone which has inlet and outlet dimensions that differ from the hoses. (See the blue jobbies in pic 2).

I'd reiterate that to my mind, the main challenge for anyone with a large enough screw-cutting lathe would be work-holding the former. The pipe will need to be absolutely centred and checked with a dial test indicator to make sure that there's absolutely no 'run-out' along the full length or the 'thread' depth will differ from one side of the former to the other. That probably means a chuck of sufficient capacity that has four independent jaws rather than the more usual 3 or 4 jaw scroll chuck, which tend to have some run-out, even if to a small degree.

A nice challenge for anyone with the equipment, skills, time and inclination.

(I only have a small ancient watchmakers' lathe which doesn't have screw-cutting capabilities).

I hope that someone picks up the baton and runs with it!
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Old 9th Oct 2017, 2:01 pm   #34
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Default Re: Large inductor lathe wound to a spec?

Quote:
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Brown (below ground) types should have suppliers data sheets on the web that detail the materials.
Hi Ed, the underground pipe material is UPVC and this is apparently fine for my purposes. Thank you! Also thank you to David for having the idea.

Everyone: would anyone now be interested in undertaking this job for me and how can we negotiate terms within the forum rules please, mods?

Here is the job:

1) Receive an offcut of brown UPVC underground pipe with a wall thickness of around 3.2mm. The pipe will be 110mm (11cm) diameter and 50cm long

2) Receive an appropriate bobbin of 27SWG wire (0.4166mm diameter) at least 350m long

3) Lathe a tight continuous spiral groove for 1000turns of copper wire calculated by
1000t x0.4166 =41.66cm plus say 0.02mm clearance between turns = plus 2cm, for a total winding length of 43.6cm.

There will be a space of a few centimetres at one end and around 1cm at the other end: a sketch will make this clear.

4) Using whatever appropriate method,wind under tension the bobbin of wire to the full length of the groove, without contaminating or kinking the wire. Any kinks will render the coil useless. Leave a couple of inches of uncoiled wire at each end. This page may be helpful.

5) Seal the coil with an initial two protective coats of polyeurethane (not acrylic) gloss varnish to keep the turns in position, allowing adequate time for the first coat to go off. I will continue to add further coats once I receive the coil back here. - these are just for safety.

6) If you have the capability to do so, measure and record the inductance of the as-built coil, unloaded. The design inductance is 24.6mH. This is just a 'nice to have' - I will also measure it but two independent measurements are better than one.

6) Protect the coil with the original packing materials provided by me and then return by signed for Royal Mail, explicitly not any courier.

I am just ordering the wire in the next few days but I'd love to hear any expressions of interest, of course according to the forum rules. If I have to offer a fixed price then mods please let me know how to agree this.

Many thanks!
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Old 10th Oct 2017, 8:04 am   #35
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Default Re: Large inductor lathe wound to a spec?

The cost of doing the work would be a matter of negotiation between you and the person doing the work. Any discussion regarding this should take place in PM's
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Old 10th Oct 2017, 8:50 am   #36
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Default Re: Large inductor lathe wound to a spec?

It sounds to me that more than one pair of hands would be useful to manage this, so the best approach would be to find someone reasonably local to you who has a suitable lathe and then you visit with the materials. If somebody is effectively taking on responsibility for the job I would expect them to charge a commercial rate to cover the risk of damage and scrap materials, whereas if you are there it remains your risk!

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Old 10th Oct 2017, 9:20 am   #37
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Default Re: Large inductor lathe wound to a spec?

Ref job spec 1)...Might be worth supplying at least a couple of lengths of pipe, one to get things right on and one for the finished article, the pipe might need skimming first.

What happens if the job man kinks the wire....who pays??

Lawrence.
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Old 10th Oct 2017, 1:30 pm   #38
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Default Re: Large inductor lathe wound to a spec?

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It sounds to me that more than one pair of hands would be useful to manage this, so the best approach would be to find someone reasonably local to you who has a suitable lathe and then you visit with the materials.
Hi Andy, sounds like a great idea theoretically, but I'm not in an area abundant with lathes. It's a gentrified part of London with all traces of industry extirpated over the last 20 years.

The job is actually quite simple for one person with a lathe. Like this

I've even done it manually several times in the past (not the machining) but definitely don't have the capacity to do that again.

Quote:
Originally Posted by David G4EBT View Post
Hopefully the first pic below might dissipate any doubts and speculation.

As can be seen, this length of 110mm diam underground drain is 3.5mm wall thickness, which I think is standard, whatever the make. The extrusion is perfectly round and though it cuts cleanly and easily, it's very rigid and doesn't deform easily.

I'd reiterate that to my mind, the main challenge for anyone with a large enough screw-cutting lathe would be work-holding the former. The pipe will need to be absolutely centred and checked with a dial test indicator to make sure that there's absolutely no 'run-out' along the full length or the 'thread' depth will differ from one side of the former to the other. That probably means a chuck of sufficient capacity that has four independent jaws rather than the more usual 3 or 4 jaw scroll chuck, which tend to have some run-out, even if to a small degree.


I hope that someone picks up the baton and runs with it!
Thank you, David. This is so helpful.

Is anyone on the forum intrigued enough to have a go at doing this, at least theoretically? If so, please would you indicate that on this post and also by sending me a PM so we could negotiate what it would cost me.

No more posts on viability, please. Thank you.
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Old 10th Oct 2017, 3:34 pm   #39
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Default Re: Large inductor lathe wound to a spec?

Hi Al, just a thought about the "gentrified " area, the society of model and experimental engineers (SMEE) was set up by gentlemen hobbyists many years ago. Try contacting them to see if they or their members will do it. There is also a Model Engineer forum where you may be able to get help.

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Old 10th Oct 2017, 3:52 pm   #40
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Default Re: Large inductor lathe wound to a spec?

I'd have a go, but my lathe isn't big enough.
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