That's why instead they use a "marking knife". Basically a knife with a very sharp point that is only sharpened on one side. (That way the back is flat to ride against the ruler, square, or whatnot.)
The problem is that these knives generally cost between $10 and $50, and a good toolbox would be stocked with 2-3 of them. (Left, right, and ambidextrous)That certainly adds up! (Here's an example from a popular catalog for comparison.)
The solution I came up with is not only made 100% free using stuff you'd normally throw in the trash, but the blades are replaceable, the handle is nice and big for people with arthritis, AND the whole thing can safely go with you in your pocket!
So what's the big secret, then? Re-engineer your spent utility knife blades!!!
Step 1: SAFETY WARNING
This project involves potentially rusty razor sharp objects, modifying tools beyond their intended purpose, hazardous chemicals, flying sparks, and possibly feeding a Mogwai after midnight. If you do not feel qualified or safe doing any of the above, please surf away from this page (and probably this entire site) and take up a more relaxed hobby like perhaps macrome or counting the number of leaves on your front porch.
If, however, you're willing to take responsibility for your own foolishness in copycatting frightening stuff you find on the internet, then you have come to the right place. Welcome aboard, my friend!
Step 2: Choose your weapon!
1) The oldschool fixed-blade knife. These used to be everywhere, so you've probably got one laying around. Do you ever use it anymore? No you don't. You try to go anywhere with it and you're pretty much guaranteed to poke yourself at least once. You use your retractable blade knife for everything, and this just lies there in the bottom of your toolbox. Forgotten and unloved. :_(
If you're only using your marking knife around the shop, why not breathe some life into this old utility knife by swapping out the blade and turning it into a marking knife?
2) The new pocket knife style blades. These are great! (Just maybe not for this purpose.) They fold up so convenient and the blade can be swapped out with the touch of the button. They do have a couple of drawbacks for this project however: First, there is no place to store extra blades like the other knives. Second, you can't even use an ambidextrous grind on the blade because the back would still be able to cut you when folded.
3) The "slide-switch" style utility knife. This is probably your best bet for something portable. Though it doesn't fold as small as #2 above, it does completely shield the blade when closed and it also has storage for your extras in the handle.
(I was going to take a picture of this too, but I couldn't find where I left it. I'll update this when it is rediscovered.)
Finally, what if you don't have a utility knife at all? (Frankly, I find that a little hard to believe, but...) Well, if you have access to a 3D printer, you can just print one out!
Step 3: Get some old razor blades.
Step 4: Wash up.
I used engine de-greaser, but you could just as easily use brake cleaner, paint thinner, etc. As strong as you please -- don't worry about it. This is steel we're cleaning!
Step 5: Grind the edge flat.
I used a mini grinding wheel for this. You could use something bigger, but don't try to tackle this with a file. I tried. It just laughed at me. :(
When you use the grinding wheel, remember all of your safety precautions, of course. Also keep that blade good and wet, keep it moving, and above all keep it cool! If it gets hot and changes colors then you're sunk. You just wrecked the temper.
Oh, and one more thing. Only grind down half. This whole instructible fails if you try to get greedy and do the whole blade. More on that later. Suffice it to say, DO NOT grind further than the notches on the top!!!
Step 6: Grind the angle.
Step 7: Admire your work so far.
Notice also that it's REALLY difficult and dangerous to grind the left-hand blade unless your grinder has a reverse or you're using a big bench grinder. You may want to start with just the ambidextrous blade first. That should keep you in fun for awhile anyway.
Step 8: Polish till your eyes bug out.
I only took one picture of this, but don't let that fool you. You're looking at the bulk of the work right here.
Step 9: Scary sharp!
Step 10: The test.
Oh yeah. There are pretty heavy pencil lines in between those marking knife lines.
I'm happy to finally have a marking knife around. :)
Step 11: The big oops.
Well, not really. It turns out that these knives rely on the width of the razor blade to hold things securely. You can see how big a gap there is by the picture!
In my fixed-blade knife pictured there's a lot of slop now that the blade is smaller. In my blue folding blade knife it won't even hang on to it anymore!
This is easy enough to avoid. Simply leave half of the original blade intact. You don't get the cool flippabilty that way, but oh well. You gotta do what you gotta do.