Originally Posted by

**Mcgyver**
k, i'll go first.

Diametrical pitch in its simplest way is just saying how big the gear tooth is. if you have 16 DP gears for example, its describing the size of the tooth. The actual profile/shape of the tooth will change as the number of teeth on gear changes (obviously with the same size teeth, a gear with a lot of teeth is much bigger in diameter than one with few), but they will all mesh perfectly and be the same size. So, to talk about gears, we use the unit diametrical pitch. Thinking that way lets us define all gear by their tooth size so our stuff meshes.....and it avoids us trying to cutt a gear of say 32.5 teeth.

Gear sizes are determined by the number of teeth on them of a given diametrical pitch

Of course DP = the number of teeth you can get on gear of 1" pitch diameter. The list bit, pitch diameter makes it a bit awkward, but its just the imaginary circular running through the middle of the teeth of the gear - ie if the gears were wheels instead of gears it would be the diameters where they both meet.

So, 16DP, means on a gear 1" diameter of the imaginary line through the middle of the teeth, has 16 teeth on it.

You could express that gear as having a 16 teeth around the imaginary line of 3.14" (pitch dia of 1 x PI). Or, each tooth have a lead of 0.196" (total length of 3.14" / 16 teeth).

So here's the part where we really blow your mind......a rack or a worm is like a gear of infinite diameter. If you measure the lead between two helix, and its .196, its a 16DP worm! DP of the worm = PI/measured lead, in this case 3.14/.196 = 16. Therefore to cut a 16DP worm (or a hob) you a lathe that'll cut a .196 lead (or in your case .392, double the .196 for the two start). Fortunatly you have a DSG which will easily do that as well as the double start.

The double start matters a great deal toward machining it, but not to determining the DP (afaik). Its like a thread profile - it doesn't change as the number of starts changes, only the lead changes.

Since you have to make a cutter, I'd make a new worm as well. It's the identical set up

I would