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Grinding Ferrite

S

Syd Rumpo

Jan 1, 1970
0
I need to gap a core, specifically, taking 3mm off the centre pole of an
RM12 core (or 1.5mm of each half).

I could use spacers to gap the whole thing - which would be easy - but
if grinding isn't too hard, I'd prefer it. Total quantity < 50 cores.

Anyone done this?

Cheers
 
D

DecadentLinuxUserNumeroUno

Jan 1, 1970
0
I need to gap a core, specifically, taking 3mm off the centre pole of an
RM12 core (or 1.5mm of each half).

I could use spacers to gap the whole thing - which would be easy - but
if grinding isn't too hard, I'd prefer it. Total quantity < 50 cores.

Anyone done this?

Cheers


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.
 
S

Syd Rumpo

Jan 1, 1970
0
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
 
B

Bill Sloman

Jan 1, 1970
0
I need to gap a core, specifically, taking 3mm off the centre pole of an

RM12 core (or 1.5mm of each half).

I could use spacers to gap the whole thing - which would be easy - but
if grinding isn't too hard, I'd prefer it. Total quantity < 50 cores.

Anyone done this?

EPCOS

http://www.farnell.com/datasheets/486642.pdf

https://www1.elfa.se/data1/wwwroot/assets/datasheets/hz330900.pdf

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).
 
S

Syd Rumpo

Jan 1, 1970
0
D

DecadentLinuxUserNumeroUno

Jan 1, 1970
0
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


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.
 
R

Robert Macy

Jan 1, 1970
0
I need to gap a core, specifically, taking 3mm off the centre pole of an
RM12 core (or 1.5mm of each half).

I could use spacers to gap the whole thing - which would be easy - but
if grinding isn't too hard, I'd prefer it.  Total quantity < 50 cores.

Anyone done this?

Cheers

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.
 
S

Spehro Pefhany

Jan 1, 1970
0
I need to gap a core, specifically, taking 3mm off the centre pole of an
RM12 core (or 1.5mm of each half).

I could use spacers to gap the whole thing - which would be easy - but
if grinding isn't too hard, I'd prefer it. Total quantity < 50 cores.

Anyone done this?

Cheers

3mm is a pretty huge gap.

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.
 
S

Syd Rumpo

Jan 1, 1970
0
3mm is a pretty huge gap.

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
 
F

Fred Abse

Jan 1, 1970
0
I need to gap a core, specifically, taking 3mm off the centre pole of an
RM12 core (or 1.5mm of each half).

I could use spacers to gap the whole thing - which would be easy - but if
grinding isn't too hard, I'd prefer it. Total quantity < 50 cores.


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
 
S

Spehro Pefhany

Jan 1, 1970
0
Can't use some sort of toroidal powder core? All that air gap makes
the ferrite sort of irrevelant.

Shielding for the air gap?
 
S

Syd Rumpo

Jan 1, 1970
0
Can't use some sort of toroidal powder core? All that air gap makes
the ferrite sort of irrevelant.
(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
 
B

Bill Sloman

Jan 1, 1970
0
Yep. Way back in slow-modem-land I used pot cores with
screw-adjustable gaps to tune the filters ;-)

Must have been a high-frequency modem. Mullard used to sell Nickel-Zinc ferrite 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 sothat 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.
 
R

Robert Macy

Jan 1, 1970
0
Yep.  Way back in slow-modem-land I used pot cores with
screw-adjustable gaps to tune the filters ;-)

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I love to cook with wine.     Sometimes I even put it in the food.

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.
 
L

legg

Jan 1, 1970
0
I need to gap a core, specifically, taking 3mm off the centre pole of an
RM12 core (or 1.5mm of each half).

I could use spacers to gap the whole thing - which would be easy - but
if grinding isn't too hard, I'd prefer it. Total quantity < 50 cores.

Anyone done this?

Cheers

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

http://www.stallingsglass.com/store/product.php?productid=20427&page=2

RL
 
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