Drilling a hole through the centre of a bar is not as simple as it appears. Even when you have filled a flat on the bar, the drill may still wander, and I am sure that all of us have drilled a "central" hole only to find that it is not quite where we expected.

This simple device works well and saves time (and mistakes).
Useful to almost everyone and especially for the novice, for whom there is a description as well as the sketches

Such a simple little gadget that it takes almost as long to describe as it does to make.
It has been described many times, and often seems too simple to bother with, but once having made one, you will find it
invaluable and it will amply repay the few minutes that it takes to make.

Neither the materials, nor the dimensions are critical.
The only parts that need particular care are the alignment of the threaded hole and scribed line.

You need,
1). a 3" length of 3/8" diameter rod,
2). a piece of sheet to make the pointer, about 2-1/4" square or 2-1/2" by 1-3/4" will do nicely.
3). A 4 BA. screw about ½" long (or ¾" if drilling a through hole)
4). A 4 BA. washer
and possibly a 4 BA. nut.
For preference, use steel, but all parts can be of almost anything that you have in the scrap box.

Take the 3" length of rod, clean up the ends, and then carefully file a flat on one side, 1-5/8" long until the flat is about ¼" width. Finish with successively smoother files, or a piece of emery cloth, so that you have a smooth flat surface.

The next part is a critical item.
Mark a line along the length of the flat, EXACTLY IN THE CENTRE. When you are certain that it is the right place, increase the depth of the scribe mark for the inch or so farthest from the end.
This line needs to be clearly seen in use, and therefore must be deep enough that it remains visible when dirty, or can be filled with paint.
Then, ¼" inch from the end of the flat, (or bottom of the tool, as it will be) mark a second line.
Where the lines cross, make a small centre pop, and when you are certain that it is in the right place, a deeper one. Then drill No. 31, about ¼" deep.

This hole is going to be tapped for the fixing screw, and being a blind hole will require a plug tap. If you don't have a plug tap or are unhappy about tapping into a blind hole, just drill all the way through and use a 2nd tap.

The second part is made from sheet material. The thickness is not critical, and anything between about 16 and 24 swg will suit. I made mine from brass, but anything that is to hand will suffice.

It is a simple marking out job.
The end of the pointer, the hole and the middle of the square must all be on the same line. (Critical.)
Then cut out as per the drawing.

Fix the plate to the rod with a washer between the screw head and the plate.

If you have made the rod with a blind hole, you will need to cut the screw down so that it bottoms in the hole when the plate is free to move but not too slack.
If you have made it with a through hole, then screw up until it is a slack fit on the plate, and the latter will swing to-and-fro easily. Then fit a locknut on the back to prevent it undoing.

IN USE. Fit the rod in the drill chuck and the work in the vise. Adjust so that the work fits into the square, and as you move the work, when it is exactly beneath the centre of the drill chuck the pointer will line up with the marked line.
Remove rod, fit drill, and hey presto!

METRIC. The drawing below is for those who prefer working in metric. The screw can be almost anything suitable, possibly M3.
SIZE. If all your work is with small sizes you may wish to make a "junior" version. As long as the piece of the plate below the line is heavier than the part above it will work OK.