Plane blade hollowed out in center
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I’m trying to figure out a sharpening issue that I’ve not seen before. My plane blades are tending to sharpen with the center (side to side) hollowed out as shown in the attached imaged, which is greatly exaggerated. I detect this by holding the blade up to a straight edge and viewing a light in the background. It affects the work in that, at the lightest setting, the shaving starts as two thin strips out at the edge of the blade, nothing in the middle, so this limits how fine my shaving can be. (I’ve simplified the drawing. The actual shape is like what I show, but then the corners of the blade are relieved to avoid tracks.)
I’m wondering if my diamond plates are worn so that the edges cut more slowly than the middle or are even worn down. Paul sharpens with his blades skewed, but I tend to have them square across the width of the stone. Maybe being skewed helps protect against this? I’m much more comfortable with the blade square to the stone, as it seems I can feel what is happening better. I always assumed that Paul grew up with narrower stones, but maybe there’s more to this choice?
My plates are probably about 10 years old. There’s nothing obvious when holding a straight edge across them.
I used to have the opposite issue until I moved away from water stones to diamond stones. My water stones always hollowed out no matter how hard I tried to flatten them.
Could it be that you are putting extra pressure at the centre of the blade while sharpening?Selva
There are a few ways I see this could happen.
1. You hand sharpen without a jig/fixture and flex the blade at the center, pressing hard.
2. The temper is damaged at the center of the blade making it soft and cut faster.
3. The diamonds are worn off on the perimeter of the plates.Are these thin plates? They may be flexing under pressure.
Thanks for the replies. More info:
-I don’t tend to use a lot of pressure when sharpening.
-Temper should be fine. Also, it isn’t just one blade.
-Plates are EZ lap and DMT, so good thickness.
Ed, you could also have some warp/twist in the iron – something to look into if you haven’t already. Similarly, you could have an issue in the frog/bed which warps the iron when you tension it down under the cap, but if you’re saying it comes off your stones like that then the problem is either the iron is warped or your stones are worn. You don’t need a honing guide – applying even pressure will always give you a straight edge as long as your plates are in good condition and your iron is flat. Don’t blow money on new diamonds right away – maybe pick up a low cost double-sided oil stone or use a friend’s to experiment first.
So, I did some exploring with feeler gauges. Everything is flat side to side within 0.001″ except for one plate (superfine) that had a hump across the width, but the hump was only 0.0015″. Two of the plates were flat to <0.001″ end to end, one had 0.0015″ hollow end to end, and one had a fall off of 0.002″ at one end (not side to side).
Thus, the plates are all mechanically fat in the dimension that matters. Since the plates are 10 years old, though, it may be that the edges are less abrasive than the center, e.g., from flattening the backs of tools.
One thing I did find, though, is that the cheap square I was using to assess the blade is *not* straight and explains some or even most of the hollow. So, much of this is a measurement phantom.
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