If you have ever drilled acrylic with a standard twist drill you've probably noticed that it is less than ideal. The angle is too shallow and the drill bit tends to break out the backside and chip the material. Here is an easy way to put a much steeper angle on the bit that leaves both flutes the same length (on center) and gives you a ridiculously sharp edge (where it counts).
- I am about to show you a few things that are definately not safe / OSHA approved.
- Try this at your own risk.
- If you blow up a grinding wheel or burn yourself or in any other way damage yourself, anyone else or your tools or equipment, you've been WARNED!!!
- So anyone who feels like leaving safety related comments, please leave them only if you have a safer way of doing this. I already know it is dangerous.
That being said...On to the show!
Step 1: Tools Needed...
The drill bit you want sharpened (of course)
A small (high speed preferably) electric hand drill
A bench or pedestal grinder
Step 2: As You Surmised...
Chuck up the drill bit in the electric hand drill.
Step 3: Secret Weapon!
Set the electric hand drill to reverse!
Step 4: Pre Grinding
Hold the drill about like yay...
Step 5: Pull the Trigger And...
I am a righty so this works for me, lefty's please swap hands..
Run the electric hand drill up to high speed, stablize the rotating chuck with your left hand and apply LIGHT pressure.
(I know, I know. Grabbing a rotating chuck with your hand might not be best / safest set-up on the planet, but it works)
Step 6: Finishing Touches
After using light pressure inspect the drill bit. You should have the roughed in angle complet with relief (relief is the "back angle" that lets the cutting edge bite / dig in without rubbing.
Finish sharpeneing the bit by applying VERY, VERY LIGHT PRESSURE. You should only see minimal sparks. This is taking the high point of the drill bit and turning it into a cutting edge.
Step 7: How and Why It Works...
Two of the more difficult parts of sharpening a drill bit are:
1. Keeping the silly thing on center (both flutes / cutting edges the same length)
2. Grinding the relief angle (so the bit cuts and does not rub)
Spinning the drill bit in an electric hand drill solves the first problem; you will always be on center.
Turning the bit in reverse automatically solves the second problem. By spinning backwards the first part of the drill that touches the wheel is the backside of the flute, precisely where you have to take off slightly more for the relief angle. Using light pressure allows the drill to “bounce up” and remove a tad bit less material from the cutting edge. The drill bit then comes down again on the opposite flute and repeats the process.
Using “VERY, VERY LIGHT PRESSURE” allows the drill to touch on the high spots (the cutting edge) putting a whole bunch of sharp where you need it.
Step 8: In Operation...
You've probably noticed that the tip of the drill has a slightly rounded, very sharp "chisel point" with a bit of negative rake. In operation this seems to work out just fine. The drill bit doesn't walk, it cuts the acrylic fairly slowly at first (just like any other drill bit) and as the more positive raked parts of the flute come into play, cuts smoothly without vibration (on center) and leave a very smooth surface with no chip-out on the backside.
These holes were dilled with no backing plate, just air on the backside.
Why didn't I think of this years ago? *slaps forehead*
Step 9: The Finale
Sorry I could not get any closer to show the amazing surface finish inside these silly holes. The macro setting kept losing it mind as it couldn't figure out on what to focus.
The impetus for this instructable came from my son who is building a seven bay reptile enclosure and has to drill a whole bunch of holes in acrylic. 10:30 at night seemed the right time to come up with a new way of sharpening a drill.
I actually ran the drill foward first to dress the angle on the end. I figured I'd then grind in the relief by hand (like we usually do) but I noticed the "negative relief" left by the clockwise motion of the drill on the grinder. It was a pretty small leap to figure reversing this might give us what we needed and lo and behold...
... It worked.
Thanks for putting up with my rambling (It is midnight y'know) and like I said, If you figure a way to make this safer, please feel free to comment.