Stator Rings

[kitestrings]

For our turbine we have individually potted coils.  Every other coil sector (six, of a twelve coil stator) attaches to the stator bracket.  The sectors engage one another with a T& G fit at the edges.  We then have these polycarbonate rings that sandwich the assembly, and bridge support between the six “arms” of the stator bracket.  They are made out of a material called Makrolon:
https://www.tapplastics.com/uploads/pdf/Polycarbonate%20AR%20Product%20Data.pdf






It has all been working very well, but I'm noticing some cracks developing near, and radiating out from the fasteners.  I'm sure this will not improve over time, so I want to be thinking about eventually replacing them with a more durable material.  The rings are ¼” x 2” IIRC.  I wondered if anyone here has suggestions.

Originally we were trying to keep any metal away from stator, but I don't have a strong sense of how close is too close to the magnets, where eddies become an issue.  Is it possible, for example that this material could be stainless angle stock or a ridged band, and still be sufficiently clear of the “fringe field” (as I think it is called)?

[Mary B]

Fiberglass sheet, this is oen example(and a decent price for cut to size...) https://www.ebay.com/itm/Fiberglass-Panel-Sheet-0-25-1-4-inch-thick-Beige-Extren-/302828084798?_trksid=p2385738.m4383.l4275.c10 this is way more resistant to cracking around holes and I have used it in the past to mount heavy high voltage components in power supplies.

[CraigM]

Look up phenolic sheet, many different grades of this material. G10 is layers of fiberglass cloth with epoxy resins formed with heat and pressure. Very strong, high modulus material.

I worked at a plastics machine shop for many years and G10 will get it done ✅

[CraigM]

Makrolon is the trade name for the brand of polycarbonate sheet you're using. Most people are familiar with Lexan polycarb... same stuff.

The cracks you're seeing could be “crazing” which are surface cracks that don't extend through the entire thickness of the material. Crazing is a sign of stressed material and deeper cracks will develop with time, but this is usually a slow process.

Polycarbonate is somewhat soft and used as bulletproof glass. This softness absorbs shock whereas something harder (brittle) would shatter.

[SparWeb]

I take it the clear sheets are clamped by the stator mounting bolts.  Distributing the clamping pressure are the washers.
I agree with Craig's suggestion that it's crazing.  The clamping pressure is high, and combined with vibration and thermal stress.

I have a sneaking suspicion that a replacement like phenolic could suffer the same fate or worse.  In any reinforced plastic, the fibers aren't oriented in a way to resist the clamping pressure.  Another point against phenolic is that it's a poor heat conductor.  Or rather, it's an excellent insulator for both electricity (good) and heat (bad).  One last point against reinforced plastics is that they can deform when they are heated.  You just have to expect that the stator will get hot when running at high power.  Lexan/Makrolon was a good choice, being resistant to warping.  Lexan is least likely to warp in the combined heat and clamping at discrete locations, where I would expect reinforced polymers to be more susceptible.

So stick with Lexan, but I think there's a way to modify the fasteners to reduce the clamping pressure on it.
For instance, you could cut a bigger hole in the lexan, and insert a metal sleeve into the hole.  Much like the bushings through your stator segments, you would clamp the bolts against these steel sleeves.  Once the bolts are tight, any further tightening would add no more compression to the lexan.  You would still use washers to clamp the lexan, bigger than the holes and sleeves, so the pressure on the lexan is still applied over a large area.  If the sleeves are exactly the same thickness as the lexan, then there's no compression, so the sleeves need to be slightly thinner to get the desired effect.  You don't need much pressure to hold the lexan in place, and less pressure at a discrete point reduces the chance of warping under heat.  Win-win.

[kitestrings]

Thanks for the feedback and suggestions.  I think it is more than surface cracks, but I'll confirm next trip up there.  I may have just over torqued them.

We molded the coil sectors so that they are all identical.  The T &G is very close tolerance, which largely holds them in one plane.


The bushings in the coil sectors are similarly, very close tolerance 400C shim stock


On the alternate sectors - those not attaching to the stator bracket - I put in ultem spacer/bushings that reduce from 1/2" to 1/4".  They may be "squishing" too much.  These are insulator bushings for the wire terminals, but it is the same stock and OD to these:


So we could go up to a 1/2" dia. fastener, with a larger ss spacer, like the shim stock above, which would take/distribute the compression of the thru-fastener.

When we choose this material, a short list of the criteria was strong, rigid, UV resistance, water impenetrable.  How do G10 and fiberglass compare on those fronts?

[MagnetJuice]

I have done some research on thermoplastics because I think that for some applications it could be used in the construction of stators instead of using the vinyl ester resins. The main reason for using them would be to be able to replace individual coils if they get damaged, similar to your setup.

Here is a table that I had, I added Fiberglass for comparison purposes:


I agree with SparWeb that you are already using the best material, even though the temperatures of Fiberglass looks very good.

I think that what caused the crazing is because the polycarbonate ring is getting hit by what I would call a tiny jackhammer. The jackhammer is the vibrations from the coils. And I think those metal bushings on the outside edge of the stator is also contributing another set of vibrations. During the expansion and contraction caused by extreme temperatures, the polycarbonate rings could be hitting the stator mounting bolts. If that happens during very cold temperatures, the polycarbonate ring is near the brittle point and more susceptible to cracking.

Concerning eddy currents, I have the exact size magnet as you have in your stator. I did a test using a steel washer to see at what distance from the end of the magnet the attraction starts. It starts attracting at about 2 inches, and at 1 ½ inches there is a very strong attraction. So I think that any metal that is at that distance from the magnets will induce eddy currents. That will cause additional vibrations, besides decreasing the efficiency of the alternator by a small amount.

Also it is possible that over-torquing the bolts contributed to the crazing. I know that at one time you had problems with the T&G sliding. Maybe you tightened the bolts a bit too much at that time.

During periods of extreme heat and cold, the expansion and contraction of the materials could be causing overtightening of the bolts. That could be prevented by using Belleville washers on the bolts. I have bought a lot of hardware from here:

https://www.marshallshardware.com

Regardless of the material you use to replace the ring, I think you should replace all the bushings and bolts with nonmetallic, maybe with the exception of the stator bracket bolts. You can provide a shock absorber by placing 1 or 2 layers of heat shrinking tubing over the area of the bolts that is in contact with the ring.

Also if the ring is attached with a bolt to all 12 coils, that will keep the ring flat against the stator and also locks the coils so they don't slide.

Ed

[CraigM]

When we choose this material, a short list of the criteria was strong, rigid, UV resistance, water impenetrable.  How do G10 and fiberglass compare on those fronts?

Kitestrings,
Believe you made a good choice going with Ultem PEI because it's a high temp (350 F) material. Not sure how warm the stator gets. G10 is good to around 250 F range.

Only other option with PEI or other high end plastics is to get 30% glass filled material.

CM

[kitestrings]

I think that what caused the crazing is because the polycarbonate ring is getting hit by what I would call a tiny jackhammer. The jackhammer is the vibrations from the coils.

I would agree.  I don't know if this is true with a direct-tie, but with the controllers we're using we get a pronounced waveform clipping effect at cut-in.  It appears as a growl or resonance audibly, but you need only put you ear to a tower leg to know if is a pulsing force acting on the stator.

Also it is possible that over-torquing the bolts contributed to the crazing. I know that at one time you had problems with the T&G sliding. Maybe you tightened the bolts a bit too much at that time.

Yes.  I think this is spot on.  We pretty much resolved the slippage issue by drilling another set of holes in six of the twelve sectors, but I may have been too aggressive with tightening them (prior to that).


Belleville washers would seem to concentrate the compressive force at the perimeter of the washer, so I'm not sure this moves us in the right direction.  I initially tried sealing washers that had a neoprene washer bonded to the stainless outer washer, but I wasn't happy with them; mainly due to the fact that the OD was a bit smaller than those we used, and the metal potion was thin.  So far I like Spar's suggestion of using a larger hole and controlling the pressure with an insert.

Thanks for the tip on the hardware source.  One we've had great results with is McMaster-Carr.

Regardless of the material you use to replace the ring, I think you should replace all the bushings and bolts with nonmetallic

The 12 bushings in the stator sectors are cast in place, so there is no practical option there.  Maybe I'm not understanding you here.  I'm not sure there is an issue here though - we're in similar proximity to accepted axial designs, and I'm not convinced eddy effect is as pronounce as ferrous, non-stainless materials might be.

Believe you made a good choice going with Ultem PEI because it's a high temp (350 F) material.

We used this material for the terminal insulators, and as a means of holding the rubber boot covers.  It is nice material to work with, and seems to have excellent electrical properties.

Again, thank you for all the constructive suggestions.

[Mary B]

I used to use acrylic for antenna insulators... used to being key... it breaks down under UV no matter what they claim and starts to crack. 3-5 years and it would fail...

[JW]

I would use JB Weld epoxy

[MagnetJuice]

The vibrations from the coils and therefore the stator, will always be there when AC waves are being generated. So there is a chance, however small, that some of the crazing is being caused by the polycarbonate ring touching a vibrating bolt. I realize that it would be difficult to verify if this is really happening. That is why I suggested the isolation technique using heat shrinking tubing. That would be an inexpensive preventive measure that is easy to implement.

I can see your concern with the Belleville washers applying the compression at the perimeter, especially when being used with a soft material. You can take care of that problem by placing a regular flat stainless steel washer between the Belleville and the ring.

Most of us agree that excessive compression could be the main cause of the crazing. And that excessive compression is happening not so much by the bolts being too tight but by the expansion of the stator as it gets hot.

That is why I see the use of the Belleville washers as an important part of addressing the problem with crazing. Marshalls hardware has the Belleville in stainless.

Concerning eddy currents, if the bushings are cast into the stator, there is nothing that can be done. Besides, the amount of metal is not that much and they are not so close to the magnets to be a major concern.

Ed

[JW]

MJ im not quite sure about that. I build some very high power solenoid actuators. I have to make the sleeve out of 316L and it operates because the SS is non-magnetic. the issue of eddy currents is negligible in my application. because its liner and not at a right angle, by nature. Never the less there is back emf and I have to use transient diodes because the reluctor and stator are steel. One thing I learned the hard way is that the two steel parts are non-magnetic,  and once they are used the first time, become polarized (magnetized) so the conductors will be polarized must be observed after that first use.

In a perfect world aluminum does not have eddy currents because its non-magnetic but we all know that's not the case..

the issue of wire in hand helps to reduce eddy currents by using several smaller conductors winded that way.               

[SparWeb]

Hey I just realized that it's not necessary to make the stator mounting bolts do the additional job of holding on the polycarbonate.
You have other holes, and for these light sheets it's hardly necessary to use such large holes - a series of 3/16" screws would do.
You could pot in small fasteners near the edges, match-drill the lexan sheet and fasten it with the screws. 
Those screws can be out near the edge or deep inside near the inner diameter, both far from the magnets.
The kind of pot-in inserts I'm thinking of are threaded, meaning you don't need to have screw heads and nuts sticking up.

Then, where there are mounting bolts, no need to have the lexan clamped between - just cut around the pads that are in contact with the structural bushing.
A bit of RTV to seal those areas would be welcome.

[SparWeb]

Such as...   https://www.mcmaster.com/94459a380

[kitestrings]

Hey I just realized that it's not necessary to make the stator mounting bolts do the additional job of holding on the polycarbonate.  You have other holes, and for these light sheets it's hardly necessary to use such large holes - a series of 3/16" screws would do.

This is probably true.  Every other fastener is actually only a 1/4-20 thru-bolt, and with the ones I added there are two of them in every other sector.  However, where the larger (1/2") bracket bolts are located, the bolt has to pass through these rings.  If it were big enough to clear the coupling nut and strut there would not be much material left.  It would crack at these locations.  Perhaps just over-sizing the holes a bit to reduce the potential contact that Ed is suggesting.

Regarding magnet metals and eddy.  I don't think I have anything unique... and all of our fasteners are stainless.  Consider these (credits to Stephen Fahey):

The vibrations from the coils and therefore the stator, will always be there when AC waves are being generated.

I'd thought is this was mostly due to the 'flat-lining' of the waveform when the the controller is cutting-in the alternator.  It seems to subside above this speed.  Are you saying it is present in a fully loaded application with - or without the effects of the MPPT controller?

[joestue]

Actually you can optimize the shape of the coil and magnet to achieve constant power flow into a dumb rectifier and a stiff battery load, but that might not be easy with an air core alternator, and rectangular magnets. Pie shaped would be better.

An alternative is add an inductor after the rectifier, but that is expensive and adds losses.

Another alternative is add a boost converter designed for the task. -it would be configured to draw power out of the turbine below cut in, and afterwards.. but not used significantly above cut in. so there is no efficiency hit and it increases power output.

The vibration at cut in is because of torque ripple, which is because the power flow into the battery is not constant, the current is only flowing for a brief time at the peak of the waveform

[MagnetJuice]

It is my understanding that the vibrations in the stator are there as long as there is a magnet passing over the coils. This is because of Lenz Law. When the magnet passes over the coil it induces a voltage in the coil. Then according to Lenz Law, an electromagnetic force of the same polarity as the magnet is produced in the coil. Imagine one magnet passing over another magnet of the same polarity and you can see why there is vibration.

Before cut-in, the vibrations are hardly noticeable because there is voltage, but little or no current flowing. When current starts flowing at cut-in, vibrations are more noticeable for the reasons that joestue described. That is what makes the towers rumble during cut-in. After cut-in, the vibrations are still there, just not as noticeable.

Ed

[joestue]

It is my understanding that the vibrations in the stator are there as long as there is a magnet passing over the coils. This is because of Lenz Law. When the magnet passes over the coil it induces a voltage in the coil. Then according to Lenz Law, an electromagnetic force of the same polarity as the magnet is produced in the coil.

only if current is flowing will the coil repel the magnet and work is done.

if the sum total of all the coils is constant work/power into the load, then torque is constant and there is no vibration.

[MagnetJuice]

"if the sum total of all the coils is constant work/power into the load, then torque is constant and there is no vibration."


When you say that, are you referring to a motor or to an axial flux alternator?
And could you expand on that a little bit?

Ed

[joestue]

"if the sum total of all the coils is constant work/power into the load, then torque is constant and there is no vibration."


When you say that, are you referring to a motor or to an axial flux alternator?
And could you expand on that a little bit?

Ed

both motors and generators of all types have this problem.

cogging torque is associated with the iron core of the rotor interacting with the magnets. this can be substantially reduced by skewing the core or the magnets. but due to saturation once current starts flowing the cogging torque may return. in a typical motor with 24 or 36 slots this cogging torque is fairly high frequency.

a generator however, whether or not it produces a sine wave, when loaded by a battery after the 6 diode rectifier, the power flow into the battery is not constant, therefore the torque at the motor/generator shaft isn't constant either, and that is what excites the resonance problems at cut in.
https://www.motioncontroltips.com/faq-trapezoidal-back-emf/

for an alternator producing a 3 phase sine wave, one way to get rid of this problem is to use a second WYE-delta transformer wound for a 1:1 ratio (coils wound for a 1:1.73 ratio) and rectify the output of that transformer. then you put that rectifier in series with another rectifier on the output of the turbine directly, and put them in either series, or parallel with an inductor.
https://qph.fs.quoracdn.net/main-qimg-953d370db9b5ae9e3e764c29c9b095ac
this will then produce much more constant DC current from a 3 phase sine wave input.

[kitestrings]

Thanks for expanding on this; I'm most interested.  When I look at the pros and cons of an axial, clearly one of the things I dislike the most is the noise & resonance at cut-in.  Once it is above a certain speed it much less pronounced, but particularly when the wind is just hovering around cut-in it is an annoyance.

Many of the older turbines that I saw or worked on, including our old Sencenbaugh, over the years were comparatively much quieter - you pretty much had to look up to notice them until/unless there was blade noise at higher speeds.

I don't know how soon I'll get to another design, or a "Kitewind 2.0" upgrade of this one ('Kitewind' is the handle I gave our turbine), but when I do I'd like to engineering this out this feature.

Nowadays you would think there would be an electronic solution...a means of rectifying without the growl.

[MagnetJuice]

On the subject of alternator vibration. This subject is not my main area of expertise, so if I err someone please correct me because my main concern is that the information that we post here is accurate.

joestue said:

"if the sum total of all the coils is constant work/power into the load, then torque is constant and there is no vibration."

I believe that the above statement would be true when the source of power to the motor (voltage) is constant and the load is also constant. In the case of an axial flux alternator that is powered by the wind, the source of power to the alternator is not constant because the wind is constantly changing speed and direction, and the speed of the alternator is fluctuating. Because of this, there will always be some vibrations even after cut-in.

KS, those Sencenbaugh turbines had Ferrite magnets, that is why they were quiet. Since Neos are a lot more powerful, we can build smaller and lighter turbines for the same power as the Ferrites. The Neos turbines don't have the mass to help damp some of the vibrations as the large heavy Ferrites.

A while back I mentioned that I was working on a system to keep most of the vibrations of the alternator from transferring to the tower and offered to post it, but there was no interest so I filed it away.

Ed

[joestue]

In the case of an axial flux alternator that is powered by the wind, the source of power to the alternator is not constant because the wind is constantly changing speed and direction, and the speed of the alternator is fluctuating. Because of this, there will always be some vibrations even after cut-in.

yeah well those vibrations are on the order of seconds peak to peak.

the torque ripple of a 3 phase alternator is 6 times the ac frequency. so in the case of a 9 coil 12 magnet machine, on the order of 12 times per revolution.

[joestue]

Nowadays you would think there would be an electronic solution...a means of rectifying without the growl.

given that there is an optimization to be made between the voltage of the generator vs battery voltage and cut in.. i would look at using a capacitive voltage doubler combined with some resistance to load the turbine before cut in. the resistor limits the current, in conjunction with the capacitors sized accordingly. maybe use a non linear resistor such as a 12v 100 watt halogen lamp.

an inductor is then placed after the main rectifier, it both smooths out the current and adds resistance which reduces the load on the turbine. this is important to smooth out the cut in zone. once the current exceeds some value, a relay shorts out the inductor. said relay could be a standard $10 30 amp contactor but the 24vac coil replaced with say, 1 to 10 turns of wire, in series with the main rectifier on the dc side. no electronics needed just change the number of turns to set the trip point. since the relay is merely shorting out the inductor there is no energy to be dissipated in the relay upon shorting out the inductor. upon opening the relay the maximum voltage it sees is the battery voltage plus two diode drops (neglecting the energy stored in the parasitic inductance of the turbine, but that should be absorbed by the capacitive voltage doubler). so it should last a very long time.

[Mary B]

Thanks for expanding on this; I'm most interested.  When I look at the pros and cons of an axial, clearly one of the things I dislike the most is the noise & resonance at cut-in.  Once it is above a certain speed it much less pronounced, but particularly when the wind is just hovering around cut-in it is an annoyance.

Many of the older turbines that I saw or worked on, including our old Sencenbaugh, over the years were comparatively much quieter - you pretty much had to look up to notice them until/unless there was blade noise at higher speeds.

I don't know how soon I'll get to another design, or a "Kitewind 2.0" upgrade of this one ('Kitewind' is the handle I gave our turbine), but when I do I'd like to engineering this out this feature.

Nowadays you would think there would be an electronic solution...a means of rectifying without the growl.

Interesting problem... you could monitor all three phases and use a switching power supply back to the batteries that is controlled by the phase form each set of coils... a lot of extra electronics and a lot of extra failure points...

[kitestrings]

Yes, most of the solutions would appear to trade simplicity for any sort of fix.

KS, those Sencenbaugh turbines had Ferrite magnets, that is why they were quiet. Since Neos are a lot more powerful, we can build smaller and lighter turbines for the same power as the Ferrites. The Neos turbines don't have the mass to help damp some of the vibrations as the large heavy Ferrites.

It is probably the same logic, but actually the Sencenbaugh used a field-wound 12-pole alternator driven through a 3:1 gearbox.  It spun up unloaded to about 175 rpm (rotor speed).  The controller monitored the frequency and started to put field current to the thing at this speed, roughly 52 Hz.  In addition, the alternator was tucked away inside a 365T6 cast alloy housing that housed the slip-rings and tail hinge.

[kitestrings]

for an alternator producing a 3 phase sine wave, one way to get rid of this problem is to use a second WYE-delta transformer wound for a 1:1 ratio (coils wound for a 1:1.73 ratio) and rectify the output of that transformer. then you put that rectifier in series with another rectifier on the output of the turbine directly, and put them in either series, or parallel with an inductor.
https://qph.fs.quoracdn.net/main-qimg-953d370db9b5ae9e3e764c29c9b095ac
this will then produce much more constant DC current from a 3 phase sine wave input.

joe, I have a few questions on this approach:

Does the output split somewhat evenly across the two rectifiers in this configuration?  Is that the intent of the compensating reactors?

What sort of losses does this introduce?  Have you, or do you know of anyone here that has tried it at this scale?

Thanks for any feedback.  ~ks

[joestue]

for an alternator producing a 3 phase sine wave, one way to get rid of this problem is to use a second WYE-delta transformer wound for a 1:1 ratio (coils wound for a 1:1.73 ratio) and rectify the output of that transformer. then you put that rectifier in series with another rectifier on the output of the turbine directly, and put them in either series, or parallel with an inductor.
https://qph.fs.quoracdn.net/main-qimg-953d370db9b5ae9e3e764c29c9b095ac
this will then produce much more constant DC current from a 3 phase sine wave input.

joe, I have a few questions on this approach:

Does the output split somewhat evenly across the two rectifiers in this configuration?  Is that the intent of the compensating reactors?

What sort of losses does this introduce?  Have you, or do you know of anyone here that has tried it at this scale?

Thanks for any feedback.  ~ks

if you run the two rectifiers in series (which would double your output voltage) you don't need compensating reactors because the current is equal in both. the added losses are simply the transformer losses and, the transformer needs to be rated for 57% of the total output power.

obviously you can run it without any compensating reactors and see what you get, maybe 30% effective.

i don't know how bad your vibration problem is to know if this would solve it, if you get the transformer sized right and the compensating reactors properly sized, the vibration frequency would double, and reduce in amplitude by something like 50%. problem with turbines and transformers is the low frequency requires a rather large transformer.

[MagnetJuice]

KS, I saw that image that joestue posted of the schematic of the 12-pulse rectifier in a discussion.

I don't know if this will help you or not. Here is the link anyways:

https://www.quora.com/What-are-the-applications-of-the-12-pulse-series-type-diode-rectifier

https://qph.fs.quoracdn.net/main-qimg-953d370db9b5ae9e3e764c29c9b095ac

Ed

JW-
joestue wrote

yeah so this schematic is even better

no need for a transformer rated for 57% of the output, just an auto transformer.. maybe 20% of rated output needed.
https://d2o7bfz2il9cb7.cloudfront.net/main-qimg-4fe319a1b209d86e516a8200b0dadbd1


anyhow when the rectifiers are in series (note that does not work with the above schematic) you don't need inductors (called interphase reactors in the above schematic) to help the two rectifiers share the current because the current is forced to flow through both rectifiers at the same time. when they are in parallel you do, and they would be better to be wound on one core as shown in the above schematic.


if you have a Y to Delta transformer to supply a second rectifier in parallel with a rectifier that is connected straight through.. then you need compensating reactors to take the place of the missing transformer.. the transformer itself acts as say, a 5% inductor (5% voltage drop at full load) and 5% loss in power, these combine vectorially for 7-8%. so, the transformer supplying the extra rectifier wouln't be "stiff" enough to push any power through the rectifier and it would not solve your vibration problem. it might still help, but how effective it would be i don't know.

you don't need the compensating reactors if you have a single transformer with both equal voltage delta and Y outputs because both the D and the Y coil would be on the same core and have the same resistance and the same leakage inductance. but if the rectifiers are in parallel then you need the inter phase reactor to parallel the two rectifiers. as far as the kva requirement of that interphase reactor i think its rather low.

 joestue wrote
if you run the two rectifiers in series (which would double your output voltage) you don't need compensating reactors because the current is equal in both. the added losses are simply the transformer losses and, the transformer needs to be rated for 57% of the total output power.

[kitestrings]

I'm trying to decide on the stator rings.  Polycarb with spacers and isolation should be an improvement., but I was surprised how degraded they were from UV.  They are brittle beyond my liking.  I broke one, with worst cracks, just tying it off.

I'm still contemplating G10.  Has anyone cut drilled, machined this material?  I understand it to be a lot harder, but have no experience with it.  Does it have to be cut by an equipped manuf/shop to get acceptable results?

Other?  Ultem?  Peek?

[mmurray70]

I'm still contemplating G10.  Has anyone cut drilled, machined this material?  I understand it to be a lot harder, but have no experience with it.  Does it have to be cut by an equipped manuf/shop to get acceptable results?

Other?  Ultem?  Peek?

I machined some parts from it a few years ago. Seemed to machine fine, a little hard on tools. We made some parts out of thicker sections and had some issues with small islands cracking off (layers separating) but that wont be a problem for you.

Not sure if drill would tend to grab and cause problems when feeding by hand. Should be fine if sandwiched between layers of wood.

[kitestrings]

Thx Mm.  The only other concern with G10, or most of the engineered composites, is that I can't seem to find anything UV properties.  Many products simply are checked "no" for outdoor usage.

I've been sleeping on this more, and thinking about increasing the size or net width of the rings.  Aside from restricting cooling, there really is no reason that the rings could have a slightly smaller ID, and a slightly larger OD.  I'll try to post a pic to illustrate the thought.