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If you want to make a disc and you have an odd shaped piece of wood, metal or plasticwhich is big enough this is one way to do it.

Materials:
  1. piece of plywood for base board. long enough to be about twice as long as the radius of the largest disc you wish to make.
  2. Piece of wood to make the pencil arm, riser, wedge and other parts to lock the pencil arm in place.
  3. A few wood screws + nuts and bolts.
  4. 4 Base board feet - rubber or felt if you want it to be non-slip.
  5. A long thin piece of card.
Tools:
  1. Jigsaw.
  2. Hand saw.
  3. Steel rule.
  4. Set square.
  5. Screwdriver.
  6. Wrench or spanners.
  7. Pencil.

Step 1: Make the Device Part 1

Suppose you wish to make a number of discs of various sizes and for example the largest will be 2' diameter. Let's assume that large discs will need a large center hole.
See the attached diagram to get an idea of the construction.

Your base board should be about 2' long and about 8" - 10" wide.
Draw a line along it's length half way across.
Near one end on this line drill a hole to be a fairly tight fit on one of the larger bolts.
Further along drill another hole for a slightly smaller diameter bolt.
Do the same further along as far as you wish.
Fix your feet to the bottom of the base board.
Cut a piece of timber to form the pencil riser.
Fix it to the end of the board furthest from the first hole you drilled. 

Step 2: Make the Device Part 2

Cut a piece of timber to form the pencil holder.
Mark a line at one end centrally along the length of it for about 3"

About 1½" from that end and on the line drill a hole
slightly smaller than the pencil you will use.

Drill a hole about ½" from the same end across the timber.
Cut a slot along the center line you drew.

 


Step 3: Make the Device Part 3

Put the pencil in the holder and tighten the bolt.
Lay it on the riser so the pencil is on the center line of the base board.
Adjust the holder so it is at a right angle to the riser.
You must fix one piece of wood to the top of the riser so it will be tight against the holder.
At the other side of the holder you need to fix a wooden wedge ( blue in the drawing ) fairly close to the holder but not touching it.
The other piece of timber ( red ) which is again a wedge is used to keep the pencil
holder in place.

That's the device finished.

Step 4: Using the Device.

See the attached example.
Pick you odd shaped piece of timber.
Cut a piece of stiff card which is about 1/2" longer than the diameter of the disc you want to make.
Find it's center and put a pin through that point.
Place it on your board with the pin in the position you expect the center can be.
Rotate the card to check that the disc can be cut out there. If it cannot you should be able to move the pin slightly until you find a place where the disc will fit.
Keep the pin steady, slide the card up it and make a mark on your material at the point where the pin goes in.
Draw a straight line passing through the mark.
Measure from the mark, along the line to the same size as the radius of the disc you want to make and put another mark on the line.

Now drill the center hole at the center point of the line you marked.
Next you need to put a suitable bolt with it's head on the underside of the base board through one of the holes in it.
Put you piece of wood for the disc on the board so the bolt passes through the center hole.
Rotate in a full circle to make sure it can swing all the way round.
If it cannot then just roughly trim some of the excess off.
Line up the line on your wood with the center line of the base board. Position your holder with the pencil in it so the tip of the pencil is on the line at the point you marked as the radius of the disc you wish to make.
Hold it very still while you tap the red wedge to secure the holder in place.
Rotate your piece of wood until a complete circle is drawn on it.
Remove it from the device and cut it out with your jigsaw.
If it is all done carefully the disc will be complete wit the hole exactly in it's center.
If you want to make another disc the same size leave the pencil set up, get another piece of wood / metal / plastic and repeat.
All done.

Beats my piece of string, nail and a pencil!

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Bio: Did HNC Chemical Engineering ( many years ago ) worked in industry as a chemist for 25 years then in IT as network admin before i retired. More »
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