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Need to replace a worm wheel gear

Charlesc

Aluminum
Joined
Mar 4, 2020
The plastic cam gear broke (worm wheel I guess would be the correct term) which rotates a cam on a sewing machine driven by the worm on the main shaft. The gear is 25mm dia. and has 18 teeth. The depth of the tooth is 2mm. i want to go with brass or aluminum. There is virtually no load on the gears. But the gear is helical of course and threads on to the shaft which isn't a problem.

What is the angle I need to set for milling the teeth? My guess it's close to 5 degrees.

TIA
 

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The pitch (as stated above), along with the outside diameter. Is the thread (tooth) a single start or multi-start? If multi-start, how many?
 
The pitch (as stated above), along with the outside diameter. Is the thread (tooth) a single start or multi-start? If multi-start, how many?
Had to google that up LOL. Yes single start. I'm not very good a gears, an amateur machinist as you can tell and my math is 60+ years old. :( You'll have to bear with me.
 
Will check can I cut a pitch that large on my lathe? Made metric transposing gears a few years ago. Will make a test tomorrow.
 
Measure the worm lead angle with a protractor, hand grind a flycutter tool to match the worm profile and just gash a new worm blank using a dividing head with the cutter centerline rotated to the lead angle- crude, but inexpensive, and will take some trial and error. Or we can cut you a nice new worm gear for $300-$400. Don't need a hob as we can tangentially hob it with a single tooth fly tool. I suspect for your purpose, the gashed worm wheel will work just fine.
 
Measured the worm with calipers as best I could and it seems to be 4mm per turn. Guess that's axial pitch from google. Dia is 20mm. I can handle 4 tpi on my lathe. From my conversion it should work if I decide to make a hob.

"Don't need a hob as we can tangentially hob it with a single tooth fly tool. I suspect for your purpose, the gashed worm wheel will work just fine."

That sounds like the method I used to make the transposing spur gears for my lathe. Used my milling attachment and a dividing wheel found on the net. Turned out Ok. Only cut a couple metric threads since in 2-3 years. This time I was concerned with the helix angle.
 
Was hoping I might have a bit of luck on the threads for the gear. No so. 13.12mm dia. OD. with 1 mm pitch.

Used a digital protractor on the worm shaft . Somewhere around 2 - 3 degrees helix.
 

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the thread is probably proprietary to the machine manufacturer. M13 x 1 is not an ISO thread, though you can buy taps and dies. For M13, 13.12mm is too big for the calculated major diameter and I don't think to corresponds to any common fraction of an inch.
The hobbing vs gashing is very similar. Both need good setup, hobbing uses a more complex self feeding cutter, gashing uses a simple cutter with a lot of work with a dividing head while cutting. Totally agree with Dan that gashing would be fine for this application.
 
Update. Cut two blanks from brass. (Only used one so far). Inside threads went well (1.0mm). Used the gashing method suggested for the teeth as I have a jig built already for my milling attachment. I set the helix angle to about 2.5 degrees. The gear is installed and works, well, so so.

The gear turned out a bit off center with good mesh on one side and a bit loose on opposite side. The machine is a bit noisy as the cam rotates when the follower arm comes off the high spot of the cam. It's tolerable. The whole milling setup is just not sturdy or accurate enough. :(

I really do need a dividing head for my milling machine. One of these days. I didn't bother trying to complete the second blank. Saving it for another day.

Thanks for all the input by the members.
 








 
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