Getting am edge on a blade...

[paulc]

So I hate to be an idiot in public but I can't get an edge on this knife... :-(  I can get a ok edge w my file but when I san out the tool marks so goes the cutting edge. Been trying this morning just running it over and over on a 180 grit piece of sand paper,  holding it at consistent angle to the paper...still no meaningful edge.

What am I doing wrong?

Thanks, Paul

[Morgan]

Hard to say without seeing what you are doing. I’ve put a lot of edges on knives with sandpaper and a marble tile for a flat backing, but sandpaper isn’t my favorite for a flat edge. It is my favorite for a convex edge using a foam “yoga” mat for backing. 180 grit is a very low grit for me unless the edge is completely blunt. When using paper, I’ll use 220 if the edge is in rough shape and go through to 600 and then strop with leather and green polishing compound. Angle is important and if you aren’t getting to the point where you have a thin burr edge, then you either aren’t spending enough time to apex the edge, or you’re unknowingly changing angles and rolling the edge. Steel plays a part, but you can get a shaving edge with mild steel, it just won’t hold.

[Mesophilic]

To echo what Morgan said, hard to know without seeing what's gping on.

Use a finger nail to check for a bur?

Do you work one side for a while, before switching to the other side? Possible the bur is rolling to one side under that circumstance...making it not feel sharp.

[mmattockx]

So I hate to be an idiot in public

It's OK, you're among friends. 

Some (possibly) stupid questions:

1) Is this a home made knife or commercial? Has it ever had a good edge on it? If you can file it that seems pretty soft to me for a cutting edge.
2) Are you able to get a good edge on other commercial knives you have?
3) I don't think you will ever get an actual edge with 180grit sandpaper, that is very coarse for sharpening. I don't think I have never used more than 320grit wet to clean up a damaged or funky edge, then the actual sharpening happens with 600 grit and finer.

I'm with Morgan, sandpaper isn't that great for a flat edge but works well for convex edges with a soft backing. Stones work much better for flat edges if you have that option.


Mark

[Hawkdancer]

I learned sharpening on stones with no guide other than tilt and cut into the stone, back in the long ago.  Coarse stone then medium then fine!  I prefer my Lansky with diamond stone and guide now, work smarter not harder.  If the metal is too soft, you won't get a good edge.  Some blades won't hold an edge, period.
Hawkdancer

[Mr. Woolery]

Late answer, I know. 

I used to have a guy coming over often.  He'd use my shop and steel (not complaining, just perspective for what I'm about to say).  He finally decided he knew more about heat treating than I did.  And couldn't get his knives sharp anymore. 

So what changed?  Not the steel.  Not the tools.  Not the quench, the grinder, the abrasives.  But something changed. 

He bought new stones and a miniature belt grinder made for sharpening knives.  Still couldn't get the fine edge he got when he followed my heat treating formula. 

Any guesses?

I'll just tell you.

He stopped using a magnet.  He had decided he could tell when the steel was hot enough.  I have been making knives for 32 years and I still come up to critical temperature slowly, checking with a magnet often.  I do two normalizing heats.  And finish with a quench in preheated oil.  And I use a magnet each and every time I heat each and every blade.

Because if I overheat the steel, getting it up near forging temperature instead of creeping up to critical, I get a weaker blade that has such large grain growth that I can't reliably get a nice edge on it.  I discovered this many years ago and now I still use a magnet.

If you are normally able to sharpen a blade well, I suggest you look at what you may have done differently with heat treating this one. 

-Patrick