Grinding Ferrite

Use transformer tape at 1.5 mils and gap the entire core. You end up with the same gap as twice the number on the center pole. and no, it does NOT affect coupling to not have one mating face. The whole reason you are gapping it IS to "affect coupling".

You tape off one core face. It is easy to trim with a nice scalpel.

You can get 1 mil and 1.5 mil transformer tapes in short qtys no problem.

Trimming out 50 core faces doesn't take that long, and the number is exact because the tape is consistent.

You then have to remember to pair an untaped core and a taped core together in the build.

Grinding works, but you can call Farnell or the like and get them already gapped too. So you could find and buy 100 pre-gapped cores and have 100 sets to use. As to the grinding, abrasive cloth on a rubber stab works, but precision is a grind-test-grind-test matching and culling hand operation. Some transformer houses might sell you a gapped core.

Thanks, but that's 1.5 millimetres (or 59 mils (which we call 'thous')). That means there's some flux leakage around the outside which might conceivably couple to adjacent wiring. Probably not an issue if I'm careful with wiring (although there's not much room), but gapping the centre only would be more certain.

Cheers

EPCOS

DigiKey sells the EPCOS parts too.

Only a 1.3mm gap but they are off-the-shelf. Using transformer tape to gap the outer faces works, but there is more external leakage inductance (though not a lot).

outer faces works, but there is more external leakage inductance (though not a lot).

Yes, thanks, I used those (or similar) for prototyping, but the high temperature cores I need aren't gapped and I need an Al of 160nH or thereabouts.

Cheers

Wrap it with a copper strip. Still worried? Use Mu metal strip.

Switchers use gapped cores all the time and the gap is of this nature.

Some go through the trouble of moving to a hard gapped core set, but tape gaps have been the std for a long time. Your gap is huge though. So not knowing the application I can't comment about radiance yet.

yes, had a machine shop do it, but single quantity.

the guy had to learn how to work with the ceramic material, but in the end, nice grind job. He learned to grind it off just like grinding off any ceramic lamp base, or such. found out it was SLOW, and he broke a few.

A few weeks ago I tried a diamond tool in a fairly rigid benchtop mill (450lbs) and got results that were a bit underwhelming.. possibly due in part to the limited spindle RPM (only 1800 RPM). Surface finish was similar to the raw parts of the core, not the mirror finish on the ground spots where they touch.

At >= 1mm, I'm sure it would be fine. If you could make a fixture, a Dremel would probably work quite well, go for a fine grit and maybe some mist coolant.

Downside is that it leaves bits of abrasive ferrite and probably a few diamond bits on the mill ways, so you will want to cover them and clean up well afterward. A magnet will help suck up the ferrite bits, but not the diamond obviously.

Suggest clamping with some urethane sheet to help spread out the forces and reduce the number of broken cores.

My mistake, it's 1.5mm total or 2 x 0.75mm. Thanks for that, I'll have a go and see just how hard it is.

Cheers

RM 12 cores have enough open side to get a half inch wheel onto the center limb. Find someone with a proper surface grinder and a magnetic chuck. It's a toolroom job, expect to pay

Can't use some sort of toroidal powder core? All that air gap makes the ferrite sort of irrevelant.

Shielding for the air gap?

Yep. Way back in slow-modem-land I used pot cores with screw-adjustable gaps to tune the filters ;-) ...Jim Thompson

(My mistake, total gap is 1.5mm or 2 x 0.75mm) Yeah, powder toroids are an option, but an RM12 makes good use of the space I have. Easier to wind, too, and I can easily get them with the required high temperature rating. And I need decent isolation which is a bit easier to achieve.

Cheers

an

Must have been a high-frequency modem. Mullard used to sell Nickel-Zinc fer rite RM and pot cores for use between roughly 1MHz and 10MHz which came wit h a hole down the middle into which you could plug an adjustable bridging s lug, which you could indeed move up and down with an adjusting screw to fin e-tune the inductance. Used one once to fine-tune a nominally 7.5MHz VCO so that the control voltage sat bang in the middle of it's range. It was vara ctor tuned, so if the control voltage got too far away from nominal, the fe ed-back loop dynamics stopped being dead-beat.

RM and pot cores for use between roughly 1MHz and 10MHz which came with a hole down the middle into which you could plug an adjustable bridging slug, which you could indeed move up and down with an adjusting screw to fine-tune the inductance. Used one once to fine-tune a nominally 7.5MHz VCO so that the control voltage sat bang in the middle of it's range. It was varactor tuned, so if the control voltage got too far away from nominal, the feed-back loop dynamics stopped being dead-beat.

Been ~30 years ;-) 1200 Baud modem, tunable to US or European frequencies... they were slightly different back then. I can't remember the brand of core anymore. ...Jim Thompson

an

s.

Gotta watch those machine shop people and be SPECIFIC!

I had 1 inch square about 6 inch long I core that I had turned to be round. When the guy got done with turning, he cut the ends off to remove the 'unsightly' notch he'd made in the ends to hold it in the turning spindle! arrrgggg start over guy! ignore notch, needed the length.

Thr Gryphon offers adjustible diameter grinding heads for center leg work, and water to cut down on dust and heat.

RL