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Old 22nd Nov 2017, 5:48 pm   #1
astral highway
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Default Cutting (router) a circular flashover guard for me?

Hello, as some of you will know, I have very limited 'workshop' facilities (in fact, none!) and do most of my cutting, brazing and prepping on a small picnic table on the balcony outside.

I need to build a corona/flashover guard of appropriately high-dialectric strength and am minded to put this job out for an appropriate fee.

It would need to be in two halves, inside diameter 22cm, outside diameter 24cm. Just a circle of those dimensions. If you have this material, which I guess could be fibreglass or similar, so much the better.

(I don't know if that would be a suitable material, easy to cut and clean, but that's the scope of the job.)

Please indicate here and PM if you're interested and please state what you think the job is worth.

This isn't laziness; I realise I could go out and buy a fretsaw and clamp the thing to the table etc, but it would be a fretsaw that I never will probably use again, given the environment here, and the finish wouldn't be nearly as good as that from a router, as I'm well aware.

Thanks for your understanding of my circumstances and I hope someone can help out!

Cheers,
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Old 22nd Nov 2017, 9:03 pm   #2
David G4EBT
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Default Re: Cutting (router) a circular flashover guard for me?

I'm not volunteering my services, but I hope the following observations might help a little.

First Al, you're right in thinking that a router would be the neatest way of cutting the two circles. It would require a circle cutting jig - an accessory which most router users would make. I don't have such a jig, nor the time to make one, but someone on the forum may well have one and prepared to undertake the project. Here's a typical example, which illustrates what you're wanting to do:

https://www.bing.com/images/search?v...=16&ajaxhist=0

Here is a two-part video on how to make a DIY circle cutting jig, for anyone who might be so minded:

https://www.bing.com/videos/search?q...er&Form=VQFRVP

https://www.bing.com/videos/search?q...92&FORM=VRDGAR

As to material, I'd suggest that FR4 fibreglass sheet would be a good choice - basically, it's the same material as the substrate on fibreglass PCBs. ('FR' stands for 'flame retardant').

To quote Wikipedia (instant erudition for the hard of thinking!):

FR-4 glass epoxy is a popular and versatile high-pressure thermoset plastic laminate grade with good strength to weight ratios. With near zero water absorption, FR-4 is most commonly used as an electrical insulator possessing considerable mechanical strength. The material is known to retain its high mechanical values and electrical insulating qualities in both dry and humid conditions. These attributes, along with good fabrication characteristics, lend utility to this grade for a wide variety of electrical and mechanical applications.

Source: https://en.wikipedia.org/wiki/FR-4

You don't mention the thickness of material you wish to have - I guess you may feel 'the thicker the better, but obviously the thicker you use, the more expensive it will be. Ideally, a piece 12" square (30cms) would be the optimum, but suppliers seem to supply A3, A4 and A5 sizes or other oblong sizes. Here are a few examples. The first is 2.4mm thick (as with PCBs):

FR4 fibreglass sheet 2.4mm x 16Ē (40cms) x 12Ē (30cms). £17.99 plus £3.99 P&P:

https://www.ebay.co.uk/itm/2-4mm-16-...-/292338986441

This source can supply 3mm/5mm/10mm thick and youíd need an A3 sheet, which - if 5mm thick - would be £22.00 plus £3.00 del. (10mm thick would be £43.00):

https://www.ebay.co.uk/itm/Epoxy-Gla...sUmapKsAxiOkvw

When you say 'it needs to be in two halves' just for clarification, do you mean as shown in the sketch below? If so, I just wondered why, and how it would be mounted.

As I say, I'm unable to help, but I hope these observations might move things along a little for you or anyone who might wish to undertake the task.

Every good wish with the project.
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Old 22nd Nov 2017, 10:27 pm   #3
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Default Re: Cutting (router) a circular flashover guard for me?

Hello David,

Thank you so much for your thoughtful reply, full of useful pointers.

FR4 does seem to be ideal - I am familiar with it as a substrate but it hadnít occurred to me as a candidate.

Your sketch is correct and the reason for the two halves is simply so I can mount it around existing windings with protruding, inflexible tapping points. It isnít beyond me to cut it in half myself, however.

I have back-up ideas for the corona guard but this is the best looking solution!

Thanks again for taking the time to bring such a detailed response,
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Old 23rd Nov 2017, 12:29 am   #4
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Default Re: Cutting (router) a circular flashover guard for me?

Designing dielectric profiles to shape electric fields and/or inhibit either air breakdown or surface tracking (two entirely different things) does take some care and background knowledge. If your approach is one of trial and error then you can punch embarrassing holes in lots of pieces of plastic before you find a design which works, so it's worth choosing a material which is cheap to buy and quick and easy to machine.

Seams are a particular problem. Were you hoping to join the two halves of your annulus with a seam which would be as electrically strong as the bulk material ? If you're careful then it is possible to make high-dielectric-strength epoxy joints to glass-reinforced plastic. But you need to make sure the surfaces are clean and relatively flat as you can't afford to have significant dielectric discontinuities (these can act as field concentrators).

The AWE pulsed power group became wonderfully capable at building extremely high-voltage equipment quickly and inexpensively over a period stretching from the 1950s to at least the 1980s. I was lucky enough to learn a lot from them when I was a student and then an early researcher. Their preferred insulator was acrylic (Perspex) joined using a relatively quick-setting two-part compound called Simplex Rapid (a liquid mixed with a powder - we always used the ivory coloured powder although I'm not sure whether the colour made any difference). This stuff was developed for use mostly by dental technicians http://www.kemdent.co.uk/simplex-rapid-liquid-1l. But it was electrically pretty strong too, and quite resistant to surface tracking.

Are you sure you will actually need a guard ? Quite a lot can be achieved to reduce tracking and, in particular, corona using spray-on conformal coating http://cpc.farnell.com/ambersil/6130...SABEgKVuPD_BwE.

Cheers,

GJ
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Old 23rd Nov 2017, 2:16 am   #5
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Default Re: Cutting (router) a circular flashover guard for me?

Al,

I would go with the idea of using acrylic sheet, its economic and its very hard to electrically punch through it if its 5mm or more thick, even 2mm sheet is good. The good thing too its easy to cut and work with than fiberglass, but fiberglass is stronger for thin sheets. Polycarbonate sheet also is good and has a high dielectric strength.

Many plastics companies that work with acrylic are all tooled up to cut it with nice polished edges to your liking and assemble up shapes to make large formers or any other shape you want. For these sorts of job I send the design to the local plastics company and they whip it up in a few days.

For my high voltage probe I used a material called Bramite, a unique AU product, but it has very similar properties to American insulating panels called Garolite which is a glass filled phenolic sheet, like this (sorry about the ebay reference but it shows a photo):

https://www.ebay.com/p/G-10-Phenolic...d=202010536860

(I think G-10 is pretty much like FR4)

Still these sorts of products are more expensive than acrylic or polycarbonate and usually more difficult to work with, with limited tooling.

It could possibly be worth looking into the readily available microwave oven mica sheet, easy to cut with hand shears:

https://www.ebay.com/itm/5-x-Microwa...YAAOSwuLZY2iUV

Last edited by Argus25; 23rd Nov 2017 at 2:33 am. Reason: add info
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Old 23rd Nov 2017, 10:30 am   #6
mark_in_manc
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Default Re: Cutting (router) a circular flashover guard for me?

hi Al

If you have room for it (and want to pay the postage back to you ), I can probably find a piece of thick transparent polycarb at work (sometimes up to an inch thick). I have a lash-up circle cutter on my router.

I like your project and I'm happy to do it for nothing. If you want to put something in your own charity box or send me something for Christian Aid (who have one of those DFID donation-matching things on the go again at the moment) then so much the better.

cheers
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Old 25th Nov 2017, 5:19 pm   #7
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Default Re: Cutting (router) a circular flashover guard for me?

hey Mark,

Thank you, that's very kind of you. In the end, I've decided to try an alternative technique - several turns of silicon sleeving, secured with lashings of epoxy resin, and then over-painted with my new test solution of transformer oil and turpentine. (There's another thread about this). The transformer oil should be left behind as a thin residue that can be overpainted with gloss (marine, oil-based) varnish.

I really appreciate your offer and if there are other bits of circular fettling that are beyond me, perhaps I could ask you in future?

Thanks again!
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Old 25th Nov 2017, 6:50 pm   #8
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Default Re: Cutting (router) a circular flashover guard for me?

You could use a lathe to turn out the centre and then turn out the ring. This could be done by hand if extreme accuracy is not required.

I would avoid glass fibre material from a health point of view.
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Old 25th Nov 2017, 9:02 pm   #9
mark_in_manc
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Default Re: Cutting (router) a circular flashover guard for me?

hi Al

Sure, come again any time. Especially if scraps of polycarb are involved - we generate a good few offcuts and it can be expensive stuff to buy.
cheers
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Old 26th Nov 2017, 12:34 pm   #10
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Default Re: Cutting (router) a circular flashover guard for me?

GJ:

Thank you for your insightful comments. You have raised the distinction between surface tracking and other unwanted arcing. On reflection , I think that surface tracking is more the culprit. At EHT HF RF and apparently well over the design voltage in an transient fault condition, the pulses in the primary coil are extremely energetic and arcs can pop up between coupled inductors with savage destructive force. They can easily punch holes in insulators very widely spaced apart if the surface provides a temporary conductive pathway. I wish I had high-speed camera triggering to record the exact mechanism.

The best approach is for me to avoid a fault condition, caused by poor tuning. The support to this is to consider the surfaces that are becoming a conductive pathway. Building up a ridge of non-conductors across this pathway seems a good idea and so does your idea of adding a retardant reagent such as the one you have linked to.

Argus25: Thank you : really good suggestions there on the materials side of things. The mica looks like a great candidate for insulating the tank circuit from adjacent conductors.

Mark: also thank you! Iíll PM you as some offcuts of polycarbonate would be useful!!
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Old 26th Nov 2017, 2:46 pm   #11
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Default Re: Cutting (router) a circular flashover guard for me?

Quote:
The best approach is for me to avoid a fault condition, caused by poor tuning. The support to this is to consider the surfaces that are becoming a conductive pathway.
Al,

At least with your planned design I assume you can bring the drive to the tubes or the supply voltage up slowly, so that might give you the ability to find any trouble spots before you push the assembly to full power. That will be an exciting time.

Hugo.
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Old 26th Nov 2017, 5:01 pm   #12
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Default Re: Cutting (router) a circular flashover guard for me?

Hey Hugo,

Good point. Conventionally, this incremental voltage stressing is done with a variac; something I no longer have. However, I have another method lined up which should offer me the same level of control and insight. As you say, that will be an exciting time!
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