Find Exact Center and Drill Concentric Holes in Metal Rods

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About: I'm a retired mechanical engineer, woodworker, boater, and inventor. Now I'm getting into wood turning, and have found that all my wood projects need not be flat and square.

Intro: Find Exact Center and Drill Concentric Holes in Metal Rods

Finding and marking the exact center of a small metal rod or dowel is more difficult than finding the center of a larger circle. I recently wanted to mark the center of a 1" long x 3/8" diameter brass rod, then drill a 1/8" concentric hole the entire length. Yes, just a brass rod with a hole down the center. A professional machinist with a metal working lathe might have no problem with this, but for the average garage shop guy (me) it was no easy task. I learned a few tricks and made a jig that made the job easier and the results better. 

With apologies to my friends in metric countries (everybody except U.S.), my example is in Imperial units. Everything applies to S.I. components and drill bits.

 

Step 1: Find the Center of the Rod - Method # 1

Method # 1 (not necessarily the best).

The classic method of finding the center of a round object is to use a center finder head attached to a standard metal rule, such as used in a combination square. You hold the round item in the V-notch and scribe a fine line. That line is a diameter of the circle. Rotate the piece, and scribe another line. Where the two lines intersect is the center. Easy and accurate for large circles (say over 1" diameter), not so easy for a 3/8" rod. Forget about using a pencil; use a sharp knife point, like an Exacto knife. Even with a sharp knife, the line can be a bit off center.

 

 

Step 2: Find the Center of the Rod - Method # 2

Method #2 

Use a Forstner or brad point drill bit the same size as the rod. The point at the center will be reliably dead center. Place the rod and drill bit on a hard flat surface, and with the point of the bit scribe a couple of lines on the rod end. Where the lines intersect will be the center. 

Step 3: Find the Center of the Rod - Method # 3

Method #3.

Insert the rod into the chuck of a power drill. Generally the chuck will accept up to 1/2" diameter. A drill chuck should center the rod exactly, but run the drill slowly to confirm there is no wobble or runout. Now you will need to hold the drill firmly; you may need a helper, or clamp the drill down somehow.With a hard tip scribe or punch, move the point slowly toward center of the rod. As you approach center, the tip will tend to go around in smaller and smaller circles. At dead center, if you press the tip to the rotating rod, it will not move at all. So, you found the center without marking any lines - and now you can mark the center point by pressing the tip into the rod or tapping the scribe with a mallet.

Step 4: Center Punch a Dimple

Marking the exact center with a center punch can be tricky; I have to use a magnifying lens to get the punch at the intersecting lines. That is the beauty of Method #3; you get at least a small punch mark when you locate the center. I bought a fancy (and expensive) optical center punch from Lee Valley tools, shown in the photo. With this you can locate the intersection of center lines with the optical cross hairs, then insert the steel punch to mark center.

You need a center punched dimple to drill into any metal object.

Step 5: Make the Jig

This jig will make it possible to drill your concentric hole straight down the axis of the rod.

 

Start with a block of softwood scrap, like the piece of 2x4 in the photo, clamped to the table of a drill press. Then drill a hole of the same diameter as the rod through the wood block; 3/8" diameter in this example.   Remove the drill bit and insert the rod into the hole you just drilled. It should be a snug fit. It is best to not remove the block; leaving it securely clamped will assure that everything remains aligned.

 

 

 

 

 

 

 

 

 

 

 

Step 6: Drill the Concentric Hole

Insert the smaller bit, 1/8" in this example, and ensure that the tip is lined up with the center punch mark on the rod. The smaller bit will automatically be aligned parallel with the dowel, so will exit at the center.  If a through hole is not needed, the rod can be removed from the block, reversed, and a centered hole drilled in the other end.

 

 

 

 

 

 

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    35 Discussions

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    DavidT618

    Tip 8 weeks ago on Step 6

    Here is a method which I think is much less prone to errors. Required: thick block of wood, a transfer punch with a small diameter compared to the rod, a drill bit bit the same diameter as the transfer punch, and a drill the same diameter as the rod.

    1: using the smaller drill, drill a pilot hole clear through the block of wood.

    2: using the larger drill, drill out the pilot hole to about half the depth of the wood.

    3: insert the the rod into larger hole and transfer punch into the smaller hole tip first.

    (both holes should be pretty tight fitting to maintain good accuracy)

    4: Stand block on a hard surface with the exposed end of the rod in contact with and perpendicular to the table surface. The transfer punch will now be on top. Tap it with a hammer, remove the rod, and voila!

    3 replies
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    Bill WWDavidT618

    Reply 8 weeks ago

    Thank you David, will definitely try your method!

    Forgot I did this Instructable. Wow, look at my finger and the Exacto knife in step#1! Someone should have yelled at me about that!

    Bill

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    DavidT618Bill WW

    Reply 8 weeks ago

    Yikes! You got that right, Bill. After puberty the human body will not re-grow a missing fingertip. Forgot to mention in my method that the holes should probably be drilled on a drill press, though it might work with a hand drill as well. As I am sure you know, once the pilot hole is started, it's pretty hard to put enough force on a hand drill to change its direction. Same with the bigger hole, once the big drill enters the pilot hole it stays quite parallel. Accutracy may be a bit less than on a drill press, I would imagine.

    Dave

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    Bill WWDavidT618

    Reply 8 weeks ago

    Yes, I assumed a drill press, but maybe others may not.

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    TravisM105

    1 year ago

    I just ordered a brass Boston Gear Pinion Wire to make new spur gears for my son's 12 volt four-wheeler because I converted it to 24 volt and needed fresh gears. My only concern was how to ensure the hole for the shaft was perfectly centered... a quick google search brought me to your article. My concerns are gone and I'm looking forward to giving it a try. Thank you sir!

    1 reply
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    Bill WWTravisM105

    Reply 1 year ago

    Glad one of my Instructables could help you.

    Your son could climb a big hill with a 24 volt drive!

    Bill

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    Bill WWOffBeatSocker

    Reply 3 years ago on Introduction

    Very good question, Offbeat, and not a simple problem.
    A commercially made jig is made to do this: "V-drill guide", www.biggatortools.com. You don't have to buy one of these, but toe jig demonstrates a solution to the problem. You need to drill thru the center of the rod, and prevent the drill from slipping off the metal rod.
    A few things you could do:
    - Center punch the rod.
    -Make a "V" shaped jig to hold the rod (just a v shaped groove in a strip of wood).
    -Drill a hole, the same diameter as the rod, in a piece of scrap wood. Insert the rod into the hole, then drill thru wood and steel rod.

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    vincent7520

    5 years ago on Introduction

    Step 5 and 6 is great.
    But how do you keep the rod fixed so it won't turn with the bit ?…
    Thanks for posting !!…

    3 replies
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    Bill WWvincent7520

    Reply 5 years ago on Introduction

    Yes, keeping the rod fixed is sometimes a problem.

    As with all we do, at times we must be creative. It is best for the rod to fit tightly into the hole. I have at times wrapped electrical tape around the rod to create a tight fit. Also. use oil on the drill bit and drill slowly.

    Merci.

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    vincent7520Bill WW

    Reply 5 years ago on Introduction

    Aaah ! So you ask us to make an effort too !…
    Nobody can blame you for that.
    Thank you for posting.
    Have a great week.

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    cepterbi

    5 years ago on Step 6

    that's some smart thinking with the last one.....I wouldn't come up with something so simple yet effective. Happy to have learned this as it will certainly come in handy in the future. Tnx

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    Bill WWcepterbi

    Reply 5 years ago on Step 6

    Thanks for the comment.
    For small work pieces, and a small drill bit, as in the photo, it is still a bit tricky.

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    Bill WW

    5 years ago on Step 6

    Thanks for the comment.
    For small work pieces, and a small drill bit, as in the photo, it is still a bit tricky.

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    isacco

    5 years ago on Introduction

    I like this instructable very much. It is useful, clear and well illustrated. It will be of great help for my workshop.
    Congratulations.

    1 reply
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    fagnelli

    5 years ago on Introduction

    For small jobs, you can use the "pretend machinist" method:
    1. Chuck the bit upside down.
    2. Lower it into the drill press vice and secure it.
    3. Unchuck the bit. Now you have the bit secured in the vice and pointing up.
    4. Chuck the rod you need to drill, start the rotation and lower it on the stationary bit.

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    jexter

    5 years ago on Introduction

    Great Instructable. When I saw the first image I thought the idea was to:
    1) Clamp the block of wood to the drill base
    2) Drill hole the size of the rod
    3) Put rod in hole (don't move wood block)
    4) Switch drill bit to the size you want in the rod
    5) drill hole through rod

    Might work I guess, as long as there isn't any play in your drill press. I like your idea better!

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    Bill WWjexter

    Reply 5 years ago on Introduction

    Thanks for the comment, jexter.

    Yes, I tried your idea also. In theory, and in practice most of the time, your idea should work perfectly. For some drill presses there is not too much room for switching drill bits, and having a top quality drill press would help too. You know, a nice big solid machine.