Worm gear

[Gordon]

I am trying to make a worm and worm gear on my lathe using a hob. I can determine the dimensions such as OD, PD etc from the W.M. Berg site or several other sites but I am still confused as to how I determine how deep to cut the "threads" on the hob or rather how to determine when I am deep enough. Basically depth of tooth is addendum plus dedendum. The compound is set at 19° for a 20° pressure angle so other than calculating using trig which seems rather imprecise how do I know when I am deep enough?

I am making a 4" turn table and using 1/10 DP with 72 tooth worm for rotating the turn table. Theoretically the depth should be .072 on the worm and .069 on the worm gear.

In looking at some other videos etc I see some folks start out with a concave surface on the worm gear. Is this necessary or it is just saving time on the cutting? Everything I look at seems to show the finished product but not how we got there.

 

[canadianhorsepower]

I am trying to make a worm and worm gear on my lathe using a hob. I can determine the dimensions such as OD, PD etc from the W.M. Berg site or several other sites but I am still confused as to how I determine how deep to cut the "threads" on the hob or rather how to determine when I am deep enough. Basically depth of tooth is addendum plus dedendum. The compound is set at 19° for a 20° pressure angle so other than calculating using trig which seems rather imprecise how do I know when I am deep enough?

I am making a 4" turn table and using 1/10 DP with 72 tooth worm for rotating the turn table. Theoretically the depth should be .072 on the worm and .069 on the worm gear.

In looking at some other videos etc I see some folks start out with a concave surface on the worm gear. Is this necessary or it is just saving time on the cutting? Everything I look at seems to show the finished product but not how we got there.

this might help

http://www.model-engineer.co.uk/forums/postings.asp?th=88040

 

[RonGinger]

Why do you think trig would be imprecise? If you use a good table, or a good calculator you will get a result more accurate than your lathe dials will measure.

 

[Gordon]

At this point I have made the worm and the hob for cutting the gear by cutting a shoulder on each end of the actual teeth cut to the root diameter of the teeth. When the tool touched the shoulder it also had advanced the compound by the calculated value so that looks to be good. I still have to harden the hob and actually cut the teeth on the gear. If the center to center distance between the gear and the worm come out to be the calculated value I am going to be surprised. I still have options to adjust for a different center distance.

 

[Wizard69]

At this point I have made the worm and the hob for cutting the gear by cutting a shoulder on each end of the actual teeth cut to the root diameter of the teeth. When the tool touched the shoulder it also had advanced the compound by the calculated value so that looks to be good. I still have to harden the hob and actually cut the teeth on the gear. If the center to center distance between the gear and the worm come out to be the calculated value I am going to be surprised. I still have options to adjust for a different center distance.

If you do your calculations correctly your results should be precise beyond what your machines can do.

As for center distances, it is very important to get these right. If you don't you will end up with excessive wear and damage to the gears, this especially so in some ur gears but holds also for worms. Depending upon the gears even a thousands of an inch error in spacing can impact gear life.

The other thing here is backlash which you will likely want to minimize in a rotary table. This is where adjustability is useful.

 

[Gordon]

At this point I have completed the worm and the gear and they seem to be working well. My main problem was trying to keep everything accurate so that center distance was correct. When I cut the gear with the hob I meshed the worm in the gear and then measured the over all dimension on the set. Since I knew the gear OD and the worm OD and the theoretical pitch diameter it was a matter of calculating what the over all should be. Even if the PD on the gear or the worm was off by a few thousands the errors offset each other. The good news is that I learned something in the process. The bad news is that at 76 I will forget it before I need it next time.