edge bends during grind, uneven bevel results. with pics!

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EGA

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Apr 12, 2018
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Hi all,
I've made a handful of blades at this point - about 5, and I think I've realized the source of a problem I've been having. I'm putting full grinds on these knives, and they kind of have the appearance that the grind is steeper on one side than another. I'm going to illustrate what I think is happening. Can anyone tell me if I'm right, if this has happened to them, and if there is a solution? As it stands, I grind one side, then the other. Is it reasonable to just alternate, do a it on the left, bit on the right? It might help with the other sort of warp as well, where the blade cups along its length. See below. not to scale, and I can't see any of this bending while I'm working on the knife.
Nl7iI7U.png

I think this is what's happening, though I don't really have an explanation for why it does bend while I grind the other side (figure 4) - perhaps just because there's less to grind, so I apply less total pressure. I'll add that it does seem to be the case that the steeper grind seems to be along the first side I grind.
 
Make sure you're using fresh sharp belts to reduce heat build up and alternate sides more frequently.
 
Ok, but then I find your description of what's happening very unlikely. How do you know you're using the same bevel angle on both sides?

To answer your other question about alternating, yes, I think that's a good practice. You don't have to alternate every pass, but I like to chamfer both edges to my scribed line, then grind 1/4 of the height increments on either side. It may not prevent warping, but relieving material in a close to even manner reduces the likelihood of extreme warp impacting the geometry of one side more than the other.
 
Use brand new belts and do not push so hard, let the 50 grit do most the work.
 
kuraki said:
Ok, but then I find your description of what's happening very unlikely. How do you know you're using the same bevel angle on both sides?
Click to expand...
Reasonable question - I don't. But i'm putting on a full grind, connecting the spine to a scribed line on the edge (not centerline, just a bit off), so if the knife weren't bending, those would be identical angles.
 
EGA said:
Reasonable question - I don't. But i'm putting on a full grind, connecting the spine to a scribed line on the edge (not centerline, just a bit off), so if the knife weren't bending, those would be identical angles.
Click to expand...

Theoretically, yes. Maybe we have a terminology problem. When you say "bending" to me that means you think the force you're exerting on the blade is pushing your centerline over relative to it's nominal position. That can happen when grinding in a jig but is unlikely in a freehand grind where you're likely using less total pressure and backing up the grind with your off hand.

However, it's certainly possible that you're relieving stress unevenly by grinding one entire side before starting the other, and the blade is warping that way. Although generally I expect it to warp the opposite directing and curl towards where material is being removed.

Back to what I said "theoretically." Theoretically yes, assuming you're starting with a piece of stock that is truly flat and parallel on both sides, which may not be the case.

Even if you can't pinpoint exactly the root cause of what's happening I believe you can prevent it by alternating sides, even if only because it gives you more opportunity to make them match, which you have no chance of when 1 side is completely finished first.
 
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EGA said:
you do alternate sides then? I've been grinding one full side at a time.
Click to expand...
I alternate sides, not every pass like Kuraki said but every couple passes I'll switch sides walking the grind up towards the spine.
 
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can you verify the stock is actually bending? in illustration #3 can you actually put a straight edge on the right hand side and see the concave curve ? what steel are you using ?
 
john april said:
can you verify the stock is actually bending? in illustration #3 can you actually put a straight edge on the right hand side and see the concave curve ? what steel are you using ?
Click to expand...
you know I only ended up doing this after I had thought through what was happening, and to the extent possible, yes, it seems like I can see it. Since I'm doing a full grind, by the time I'm done, there isn't a part of the blade that is parallel to the original, pre-grinding sides, other than the handle. So when I set the knife down on a true-flat surface, some granite, and hold it down by the flat handle, it seems like the edge is up a bit higher on one side than the other
 
kuraki said:
Even if you can't pinpoint exactly the root cause of what's happening I believe you can prevent it by alternating sides, even if only because it gives you more opportunity to make them match, which you have no chance of when 1 side is completely finished first.
Click to expand...

This is all great, thanks very much. I'll give alternating sides a try and see what happens. The reality is that the effect is quite small, but it's bugging me and I think I should fix it.
 
AVigil said:
Use brand new belts and do not push so hard, let the 50 grit do most the work.
Click to expand...
Maybe that's it- I use belts longer than I should, and on the last blade, I was pushing hard ernough to stop the motor at some point - I think 2/3hp.
 
Grinds on lefthand side for right handed makers can be different. It takes practice to make both sides the same. Its just something to look for. Also I have found that small mistakes can really let low grit belts dig more than expected. I find grinding the blade after hardening to be more predictable.
 
If you are stopping the motor you are pushing too hard, especially with 1/8" stock. The reason we use power tools is to let THEM do the work. As others gave said, alternate sides as you go, maybe 5-10 passes per side. Keep the blade cool, even before heat treatment. It's good practice. If you have a good centerline established then the geometry should take care of itself as long as the grind heights and shapes are the same on both sides. If the blade starts to warp a bit don't worry. Grind to your centerline and fix the warp later.
 
EGA said:
you know I only ended up doing this after I had thought through what was happening, and to the extent possible, yes, it seems like I can see it. Since I'm doing a full grind, by the time I'm done, there isn't a part of the blade that is parallel to the original, pre-grinding sides, other than the handle. So when I set the knife down on a true-flat surface, some granite, and hold it down by the flat handle, it seems like the edge is up a bit higher on one side than the other
Click to expand...

If you grind one side totally then the other, then what John said should hold true: the flat unground side against a reference straightedge will show whether you induced warping as long as you started out with it dead flat on both sides.

I alternate every two or three passes and avoid heating up the steel too much, but I know my off hand side is a little less steady than my dominant hand, so I stop and check my work often.
 
Are you using steel that has been sheared from a larger sheet by any chance? My experience with steel that has been sheared is that the edges bend down exactly like your drawing.

If that's the case you should try some Precision ground stock or you need to make sure you are flattening your piece before you start working on it.
 
EGA said:
This is all great, thanks very much. I'll give alternating sides a try and see what happens. The reality is that the effect is quite small, but it's bugging me and I think I should fix it.
Click to expand...

Yes you need to do this. I do a lot ( for the little I do ) with AEB-L and pre HT that stuff does like to warp with the grind. So I alternate and maybe bend a bit by hand here and there. It did take some time for me to realize why the F I wa shaving such a hard time chasing the grinds on the sides.

I have some coming in where I am going to grind them post HT with a spray cooling system. That I would think will help the warpage. But doing things the hard way first makes the rest easier right?
 
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