Drilling Just the Right 1/2″ Hole: More Complicated than it Sounds!

So seriously, how hard can drilling a hole be? Generally, it’s not much of a challenge, but with a project I’ve been working on I need to get a hole in polycarbonate that a 1/2″ rod can slide though, but not be too loose. This holes have already been cut with a waterjet machine, but need to be slightly bigger.

Just have it cut correctly!

If you’re reading this—and obviously you are—you’re probably thinking, “just have it cut to size.” In an ideal world this would be done, but I was told that waterjet machine generally cuts on the low side, and if there was some variation I definitely wanted the holes too small and not too big. After all, it’s much easier to take material off than put it on. So the parts are slightly undersize, which is a good thing.

I could use a file on these parts to open them up a little, but that’s not exactly a precision method of doing this, and besides, it would take forever. Better to use a drill bit.

1/2 Inch Drill Bit, No Problem!

So if it’s not quite 1/2 inch, then I could use a drill bit. Unfortunately there’s some variation on this as well, and the holes tend to be slightly undersized. As seen here, I can probably force things together, but they won’t move smoothly afterwards.

.501 Reamer

In theory, this should work quite well, and I reamed several parts with my new “slip fit” reamer (Amazon). However, even after attempting to freeze my parts to keep them from melting during cutting, they didn’t slide onto my ground aluminum rods well. I mean, as seen here, they could slide on, but the point is to allow things to rotate smoothly. I needed to take out a few thousandths of an inch more! So what’s to be done?

Go Metric

Being an American, for better or worse my first instinct is to see what’s available in the English System. 33/64 bits can be had from Amazon, but that’s 1/64, or .016″ larger than the hole I’m aiming for. Though that sounds minuscule, a few thousandths (of an inch) of slop can make a big difference in this type of application. Which got me thinking…

If 25.4mm is an inch, then a 13mm bit might just be closer to a 1/2 inch than a 33/64 bit. In fact it is, and works out to be .512″ or .012″ larger than my shaft. This still seemed like a lot of slop to me, but since the .501 reamer wasn’t cutting it, this seemed like the best choice.


After drilling quite a few pieces of polycarbonate I’ve found so far that this 13mm bit is a great fit for the application. I can slide the 1/2 inch shafts on easily, but there’s not enough slop to be extremely noticeable. I suppose I could have started there, but as noted before, if I had gone too large, there would really be no going back.

Although engineering is rooted in science, how things are applied sometimes take a bit of trial and error. In my view, if a mistake could mean a ruined part, it’s definitely a better idea to err on the side of caution, even if it take a little longer!


Speaking of taking a bit longer, the point of going through this exercise was to save time. Another thing that helped me do this well, at least with the small round parts that are difficult to hold was to build a fixture. This took some time to make, but now when I want to drill a new part, I just place it (two actually in the slot slot, then align the fixture with the edge of my vise, tighten it down, and drill. This can be repeated over and over.

I would absolutely recommend this vise I now use—here it is on Amazon—over the cheap drill press vise I was using before; it makes things so much easier and repeatable. Definitely worth the $100 bucks or so. If this fixture looks suspiciously familiar, here’s a very similar version I made out of aluminum for smaller plugs:

Another Option: Adjustable Reamer

If this post wasn’t long enough already, I’ll point out another option that I didn’t try, but may have been a better choice—an adjustable reamer. I didn’t know these existed until quite recently, but for boring precision holes of varied diameters, they could be useful. Here’s a set from Amazon for around $55 plus shipping that seems to be getting quite mixed reviews. Considering, that just one costs over $100 on Mcmaster Carr, I’m sure you get what you pay for to some extent, but maybe you can live with it being a little off. Who knows.

As for my larger project, it will involve, many, many drilled holes. Here’s a “hint” as to what it will eventually become! Be sure to check back here or subscribe on YouTube to see what comes next.

Leave a comment ?


  1. Good post! Sometimes it is the simplest-sounding issues that end up taking the longest time to get right!

    The only thing you forgot to address was the tolerance in the shaft itself. That is, is every piece of shaft going to be *exactly* the same size or a bit smaller or larger?

    Perhaps a better solution (but costlier) would be to using bearings or bushings, press-fit into your piece. That would give you the accuracy you need for the shaft, and allow for less-than-perfect holes in your piece.

    • Hi Brian,

      Good point, I should probably have added that I am using ground aluminum rod with I believe a +-.0005″ tolerance. So quite close.

      Yeah, bearings would be better, but I’d need somewhere in the range of 100 for this project, so $$$$. I will be using some thrust bearings though to keep things straight.

  2. I run a small 3D printing hub in Winnipeg. I’ve had to explain a number of times to first time designers that you cannot put a 1/2″ peg into a 1/2″ hole. You need to allow for manufacturing tolerances. Or as Snepscheut so elegantly put it:

    “In theory, there is no difference between practice and theory. In practice, there is”

    • Great quote!

      Hadn’t thought of that, but yeah, I bet that is an issue for “print to model” businesses that cater to beginners…Guess “build to print” wouldn’t make any sense, ha.

  3. Get your self a set of adjustable reamers.

  4. You buy a drill bit at a hardware store and it ground to cut steel. If your cutting Plexiglas or worse yet Acrylic you need to reduce the rake of the cutting face to 90 degrees, reduce the web by grinding down the land of the bit so less of the bit rides on the plastic heating it up.

    I alway use a flood of emulsified oil in Plain water is fine but it rusts the tools.

    DON’T get in hurry and feed the drill bit too fast and let it screw in the plastic causing it to split rather than cutting the plastic. If you reduce the rake of the cutting lips of the bit to 90 degrees it helps a lot.

    Here is a page on bit nomenclature: http://www.mfg.mtu.edu/marc/primers/drilling/nomen.html

    • Hi OldRed, thanks for the comment—seems like you know what you’re doing better than I.

      Wow, I’ve been pleasantly surprised at all the feedback to this post!

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