[TowerTalk] Can type 31 cores be cut in half ?

Tue Jul 5 18:25:38 EDT 2016

The choke on my 75/80 meter sloping, fan dipole has lasted through quite 
a few Michigan fall, winter, and spring cycles.
Spring can be a particularly difficult time with ice storms leaving that 
choke one big glob of "heavy" ice
I've never had a core chip, or crack with temperature cycles from sub 
zero to 60 degrees.

All of my chokes are open to the elements.

I've taken two approaches to the core assembly. In the first approach, I 
epoxy, or glue the cores together using just enough glue to bond the 
face of the cores together. In the second, I space them an eighth to a 
quarter inch either using a comb like structure made of lexan, or simple 
spacers made from plastic washers, epoxied in place. This approach 
allows extra cooling. Close spacing as in one eighth inch (0.125"), or 
even one quarter inch (0.250") may allow enough water between the core. 
to force them apart when it freezes.  Typically, that won't hurt the 
cores, or the operation of the choke

NOTE: I've not seen any core surfaces that I'd call flat. All have had 
enough irregularities to allow water in between the faces. Cores with 
truly FLAT faces will stick when placed together as if they had been 
glued and it may not be possible to get them apart. Rather than going 
through the messy and labor intensive process of lapping the core faces, 
gluing is a whale of a lot easier and keeps the potentially damage 
causing moisture out. "Super Glue" works, but I prefer to fill the 
groves with epoxy to prevent water from getting a foothold where it can 
freeze, forcing the cores apart.

When using only 2 or 3 cores, I prefer to spacing the cores for 
additional cooling

With epoxy filling the groves as well as the spaces between the cores 
it's unlikely they can be disassembled. Super Glue can be removed in an 
ultrasonic cleaner with the temperature of the solvent at 130-140F.  
That requires a lot of ventilation, or do it outdoors.

I can't imagine cutting cores, but...
NOTES: on cutting cores. If sufficient wash is used, the abrasive 
particles are carried away and using a diamond abrasive cut off saw 
works just fine, nor would its life be shortened.  Lapping the faces is 
much preferable to a surface grinder. Surface grinders are messy at best 
and can be extremely dangerous. I saw one throw a work piece through a 
cinder block wall. (Never get off the end of one.)  They look simple, 
but are not for the inexperienced.
There is a bacteria that likes ceramic, or Silicon in the slurry that 
produces Hydrogen, so clean up right after finishing

Most machinists don't like to setup and cleanup for a simple one or two 
piece job and particularly one that can not be assured of tight, 
magnetic clamping.
If you are paying for their time, the set up and cleanup may cost more 
than the job time.

Lapping can be done with two or three steps in abrasive sizes. Use a 
different, flat glass plate for the aggressive compound than the one 
used for the fine and polishing compounds. The Jeweler's Rouge final 
step of polishing can probably be skipped as the fine lapping compound 
will likely be sufficient.

When bonding, use pressure and work the cores against each other to get 
the bonding layer as thin as possible.(It's similar to the proper 
installation of a heat sink on a CPU)  I use two 90 degree Al angles to 
keep the cores aligned.  The groves can be filled later

CAUTION: Many of today's ultrasonic cleaners will be damaged if more 
aggressive solvents, like acetone, are used.  Mine has a 2 gallon SS 
tank, but the valves and cover are plastic

73

Roger (K8RI)

On 7/5/2016 Tuesday 12:32 PM, Jim Brown wrote:
>
> On Tue,7/5/2016 6:58 AM, Grant Saviers wrote:
>> Yes it is possible with a diamond saw in a surface grinder. However, 
>> it is very messy and and the dust/slurry is very abrasive so the 
>> grinder owner may say "no way".   We machined a lot of ferrite many 
>> years ago for recording heads and pretty quickly the grinders were 
>> trashed.
>>
>> You need very flat and smooth mating surfaces (see a big clamp on) as 
>> this mating changes the magnetic properties of the core by inserting 
>> an air gap.  Obvious it can be made to work since #31 clamp ons are 
>> very effective if kept tightly closed.  A tungsten carbide blade will 
>> cut ferrite in your bandsaw but I think the cut will be so rough as 
>> to be useless. A diamond blade in a cut off saw may work if it last 
>> long enough without water cooling.
>
> Grant has very effectively detailed why it is a very bad idea to try 
> cutting cores in half. Emphasizing, if the mating surfaces are not 
> PERFECTLY smooth and parallel, there will be an air gap, and 
> performance will be degraded, probably a lot.
>>
>> I think you need a bigger NEMA enclosure as any of these techniques 
>> will cost a lot more with probably poor results. 
>
> I can't think of a reason for using an enclosure at all. In the 9 
> years my work has been on the internet, no one has yet told me about 
> damage of a choke by exposure to severe weather. Here along the 
> Pacific coast, all my chokes see is a LOT of rain and sun, but I'm 
> sure there are lots of ferrite chokes in parts of the world that have 
> serious winter.  The only hazard I can think of is the ferrite cores 
> cracking from freeze-thaw cycles.
>
> 73, Jim K9YC
>
>
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-- 

73

Roger (K8RI)

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