Finding the Center

Case #1:

How in the world, do you find the center of a cutoff tree member ? Say like the one shown below.

If I have a tree member (See above Photo) and I need to find the center for each end of the tree member, say to center the tree member ends on my lathe. First off, the ends are not true circles and this muddies the water, when it comes to finding a suitable pseudo center for the two ends of the tree member.

To find a suitable center, I use the tool shown in the Photo below. The Groz CS/2 tool is made in India. This tool can be purchased from Amazon for about $20. Surely other retailers carry this tool also, Amazon just happens to be the company I am familiar with.
The Groz CS/2 is capable of finding the center on stock up to 6″ in diameter. That meets my needs for turning wood.

Pictured below is the backside of the Groz CS/2.

In the Photo below there are two fence members on the backside of the Groz CS/2, that rest on the perimeter of the wood stock. With the tool positioned as shown, a line is drawn along the edge of the tool leg that extends across the wood stock.

Looking at the Photo below, multiple lines are shown across the end of the wood stock. Rotating the tool around the perimeter of the wood stock, the lines shown were drawn. There’s a triangular area in the center of the wood stock, shown shaded in. I use the center of this shaded in triangle for my center. The three end points of this triangle represent the three points where the lines drawn, were predominately intersecting.

If the wood stock were a perfect circle, all the lines shown would intersect at the same point, at the center of the circle. However, for a piece of wood coming directly from a tree to your shop, odds are it’s not going to be a perfect circle.

Case #2:
Another useful tool for finding the center of a circle is a combination square. However, the ’45 degree holder” needs to be replaced with a “center square”. Refer to “Related Links” at the bottom of this post for a Photo of a center square.

The Sketch below shows a combination square ruler and center square being used to find the center of a circle.

Notes for the Sketch above:

  • Note 1 — Where the two lines, drawn the length of the ruler edge cross, is the center of the circle.
  • Note 2 — Center Square for a combination square.
  • Note 3 — Locate ruler & center square in the approximate two positions shown, and where the two ………………lines drawn cross, is the center of the circle.

Case #3:

A carpenter’s framing square can also be used for finding the center of a circle. The two Sketches & associated notes below, demonstrate using the framing square to find the center of a circle.

Notes for the two Sketches above:

  • The Sketch above on the left shows how the framing square is positioned on the circle.
  • Line 1 is drawn across the circle as shown. The two end points for the line occur where the outer sides of the framing square legs cross the perimeter of the circle.
  • The Sketch above on the right shows the framing square located on the circle, to draw the second line.
  • Line 2 is drawn across the circle as shown. The two end points for the line occur where the outer sides of the framing square legs cross the perimeter of the circle.
  • Where lines 1 & 2 intersect, is the center of the circle.

Case #4:
A geometrically neat way for finding the center of a circle, that I like is shown in the following Sketch.

Granted there’s probably not much if any need for this technique, for woodworking. However, I have always thought it was a neat method, for finding the center of a circle.

Notes for the Sketch above:

  • Note 1 — Draw the two lines shown in Fig. 1 (These two lines are called chords).
  • Note 2 — Using a compass, draw the arc (A partial circle) shown, with a radius slightly greater than …………… 1/2 the length of the chord. With the compass swing two arcs, one from each end of the chord.
  • Note 3 — The line shown is drawn such that it passes thru where the two arcs intersect.
  • Note 4 — Center of the circle is where these two lines intersect.

Case #5:

To find the center for ends with square shaped faces. For say mounting on a wood lathe, I draw diagonal lines as shown in the Sketch below.

So if you need to find the center of a circle, a somewhat circular surface, or the center of a square surface, try out the methods discussed above.

 

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Until Next Time, Take Care
AL

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