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S. Cruse

Geometry issues

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While practicing grinding with a old file i ended up with this Knife Like Object, but the geometry is all weird. any ideas or suggestions on how to fix it? 

20170317_132920.jpg

the geometry is kinda wonky in the cross section, wide cheeks to the bevels, almost tear drop shaped. Hard to get a decent image, but you get the idea

20170317_132803.jpg

Now the cross section i was going for 

20170317_132716.jpg

 

I fixed the bend in the tang by the way.   8" long, 25% is tang 6" in the blade and still working on the shape before I start fit and finish

 

Sorry for the crappy pics, I don't even own a cell phone to take better ones with

Edited by S. Cruse

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The pics are fine. They certainly provide plenty of clarity to understand what you are asking and looking for. You are using the file and not the grinder, yes?

What you have is a type of grind called convex, or cannell, or apple-seed. What you are looking for is more of a flat grind. How you wound up with the convex instead of the flat is that you rolled over the face with your file as you went across and down the length of the blade. Common mistake when learning how to file a bevel in. Until you develop the motor control to keep the file in one plane, draw filing is going to produce a convex bevel.

Here's an idea on how to not only fix that blade, but learn how to develop the hand/motor control. Take a black sharpie and turn the bevel face completely black. Chuck that blade up in your holding jig (you do have some way to hold that blade steady and horizontal, point toward you don't you?) and start draw filing along just a little back from the edge. You should start to develop a shiny stripe of uniform width along the edge. If it starts to grow in width, you are rolling over the face. Stop, apply more sharpie, and start again concentrating on keeping the cut flat and steady. When you have a shiny stripe about 3/16-1/4" wide, it will have a sharp ridge along the spine side. blacken the shiny stripe and lay the file on top of that ridge. Start draw filing along that ridge looking to create another uniform width stripe down the blade. Keep it going until the stripe is about 3/16 - 1/4" wide again. It should have taken away some of the black on the previous flat spot and moved the bevel up the blade toward the spine.

Keep doing this over and over. Blacken the blade. Develop a flat space with a sharp ridge. Apply more black to the blade. File the ridge down until the flat spot is about twice as wide as it used to be and joined with the previous flat spot.  Repeat as needed.

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1 hour ago, Joshua States said:

You are using the file and not the grinder, yes?

Sorry, to clarify I am using the grinder on an old file for practice.  I do have other files, but so far has just been a little bit of hot work and a butt-load belt work

but thank you, you did a good job of explaining how to fix the reuleaux triangle

Edited by S. Cruse

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Are you using a platen behind the belt, or an unsupported belt?  An unsupported belt gives a convex grind no matter what.

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In that case, as Joshua said you're rolling the blade a bit.  Gotta keep it solidly flat!

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