SecKit wanted me to pay shipping and part of the material cost for replacing the bed, which I wasn’t satisfied with.
I did finally receive the replacement magnetic sheets (with the additional flex beds, so I can have one super clean for PLA, another with hairspray as a release agent for PETG, and one in reserve for whatever else I come up with). So now I feel OK trying some things out; worst case, I can recover by replacing the magnetic sheet. And if I need to do that, I can use a dial indicator on the carriage to indicate the flatness (or lack thereof) of the raw aluminum bed before I put the next sheet on.
So now I decided to experiment on the SK-Tank, since I have the ability to recover and it’s really badly out of flat. I figured the worse that happens is that I contribute to the journal of negative results, and honestly I’m getting kind of curious here.
I attached a dial indicator to the hot end and moved it around, and it’s pretty clear that what the inductive probe is reading is actually true. This is not just, say, variation in magnetic field causing the probe to read different in different parts of the bed. And what I see looks about the same whether I’m probing with the PEI spring steel sheet installed or not, so it’s not varying thickness of that sheet. It really is the substrate.
I decided not to start with the replacement nozzle small sandpaper block. It’s an interesting idea, but if I overdo it, there’s a lot of flex in the head compared to a machine tool, and if it catches an edge it could dig in instead. So I’m putting that at the back of the queue of ideas to try.
I honed the finger plane on an arkansas stone to make sure the blade had a good edge. However, the aggressive angle appropriate for wood is too “grabby” in the magnetic material. So the finger plane may be useful for a wood project but won’t be useful here.
I honed a typical ½" chisel sharp and tried a few things. The only thing that worked at all was to use it basically like a shaper, holding it straight up and down and pulling it flat-side-first rapidly across the bed while putting very consistent pressure on it. I tried clamping it to a square wood block to make sure that it was aligned, but that was an abject failure; it did nothing.
Like scraping, I identified high spots and worked them in a cross-hatch pattern to bring them down. Every once in a while I’d clean the bed, put the flex sheet on top, probe 121 points (11x11) twice each, and visualize the mesh again.
That was really slow, and I found that I was not working out all the slight gouge marks from not keeping the chisel perfectly upright. Additionally, it was really bad for the chisel; it didn’t stay sharp long under this kind of abuse for which it was not designed. So I stopped using the chisel.
A gooseneck cabinet-maker’s scraper plane might do the trick, but I don’t have one.
I attached 60-grit sandpaper to a flat block and started sanding the high spots. The resulting magnetic dust is harder to clean up than scrapings. But it is working faster than anything else I’ve tried so far. I’ve spent a few hours working on this project, and I am not done yet. I started out worrying about removing too much too fast, but that hasn’t been a real problem. The sanding block clogs with gummy lumps of magnetic surface bits within 10-15 seconds and then doesn’t remove so much. I then took several minutes to pick the lumps off the sandpaper with the tip of a nail. Sanding a 90° corner of a block of wood cleans them out in less than a minute, but it’s still one third sanding two thirds cleaning at best. And it’s still really slow going.
I know that when cutting some soft metals (like brass) you don’t want any top rake, so I tried clamping a HSS lathe tool blank with a 5° pre-ground edge to an aluminum block as a kind of plane with a 90° cutting angle and 5° relief, but that didn’t work much better, and it was back to sanding.
I’m to the point where I’m starting to worry about having enough of the magnetic sheet left to hold on, and I finally realized that I have some shim steel stock in the shop in one, two, three, and five thousands of an inch thickness. I also have empty soda cans, which are about three thousandths. The soda cans aren’t magnetic because they are aluminum, but for a layer underneath some steel I might get a sandwich that makes the bed more level.
@shorai you were on to something!
Overall trying to flatten the bed by removing some of this gummy magnetic sheet has been an exercise in frustration.