Tips and trick??? enlarging a circle with a hole saw???

All the tips and tricks in this months new issue got me wondering....

So when I drill a 2" hole with my hole saw and then decide to make it a 3" hole I will typically clamp a piece of wood to the back side and have a part clamped to a part clamped in a drill press, and a hole that is no longer is the center of the part. From there I get question # 2...

This may be a high school question but I know there are hundreds of tricks out there for working with geometric shapes. Carpenters and brick-masons know tons of them. So if I grab an object and trace around it and then want to start a hole in the center how do I find the center. Typically I would divide the diameter in half and mark that in two directions perpendicular to each other. And that gets me close, but is there a better faster easier way and most of all more accurate. Or one that does not need a ruler at all.

High school trig was a long time ago. I should just google it...

I think it was something like draw the biggest square you can in the circle. Then draw lines from corner to corner of the square making an x. Where those lines intersect is the middle.

pick any point on the circumference of the circle, and draw two straight lines from that point to any other two points on the circumference of the circle. now, at the midpoint of each of those straight lines, draw a perpendicular line through the circle. they will intersect at the center.

my way is no easier than yours, and is probably only slightly more accurate. Depends on who's doing the measuring and with what scale.

Carpenter here.

The exact center can be found with a compass or pair of dividers. Set the compass to a radius that is obviously more than half. Basically, you are going to "walk off" the dimension, without changing the setting on the compass. Start at point A- anywhere on the circumference. Make 2 marks from point A in each direction on the circumference (points B and C). From both B and C make 2 small intersecting arcs inside of the circle. The intersection is point D.

Draw a straight line (line 1) from point A through point D. You have just halved the circle.

Repeat the entire process, with a new point A (approximately 90* from the original). Your 2 straight lines will intersect at the EXACT center.

Sounds much more complicated than it is. Try it- it's very simple!

You guys would do math for that? I'd just take the hole saw apart and mount both the 2" and the 3" cutters in it. Stick the 2" in the existing hole and cut away.

I was going to mention the divider thing but SVreX beat me to it. oldopelguy's way of enlarging an existing hole will work assuming that the 2" hole saw sticks out further than the 3" saw when both are on the same mandrel.

oldopelguy wrote: You guys would do math for that? I'd just take the hole saw apart and mount both the 2" and the 3" cutters in it. Stick the 2" in the existing hole and cut away.

X2.

Just don't try it with a cordless drill, as there will be too much friction and it will overload the motor (Hmmm..... what's that smell? Uh oh...).

oldopelguy wrote: You guys would do math for that? I'd just take the hole saw apart and mount both the 2" and the 3" cutters in it. Stick the 2" in the existing hole and cut away.

No, I don't generally do math for it. I would generally drill a 2" hole in a block and clamp it to the other block, using the outside diameter as a guide.

He didn't ask if I would do math for it. He asked for the carpenter's method on HOW to find the center, so I gave him all the geometry without a BIT of the math!