So what makes this circle cutting jig improved....? Magnets!
Rare-earth magnets embedded in the bottom of the jig provide downward force while feeding in and cutting the circle. Better support of the jig allows for larger circles to be cut faster and safer. Of course, for this to work the band saw must have a cast iron table.
Now that you are interested, read on for building instructions or add magnets to one of the other abundant designs!
(Fits 14" Delta band saw or equivalent without fence, cuts up to 30" diameter circle)
- Remember to use proper blade for radius of cut
Step 1: Materials and Tools
- 3/4" plywood (about 2X2 ft)
- 2X4 board (16")
- hardwood to cut the miter slot guide strip from (17")
- 1/4-20 2" flat head screw
- 1/4-20 threaded wood insert
- 1/4" steel rod
- 4X neodymium cylinder magnets (1/2" diameter X 1/8")
- 1/4-20 threaded knob (or 3/4" aluminum bar to make knob, see instructable)
- wood glue
- 1 1/2" and 3/4" 18 gauge staples, or other fasteners
- table saw
- 18 gauge stapler and compressor if using staples
- drill press and drill bits
- something to cut the 1/4" steel rod to length if needed
- tape measure
Step 2: Cut Miter Slot Guide Strip
Create a hardwood guide strip to fit into the miter slot
- Measure the width of the miter slot (normally 3/4")
- Cut the hardwood about 1/32" narrower than the miter slot and test fit
- Measure the depth of the miter slot
- Cut strips 1/16" less than measured depth (I cut more for other projects)
Step 3: Cut Body to Shape
Cut the 3/4" plywood to 16x24"
Step 4: Cut Dovetail Slide
Create a dovetail strip in the center of the plywood running along the 24" dimension. The strip should be 3" wide on the narrow side.
- Set the table saw to 30 degrees
- Make the first cut leaving 9 1/2" left of the blade
- Take the larger of the cut pieces and place the angled cut facing down and on the left of the blade
- Make the second cut with 6 1/2" right of the blade
(using right tilting table saw and measuring on the surface of the table)
Step 5: Cut Cross Members
Cut the cross members and front stop to size.
- Measure the width (shorter dimension) of the body
- Cut the 2x4 to the measured width
- Cut the guide strip to about 1/2" longer than the width
- Cut a strip of 3/4" plywood to act as a stop (2x24")
Step 6: Install 1/4-20 Threaded Insert
Install the threaded insert in the 2x4 to line up with the center of the dovetail slide.
- Find the center of the 2x4 on the largest face
- Drill a relief for the head of the 1/4-20 screw using a 7/8" paddle bit (or equivalent) to 1/2" depth
- Drill through-hole to the proper diameter for the threaded insert (I used 5/16") in the center of the relief hole
- Install threaded insert with glue on external threads taking care not to contaminate the inner threads with glue
- Insert should not protrude into the relief hole or from the other side
Step 7: Attach Cross Members
Glue and staple the 2x4 and guide strip in place.
- Use about 6 layers of paper to form spacers for the dovetail to prevent binding
- Apply glue to the relief drilled side of the 2x4 avoiding the area that the center dovetail strip will slide on
- Attach the 2x4 with 1 1/2" staples to the narrow end of the body
- with the spacers in place, hold the body together and square while fastening (do not attach the center dovetail)
- Measure the distance from the left of the table to the miter slot and mark a line on the underside of the body at this distance
- Attach the guide strip using 3/4" staples and glue along this line (see pictures)
- The center dovetail must not be attached to the guide strip
- The body should line up with the left of the table when the guide strip is in the miter slot
Step 8: Attach Stop
Glue and staple the 2" wide plywood strip to the front of the jig. This will act as a stop against the table when feeding in to cut a circle.
Step 9: Last Cuts
- Place the guide strip in the miter slot and cut the backside of the jig flush with the blade
- Cut off 2 1/4" of the dovetail slide to act as a stop and attach to the end of the slide (see 2nd picture)
- Place the slide in the jig until the stop is reached and cut the slide flush with the blade
Step 10: Install Pivot Pin
- Place the jig on the table so it is against the front stop. Push the dovetail slide into place against its stop. With a blade in the saw that will be used for cutting circles, mark the front of the blade on the slide.
- Remove the slide and drill a 15/64" through-hole about 1 1/2" back from the end lined up with the mark
- Cut the 1/4" pin to 1 1/4"
- Roughen up 3/4" of the pin using sandpaper
- Apply glue to the rough end and press into the slide letting the pin protrude from the narrow side of the slide
Step 11: Install Magnets
See picture for approximate location of magnets. They should be on the table when the jig is against its stop.
- Drill two 31/64" through-holes for the 1/2" diameter magnets
Lightly roughen magnets using sandpaper
- With two magnets for each hole, apply glue and press into the holes with a slight recess so as not to rub on the table
Step 12: Install the Knob
- Thread the 1/4-20 screw into the insert so the head is inside of the relief
- Using a screwdriver to hold the fastener, thread on the knob until tight
- See instructions for making the knob
- Slide the dovetail into place
Step 13: Using the Jig to Cut a Circle
- Set the diameter of the circle by adjusting the distance between the pivot pin and the blade, then secure the slide
- Select a piece of material large enough to cut the desired circle from and mark its center
- Drill a 1/4" hole a little over 1/2" deep in the center of the material
- Place the material over the pivot pin with the jig slid out enough for the material to clear the blade
- Cut into the material while preventing rotation until the jig hits the stop
- Turn the material around the pivot pin until the entire circle is cut
- Turn off the saw before removing the material
Enjoy the circles!