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

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Can type 31 cores be cut in half ?
From: "Roger (K8RI) on TT" <K8RI-on-TowerTalk@tm.net>
Date: Tue, 5 Jul 2016 18:25:38 -0400
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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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