Grinding struggle - anti-belly instead of flat edge.

J

JulianF

Joined
Oct 19, 2024
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11
Relatively new to grinding and having a problem.
Trying to make a ``cheese cleaver'' of sorts. (Image of inspiration attached. Burt Foster inspired by Tai Goo.)
I wanted a flat edge. No belly.
But I keep getting an ``anti-belly''. When heal area and tip area of the edge are in contact with a flat surface, center of knife has a concave area where the edge is not in contact with the surface.
I am having trouble with getting the tip too hot. And looking at the picture that inspired the design, I see there is a plunge where I had none.
Both of these things might be contributing to the problem since they encourage an angle and unequal pressure where the belt hits to center of the knife.
But I'm not sure how to fix either issue.
Or, more importantly, if there are things I should be doing that anticipate this problem and would help avoid it.
I'm using 1084 and doing post heat treat grinding.
Appreciate any suggestions and feedback.
https://flic.kr/p/2rg7g1r
 
There is a knife maker's forum here ; maybe the mods can shoot you over there .

At a guess :
Belt too fast .
Belt not tight enough so the belt is convex .
Badly made belt.
Maybe do the last bit of edge forming by hand on a super coarse stone 36 grit 60 grit etc.
or
on a very wide slow moving grinding wheel with the stone trued / dressed properely .

Something like this but with a much more coarse wheel than it comes with ; don't ask how I know that .
1752280215590.jpeg
 
So behind the edge is thinner in the center and thicker at the heel and tip?
Are you scribing the center of the blade before grinding?
If you grind the blade back and forth on the belt you are spending twice as much time grinding the center as you are the tip or heel.
 
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Pretty typical beginner problem. I do it all the time. Essentially your just grinding too much on your ends.

It happens because you're keeping your knife in contact with the belt when you change directions.

The "correct" way to grind, if there is one, is to start at one end or the other, then grind at a constant rate to the other end. Remove it, check your progress, then do it again.

Using a marker or dye-kem to cover the bevels can help keep track of your progress.
 
It's called a recurve edge. I ran into it a lot with kitchen knives without plunge cuts before and is usually around the same width as your belt from where you start the grind. I left more length at the heel of the knife, do my plunge cuts and then grind most of the plunge out when I clean up the heel/choil area (IE shorten the blade from the heel end).

Without a plunge, you may not be grinding parallel to the center edge area. Scribe the center of the edge (or 2 center lines with a tiny gap between) and grind to that and get a nice plunge cut at a fairly steep angle. Then walk the grind back up the blade. Without a plunge cut, it's easy to do!

It could also be the 2" mark (or 1" if using a 1" wide belt). This is a common issue if you aren't grinding parallel to your platen, which often happens without plunge cuts. If your blade is at a slight angle to the platen/belt, the edge corner of the belt can gouge the blade and grind into it more aggressively than the remainder of the belt (less contact area, same pressure = more grinding power).

Keep the centered edge parallel to the platen/belt at all times to avoid this.
 
Thanks for the responses.

Blade at an angle to platen, where I start grinding, 2'' mark, changing directions all make sense. And are likely problems.

Two follow-up questions:

1) Two of you mention scribing. I understand this for blades ground from blanks. This is a forged blade. Nothing is flat, there's forge scale,... I don't understand/can't picture how one would scribe a centerline for the edge. I'm gradually working bevel down to something near a straight edge in line with handle before establishing primary bevel back from that. If this is wrongheaded, can you give me advice or reference about another approach?

2) It makes perfect sense to grind starting at one end or another and paying attention to how much time the belt spends at each location. That said, I am comfortable referencing/addressing the blade angle to the platen there. Trying to do this at the tip (where it's narrow) or the heal (where the angle is wrong or there is a plunge) feels perilous to me. Maybe in part because I'm new, but also because the area near the tip is so small and you need to navigate the exaggerated angle or plunge at the heal end. Help me around this?

Thanks again.
 
Ah, forged blades are a bit harder to scribe! You may be able to lay the blade down on something flat and use something like a height gauge or carbide tipped drill bit to scribe a line that is parallel to the flat surface the blade is on? It will help give a straighter edge line to follow, but won't be perfect.

You can also use a soap stone and color the edge white (or grind the edge clean and use a sharpie) to help with a visual and make the scribed lines stand out more.

I would start with a steeper bevel to get down to a thicker straight center line with the forged blade and then walk it back up the blade. This will leave you room for correction, too. If you grind the center edge too thin and realize it's wandering a bit, you will have less meet to remove at the edge and the side bevels to straighten it out.

With a forged blade, and bevels hammered in, it's harder to grind it evenly and straight. It will take a bit more practice, but keep watching where the edge is and what it's doing.

When I grind blades, I true up the sides first, flatten everything out, get everything at a clean 400-600 grit finish running lengthwise and then mark and cut the bevels in, but I don't leave forged finish on the blades typically. I think I did it on 1 blade and it was a PITA, but turned out OK. I really had to focus on hammering the bevels in clean and establishing the plunge cut/ricasso and used a flatter to keep the bevels and other surfaces flatter, as well as using lighter hammer strikes and planishing the surface so I didn't have deeper hammer marks to grind out.

I would get a Carbide file cut guide. These are metal pieces with carbide surfaces on them that are trued up. You put the blade in between the 2 carbide faced bars and clamp the bars on the blade. This prevents you from over grinding back further into the ricasso and lines up your plunge cuts for you. I started using mine again when rough grinding until I establish a good plunge and then remove it when using the higher belt grits. Once a good, even plunge is established, it's easier to keep it going as the bevel walks up the blade and the grits progress. I usually take it off when the bevel grind is around halfway up the blade or so. The carbide file guide will let you start further up the blade and bring the cuts back to the guide, or start right at the guide and move forward. I usually start like 1/8"-1/4" in front of the guide with the grind, and then grind back into the guide to start it.
 
One of my first lessons I give to apprentices is that when tuning your grind DO NOT grind the whole bevel on each pass. Focus on the high spots only, then once the spot is fixed do a blending pass on the whole bevel.

By hitting the whole bevel each time you only re-inforce the problem.

To help focus on only the high spots use a crayon to mark the problem area. Make a pass or two so the blade is warm first. This will allow the crayon to melt onto the spot. This mark will have a thickness that can be seen from above as you approach the grinder. Only touch that spot to the grinder. This will allow you to focus metal removal from specific problem spots. Then blend.

Your short blade means that as you're we working your levels the middle gets reduced constantly while the plunge area and tip alternate. Hence your 'recurve' problem.
 
Much appreciate the help from everyone.
Much, much better the last two days after thinking about all of the suggestions.
They have helped me be much more intentional where I address the blade to the belt, how much time I spend in each area, angle difference at blade/handle juncture (is it called a ricasso in this context?), staying away from the tip until the later grinds,...
Fixed the one that I was having so much trouble with.
Ground a second in less that two hours today.
Both pictured here.
Feels like lots of progress.
Thanks again for the help.
(I know neither of these is these blades are done or perfect. I'm just learning. I've taken classes over the last few years. I finally got a forge in spring 2024. Forged and hardened a dozen knives and a bunch of other stuff. I only got a grinder recently. My experience with the grinder has taught me a number of things about how I was forging. In particular, I was trying to forge the shape and bevels much too close to desired final form. Really tough to grind out the hammer marks, waves and other imperfections with so little extra stock. My grinding experience will certainly inform how I am forging.)
https://flic.kr/p/2rgWzwq
https://flic.kr/p/2rgVr7p
 
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