Helix angle needed

[Oldchipmaker]

So I have a busted nylon gear (worm wheel) from a sewing machine. I hope to machine one from brass or possibly aluminum. The work load is minimal since it only drives a cam for various stitch patterns. Very common issue with this particular series of machines if you screw up reversing the feed of the material being sewed.

I don't have a dividing head for my mill (yet). Previous gears I cut was on my lathe with a milling attachment using a dividing wheel jig. I could do the same this time but why not use the mill? I'll need to make a jig of course. What angle will be required? I have a digital angle gauge. BTW, this gear threads (metric) onto the hub. To get a good mesh with the worm and worm wheel I may have to remount the gear for touch ups. That should be easy I would guess. Hopefully there is enough information in the pictures.

Thanks

 

[o0norton0o]

I just recently researched helical gears because my thread dial was missing 2 gears (metric lathe)

I used this website to design the gears- https://www.stlgears.com/generators/3dprint

I used this website to view the gears I designed to make sure they looked correct - https://www.viewstl.com/

then I went to https://www.shapeways.com and ordered my gears in nylon for about $7. per gear...

It was more expensive for gears made in other materials. I think my gears in metal were around $60. so maybe if you must have metal gears, you can make them cheaper than shapeways, but their printed nylon gears are dirt cheap... and in my case the thread dial doesn't really carry any force load so nylon should work fine for my application...

 

[KellyJones]

It's pretty easy to figure out the required helix angle. The pitch of the worm must match the pitch of the mating gear. The pitch is simply the distance from one tooth to the next, just like screw threads. The circular pitch of the gear is pi divided by the diametral pitch (or multiplied by the module for metric). Alternatively, you could just measure the pitch on the existing worn worm gear, I suppose.

Once you have the pitch, the tangent of the helix angle is the pi times the diameter, divided by the the axial pitch. The pitch has to be measured at the same diameter you use in the calculation (e.g. the OD), and the pitch is measured along the axis of rotation, not normal to the tooth profile.

Good luck.

 

[Oldchipmaker]

I pressed a piece of paper on the worm. The grease mark of the three teeth measured 12mm. Pitch would be 4mm. That worm measured as close as I can tell is 20mm. So pi x 20 = 62.8. Tan is 4mm/62.8mm = 0.064. From trig table that works out to just under 4 degrees. Am I thinking correctly? Math and trig were my better subjects but that's decades ago and never used any trig since. Sad how much a person forgets.