Tips for gapping Ferrite cores on a surface grinder? - Page 1
Forum section: electronics / projects

author: SuperFungus
author_rank: Contributor
timestamp: September 09, 2022, 04:57:30 pm
content: Does anyone have any experience gapping ferrite cores on a surface grinder? It seems like it should work, but our first attempts were not successful. None of us are really surface grinder experts nor have experience machining Ferrite. We used coolant, held the part with the mag chuck and took about 2 micron passes at a time with an Aluminum Oxide grinding wheel (100 grit I think). Our first couple attempts all eventually resulted in terrible fracturing. I have a couple theories about what to try (no mag chuck?) but was hoping I could find a process others had used successfully first but my google searches thus far have come up empty. Does anyone have any experience with this they can share?I'm aware of the gap tape that can be used, but for our application there are reasons we want to have an actual ground gap. We were trying to use the surface grinder because it's a tool we have which seemed like it could work (precise, easy workholding, low cutting force) but if there's a reason it's a totally inappropriate machine we're interested to hear that too.

author: strawberry
author_rank: Super Contributor
timestamp: September 09, 2022, 05:22:08 pm
content: finer grit better, less rough edges, wet grinding,plane grinder (easy to adjust gap length)rotation and feed direction is important (pressure on last edge/mm could brake chunk out of core)

author: SuperFungus
author_rank: Contributor
timestamp: September 09, 2022, 05:36:16 pm
content: Thanks for the input!Can you be more specific? What grit? What grinding disk material? What depth of cut? What grinding direction is best? Is a "plane grinder" different from a "surface grinder" google isn't helping much here...

author: strawberry
author_rank: Super Contributor
timestamp: September 09, 2022, 05:51:44 pm
content: I have done it manually with 1000 whetstone custom ~0.5mm gap with flatness ~0.1mm (hand precision but looks and works good)

author: jpanhalt
author_rank: Super Contributor
timestamp: September 09, 2022, 06:28:21 pm
content: Ferrite works much like glass. I did department glassblowing part time in grad school. Water is key. Attached is a photo of a gapped toroid I made that was used for a current sensor. I cut the original toroid with a Dremel tool and a Dremel silicon carbide cut-off disk -- not the fiber reinforced one. I don't have a clue of the grit. It's relatively coarse -- maybe 120 to 220. Lapping was done with wet-or-dry sandpaper on a glass plate (plate glass is quite flat). The grit was probably 320 to 600. Nothing superfine. When I needed both flatness and finish for some glassy carbon, I used 3M microfine papers. They go to sub-micron grits and give a very fine polish.For the Dremel cut, I just put a piece of wet sponge in the side going into the ferrite and cut through both simultaneously. None broke. If you look at a glassblower's cutoff wheel, it is also relatively coarse. You need some clearance around the kerf to avoid a wedge effect. In other words, too fine could be a disadvantage. Then flatten the thing after it is cut.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 09, 2022, 06:55:26 pm
content: I've never done ferrite on my SG but have gapped it by hand. I'd block it up in an adhesive "wax" like Aquabond-https://aquabondtechnologies.com/thermoplastic-adhesives/(that site is impossible to find unless you know what you're looking for!) Choose one of the stronger ones so the part doesn't launch out of the SG. Probably needs side support, not just one face.IMO, you have to use a diamond wheel.Maybe a green wheel would work poorly, but definitely not aluminum oxide. Some coolant, could be sprayed by hand (my SG has no provision for flood). The stuff is usually harder than glass, but there are many grades.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 09, 2022, 10:41:53 pm
content: Hm, as I understand it, ferrites are usually softer than glass; though not by much (Mohs 5-6, vs. ~6.5).Interestingly, these materials can even exhibit ductile cutting (smooth rather than fractured grooves left in the surface), so it really is a soft material on a microscopic level; in bulk however, it's not nearly enough to be useful, and indeed is about as brittle as glass -- and not nearly as strong. So yeah, support is a good idea, and low feedrate.I don't know that diamond is required, alumina is certainly hard enough to do the job by itself (Mohs 9), at least technically; but it could still be the heat generation or wear is just too much for practical purposes. Carbide should be fine, I think most commercial shops use it?For my part, "wet/dry" SiC sandpaper works fine. But, again, tons of hand work; I can't recommend that for production purposes, even of just a few units, heh.I also wouldn't be surprised if sharp end mills can be used -- but requiring high speeds and extremely low feeds, and again, any slight misstep, causing excess sideforce or whatever, *tink*, part is ruined.Maybesomething to try if you have plenty of parts to spare, and a nice stiff, well tuned mill or engraver with carbide bits. (When you need to freely machine ferrite parts, the preferred way is to obtain "green" (unfired) body from the manufacturer, machine it in that form (probably HSS is fine), then send it back for finishing. But needless to say, not something you'd do just for gapping...)Tim

author: TimNJ
author_rank: Super Contributor
timestamp: September 09, 2022, 11:32:11 pm
content: Some smaller manufacturers indeed use a surface grinder to process cores in small-ish batches. Gang as many up in a line as you'd like (or can fit) in your vise. High volume production will use some sort of continuous feed-through processors, that doesn't need to be loaded/unloaded, re-fixtured, etc.You need a diamond or CBN wheel.https://www.nortonabrasives.com/en-us/diamond-cbn-productsI thought this was an interesting idea for grinding one off samples..https://www.youtube.com/shorts/RpErxALlv3QAt work, we just use some small diamond Dremel wheels from AliExpress and a diamond file. Doing it with a Dremel is a little dicey, as you can take a chunk out of it pretty quickly if you're not careful, but not too bad. Depends how many you need to do, and what type of quality.

author: Someone
author_rank: Super Contributor
timestamp: September 10, 2022, 01:35:37 am
content: Quote from: T3sl4co1l on September 09, 2022, 10:41:53 pmI have some recent production parts here that have such a regular pattern to their surface they look like they were milled, but the radius of the cutting path is too large (still matches up with some sort of circular grinding). Perhaps there are some structured/designed cutting pads for this purpose?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 10, 2022, 02:08:39 am
content: Yeah all my planar cores I think have cutting marks like that. Kind of amusing (or scary..) to imagine a big ass fly cutter doing a whole bunch at once; but a cup/face grinding tool could have very similar results.Tim

author: SuperFungus
author_rank: Contributor
timestamp: September 10, 2022, 04:47:52 am
content: Sounds like the consensus is I need a better wheel. I'll see what I can do about getting one and hopefully we'll have better results. Thanks for the input all.

author: jonpaul
author_rank: Super Contributor
timestamp: September 10, 2022, 05:57:25 am
content: Hello the ferrite manufacturer uses a diamond charged tool, and special lapp or elliptical machines to achieve a flat or mirror polished surface.We have never seen a DIY gapped core.Suggest that you order a stock gapped core from a manufacturer or Ferrite fabrication like ELNA labs in Woodstock NY, or simply gap between the two cores with fish paper or fiberglass sheet of the required thickness.Those methods served us well for decades of power supply design and manufacture.Bon courageJon

author: mag_therm
author_rank: Frequent Contributor
timestamp: September 10, 2022, 06:12:47 am
content: In my old reference book Snelling: "Soft Ferrites" which covers the magnetics side, there is some info on post sintering finishing including grinding media, methods, cutting speeds etc.More focused and recent,Search " Grinding of Ferrites", shows some interesting ones, including one about effects on magnetic properties near the ground surface, and how to mitigate.All paywalled.I know that the high frequency GOSS c cores are annealed after winding and cutting, then finish ground. That grinding does increase losses slightly.

author: LaserSteve
author_rank: Super Contributor
timestamp: September 10, 2022, 07:46:01 am
content: You need a lapidary fauceting machine for proyyping. . Takes the material off at grain level with a gentle shear force. .For large production you would use a optical lap or polishing machine.The key here is using a gentle force in compression.Steve

author: geggi1
author_rank: Frequent Contributor
timestamp: September 10, 2022, 09:32:10 am
content: You should look in to the kind of machines used for grinding stones for jewelry.They are usually called gem facing machine,https://www.amazon.com/faceting-machine-polishing-professional-processing/dp/B08LK5GB73

author: beanflying
author_rank: Super Contributor
timestamp: September 10, 2022, 10:34:02 am
content: Take a bit of a look through some on Tom's videos on Lapping in particular what to make plates from for Harder or Softer materials to Lap.https://www.youtube.com/user/oxtoolco/search?query=lappingThere is also some low speed powered Lapping options including a DIY one below using some very cheap plates off evilbay. anything high speed (like a Surface Grinder) is going to cause more fractures than it removes. All the industrial Lapping machines I have worked on for seals (SiC or Carbon generally) and Hydraulic Valves were all low speed.Graphite can be turned but it is very grade dependant and very sharp tools to reduce the fracture issue as much as possible same deal low speed. Not something I have done a lot of but a quick check also suggests machining DRY without lubricant but maybe try IPA rather than a soluble oil just to keep it cleaner near the cut.

author: nctnico
author_rank: Super Contributor
timestamp: September 10, 2022, 07:04:07 pm
content: I'd create a gap using some kind of filler. Like polyimide tape which has a nice thickness to it.

author: IDEngineer
author_rank: Super Contributor
timestamp: September 11, 2022, 12:25:41 am
content: Somewhere in the past I read a writeup of how Tektronix cut and polished the cores they used in their clip-on current probes. I wonder if that's still around somewhere....

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: September 11, 2022, 04:41:11 pm
content: Please share when you do!

author: TimNJ
author_rank: Super Contributor
timestamp: September 11, 2022, 05:01:10 pm
content: Quote from: mag_therm on September 10, 2022, 06:12:47 amNice book. Thanks!https://ia803405.us.archive.org/1/items/SNELLING__SOFT-FERRITES__1969/SNELLING__SOFT-FERRITES__1969.pdf

author: SuperFungus
author_rank: Contributor
timestamp: September 15, 2022, 02:43:03 am
content: Got a diamond disk, works great. Thanks for the help all.