What am I doing wrong? I’m trying to flatten a new plane iron. Starting out with coarse sandpaper and working my way up to 600 grit then moving to my wet stones 1000 to 8000. After the coarse sandpaper the back of the iron is flat. By the time I get to the 8000 wet stone it has a hump in the middle. What’s up?
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Replies
If your plane is new, why are you flattening it? It should be machined flat to a fairly high degree of accuracy. Was there something wrong with it?
It wasn't flat.
Unless you are buying premium brands like Lie-Nielsen, Veritas, etc it would be a mistake to assume the blade is flat. They usually aren't for the Stanley, WoodRiver, Benchdog, etc. ilk.
Probably you are putting uneven pressure on the blade - it'll only be a microscopic curve.
There is no need to flatten the whole back of a plane blade in any case. It needs to be flat, but not polished - only the bit right at the edge needs to be perfect.
Even chisels don't need to be absolutely flat - whilst that is the theoretical ideal, being a few microns out isn't going to upset anything.
I don't feel it's possible to give a definitive answer because of unknown variables but here a few possible causes.
The surface substrate for you sandpaper is not perfectly flat.
Some or all of your stones are not perfectly flat. This I feel is most likely, especially with coarse stones which can dish easily. If you are stroking the length of the blade back and forth over the length of a dished stone creating a hump in the middle is exactly what you would do. It is good practice to always flatten your stones before using them to flatten the backs of a tool.
Don't polish the back. Incredible waste of time. Get it reasonably flat on 1,000ish grit, then polish with the ruler trick.
I am thinking that the hump in the middle was created by the flattening method using sandpaper. Even if the surface on which the sandpaper was laid was flat and hard, the sandpaper backing itself is soft and as we all experienced, edges wear away faster than the center.
Gulfstar-I have successfully used sandpaper on a large granite machinist plate to flatten large plane soles and such so I feel given a proper base, plate glass, honed granite, a quality tool top, etc. it can be done. Obviously anything wood would not comply so no benchtops, plywood or MDF scraps, etc.
I really think the stones are the likely culprits. All stones get dished because most sharpening takes place in the middle ⅔ of the stone leaving the edges raised. If you were to run the blade parallel to the long edge of the stone the edges of the blade would be ground down leaving a hump in the middle. Most newer wood workers don't think about flattening their stones nearly often enough.
The substrate over which you're using sandpaper and the surface of your honing stones differ in flatness. One is ruining the other, most likely the sandpaper.
Thank you all for your comments. The sandpaper is on a granite block and I flatten the stones before each use. I’m using a strong magnet to hold the iron in an effort to get even pressure and after the sandpaper the iron is flat. Nonetheless, I suspect it’s the stones getting dished. Is there a consensus on an alternate to wet stones?
Although I'm not a fan of them for edge honing I do use quality Diamond Plates for tasks such as flattening backs. As previously mentioned going much beyond 2-3000k is overkill for flattening the back. My goal is two fold for back flattening, obviously to insure the back is flat but also to make sure the scratches along the edge are fine enough to insure the edge is strong enough to hold it's edge for awhile. Course scratches weaken edges and lead to more chipping and shorter edge life in my experience. I find something around 3k is a good choice without overdoing it.
"Wet stones", especially the very wet water stones, are notorious for quickly wearing as they're used. Observing various expert users in the 1001 vids about water stones, it's noticeable that they stop sharpening to re-flatten their wet stones every minute or so! (This generally seems to require a large and expensive diamond plate).
When flattening large areas like blade backs or plane soles, you're at it quite a long time so those wet stones perhaps wear more than when used just a little to put on final micro-bevels or similar.
Personally I've long dispensed with the mucky wet stone in favour 0f 3M microgrit plastic-backed "papers" on dead-flat float glass plate. I have a large and thick sheet of float glass with various grades (from coarse to ultrafine) of 3M microgrit strips stuck on, itself used on the flat cast iron top of my table saw with a very thin piece of non-slip stuff between the two. (Even thick float glass can bend a significant amount under downward pressure if it's sitting on a non-flat surface).
In short, thick float glass resting on another very flat surface such as a cast iron machine table will remain dead flat, for years perhaps, unlike the wet stones which stay flat for just a minute of use, maybe less.
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The other thing to be wary of is that very thin things being flattened can distort and bend under the pressure of you pressing them on to the sharpening surface. Thin plane blades, for example, bend easily. You think its flat - and it was when you were pressing it on to the sandpaper. But when you stop pressing, it might just unflatten itself.
Using a magnet, as you describe, can help but you also have to apply only light pressure and let the sandpaper do the cutting, which it will albeit a bit slower than with more pressure, perhaps.
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However, as others note, a plane blade back doesn't need to be dead flat & polished from one end to the other. So, a small digression ..... . :-)
The end with the bevel on needs to be flat and polished for just a little way behind the edge. The main thing is to avoid any distortion or scratches being "caught" in you final cutting edge, which will result in a non-straight edge with teeny scratch-grooves in it.
If you use a back bevel, such as the smallest possible given by the ruler trick, the flat and polished area of the blade back behind the cutting edge can be quite small. Even a 1/4" back from the edge will do.
A back bevel (like the front bevel/micro-bevel) is a small area from which scratches from coarser sharpening grits can be very quickly polished out. Using a back bevel is a safeguard ensuring that if your sharpening of the main bevel doesn't deal with wear and scratches that have also been made on the back of the blade, the back bevel will remove any such wear remaining.
After all, an edge is not one sided but the meeting of two flat surfaces at an angle. Both those surfaces need to be dead flat and scratch-free. That needs two bevels - or a blade back without a bevel that remains flat and polished right up to the edge without further treatment - which is what the traditional flattening of a blade back from end to end is supposed to (but doesn't, actually) achieve.
Lataxe
For what you're trying to do I'd suggest diamond stones if you are set on spending money.
One last Q: How big are your stones? As you lap the back of the plane iron are you sliding off the ends? That could account for the uneven wear.
You could just work up the grits with finer sandpaper until you reach honing time. You seem to be having success with the granite block.
What brand of plane is this? How much of the back are you trying to flatten? You only need a 1/8" or so b/c the cap iron will never be further than 1/16". Try doing a back bevel aka the ruler trick.
Bottom line, this is an issue for all of us. I believe there are some irons that have stress in them and you will never get flat.
If you think that's what's going on, then at some point you call the seller and see what they can do or purchase an aftermarket iron.
I think the stones are becoming cupped WHILE you're using them.
Cupping occurs when you rub two things together, but which side becomes cupped depends on the relative size of the tool and the stone. If the tool is smaller, the stone becomes cupped. (If they're about the same size, whatever is on top becomes cupped, and in fact that's how telescope mirrors were/are ground. It can be done by hand, and done properly, it can result in an almost perfectly spherical profile (and I've done it.))
So like others are saying, I would stick with sandpaper on the granite block, and just get finer sandpaper. Good luck!