Spirals By Steve

Frequently Asked Questions

Q: What about the special blade mentioned in the manual?
A: I only recommend regular tablesaw blades for use in solid woods. I use a fine crosscut blade with good results. The carbide grit blade is great for plywood but has a tendency to load up when cutting hardwood. Use only a BRASS wirebrush to clean it (not while it's spinning). This type of blade is manufactured in Georgia by Tunco. I recommend using an 8" blade because the 10" blade plate they have has a 1" bore. 36 grit is probably the best balance between cutting performance and finish. Ask for Julie when you call. http://www.tuncomfg.com/index.html

Q: How do I determine the width of the guide marks on the fence?
A: before drilling out the center on your gear blank, use a compass to draw a circle showing the pitch diameter. Cut two adjacent teeth with only one pass for each notch removed. This will leave you with teeth that are too wide. Now measure the tooth thickness where the pitch circle intersects the tooth profile and compare that measurement with the tooth thickness from the gear calculator. The difference between these two measurements should be the width of the mark for gears with zero backlash. Add a little bit more width to the mark to give the gear some backlash space. The tooth thickness is simply the pitch diameter multiplied by PI and divided by two times the number of teeth. The tooth thickness measurement will remain constant no matter what the pressure angle is. It essentially divides the pitch circumference so the teeth are exactly as wide as the gap they fit into along the pitch diameter. This distance is an arc distance along the curve rather than straight line distance. There won't be much difference between arc distance and chordal distance in normal sized gears, but it becomes more of a factor as the number of teeth decreases. Ideally a pinion and gear should mesh properly where the distance between centers is equal to half of their combined pitch diameters - the pitch circles should just barely touch and not overlap or not be touching.

Q: I made a pair of 45° same-hand helical gears to be meshed at right angles like those on the blinds, why don't they mesh at a 90° angle?
A: Good question!! I have wondered about that myself. I think this effect will be more noticable in gears with larger teeth. If you look at one of the gears with a tooth pointing straight at you, it will appear to twist slightly from the tip to the root. I don't know how to put that into Excel, but I have found by trial and error that approximately 48° between fence and blade compensates for this effect for the gear parameters I used: 5 pitch, 25° pressure angle, 45° helix angle, 9 and 16 teeth. I wish I had an equation for that.

Q: Can this technique be used to make bevel gears?
A: Yes, and I am working on figuring that out. It is going to be a little more complex though.

Q: The gears I have figured have profiles that cut inside the base circle, how do I keep from ruining the base circle guide?
A: If you are using a scrollsaw you can remove the base circle guide before cutting along the root diameter to remove each notch. If you are using a tablesaw, you can avoid this by making the base circle guide thicker. Another solution for either saw is to make helical gears instead. The notch cut into the guide will be at an angle so the notch will be bridged at a different distance from the faces of the guide if the gap isn't too wide. You can narrow the notch and land width by increasing the pressure angle. Just remember that the mating gear must also have the same pressure angle. Another option is to make the gears on a router table set up like a scrollsaw except the straight bit can be set at a height where it does not cut into the base circle guide.

Q: How do I make the very small pinion gears on a tablesaw?

A: Use another straight board around 3 or 4 inches wide and maybe a foot long with a strip of sandpaper glued to one of the edges to press down against the base circle guide with both hands. This sandwiches the guide between two edges and lets you put a lot more pressure on the guide to prevent slipping - and keeps your fingers a safe distance from the blade.

Wood Gears

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