Worm Drive Dividing Attachment for Mini Lathe

[old_toolmaker]

I had a need to divide a micrometer dial into 360 divisions so I built a worm drive dividing attachment to fit my 7 X 16 mini lathe. This design is a collection of ideas gleaned from the internet and re-engineered to suit me on my CAD system. (You know how we "Builder Types" always have a perceived need to improve things!)
It has an 8:1 ratio and the steel worm meshes with an idler gear (a plastic mini lathe change gear) which in turn drives the standard metal 45 tooth spindle gear. The index plate has three hole circles having 35, 40 and 45 holes With this arrangement it can index for most all divisions I am likely to need including 360 degrees which is why I built it in the first place. The worm is canted at an angle to better mesh with the plastic idler spur gear. A worm wheel would certainly be better than the plastic spur gear but for a low speed and seldom used device it works well enough.

[rossbotics]

Looks great, I want to see the project your going to make using this devise

Doug

[old_toolmaker]

Hi Doug,

I used the dividing attachment to engrave the lines onto the 360 degree ring on the work head and rotating table of the Quorn Tool and Cutter Grinder I built. I have attached a photo.

[rebuilder1954]

Beautiful work indeed!

i have a question, because i want to build something similar for a spindex. regarding the 8:1 ratio, how is that calculated? the worm looks like a single start, as best i can tell, and counting roughly half the teeth on the spur gear makes it look like a 40 tooth gear, more or less.

Many thanks,

Bob R

[Frank S]

Rob R calculating the ratio of a worm gear drive is done the same way you would calculate any gear train. divide the number of teeth on the worm wheel by the number of teeth on the worm only on a worm the teeth number is the number of tooth or thread starts the most common worms have 1 to 4 starts so to get and 8 to 1 it would mean a worm wheel would have 48 teeth and the worm would have to have 6 starts, if you have an 80 tooth gear and a worm with 1 start you would have an 80 to 1 with 2 starts it would be 40 to 1, with 4 starts 20-1 so to get 8 to 1 without adding an additional gear to a train the worm would have to have 10 starts. Not very practical as the pitch angle of the threads on the worm to match as close as possible the circular or modular pitch of the worm wheel or in your case a spur gear are going to dictate the diameter of the worm plus the angle that the worm must be mounted in relationship to the spur gear
worm gear drives using either spur gears or helical gears and worms instead of a worm wheel have been done for many years but there is a trade off in in contact area and mounting angles

[old_toolmaker]

rebuilder1954,

I made an error stating an 8:1 ratio. What I meant to say is that the worm is 8 threads per inch. That actual ratio in my case is 45:1 (45 turns of the worm = 1 revolution of the spindle). The worm turns the 40 tooth idler gear which does not change the ratio but only the direction. The ratio is determined by the spindle gear vs worm pitch. The circular pitch of or actual tooth spacing for a 1 module gear is .12368". This is very close to .125" so the 8 tpi worm will mesh close enough with the metric module 1 gear. I suppose I could have cut an exact metric pitch but in use I have not noticed any visible error when marking dials One turn of the worm advances the spindle gear by one tooth. I hope this helps clear that up.

[DIYer]

Thanks old_toolmaker! We've added your Worm Drive Dividing Attachment to our Lathe Accessories category,
as well as to your builder page: old_toolmaker's Homemade Tools. Your receipt:

Worm Drive Dividing Attachment by old_toolmaker

tags: lathe, divider, worm gear

[Stevohdee]

Nice work dick, unique idea, well done

[old_toolmaker]

Stevohdee,

Thank you very much for the nice comment.