Technical - Workshop Notes
“Making Cogs on a table saw"
An interview with Andre Cook, Project Officer KDWC
This article shares a method used by Andre Cook for making COGS. Rather than cutting each cog on something like a scrollsaw, Andre has adopted an idea seen elsewhere, involving a template JIG in the form of a table saw sled, to make very accurate teeth.
A misconception obout cogs, is that the diameter is important. Not so, it is the number of teeth involved, that determines the rotational relationship between any two cogs (of any size).
JIGS, TEMPLATES and MATERIALS needed
To make a gear or cog for a clock or anything else on a table saw, make these items.
Make a disc “A” for your cog at a radius that you want the cog. Make a second disc “B”, dimension “A” minus 6mm.
Make a drawing on paper with the number of teeth that you require. As a guide, 9mm is a good distance between teeth. Draw lines raidiating from the circumference to the centre point of the disk, one line for each tooth. There is no need to draw a complete tooth. This drawing needs to be 100% accurate as this will determine the spacing of the teeth. A CAD program will make this easy.
Attach the disc “B” to disc “A” in the centre. The drawing will need to be attached to disc “B” with any adhesive, glue stick or similar.
Make a “sled like JIG” about the size of a kitchen cupboard door. On the sled make a fence 100 to 150 mm high across the door. In front of that another piece of timber on flat , 25 by 45 mm will do although these measurements are not important. This should be fixed the thickness of cog “A” in front of the fence.
A strip of sandpaper, cut exactly square on one end fixed on the top of the 25 by 45 mm piece. This square edge will be the reference later on. This sandpaper is used to avoid slipping during cutting.
Place the “sled”, fence side at 90 degrees to the blade on the table saw, “with the blade completely withdrawn” so that the space between the fence and the guide timber is at the uppermost point of the blade.
The square edge of the sandpaper should be approx 100 mm or thereabouts to the side of the blade. Clamp the board to the table saw so that it will not move. Now raise the blade “cutting through the board” so the top of the blade is exactly the same height as the sandpaper strip. Some sliding of the sled backwards or forward may be necessary so the top of the blade is in the centre of disc “A”.
Disc “B” will be the guide to the cutting of the cog. At the reference point hold the disc with the line on disc “B” exactly vertical, 6:00 o’clock on the square edge of the sandpaper. Attach a stopper at 9:00 o’clock to the fence to aid positioning at each pass of the cog.
The cutting of the cog will be done from the back of the table saw as the rotation of the blade will result a clean cut for the teeth.
Cutting of the teeth can now proceed. From the reference points roll the discs along the sandpaper so the saw will cut the teeth and then roll back to the reference points. Care to be taken not to slide the disk as this will result in miss cut tooth.
This action will result as an inverted “V” in disc “A”. Repeat this for each tooth by rotating the disc at the reference point to the next line on disc “B”. This is to be repeated until the entire cog is cut.
To further achieve a clean cut cog attach another tear out reducing disc to the back of cog “A” so as not to get tear out on the cog. This also can be attached with the glue stick. Attach this third disc prior to cutting the disc blank so that a perfect circle is achieved. Rember to adjust the gap between the fence and the sandpaper strip to accommodate both disc “A” and its tear out reducing disc.
The photos below show some of the salient points
Happy Cog Making!
Authored by Andre Cook