Gear hobbing

[blackangus]

The worm gear in my small rototiller wore out and they do not sell a replacement just the complete gearbox so I made a new gear for it using a hob. I made the hob which is from drill rod. I have include some pictures . I have a video but do not have it on line so it will have to wait. The gear works OK. I should have taken more pictures of the process ( next time ).

Thanks

[kbalch]

Thanks blackangus! I've added your Worm Gear to our Machining category, as well as to your builder page: blackangus' Homemade Tools. Your receipt:

Worm Gear by blackangus

tags: worm gear

[Al8236]

Hi Blackangus,
The helix angle looks pretty extreme is that a double start on that hob? Also how many flutes did you make on the cutter?
One more, Any pics of the completed hob?
Thanks Al

[rasta]

if need arise you can make a gear with a tap, all you have to do is calculate it as follows:
M= module
DP= pitch dia
OD= outside dia
NT= number of teeth required on gear
P= pitch of tap

M= P/3.1416
OD= N+2 * M

Hope that this helps.
Mariano

[rasta]

judging from the gear Pics, it looks like a single start...

[Manitoba Man]

Blackangus

To a very poor shade tree dabbler with a lathe this is impressive indeed. Those tiller worm gears are handy. I used one as a welding positioner.

Bob

[steamingbill]

if need arise you can make a gear with a tap, all you have to do is calculate it as follows:
M= module
DP= pitch dia
OD= outside dia
NT= number of teeth required on gear
P= pitch of tap

M= P/3.1416
OD= N+2 * M

Hope that this helps.
Mariano

Hello Mariano,

Is the difference between a tap and a hob simply that taps are relatively small and have a relatively fine thread whilst hobs are big and fat with a coarse thread ?

Are you suggesting that this job could have been done with a tap rather than a hob ? Does a tap for a big coarse thread look a bit like a hob ?

I am unclear on these questions myself hence this post - hoping for somebody to enlighten me.

Taps : Designed to go into holes and cut a relatively small groove inside the hole

Hobs : Designed to cut teeth on a gear and not be confined in a hole

Ratio of tooth size to tool diameter seems to be quite different for the two tools

Yes I am aware of videos showing worm gears for telescope alignment mechanisms being cut by a tap on a freely rotating blank.

I've never seen a video of a Hob being used to cut a thread inside a hole - am thinking about it would be a fairly massive depth of cut. Could a big machine do this or would big coarse deep threads always be cut by a single point thread cutting bit ?

Bill

[Al8236]

A hob cuts gear teeth and has no helix, A tap will cut a worm wheel and has a helix. That is the short answer without getting into tooth shapes pressure angles and a whole lot of math, A hob cuts a straight spur gear and a tap cuts an angled concave gear that will run against a worm.

[Ralphxyz]

How did you index the gear? All I see are the pillow block bearings holding the mandrel. Some of the teeth seem to have a concave cut in the center others look good.

Ralph (brand new to machining so really curious)

[Frank S]

Ralph, here are a few unique points about worm gear drives
# 1 there really is not a hob per-say used to make the worm wheel Some call it a hob but actually the cutter that makes the worm wheel is an exact profile of the worm screw itself.
#2 if you are making your own worm drive from scratch you can design your worm screw and wheel pretty much as you like with a few limitations.
#3 you can achieve an extremely high reduction with only 1 worm and wheel.
#4 you can also get a reasonably low ratio as well by creating a 2, 3, 4 or more tooth starts or threads on the worm screw
#5 just like a helical cut gear set you can design a worm gear set to be several degrees off from 90°
#6 you can have multiple worm wheels being run from a single worm screw with shafts leading off at odd angles to each other
#7 the worm screw can have a large diameter while the wheel is quite narrow or you can have the wheel wider than the screw
If re manufacturing an existing wheel then you would first duplicate the screw then cut groves in it to resemble a tap then add in a bit of relief grind chuck you newly made cutter into either a lathe or a mill place your blank on a spindle and cut your gear That is over simplifying it a bit but the grist of the process is there
I left out the part of needing to do the math to determine the number of teeth and in many cases it helps to pre-cut a radius profile then gnash the starter profile of the teeth with a hob and while indexing the wheel effectively making what would look like the beginnings of a spur gear with a concave face