worm gear calculations

[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 

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


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 .

[velocette]

Hi Bertie
following this posting I trolled through my "Filed Away For Future Action" and found this file on "worm wheel hobbing"
This may be of some use as it is a very detailed commentary on cutting wormwheels on a small lathe.
A quick read through it and it can be translated into using a tap for the hobbing tool.
Forwarded as a plain text file this loses some of the original format.
Hope this helps a little more

Eric

[bertie_bassett]

well gave things another go this evening,

made it a bit smaller this time and gashed it deeper, stuck it in the mill. 

and . . . .

got 73!

skimmed it a bit and re-cut the slots for 72, tried again but no joy

[Sea.dog]

Velocette, your hobbing text is certainly comprehensive. If anyone is having trouble reading it and wants to make it clearer, and  has Microsoft Office, then copy and paste into a new Word document, it's a damn sight easier to read.

[Pete.]

Or read the generating page here:

http://www.mini-lathe.org.uk/making-telescope-worm-wheels-gears-mini-lathe.shtml