Foolproof, Easy, Drill Bit Sharpening for Acrylic




About: I teach High School Welding and Video Game Development (currently) and have taught everything in the Industrial Technology area. I also currently teach Welding at the Community College. I love designing ...

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! 

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Step 1: Tools Needed...

You'll need:

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. 


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    22 Discussions


    Tip 3 months ago

    The quickest way to do this without wasting so much of your drill is just change the cutting edge on the tip of the drill is just touch the drill cutting edge on your grinder to give it a negative cutting angle this will stop the drill grabbing the plastic but still give a neat hole


    2 years ago

    This was awesome,

    And given that I was drilling and tapping 6 holes in the top of a $500 acrylic podium... it was SUPER helpful!

    Great idea, but my first thought was that with the direction of rotation shown, it looks awfully easy to accidentally have the grinder catch the end of the bit. If you could set the grinder or bit so it was spinning away from you, seems like it would be significantly safer.


    4 years ago on Introduction

    Brilliant Idea !

    I tested out on one of my weekend projects and it worked perfectly.


    5 years ago on Step 7

    Thanks fella, great tip, I have recently started working with acrylic & as Susan(sola0005) mentioned acrylic is too expensive for trial & error when error is winning :) Only one thing, my drill has a plastic chuck & isn't suitable for holding while it's turning as it burns your skin very quickly so I use a garden glove but the end result is perfect. Cheers BEERS :D


    Reply 6 years ago on Step 7

    I meant, is the grinding wheel turned on as well?

    Thanks for the tip!

    Mikey Dsola0005

    Reply 6 years ago on Step 7

    Yes Maam. Light pressure is the key - Thanks for asking. - - - Mikey

    sola0005Mikey D

    Reply 6 years ago on Step 7


    I have used your suggestion several times now and it works great! Thanks very much for the tip. Acrylic is too expensive to crack or break!



    8 years ago on Introduction

    in my experience drilling into acrylic when using a bigger drill bit caused it to want to crack. did you notice if this way of sharpening the bit solved that problem or toned it down? also i thought that drill bits had a temper in them, won't setting them against the grind wheel make them lose it?

    2 replies

    Reply 8 years ago on Introduction

    again about the temper, have a container of water or other non-flammable coolant handy to cool the bit every now and again between grinding, that way you wont get it past the steels critical temperature

    Mikey Dwaldosan

    Reply 8 years ago on Introduction

    Excellent quetions! The angle on the drill bit solves exactly that problem. because it gradually enlarges the hole it minimizes the cracking problem.

    If you need a larger hole than you have a twist drill for, a spade or paddle bit will also work. It however does not leave such a smooth finish inside the hole; but a small drum sander on a "Dremel" type tool can solve that. Also the spade bits with "spurs" on them work better than the flat ones.

    Concerning the temper in drill bits: They should be hardened (from the factory) that's why they will shatter or snap rather than bend. If you grind slowly (lightly) and don't burn (blacken the steel) you will be just fine.

    Thanks for the comments!



    8 years ago on Step 9

    That's really neat, thanks for posting it. I'm going to give it a try. Instead of holding onto the rotating chuck (!), I think I'll use my drill's handle, which clamps onto the collar just back from the chuck.


    8 years ago on Step 8

    my shop teacher puts some type of burr deal on the end of his, and it seems to cut fine. He also uses the bits for brass too.

    1 reply
    Mikey Ddaftcloud

    Reply 8 years ago on Step 8

    Ideally drill bits for brass should be sharpened at a 155 degree angle. The point is almost not a point but is 25 degrees from being flat. This is because brass is a verry "grabby" material and likes to hang onto cutting tools and bind them up.

    I'd like to see the "burr" to which you are referring. Cabinet makers put a burr on a flat (or sometimes curved) piece fo steel and call it a cabinet scraper. It leaves an exceptionally fine finish that needs little or no sanading.

    Thank you for the comments



    8 years ago on Introduction

    While the danger is just a "fact of doing business", an excellent rule of thumb is " objects in motion tend to stay in motion". Yeah,so? You ask? Well, If you realize the "plane of motion" and keep your body away from said plane, anything which strikes you has already scrubbed off considerable speed and energy by being the result of a ricochet and not a direct hit. Still not ideal, but less is, well, less.

    1 reply
    Mikey Dburnerjack01

    Reply 8 years ago on Introduction

    I guess I reflexivly stand out of the "line of fire" whenever I use a grinding wheel. Great point though - thanks!