On Wed, 29 Apr 2009 00:12:33 -0700, "BEI Design"
wrote:

Square Peg wrote:
I would like to make several targets for practicing golf
shots. These would be circles of various sizes from from
about a 1-yard radius up to a 5-yard radius -- possibly a
bit larger.

36" radius = 72" circles. That could be cut from a single
width of "double wide" fabric, but I don't know if canvas or
ripstop nylon (or ballistric nylon) comes that wide.

5 yard radius = 360" diameter. You would first have to join
several 10 yard long lengths of fabric side-by-side, than
cut the circle. Or a whole bunch of pie-shaped pieces and
join them.

I see that I wasn't as clear as I should have been. When I said
"circle", I should have said "ring". The targets won't be solid disks,
but circular rings. For the smaller targets, the rings would be maybe
2-3" wide. For larger targets, wider.

Are you planning on leaving these out in the weather or
packing them away every day?

I would probably pack them up most of the time, but I'd want them to
be able to tolerate a little weather.

I have been using rope, which works fairly well, but it's
difficult to lay out the rope in a nice circle and keep
it that way. It occurred to me that if I could make
circles out of some heavy fabric like a canvas, it might
retain the circle shape a little better.

It would retain its shape until a stiff wind blows... or
someone walks on it... or......

My plan is be to get a piece of fabric, cut the fabric
into arcs of the correct radius, then sew then together
into a circle.

I don't understand what you mean by "...cut the fabric
into arcs of the correct radius...", so I don't know if
yur plan would work or not. I'm not a mathmatician. :-}

Suppose I have some fabric that is 2 yards (72") wide. I can make
targets up to 1 yard in radius in a single piece (no sewing).

To make a 1-yard (radius) target, I would draw a circle with a 1-yard
(36") radius on one end of the fabric. The circle would touch both
sides and one end. When I cut it out, I'd have a solid circle with a
radius of 1 yard (diameter = 2 yards or 72").

But I don't want a solid target, I want a ring. If I want the edge of
the ring to be 3" wide, I'd draw another circle inside this circle
with a radius that is 3" shorter, or 33" in this case. When I cut out
this smaller circle, I will have a ring with a 36" radius and an edge
that is 3" wide and a small circle with a radius of 33" (which is
probably waste).

Is that any clearer?

For targets that are larger than the width of the fabric (most of my
targets), it will be necessary to cut the ring in pieces (segments)
and then sew them together to make the complete ring.

To make a target with a radius of 3 yards (diameter = 6 yards), I'd
need fabric 6 yards (216") wide. Even if this were available, I
wouldn't want it, because most of it would be waste. I don't want a
solid circle, just a ring a few inches wide.

I called a canvas store. Most canvas material comes in rolls that are
5 yards (60") wide. Here's where the mathematics comes in. ;-)

To make a target with a 5-yard radius (10 yard diameter), I'll need 6
pieces cut from a roll of fabric that is 5 yards wide. (I show the
calculations below.)

Here's the procedure to make a ring 3" wide with a radius of 5 yards.
This will work on fabric of any width.

1. Get a piece of string 5 yards long with a pin on one end and a
pencil on the other.

2. Lay the fabric on a solid surface. Pin one end of the string to the
center of the fabric 5 yards from one end.

3. Draw an arc (radius = 5 yards) on the end of the fabric. The arc
will touch the end of the fabric and go to both sides.

4. Move the pencil 3" down the string (radius = 3" less) and draw
another arc. This is one segment of the ring.

The narrower the fabric, the more segments will be needed. The number
of pieces can be calculated by first calculating the angle of the
wedge drawn in Step 3 above. The formula is:

Angle = 2 x arcsin((Fabric Width/2)/Target Radius)

For the example above,

Angle = 2 x arcsin((5/2)/5) = 60°

(I have done some conversions from radians to degrees that is now
shown.)

Now all that is needed is to calculate how many 60° segments there are
in a whole circle. Since a circle has 360°, the formula is:

Number of segments = 360 / Angle

In this case, 360 / 60 = 6. We will need 6 segments to make a target
with a radius of 5 yards out of fabric that is 5 yards wide.

Here's a table showing how many pieces are needed for targets with
radii up to 10 yards from fabric 5 yards wide:

Target Angle Angle
Radius Radians Degrees Pieces
3 yd 1.97 rad 112.89° 3.2
4 yd 1.35 rad 77.36° 4.7
5 yd 1.05 rad 60.00° 6.0
6 yd 0.86 rad 49.25° 7.3
7 yd 0.73 rad 41.85° 8.6
8 yd 0.64 rad 36.42° 9.9
9 yd 0.56 rad 32.26° 11.2
10 yd 0.51 rad 28.96° 12.4

Now I am quite sure that is way more information than you needed or
wanted. ;-)


In any case, I need a fabric that is stiff enough to hold its shape
when formed into rings of this size, can be sewn on a home sewing
machine, is more or less outdoor safe, and comes in bright colors.