Help flattening blanks before grinding bevel

P

PredHunter

Joined
Dec 2, 2013
Messages
115
Hello,
I am pretty new to knife making and am having trouble successfully getting my blade blanks adequately flat. I have been flattening my blanks by using a magnet handle to hold the blade flat on the platten of the grinder. The problem I am consistently having is that I always end up with a blank that is thinner at the tip of the blade and at the pommel of the handle as well as a low spot in the middle of the blade. Any suggestions? Thank you for any help.
 
Do you have a glass platen? I was having similar issues and recently got one, which it seemed to help a lot.
 
Well, this won't be near all of it but if I can help.
First do you soak your blades in vinegar or a weak solution of muriatic acid (HCL) to get rid of the scale if there is scale there? You want to make sure you are not using up the belt just to do that. If the blade isn't straight you must make it so or close BEFORE you start the grinding. Get out a hammer to help if necessary. Are you starting with say a 60 grit or close to that? Are you using some guide lines for edge thickness? Following these will tell you where to go or where not to go. Please if you haven't read the stickies above on this page.
Frank
 
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you have this result because the piece of steel must first be straightened. You can do it using the hammer and the anvil and also using a good vice. Often some steel pieces besides being curved longitudinally are also slightly transversely twisted. To recover this second problem I help with the vice.
 
A couple of additional points.
Do you flip the knife top to bottom and spend about the same amount of time on each flip? If you just stick the blade against the platen and push it isn't going to evenly remove material.
The low spot in the middle may be due to too much pressure. Too much pressure in a single spot will remove more metal from that spot.
Take it slow, move the blade around a bit, and try to put pressure where you need it.
A caliper will help inform you where you are high or low.
 
Is this pre-HT or post HT grinding? The blade needs to be pretty darn straight before grinding -
 
your tip gets thinner because there is less metal there and it will sand off with less pressure as compared to the rest of the blade. with your method ( i use the same because i do not have a surface grinder) put the blade flat on the belt before turning the machine on. you can not lower a blade onto a moving belt and keep it flat by hand, one spot will always touch first and get sanded away more than the rest of the knife. make sure you are only applying downward pressure in the center of the blade, not near the point. it will not be perfect but you might get better results than you have been.
 
Wow, thank you all very much for the quick replies. There is no scale on the blade as I am grinding it flat pre-heat treat and am using the stock removal method so I am starting with typical flat bar blade stock. I have not been trying to straightening the blank at all before grinding so I think you guys may be right about that being the problem. I will also try holding the blank on the belt before turning the grinder on.
Thanks again for all the help guys.

Edit: I can't seem to get the problem to show up in pictures very well, sorry.
 
Frank Niro said:
The scale I'm talking about often comes with the piece of steel when you buy it.
Frank
Click to expand...
Oh my mistake. Like mill scale? I have not been soaking prior to flattening. I will try that as well. I have been grinding with a 120 grit belt for fear of thinning it out too much with a coarser belt. Could this also be part of my problem?
 
If you're doing stock removal it's worth the money to buy precision ground.
 
I flatten every piece of bar stock before I start any stock removal blades. It's not really that necessary to straighten a 4' long bar because it's going to get bent again. But after I make the template for whatever blades I'm making, I cut appropriate lengths of the bar and straighten those. Smaller lengths won't easily bend again after straightening (unless from uneven heat during grinding) and I can more easily check the flatness of smaller pieces on my surface plate.

I use a cheap 20 ton shop press from Harbor Freight. I have a couple of 1/2" thick steel bars that I lay across the horizontal beam of press. The hydraulic ram and those two bars essentially make a big 3 point jig. The beauty of this is the variety of ways I can make corrections and the subtlety of force I can apply when/where needed. Depending upon how far I spread the bars I can achieve a more gradual bend or a tighter bend. If the bend takes place closer to one end of the bar I shift the two bottom bars over so that the ram comes down closer to the bent end. Sometimes a bar of steel has more curve along one edge than the other. By placing the ram closer to one edge I can create more of a correction on that side than the other. If the bar has a slight twist I go to the vise and use a big honkin' crescent wrench and dial in the counter-twist. I keep checking against the surface plate until it is STRAIGHT.
 
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