Precision hole drilling?

One of the things that our team is going to focus on this year is precision in getting holes in our aluminum tubing – getting the right size hole into exactly the right place on the tube. And, no, we do not have a mill or a CNC router.

Step one for us was replacing our old 8" Harbor Freight drill press. It has a number of problems that hurt the precision: first, the runout is just horrible, which means that holes are either oversize or (more frequently) just not in the right place. Second, the table isn’t exactly square with the upright, making it hard to drill parallel holes. Third, the table just clamps on to the upright – there is nothing preventing the table from moving side-to-side when you only want to move it vertically. (So, “Ok, drop the table to move to a larger bit, then raise it back up” is a lot harder than it should be.)

Step two is clamping: We started using a drill press vise last season (as opposed to just clamping material to the table), and it really helped keep the work in place.

Step three, I think, is technique: shallow holes should use short drill bits, and the table should be as close as possible to the retracted spindle. Since distance magnifies runout, this will reduce the amount of runout that’s there. Is this right?

And, what else am I missing? How do we measure and mark precisely so we are putting holes in the correct place? Do we get a sliding vise? (The cheap ones don’t seem to be all that precise and the expensive ones are, well, expensive. ) Should we get a center finder (like what milling machines use)?

First and most obvious question: How are you measuring and marking your pieces so you know where to drill? When I was making my Fight Night robot this summer, I was able to achieve acceptable precision (enough to mount hubs and BaneBots gearboxes to plastic and cardboard, and to join pieces of cutting board with finger joints) with rulers and cheap plastic calipers. Your process there, whether you’re using templates or measuring out each spot, may also be an opportunity for improvement.

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I think a pair of calipers–particularly a long pair, say 12" or 24"–may be more useful than an edge finder; if you have a sliding vice then an edge finder might be useful for smaller parts.

Also some marking fluid. Lay down marking fluid, mark hole center in dried marking fluid. Use a center drill to locate center with respect to drill chuck, and drill a starter point. Then step up to the size you want. Depending how accurate you want your final hole diameter, consider using a reamer (bolt holes probably don’t need this). Go slower RPM as the bit gets bigger.


We use a an automatic center punch ($8 at Menards) and a pair of low cost harbor freight calipers for 1off hole layouts on parts. For hole patterns I’ll make a 3D printed tool with the holes (.125" pilot) in the correct locations. All holes get clamped and drilled to size using a drill press.

If you’re getting a lot of run-out drilling through box tube, we sometimes drill both sides individually and team both holes together.

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Beyond just marking the holes right you need to control wandering when first drilling. First use a punch to mark the location. Second use a smaller bit to do a pilot hole, as they are easier to control and the pilot hole will control the larger bit(s). Finally if manually drilling, to make sure it’s a straight hole I would recommend a drill guide like this just make sure to clamp it in place.


CAD drawings printed 1:1 spray glued to material with holes centers marked is a nice way to get the holes where they need to be.

Scribing with a caliber with or without markings (fluid or marker) is another way to get the locations right.

Then locating the centers with a center punch. Then pilot drilling and working up to the final size.

Then check work and improve from their. Do precision drilling on press and try to encourage that over hand drills when possible.


I would comment that a properly trained user of a hand drill can be just as effective at precise hole-drilling as a user with a drill press. But that’s a level above what I’d expect a team of high school students to do.

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Marking is important. If you’re using holes from an existing part I highly recommend a set of transfer punches.

As for drill bits - stub length bits may help minimize wobble (available other places too but who doesn’t love MCM)

I would definitely suggest some machinist center drills. They will help make sure your hole goes exactly where you want it without the drill bit walking.

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The first project for your team is take some time to tune up the drill press. Most drill presses have adjustment screws for the quill allowing you to take some of the slop out of the system. This will not totally solve your runout problem but should definitely help. You will also want to tune in the table to get to be as square as possible to your quill.

Next y’all need to practice laying out your drill points. A cheap pair of HF calipers, sharpies and patience are all that is need for this. Printing your designs out 1:1 as @ngreen mentioned is also a great technique.

Once your points are layed out center punching is mandatory for accurate holes. I am a fan of these center punches:

Having a manual center punch is also helpful to have around:

As @JacobD mentioned center drills are a great way to actually pilot holes.

The last tip I would recommend is to tune in your hole sizes. If after all of these steps your holes are still coming out over sized take some time to test multiple drill bit sizes to find the size that gives you the correct final hole size regardless of what size on tin says. Having a complete set of numbered, lettered, fractional and metric drill bits will help with this. The drill bits you discover give you the closest to correct size should then become your “golden” drill bits only use these drill bit when you are dealing with a critical part or hole.

To expand on that idea when in doubt drill a smaller hole than you think you need making a hole bigger is easy but making a hole smaller…is really hard…

Best of luck to your team!

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In addition to everything already mentioned about marking and punching, getting a cross slide vise for the drill press really helps.

Fixtures like this can help a ton.


I just thought of another thing: reamers.

A reamer is essentially a precise drill bit with straight teeth. The purpose of the straight teeth is to create a circle. A twist (traditional) bit will create an oval rather than a circle from the helical shape. Also, a reamer usually comes with a tolerance and can help achieve press or slip fits. Just don’t forget to use oil on the reamer and pre-drill with the correct size twist bit.


Hopefully, you are referring to the old drill press and your new one has a table that stays in place, vertically and horizontally. If it will not, all the other advice you are getting won’t help.

Before starting the drilling, think about how best to clamp your workpiece(s); using the drillpress clamp or clamping directly to the table.

If there are multiple pieces, say a pair of gearbox plates, you should also think about how to hold those pieces together, separately from how to hold them down in the drill press. Holding them together with one’s hands will allow the pieces to move relative to one another as the pieces are moved to drill multiple holes resulting in some of the holes will not line up.

Do you know of any jigs that can help for drilling holes on the 2" side of a 2x1 tube?

Not off hand, you could try reaching out to Ozzyman about making it or Amazon has some drilling jigs for working with dowel pins that might work. You could also CAD one up and contact a local machine shop or team about machining it for you.

I will echo what people have said above about center drilling a hole first and in some cases using a reamer as well. If you have the choice you should really center drill every single drilled hole that will be drilled using a non-stuby drill as they can walk quite a lot, especially if they are full on the tip.
To get a better hole I would also highly recommend to use as much coolant as possible.
In addition to this run the drill slower than you think. Generally with a hss bit in aluminum I don’t run any drill bit .1in or bigger at over 1000rpm. Drill bits really benefit from being run at slower rpms.

Few additional questions:

(1) Currently, we really just use machine oil as a cutting fluid. Is that a mistake? I’ve seen suggestions for using alcohol when cutting aluminum, but I’d think that would evaporate too quickly.

(2) When people talk about “center drilling”, is that meant using one of these : or just a small stub bit to create a pilot hole?

Rockler and Woodcraft both sell various self-centering dowelling jigs. Some kits have more accessories that you may or may not need. They are designed to guide your drill bit so it drills a hole in the center of a workpiece that is up to just over 2" wide. If you want it off-center, you can pad one side with shims. If you are only wanting to drill through one side of the tube, you don’t need to use this in a drill press since the guide block is over 1" tall. I have one somewhere in my garage and it worked very well when I built a bunch of shelving units out of hardwood.

For other hole patterns, you can make a jig using a piece of aluminum angle wide enough to contain your holes, or an appropriate subset of them. Make sure the inside corner is not radiused. Layout your hole pattern on the INSIDE of the aluminum angle IN REVERSE. To use the jig, place it over the tube and push it into the corner of the angle. Clamp the jig to the tube at both ends. Drill using a properly set up drill press (quill at rigth angles to table) if the hole has to go through both sides of the tube. Otherwise, a hand held drill will work if the hole only has to go through one side. For greater durability, the jig can be made from steel but I have rarely seen steel angle that does not have a radiused inside corner.

  1. I wouldn’t use machine oil except in emergencies. There’s a variety of cutting fluids but I’d be looking at this one or one of a number of other similar ones as a primary option. If you want to go cheap, you could use WD-40 as a cutting fluid. Smells kinda funny and is a little harder to get a precise dose but does the job.
  2. Yep, using one of those. The idea is a short and small drill bit that can mark the location for a bigger bit. Back when 1197 first started using a mill, we increased our output a little bit by using the mill to center drill all our hole locations before sending the parts to the drill press. The next year, we just did 'em on the mill… or the CNC.

As a speed note… I tend to run most drilling ops on the mill at work at about 1K RPM. I slow down for steel or large sizes or reamers. If you use a reamer: SLOW DOWN, and once you’re through the material stop rotation and back it out.