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worm gear calculations

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bertie_bassett:
im needing a worm gear for the dividing head iv cast so thought id have a go at hobbing a 72T gear with a 3/4 BSw 10TPI tap

after doing a lot of reading online and a lot of calculations I thought I had the method worked out and went with the following calcs (in imperial)

DP = Pi/(1/TPI) =PI*1 = 31.4159
Diameter = T+3/DP = 75/31.4159 = 2.38732414638

I double checked this by using a circular pitch of 10, to give a pitch diameter of 2.291. add 2x tooth depth (0.060?) = 2.411, seems close enough.


so I cut off a lump of bar and turned it to 2.387'' marked out and gashed at 5 degrees with a hacksaw for 72 teeth.



put it on the shaft of the dividing head and with the tap in a collet fired things up






turned nicely and pretty soon I had a nice set of teeth.

problem is there were 4 to many  :Doh:

so I turned the lump down and tried for a 60T, ended up with 66

so I tried for 45 - without gashing - got 44

so I then just kept pushing the blank in to see what would happen, ended up with 35 :doh:


so before I cut up another bit of ally what am I doing wrong??

velocette:
Hi Bertie
sorry to be a pita but did you use a spiral flute tap.
It appears that the outer diameter is to big for the pitch of the tap then the tips of the tap fall short of the gashing.
Recalculate for less than full depth engagement of the worm by reducing the diameter of the worm wheel to about 80% engagement.
A wider gashing will help.

"put it on the shaft of the dividing head and with the tap in a collet fired things up"

It is possible that the drag of the dividing head is creating  the problem.

Eric

chipenter:
I did exatyl the same but in bronze finished up with 73 , the PCD is right but you only need to add for 1 tooth as the PCD is the centre of the tooth , slightly different to an involute gear  this worked for me .

bertie_bassett:
hi velocette thanks for the input

no its just a normal tap, I deliberately left the blank wide so that there should have been enough contact to keep things moving. it certainly looked fine going round nice and smooth.

not sure I can gash any wider by hand, although I suppose I could file each slot?

maby ill turning a concave profile will help?

there's no real drag on things, the gear blank is spinning on its shaft so I don't think that's an issue.


chipenter

glad to know im not the only one, ill try a smaller diameter and see what happens, I wasn't sure about adding 3 on  but it seemed to work for others.

chipenter:
A ten tpi tap is 4 degrees pitch , angle your slots to that will help .

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