How to mark and drill holes in tubing that are 180 degrees apart
...........without using a drill press or drill guide!
by N4UJW

How? Go back to your school days!

How to use a protractor for marking and drilling holes
 in round tubing used for the boom of Yagi antennas or other round tubing projects.

This tip came about when a fellow ham radio operator asked me how to drill holes in the round boom of antennas if you don't have a drill press so that they would be exactly 180 degrees apart. After much searching of my limited mental faculties, this is what I came up with. This should work in most cases where you want to drill holes that are exactly 180 degrees apart on the boom of a Yagi antenna you are building, or for that matter, with any project that requires holes 180 degrees apart in round tubing. If you don't have a drill press or drill guide..... read on.......

Enter the protractor!
In case you have forgotten, a protractor is "tool" usually made from plastic that is transparent and is used in marking and drawing various angles.You probably used one in school in geometry class may years ago but had forgotten about it. It has markings on it that are in increments of 180 degrees. They also come in 360 degree versions.

 
180 degree protractor on left....360 degree version on right. (not to scale)
 

Now since tubing is usually a perfect 360 degree circle when viewed from the end, you want to be able to find a way to mark both sides of it for the Yagi antenna element holes we are using in this example that you want to drill. You know that for the antenna to perform as designed these markings must be exactly 180 degrees apart so the antenna elements will all be in the same plane with each other.

A very simple method to mark your tubing using a protractor, (either version shown above), is to simply lay the protractor on a flat surface, then set your tubing on top of it while lining up the exact outer top and bottom surface of the tubing with the line drawn from the 90 degree mark at the top of the protractor to the bottom where all of the angles intersect...this forms a 90 degree angle relative to the base of the protractor.
See FIGURE 1 below. Read the instructions carefully for it.

Figure 1.

Instructions: Refer to Figure 1. above
Using this method may get you extremely close for the holes. The red circle in the above graphic represents the end of the pipe, tubing, etc as seen from it's end, all of which is laying on a flat surface. The dark squares on the top and bottom of the red circle represent marks on tubing, pipe, etc. The tubing or pipe to be marked should rest on the exact line from the top 90 degree mark to the center at the bottom of the protractor. The blue line represents the exact center of the tubing, etc. Now mark the ends of the pipe with a marker exactly where the blue line crossed the ends....then do the same on the other end of it. Don't let the pipe or tubing move. Clamp it down if needed. Now draw a line between the marks on each end to the other end. This is the center of the tubing lengthwise. Do the same on the exact opposite side. You can now mark on the line where you want the holes drilled!

Using Figure 1 above in this example, you simply use a fine tip felt marker and mark the outer end edge of the tubing on the top and the bottom of it. See the small black squares in Figure 1 above. You now have one end marked. Do the same for the other end.

Important Hint! Since we want to mark BOTH ends exactly in the same plane, you will need to have some method of preventing the tubing from turning from its present position after you mark the first end. This method will be left up to you but you could use a heavy weight or someone to hold it to keep it from turning while you place the protractor on the other end, etc. This is the very tricky part of marking both ends exactly in the same plane. If after you mark one end AND the tubing accidentally turns, the other marks you make will be out of plane with the other end! The tubing MUST NOT TURN when marking the second end.

If you are successful then these marks on each end should be exactly in the same plane and you should be able to draw a straight line between them!

The next step is to draw a line between the end markings on BOTH oposite sides of the tubing, then measure out your drill hole lengths, center punch them exactly where the elements will go inside the boom, mark them, etc.

You know have a line on both sides of the tubing down the center of it from end to end on the tubing that is 180 degrees apart from each side where you can mark the holes to be drilled for the antenna elements! Use a small center punch in the center of your element position marks for starting the drill bit. Drill one side at a time and don't attempt to drill both at once..this is the key to drilling the holes without using a drill press or drill guide in tubing that yields you holes that are 180 degrees apart!

This tip will save you a lot of money if you were prepared to buy a drill press or drill guide for your tubing drilling project, but if you can't figure out what all this means and are in the market for either one, then check out those listed below from a good source, Amazon.com!

 

So there it is...sometimes what we learned in school comes back for good use in later life!
If you have any other hints, tips, etc for drilling holes in round tubing 180 degrees apart,
without using a drill press, please email them to us for possible publication here!
N4UJW at Hamuniverse.com