Banana Knife Syndrome

[DBDuffer]

I am forging my first knife, about eight and a half inches overall, including handle.
Working with an automobile coil spring, material about five eighths of an inch diameter.

No problem straightening the spring and flattening it.
While roughing out the cutting edge I noticed the stock is getting banana shaped.
I am leaving plenty of stock for later grinding the final profile.

I had expected this to some extent and had in fact allowed for that by forging the stock into a cresent shape before I started working on thinning the cutting edge.

Finally, the question...Are there any guidelines or rules of thumb as to how much to allow for before I start thinning the cutting edge?

Thanks

 

[Kenon Rain]

how much what to allow?

 

[CWL]

Welcome to BF!

These type of questions would be better answered if you posted them in the Blademakers' sub-forum here:
http://www.bladeforums.com/forums/forumdisplay.php/741-Shop-Talk-BladeSmith-Questions-and-Answers

 

[Vicarious Reality]

I hope you're not talking horizontally?
I like verticallly banana shaped knives, they feel natural.

 

[Southclaw]

Trial and error. for me. You can also lay it on it's back and use a good chunk of wood insted of the hammer to pound it straight.
I learned that one from Will.

 

[Will Leavitt]

Yeppers, use the "whomping stick" to correct banana blade syndrome. Just grab a piece of wood that will stand up to the abuse. I use pecan logs, I've used part of a pick handle, wooden bat, I've seen 4x4 with a handle cut into the end. Here's a picture of my student using one to correct the blade.

 

[Makermook]

Finally, the question...Are there any guidelines or rules of thumb as to how much to allow for before I start thinning the cutting edge?

Thanks

I assume you mean how much counter-curve to put in the blade before you start forging the bevel, right?

Try this formula: L = D

L is the length of the cutting edge of the blade you will be forging.

D is the diameter of a circle you will use as a template for putting in the counter-curve.

Draw a circle of that diameter on your workbench or get a piece of large pipe that diameter. Heat your blank and form it around so the "cutting edge" conforms to that diameter. Of course, you'll have to flatten the blade and remove any twist that it wants to assume when you curve it, but that curve is a good rough starting point for how much counter-curve you have to have to overcome the tendency for the blade to "banana", and end up with a more or less straight blade after forging the bevel.

Again, this is a rough rule of thumb. How much counter-curve you put in the blade depends on how much straightness you want in the final product. If you're going for a khukri with it's nose-down profile, you'll put in more counter-curve; if it's a nessmuk, then less.

Good luck, and let us see what you come up with.