On Feb 18, 1:45*pm, Grant Erwin wrote:
I have a 4" tube which I want to scribe lines on. The two lines will be
parallel to the tube axis and should be exactly 180 degrees apart, i.e.
on the opposite sides of the tube.

I know how to draw one line down a tube, but I am looking for a good trick
to locate the other line.

Ideas?

Grant

1. Wrap a sheet of paper around the tube, aligning the edges where
they overlap to square it. Mark the corners of the inner layer on the
outer layer. Remove the paper, connect the marks and cut the line.
Fold the paper in half and crease it. Put it back on the tube. The
crease is one line, the abutting edges are the other.
You'll need to use a Legal or B size sheet.

2. Stand the tube upright on paper and trace the circumference. Lay
the corner of another sheet over the drawn circle and align the corner
with the line. Mark the circle where the two edges of the top sheet
cross it. If you were careful a line between the two marks is a
diameter. Cut out the circle and slide the paper over the tube.

3. If your first line on the tube is near the end, you can use the
center finder head on a combination square to locate the second one.

Jim Wilkins