Greetings, /out/. I come to you in search of wisdom on sharp things. I generally dislike going outdoors but I love working with wood, and I've had the idea to make my own marking knife out of a drill bit.
Here's what I do know. A marking knife is simply a very small knife, held like a pen, meant to scribe lines in wood as close to a straight edge as possible. So it's single beveled, and is shaped so that a line can be scribed onto wood with either your left or right hand. And I have an assortment of spade bits. I know that the steel is some sort of high-carbon, hardened tool steel which is made to drill through wood and steel with no issue and should work well for this application in theory.
What I'm unsure of, is whether or not on typical spade bits, part A (the new bevel) is as hard as part B (the part I want to grind away). Because if the bits are manufactured so that only the cutting edges are hard, I would need to heat treat the steel, and I'm not equipped to do that. And also, I don't know what is the best way to grind this metal without affecting hardness in any way. Would it be sufficient to grind away Part B on a bench grinder while dunking in water, or do I have to do something more drastic?
If it's made for cutting steel, it's probably made of some common HSS alloy like M2. Just keep it cool while you're grinding it and you'll be fine. If you burn the edge, just grind it off and reprofile.
Maybe give it a final sharpening using progressively finer grades of sandpaper/emery cloth if you're feeling artsy.
Normally you normalize a piece of steel before grinding and harden it again afterwards. If you use a low alloyed carbon steel, you can do that with just a magnet and some really hot coals in your backyard.
If you got a HSS (Cobalt alloyed) steel, forget about it.
If you grind it without normalizing, there is a good chance that you at least punctually warm the metal over annealing temp, and that be bade for your future edge. water cooled grinding stone would be the tool of choice.
Small nitpick: M2 and M7 HSS has no cobalt, and that's what most of your everyday drill bits will be made from. "Cobalt" HSS is considered a (small) step up from normal molybdenum-based alloys because it has more heat resistance.
I went ahead and made it.
The 'wings' and bulk of the spade bit were cut with an angle grinder. Then I stuck the bit in a vise and used a very aggressive file to make the bevels. The steel was soft enough for a bastard file to cut. You might be able to see a nick in the right side of the blade. It's not a nick, just a remnant of the bit's machining that I didn't bother to grind away. It will go away with time, I guess.
Honing was accomplished with a succession of DMT diamond stones, with 1200 being the finest grit. The back was also lapped flat. This is an important feature of a marking knife.
Stropped for the final edge refinement.
It's much too steep a bevel angle right now, about Xacto knife level. So while it's very fun to watch it split hairs down the middle, I will have to put a micro-bevel on it to keep it from self destructing from normal use.
looks noice anon, I've got a 3/4" bit i could do the same with lying around, but in the meantime I use one of pic related and it's great!
Making anything good? I'll be making a new marking gauge soon for framing work, my old one is kinda crappy and the blade sucks bad.
I think I'm not going to put a handle on it, but just spray lacquer on it and call it a day. I don't see any good coming from adding bulk.
That's all. Thanks for watching.
Thanks. I mostly make my own furniture (bed and desk, tables, night stands etc next one will be a dresser but that will be the true test of my woodworking knowledge and skill) also lil jewelry boxes that I usually give away as gifts.
Never done a box with handcut dovetails before though. I figure having a marking knife is another step towards that.
so, im not taking the piss but what are you planning on using this knife for? Like it can do most "knifing" things fine, i mean all you need is a cutting edge and this thread has also learned me a few things but really. Was this just a weekend project? No criticism here mate but i just find it odd that you'd cut a drill bit where im sure you could do the same kind of project with a piece of steel thats much more /out/ functional. unless im reading the thread wrong, which is entierly feasible as ive been up for near on 30hrs.lol. i shuould go to bed
anyway, it looks good man. Diggin the low profile blade