Full convex grind - ?'s on rust and HT

M3mphis

M3mphis

Joined
Jan 13, 2011
Messages
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First - thanks to all you experts that have made this site a wealth of information.

I'm working on my first (kinda second) knife, and I have a couple questions.

This is a full convex grind out of 1/4" 5160. Has not been HT'ed yet.

1. Today I cut open a pomegranate with it (a LITTLE anxious to get using it!), and then I rinsed it with water and set it on a towel. I meant to dry it, but I finished picking apart the pomegranate and took it to my wife who was working in our home office. When I came back, there was a shocking amount of rust on the blade. Just on the surface - no real harm done, but that happened really easily. SO - is there anything you would recommend to inhibit rust? Since it is full convex, I'm a little stumped (since I will be sanding the entire blade when sharpening). Is keeping it oiled at least every sharpening my only option? Maybe I screwed up by going full convex on 5160. What do you think?

2. I think I'm going to HT it myself. Because it is full convex, I had no idea how to grind it without taking it down to an edge. So, it has an edge on it. Now, since I will be heat treating in an oxygen rich environment, how much scaling damage will I do to that edge? Just a simple clean up or will I end up regrinding the whole blade?

3. For heat treat on a full convex - better to treat the entire blade the same (since sharpening involves the entire blade) or only the third at the edge?

Thank you!!!!
 
a full convex is ok. when you heat treat a knife, it must not have any sharp edges or edge. i leave my edges about as thick as a dime. make sure to have a smooth finish on the edge and no deep scratches.

how do you plan on doing the ht?
 
When I came back, there was a shocking amount of rust on the blade
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More likely just patina, takes more than a few minutes for rust to forum.
A carbon steel blade will darken with use, let it happen, it's a good thing :)
 
Some comments about corrosion: pomegranate is acidic. Acid will mess up steel very quickly unless it is stainless. Maybe this was an issue, but it depends on things such as time and how thoroughly you rinsed the blade. I tried my first knife on a tomato and after the first cut the steel was darkened. I don't know if this happens for 5160, but for some steels, the speed of corrosion changes depending on it being HT'd and how. If you are going to use your knife in the kitchen, I suggest forcing a patina with vinegar or a potato.
 
I'm planning to heat treat with a soft flame from an oxy/acetylene torch and an oil quench and temper in my oven. I know that is considered inferior to the fancy stuff, but I think it will serve my purposes. I called around for professional HT in the area and everyone has a $50 to $75 minimum. Doesn't seem worth it to me at this point, unless we ground out several more knives from the same material.

So if you leave a dime size width on the cutting edge - does that mean that you just have a lot of grinding to do after HT? I'm just thinking this because you have to grind the whole width on a full height, right? I don't have a secondary bevel at all.

Here's a couple pics.

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P1040123.JPG
 
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Good HT will make the most of any steel's resistance to corrosion. So will a fine polish. I've seen wet, rough-ground, pre-HT O1 and 1084 blossom orange almost right before my eyes, that doesn't mean the finished knife is going to fall apart at the first hint of humidity. Relax :)
 
Un-hardened steel can rust in minutes with acidic moisture on the blade. After HT it is somewhat more resistant , but all carbon steel blades will rust and/or stain if not washed and dried soon after use. A light coating of vegetable oil works for culinary use, and a coat of Renaissance wax works well for camp choppers.

A full convex grind is no different than any other blade....the entire blade will need to be re-sanded to the final finish after HT. If the HT is done with a torch, this step is even more important. You will need to remove all the scale and decarb after HT. Prior to HT, leave the edge about .030" thick. It concerns me that you were cutting something with this blade, as that implies a sharpened edge. If so, blunt the edge back until there is a flat surface of .020-.030".

After the blade is done with HT, and properly tempered, re-sand it with progressively finer paper starting at 220 and going up to at least 800. This should bring the edge back to near sharp. Once all sanding and handle work is done you will add a small secondary bevel to get the cutting edge properly sharp. If you try and go to a convex zero edge, you will never get it properly sharp and it will dull fast.Subsequent sharpenings will only require a few strokes on the edge bevel, as with any other knife.

Unless you are doing togi on a Japanese sword, the full convex grind will still have a small secondary bevel. At each sharpening, this will get slightly larger. After many sharpenings you may have to re-sand the entire convex surface, or reshape into a small flat bevel, as the edge will thicken minutely at each sharpening. However, if the convex grind is well executed, it will take a long time to thicken the edge with just normal maintenance sharpenings.
 
Drill it for pins before HT.
 
Alright, thanks everyone. I'll go ahead and blunt the edge and do the HT. I'll let you know how it turns out.

I need to get out into the Owyhee Mountains near where I live and find some bitter brush. My dad has made a few handles from it, and it is beautiful. It looks like ironwood, and it's every bit as hard. It has rained so much lately that those hills will be a sloppy mess. Hopefully this weekend I'll get up there and find myself some handle material.

Thanks again.
 
Fiddleback said:
Drill it for pins before HT.
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Oh yeah. I also need some brass tubing. I have pins, but I'm going to do two pins and a third with a tube for a lanyard. Haven't picked up the tube yet, so I don't know what diameter to drill the third hole.

Thanks for the reminder.
 
You realize that any wood you collect now will need to be dried for six months to several years before making a knife handle.
 
Your knife is looking good:thumbup:

If you use un-seasoned wood it may shrink on you leaving the tang sticking up a little. Get extra wood and keep it in the house for 6 months to a year to use on future knives.

Have fun and post up pics of the finished knife!

Bruce
 
Yeah, I realize that I'd have to cure it. But this is high desert country and I should be able to find some very dead stuff. The only challenge is to find a piece that is not split. I think this wood would probably take extra long to cure, it is so dense and hard. I'll see what I can come up with.

Thanks for looking out.
 
bladsmth said:
If you try and go to a convex zero edge, you will never get it properly sharp and it will dull fast.
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Stacy, while it is true that when you resharpen a convex edge it will develope a secondary bevel unless steps are taken to avoid it. the statement above is totally false. You referance a Japanese sword later in you post and you and I both know that these are convex to a zero edge. I grind almost all of my knives to a zero edge and a convex grind and they are very sharp and hold their edge very well. Here is a link to another forum where one of my kitchen knives was tested. It this thread some where the tester comments that he had to use the knife for a year before it needed a complete sharpening. I would say that is good edge holding. http://www.knifeforums.com/forums/showtopic.php?tid/873616/post/2082876/hl/Burke/fromsearch/1/
 
m3mphis, i can give you more tips on heat treating with a torch over the phone if you wish to email me your number. i have free long distance on my home line. i can even send you some pictures of what your blade should look like right after it cools from the heat treat. rje196021@gmail.com

i use the same method for heat treating my knives and they hold up great.
i use 1075 though but you should still get the same results. i use what are called seperator discs from a john deere planter that have an rc of 56 to start with and i work the steel as is.
 
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Not wishing to get into a detailed discussion, but the OP is an inexperienced maker who is making a chopper. That is a world away from an experienced togishi and a kitchen slicer. With limited equipment and experience it would be a long shot for the OP to maintain a full convex grind.
 
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