10 tpi tap for making worm wheel

[Humph7ey]

I have a 10 tpi 1" BSW tap that I could use to generate a worm wheel on my lathe. Any idea what diameter of wheel I should try?
I want a 60 tooth wheel for a dividing head.
Does 10 tpi equate to a gear DP at all?
I have not tried this before but hoping it can be done. I will be casting my own blanks in ally.
Humphrey

 

[tel]

My poor ol' brain isn't up up it, but it should be somewhere in the region of 2" diameter?

 

[ShedBoy]

Don't know the correct size but this page made some interesting reading on the subject

http://www.atmsite.org/contrib/JSAPP/wormgear/wormgear.html

Brock

 

[ShedBoy]

I can't find the site at the moment but a chap used acme threaded rod to cut worms successfully in ally and all he done was machine some slices lengthways down the rod before using it. Acme thread will be stronger. Marv may have a formula somewhere. In the link above it has a formula and some suggestions.

Brock

 

[djc]

Does 10 tpi equate to a gear DP at all?

10 tpi linear pitch of your 'hob' translates to 0.1" circular pitch (CP) of your worm wheel. DP = pi / CP hence your DP will be 31.42 . Blank OD = (no. of teeth + 2) / DP.

If you are 'free hobbing' the worm, a spiral flute tap is a lot better as there is always some part of it in contact with the blank. If only a straight flute is available, it's best to pre-gash the blank to give the process a helping hand. Remember that the gashes need to be at the helix angle of the 'hob'. Try not to do left-hand gashes for a right-hand worm wheel!

 

[Ken I]

Hobs for worms have unusual pitches to some value of Pi.

Example a 48T 48D.P. gear has a 1.000" pcd therefore the hob pitch is ( 1.000 x ¶ / 48 ) = 0.06544..."

Some lathes have a Pi pitch function for this purpose or gears that provide the 22/7 ratio approximation (if you ever wondered why you have a 66T or 33T & a 21T gear in your changewheels - that's what its for).

In the above example set pitch to 24 tpi - interpose the 33/21 drive and you get ¶ x 1/48"

Regards,
Ken

 

[arnoldb]

Humprey

Yes; blank OD of 1.973" - Tel had the guesstimate pretty much spot on.

And hobbing the wheel is really not difficult; even a novice like me did it. (see what I did in the following posts)

Regards, Arnold

 

[Humph7ey]

I just tried with some ally that i turned 3.819 to get 120 teeth, but the result was 124 teeth. I did another one on the same blank next to the first and I have to say that the resulting accuracy of this method with a straight flute tap shows some inaccuracies as the cuts tend to drift from each other.
Oh well...

 

[Humph7ey]

Nice work, Arnold.
I now see that if I wanted accuracy, I would have to gash the gear first. Next time.

 

[arnoldb]

Thanks

Yes, with a straight-flute tap you will have to gash first. The cause for the extra teeth is that if you start off with a light infeed into the blank, the effective circle diameter is too big... The gear blank size is based on the number of teeth on the pitch circle diameter of the gear - and as you normally can't start by cutting at full depth you end up with too many teeth and all out of proportion and miss-formed.

The gashing helps to prevent this by forcing rotation at the actual needed amount. Also, you can effectively start cutting at a "smaller" initial diameter, further making things easier.

Show us a photo once done Thm: ;D

Kind regards, Arnold